Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition (1976)

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Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats BEEF CATTLE Protein requirements for beef cattle and general guidelines for urea usage in partially satisfying these requirements are given by the Na- tional Research Council (1970). Urea has been used successfully in diets for beef cattle over the past 35 years, and review articles covering these studies include those by Krebs (1937), Reid (1953), Lewis (1961), Briggs (1967), Loosli and McDonald (1968), Tillman and Sidhu (1969), and Helmerand Bartley (1971). It has been pointed out that, although urea oftentimes is a satisfactory supplement, nevertheless, under some conditions, it is not a satisfactory source of supplementary nitrogen for beef cattle, even though conventional recommendations for urea use are followed. These recommendations include: (1) feeding no more than 1 percent of urea in the diet dry matter consumed, (2) feeding no more than 3 percent in the grain mixture consumed, or (3) feeding no more than one-third of the diet nitrogen as urea nitrogen (Rupel et al, 1943). Helmer and Bartley (1971) state: "Because urea by definition is con- sidered to be 100 percent digestible, feed formulators often consider that all nitrogen in urea is utilized by the ruminant. The nitrogen con- tent of urea is multiplied by 6.25 to get the theoretical protein equiva- lent that urea nitrogen contributed directly and completely towards satisfying the digestible protein requirement of the animal. Obviously, that is not true because we have not yet learned how to utilize all of the nitrogen furnished by urea." 38

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 39 More recently, Burroughs et al. (197 Id, 1974a) proposed a system to quantitate urea utilization. This proposal made use of a new eval- uation of feeds based upon their estimated urea fermentation potential (UFP). The UFP was estimated on the basis of the fermentable energy of a given feed or diet as proposed by Bloomfield et al. (1964) and the amount of the feed or diet protein degraded in the reticulorumen when consumed by cattle. In the discussion of beef cattle feeding experiments that follows, the experiments were grouped on the basis of diets fed having energy densities capable of supporting either submaintenance to low, medium, or high levels of cattle productivity. Each productivity group is discussed, starting with the trial having the unsupplemented diet with the lowest level of protein and proceeding for the most part to the next trial with the unsupplemented diet with the next higher level of protein. This approximate arrangement also appears in Table 1 for the trials reported in which the experimental design included an un- supplemented or negative control diet. The primary purpose of negative control diets in these experiments was to ascertain protein adequacy or inadequacy without supplementation based upon animal performance. Thus, in each experiment where a greater animal response was observed with either urea or protein supplementation, it was assumed that the negative control diet was inadequate in protein and that supplementa- tion was needed to satisfy the animal's protein requirements. SUBMAINTENANCE AND LOW-ENERGY BEEF CATTLE DIETS Experimental evidence indicates that supplementary urea is beneficial to submaintenance as well as low-productivity beef cattle diets with less than 60 percent TON in their DM and composed almost exclusively of low-quality roughages with less than about 7-8 percent protein. This ap- pears to be true whether the urea supplement contains a readily ferment- able carbohydrate such as cane molasses or cereal grains. On the con- trary, results of experiments with higher levels of protein demonstrate the ineffectiveness of supplemental urea in diets with less than 60 per- cent TON. These will be discussed later in this chapter. Altona et al. (1960) in two trials found that supplemental urea im- proved the performance of cattle consuming on a dry matter basis veld forage containing 3.5 percent protein and 54 percent TON. Briggs et al. (1947) fed mature-weathered prairie hay containing 3.5 percent protein on a dry matter basis and observed low or negative nitrogen balances when the hay was unsupplemented. Supplementing the hay with cotton- seed meal or with urea and cottonseed meal approximately doubled the nitrogen balance. Replacing all nitrogen of the cottonseed meal with

40 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition TABLE 1 Summary of Urea Benefits in Beef Cattle Diets Varying in Energy and Protein Content Protein TDN asa% of DM Protein asa% of TDN Was SuppL Urea Useful? Was SuppL Protein Better?" asa% of DM References Low-Energy Diets (TDN < 60) 3.5 54 6. 5 Yes S. Aft. J. Agric. Set 3(1960):69-81 3.5 54 6. 5 Yes S. Afr. J. Agric. Sci . 3(1960):69-81 3. 5 54 6.5 Yes No J. Anim. Set 6(1947):445^»60 3.5 54 6. 5 Yes Yes J. Anim. Sci. 6(1947):445-460 3. 5 54 6. 5 Yes No J. Anim. Sci. 6(1947):445-460 3.5 54 6. 5 Yes Queensl. J. Agric. Sci. 16(1959):223-232 3. 5 54 6. 5 Yes FarmingS. Afr. 47(1971):7-9 3.5 54 6.5 Yes Farming S. Afr. 27(1952):453-454 3.6 54 6.7 Yes Yes J. Agric. Sci. 58(1962):173-178 3.7 54 6.9 Yes Queensl. J. Agric. Sci. 20(1 963):21 3-230 3.7 54 6.9 Yes Queensl. J. Agric. Sci. 20(1963):213-230 3.9 52 7.5 Yes Br. J. Nutr. 16(1962):115-124 4.3 57 7. 5 Yes Queensl. J. Agric. Sci. 18(1961):409-424 4.7 54 8.7 Yes Queensl. J. Agric. Sci. 15(1958b):181-194 4.9 57 8.6 Yes Queensl. J. Agric. Sci. 15(1 958a): 161 -180 5.0 54 9.3 Yes J. S. Afr. Vet. Med. Assoc. 30(1959):457^58 5.0 54 9.3 Yes Farmings. Afr. 33(1957):30-32 5.0 54 9.3 Yes Farming S. Afr. 35(1959):27-29 5.0 54 9.3 Yes Farming S. Afr. 29(1954):135-138 5.5 54 10.2 Yes Yes J. Anim. Sci. 22(1 963): 330-334 6.0 54 11.1 Yes S. Afr. J. Agric. Sci. 3(1960):69-81 6.3 58 10.9 Yes Yes J. Dairy Sci. 43(1 960):443-444 (A) 6. 5 50 13.0 Yes Yes Aust. Vet. J. 24(1 948): 197-204 6.5 50 13.0 Yes Yes Aust. Vet. J. 24(1948): 1 97-204 6.5 58 11.2 Yes Yes Aust. Vet. J. 24(1 948): 197-204 6.7 54 12.4 Yes S. Afr. J. Agric. Sci. 3(1960):69-81 6.7 55 12.2 Yes J. Anim. Sci. 33(1971):133-136 6.7 57 11.8 Yes Yes Miss. Farm Res. 7(1944):8 6.7 58 11.6 Yes Miss. Farm Res. 6(1943):8 6.8 56 12.1 Yes J. Anim. Sci. 8(1949):24-34 7.0 56 12.0 Yes ). Anim. Sci. 8(1949):24-34 7.2 54 13.3 No Okla. Agric. Exp. Stn. Misc. Publ. No. MP-43(1955):51-54 8.0 52 15.4 No Yes Am. Soc. Anim. Prod. Proc. (1939): 404-406 8.0 54 14.8 No Queensl. J. Agric. Sci. 17(1960):135-146 9. 5 54 17.6 No S. Afr. J. Agric. Sci. 3(1960):69-81 9. 5 54 17.6 No S. Afr. J. Agric. Sci. 3(1960):69-81 9.5 54 17.6 No S. Afr. J. Agric. Sci. 3(1960):69-81 Medium-Energy Diets (TDN 60-75) 4.4 60 7.3 Yes Yes Can. J. Agric. Sci. 29(1 949): 173-1 84 4.5 66 6.8 Yes Yes Iowa State Univ. Anim. Sci. Leafl. 5.0 66 7.6 Yes Yes J. Agric. Sci. 59(1 962): 125-141

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 41 TABLE 1 (Continued) Protein TDN Was Protein Suppl. as a % Urea of TON Useful? Was Suppl. Protein Better?" asa% of DM asa% of DM References 5.4 65 8.3 Yes J. Anim. Sci. 35(1972):859-864 5.9 60 9.8 Yes J. Anim. Sci. 35(1972):859-864 5.9 62 9.5 Yes J. Anim. Sci. 35(1972):859-864 6.0 60 10.0 Yes Yes J. Dairy Res. 9(1938):263-272 6.0 63 9.5 Yes Yes J. Dairy Sci. 43(1 960):890 (A) 7.4 70 10.6 Yes Yes Iowa State Univ. Anim. Sci. Leafl. R195(1974):l-9 7.7 74 10.4 Yes Yes Iowa State Univ. Anim. Sci. Leafl. R195(1974):l-9 8.4 69 12.2 Yes Yes Iowa State Univ. Anim. Sci. Leafl. R195(1974):l-9 8.4 70 12.0 Yes J. Anim. Sci. 12(1953):934 (A) 8.4 70 12.0 No Yes Proc. Assoc. S. Agric. Workers 52(1955):64 8.4 70 12.0 No Yes Proc. Assoc. S. Agric. Workers 52(1 955):64 8.9 64 13.9 Yes Yes Aust. Vet. J. 24(1 948): 197-204 9.5 74 12.8 Yes Iowa State Univ. Anim. Sci. Leafl. R195(1974):l-9 9.7 68 14.3 Yes J. Agric. Sci. 64(1 965): 343-350 10.0 67 14.9 No No J. Dairy Sci. 49(1 966):450 (A) 10.1 74 13.7 No Yes Iowa State Univ. Anim. Sci. Leafl. R195(1974):l-9 10.2 73 14.0 Yes Yes Ohio Agric. Exp. Stn. Res. Bull. 766(1955):1-20 10.3 72 14.3 Yes J. Agric. Sci. 64(1965):343-350 10.7 74 14.4 Yes J. Agric. Sci. 64(1965):343-350 11.4 60 19.0 No J. Anim. Sci. 8(1949):24-34 High-Energy Diets (TDN > 75) _ 95 _ Yes Yes Science 153(1966):1603-1614 _ 84 _ Yes Yes J. Nutr. 89(1966):385-391 4.0 82 4.9 Yes Yes J. Nutr. 25(1943):197-202 5.0 76 6.6 Yes Yes J. Dairy Sci. 22(1939):785-798 5.0 77 6.5 Yes Yes J. Dairy Sci. 22(1939):785-798 6.2 78 7.9 Yes Yes Iowa State Univ. Anim. Sci. Leafl. R192(1974b):l-12 6.2 78 7.9 Yes Yes Iowa State Univ. Anim. Sci. Leafl. R173(1973):l-17 7.3 82 8.9 Yes Yes J. Dairy Sci. 43(1 960):890 (A) 7.8 78 10.0 Yes Yes Iowa State Univ. Anim. Sci. Leafl. R192(1974b):I-12 7.8 78 10.0 Yes Yes Iowa State Univ. Anim. Sci. Leafl. R173(1973):l-17 8.3 82 10.1 Yes Yes Iowa State Univ. Anim. Sci. Leafl. PI^^MQTIVl /i IX 1 J J ( 1 ' / 1 J . 1 — f 8.4 77 10.9 Yes Yes J. Anim. Sci. 39(1974):102-107 4.8 100 8.8 Yes J. Anim. Sci. 28(1969):256-262 8.8 76 11.6 Yes Queensl. J. Agric. Anim. Sci. 25(1968):19-28 8.9 77 11.6 Yes Yes Nebr. Agric. Exp. Stn. 63d Annu. Rep. (1950):96

42 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition TABLE 1 (Continued) Was Was Protein TDN Protein Suppl. Suppl. as a % as a % as a % Urea Protein of DM of DM of TDN Useful? Better?" References 9.2 86 10.7 Yes Yes Iowa State Univ. Anim. Sci. Leafl. R152(1971):l-5 9.3 81 11.5 Yes Queensl. J. Agric. Anim. Sci. 25(1 968): 19-28 9.3 83 11.2 Yes Yes Nebr. Agric. Exp. Stn. 62d Annu. Rep. (1949):75 9.6 77 12.5 Yes Yes J. Anim. Sci. 39(1974):102-107 10.0 77 13.0 Yes J. Anim. Sci. 8(1949):24-34 10.5 78 13.5 No Yes Iowa State Univ. Anim. Sci. Leafl. R192(1974b):l-12 10.5 78 13.5 No Yes Iowa State Univ. Anim. Sci. Leafl. R173(1973):l-17 10.5 86 12.2 Yes J. Anim. Sci. 30(1 970): 297-302 10.7 77 13.9 Yes J. Anim. Sci. 8(1949):24-34 11.1 77 14.4 Yes Mich. State Univ. Cattle Rep. 645(1965): 40-47 13.0 76 17.1 No Minn. Univ. Cattle Rep. B-40(1963):l-3 13.0 76 17.1 No Minn. Univ. Cattle Rep. B-53(1964):7-10 13.8 80 17.2 No Yes Anim. Prod. 9(1967):155-165 a Blank spaces indicate no answers were possible due to limitations of experimental design. urea, however, was less effective in increasing nitrogen retention. In a wintering trial, pasturing 2-year-old steers on dry prairie grass, these authors reported equal performance between a cottonseed meal supple- ment and one containing one-third urea and two-thirds cottonseed meal nitrogen. Briggs et al. (1948) found that a low-protein, dry-range grass with a supplement of 1.3 kg of pellets per animal daily with urea pro- viding 25 percent of the nitrogen satisfactorily maintained beef bulls and pregnant cows. There is an abundance of experimental evidence to show that the inclusion of readily fermentable feeds in a urea supplement enhances the effectiveness of the supplement. Clark (1952) fed a mature veld hay low in protein to Tollier cattle. Unsupplemented cattle lost weight, while those supplemented with molasses and urea gained weight. Simi- lar results were obtained by Bishop and Wilke (1971) and Beames (1959). Beames (1960) found that the ratio of urea to molasses could be reduced from 1:8 to 1:2 without loss of effectiveness, if the mixture supplied adequate urea. Von La Chevallerie (1965) investigated the ef- fectiveness of various ratios of urea to molasses in licks placed before three groups of cattle. The results indicated that the ratio of urea to molasses is of importance if the narrow ratio lowers feed intake to the

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 43 extent that the intake of urea is too low. Smith (1962), in a short-term experiment with the feeding of low-quality veld hay containing 3.6 per- cent protein in the dry matter, observed that supplements of either urea or peanut meal increased feed intake and performance in cattle. Beames (1963) found that cattle under 18 months of age could not survive 161 days on a low-protein veld hay containing 3.5 percent pro- tein in the dry matter. Older cattle during the same period survived on the hay alone, but lost weight. Survival in the younger cattle and body weight losses in the older cattle were not materially improved by spray- ing the hay with molasses. However, the addition of urea to the molasses enabled all animals to survive with only slight loss in body weight. Campling et al. (1962) fed nonlactating cows oat straw containing 3.3-4.1 percent protein in the dry matter and daily continuously in- fused into the rumen of each cow 150 g of urea, which was in solution with or without 500 g of sucrose. They found that urea supplementa- tion with or without sucrose increased voluntary intake of straw 40 per- cent and increased organic matter digestibility 41-50 percent. Cotton strings submerged in the rumen ingesta through a fistula opening were digested nearly five times as rapidly in urea-supplemented cattle as com- pared with unsupplemented animals. These results were confirmed by Pieterse and Lesch (1963). Ryley (1961) fed sorghum silage unsupplemented and supplemented with two levels of urea to heifers in late pregnancy and early lactation. The unsupplemented diet contained an estimated 4.3 percent protein and 57 percent TON on a dry matter basis. After 24 weeks of feeding, the unsupplemented cattle lost more weight than the supplemented ones. Morris (1958a) also found that cattle performance was improved when either urea or crushed sorghum grain was added to the diet of hay containing 4.7 percent protein in its dry matter. In another study Morris (1958b) fed Hereford heifers a sorghum silage diet containing an estimated 4.9 percent protein and 57 percent TON on a dry matter basis. During the 28-week feeding period, animals without supplementation lost weight, while those receiving urea gained. Barrie and Clark (1959) confirmed the earlier work of Clark (1952), which indicated that urea and molasses supplementation improved the diets of cattle consuming a low-protein veld hay. Cattle without sup- plementation lost weight, while those supplemented gained. Bishop (1957, 1959) pastured groups of 2-and 3-year-old oxen on mature and weathered winter veld pasture of low-protein content. Cattle without supplementation lost more weight than those with supplementation. Cattle-feeding experiments by Clark and Barrie (1954) with veld hay containing 5.5 percent protein in the dry matter and supplemented

44 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition with 1.8 kg of ground cobs containing urea resulted in live-weight gains compared with no gains in unsupplemented cattle. Raleigh and Wallace (1963) mixed ground hay that contained 5.5 percent protein with dif- ferent amounts of urea, cottonseed meal, or a mixture of the two, to supply 6, 9, and 12 percent protein. The complete mixtures were then pelleted and fed to similar groups of cattle. Ammonia toxicity and death losses occurred with the 12 percent protein pellet containing only urea and no cottonseed meal. However, the lower urea levels promoted better live-weight gains than obtained in control animals. Gains were highest in animals fed cottonseed meal as the only sup- plemental nitrogen source. Intermediate live-weight gains occurred when the supplemental nitrogen was 50:50 urea and cottonseed meal. Altona et al. (1960) reported benefits from urea supplementation of veld forage containing 6 and 6.7 percent protein and 54 percent TON on a dry matter basis. Browning et al. (1960) reported a daily dry mat- ter intake of 0.91 percent of body weight in cattle receiving sorghum silage compared with 1.11 percent in animals fed 160 g of urea daily with silage estimated to contain 6.3 percent protein and 58 percent TON on a dry matter basis. When McClymont (1948) fed wheat straw with and without small additions of cereal grains, he found that urea supplementation in diets containing 6.5 percent protein and 50-58 percent TDN improved cattle gains. Similar benefits from urea supplementation were noted in re- search by Chicco et al. (1971) when they fed a green-chop elephant grass containing 6.7 percent protein and an estimated 55 percent TON in the dry matter. , Cullison (1943) fed control cows a sweet sorghum silage and John- songrass hay. The diet was estimated to contain 6.7 percent protein and 58 percent TON on a dry matter basis. The experimental cows were fed the same diet, except that 0.5 percent urea was added to the silage at the time of ensiling. After 78 days, the control cows had lost weight, while those receiving the treated silage had maintained their body weight. Means (1944, 1945), using similar diets, confirmed Cullison's results. In two metabolism trials, Dinning et al. (1949) fed a maintenance diet containing an estimated 56 percent TON and 6.8-7.0 percent pro- tein in the diet dry matter and found that urea additions increased ni- trogen balances. The performance of cattle consuming the many diets presented up to this point have been improved by urea supplementation. Most, if not all, have been improved more by protein supplementation. All the diets before supplementation were estimated to contain less than about 7 per-

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 45 cent protein and less than 60 percent TON in the diet dry matter, sup- porting submaintenance or low cattle production. By contrast, a num- ber of diets containing this low level of energy and more than 7 percent protein have been investigated, and these have not responded to urea supplementation. For example, a thick stand of native grass in Okla- homa containing 7.2 percent protein on a dry matter basis was sprayed by Pope et al. (1955) with a mixture of six parts cane molasses and one part urea during late summer. The cattle gained slightly less weight over a 62-day period on the sprayed, as compared with the unsprayed, grass. Work and Henke (1939) fed a diet containing an estimated 8 percent protein and 54 percent TON in the dry matter and received no signifi- cant benefits from urea supplementation. Similar results were obtained byO'Bryan(1960). Altonaetal. (1960) fed eragrostis hay containing 9.5 percent protein and found that urea supplementation did not improve cattle perfor- mance. In one trial no significant differences in gains were observed with 1- and 2-year-old steers fed for 85 days on eragrostis hay with licks containing molasses with or without urea. In the same trial, steers fed eragrostis hay sprayed with a solution of urea and molasses performed little differently than when a urea-molasses lick was used. In two addi- tional trials, cattle were fed eragrostis hay with additions of silage or urea, which was dissolved in water and sprayed on the hay. Neither sup- plement improved gains. In the final trial, four molasses solutions with 0-275 g of urea per 1 kg were sprayed on the eragrostis hay. No urea level produced greater gains than found in the controls. Verbeek and von La Chevallerie (1960) also supplemented eragrostis hay with either urea, peanut meal, or guano meal and found no significant differences in cattle gains. Several experiments have been reported comparing urea with protein supplementation for submaintenance and low-energy beef cattle diets in which no supplemented control diets were fed. Cattle performance in most of these experiments (Murray and Romyn, 1939; Nelson et al., 1957; Berry et al., 1958; Davidson and Purchase, 1961; Nelson and Waller, 1962;Kreft, 1963;Hornand Beeson, 1969;Williamsetal, 1969; Varner and Woods, 1970) was satisfactory but with some small advantage toward protein supplementation. In summary, cattle performance was improved when urea was added to submaintenance and low-productivity diets having less than 7 to 8 percent protein in their dry matter. Such diets contain principally for- ages such as mature and weathered pasture grasses, cereal grain straws, corncobs, corn stover, and silages. The addition of readily fermentable energy sources such as the low-protein cereal grains or molasses to these

46 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition low-productivity diets further improved animal performance when urea was added. MEDIUM-ENERGY BEEF CATTLE DIETS Medium-energy beef cattle diets are defined here as those containing 60 to 75 percent TON in the dry matter. Composition of the diets used in the research reports reviewed were quite varied. However, they ranged from high-roughage feeds with small additions of cereal grains to high- silage (whole plant sorghum and corn) feeds with additions of cereal grains. These medium-productivity diets, which contained less than 7 to 8 percent protein in the dry matter would be expected to be bene- fitted by urea supplementation, since the lower-energy diets reviewed in the preceding section with this amount of protein were benefitted by urea supplementation. This proved to be true in each of the follow- ing research reports reviewed (see Table 1). Watson et al. (1949) fed calves a diet consisting of timothy hay, oat straw, barley grain, cornstarch, and cane molasses. The diet TON and protein percentages of dry matter were 60 and 4.4, respectively. The calves receiving the basal diet failed to gain, while those receiving the basal diet supplemented with urea gained 99 kg in about 1 yr. Another group supplemented with casein gained 142 kg. Burroughs et al. (1971b) fed light-weight steer calves a basal diet containing 66 percent TON and 4.5 percent protein and composed of corncobs, corn grain, cane molasses, and beef tallow. The calves receiv- ing the basal diet lost weight, whereas similar calves fed the basal diet supplemented with urea gained 25 kg in 50 days and calves supple- mented with herring and soybean meals gained twice as much. Coombe and Tribe (1962) fed a basal diet estimated to contain 66 percent TON and 5 percent protein in the dry matter and composed of fresh sugarcane plus a small amount of corn grain. Cattle receiving the basal diet lost weight, contrasted with the cattle receiving the basal diet supplemented with urea. The latter gained a moderate amount, while cattle receiving protein (alfalfa hay) gained substantially. Chiccoetal. (1972) compared three unsupplemented diets-mature chopped Guinea grass, hay, and molasses-with three similar diets to which urea was added. The protein in the three unsupplemented diets was 5.4, 5.9, and 5.9 percent, and the TON content varied between 60 and 65 percent by feeding a constant quantity of molasses with differ- ing quantities of hay. Highly beneficial results from urea supplementa- tion were observed with each diet. Bartlett and Cotton (1938), in one of the earliest reported cattle ex-

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 47 periments with urea, fed heifers a 6 percent protein and 60 percent TON basal diet on a dry matter basis and obtained small live-weight gains. An- imals receiving a similar diet supplemented with urea gained a moderate amount, whereas those receiving a protein supplement gained more. Brown et al. (1960a) fed heifers a diet composed of corncob-corn silage and containing 6 percent protein and 63 percent TON on a dry matter basis. When the diet was supplemented with either urea or soybean meal, it was found that the urea-supplemented group gained well, but those receiving the soybean meal made 11 percent greater gains. Vetterand Burroughs (1974) fed five basal diets containing 7.4, 7.7, 8.4, 9.3, and 10.1 percent protein and from 69 to 74 percent TON to young growing calves initially weighing about 160 kg. Each diet was supple- mented with an equal amount of nitrogen from urea or protein, except the 9.3 percent protein diet, which received only the urea supplement. Protein supplementation stimulated live-weight gains with all diets tested and to a greater extent than that observed with the urea supple- ment. However, urea supplementation resulted in substantial stimula- tion in all diets except the highest protein diet (10.1 percent before supplementation). Van Arsdell et al. (1953) fed fattening steers corn silage ad libitum and supplemented it with either soybean meal or urea. The diet on a dry matter basis was estimated to contain 70 percent TON and 8.4 percent protein. Daily live-weight gains were 10 percent greater in the group receiving the soybean meal supplement. Goode et al. (1955) fed four lots of heifer calves and four lots of mature pregnant cows (1) an unsupplemented corn silage diet estimated to contain 70 percent TON and 8.4 percent protein on a dry matter basis, (2) unsupplemented corn silage treated with 0.5 percent urea, (3) corn silage supplemented with 0.23 kg soybean meal daily per ani- mal, and (4) urea-treated corn silage with 0.23 kg of soybean meal added. No benefits were obtained from the urea treatment of the silage. The addition of supplementary soybean meal, however, resulted in significantly faster live-weight gains in both the cows and heifers. Robertson and Miller (1971) supplemented a corn silage diet fed to beef calves with levels of urea nitrogen varying from 0 to 28 percent of the total supplemental nitrogen. Although no unsupplemented negative control diet was fed, the authors concluded that urea could be effec- tively utilized up to 20 percent of the supplemental nitrogen in this type of diet. McClymont (1948) fed a half-concentrate, half-wheat-straw basal diet to beef calves. The diet contained an estimated 64 percent TON and 8.9 percent protein on a dry matter basis. A second group of calves

48 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition received supplementary urea to make a 14 percent protein diet, while a third group received supplementary protein to make a 14 percent diet. The respective live-weight gains on the three diets were progressively larger, starting with the first diet. Various ratios of sorghum silage to sorghum grain (60:40, 40:60, 20:80) with and without urea supple- mentation were fed to yearling Hereford steers by Morris and O'Bryan (1965). The estimated TON percentages of the three diets were 68, 72, and 74, and the estimated protein percentages were 9.7, 10.3, and 10.7, respectively, on a dry matter basis. Average daily live-weight gains and improved feed conversions were appreciably improved by urea supple- mentation of each diet. Pope et al. (1959) fed a sorghum grain and silage finishing diet con- taining either a soybean meal or a molasses-urea supplement to heavy yearling steers. No negative control diet without supplementation was fed. The diets, before supplementation on a dry matter basis, were estimated to contain 73 percent TON and 9 percent protein. The cattle receiving the soybean meal supplement made daily live-weight gains 10 percent greater than those receiving the urea supplement. Three diets were fed to heifers by Lassiter et al. (1958a). The diets were composed of ground corncobs and grain mixtures and contained either 3, 5, or 7 percent protein equivalent from urea. Each diet, after supplementation, was approximately isonitrogenous and isocaloric, con- taining on a dry matter basis an estimated 62 percent TON and 9.2 per- cent protein. Daily live-weight gains were highest with the diet containing the least urea and were lowest with the diet containing the largest amount of urea. The diet containing intermediate amounts of protein and urea supported an intermediate rate of gain. Finishing diets containing sorghum grain and silage were supple- mented by either soybean meal or urea by Sellers et al. (1960). The diets, before supplementation, contained an estimated 10.3 percent protein and 73 percent TON in the dry matter. The urea-supplemented cattle gained almost as much (95 percent) as the soybean-meal-supple- mented cattle. In a companion experiment, in which the sorghum silage was replaced with cottonseed hulls, the urea-supplemented cattle made appreciably smaller daily live-weight gains as compared with the soy- bean-meal-supplemented cattle. This diet had a lower TON (70 per- cent) and protein (9.4 percent) content than did the sorghum grain silage diet. Bond and Oltjen (1973) studied growth and reproductive perfor- mance of beef females fed high urea-containing diets over long feeding periods. They reported that urea-fed cows performed satisfactorily and almost as well as cows receiving soybean meal supplementation.

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 49 Reddy et al. (1961) fed diets estimated to contain 65 percent TON and 9.4 percent or more protein composed of equal parts grass hay and isonitrogenous concentrate mixtures containing 0, 9, 18, or 36 percent of their nitrogen as urea nitrogen. Although no negative control diet was fed, they concluded that urea was satisfactorily used since cattle performance was approximately similar in each case. Considering the high level of protein in relation to TON in their diet, this conclusion may or may not be valid. Approximately the same situation seems to have existed in an ex- periment reported by Kirk et al. (1963), in which pangola grass hay and silage were supplemented with either cottonseed meal or a mixture of cottonseed meal, urea, and citrus pulp. The performance of the cattle was similar with the two supplements; but, again, the urea-containing diet had an estimated 9.6 percent protein and 71 percent TON content on a dry matter basis. Therefore, there was probably no need for any supplemental nitrogen in the diet. Kirk (1952) supplemented yearling steers grazing native pasture with either cottonseed meal or citrus pulp meal and urea and obtained similar live-weight gains. The protein con- tent of the pasture was not indicated. Bentley et al. (1955), in a 3-year study with steer calves, fed a corn silage and hay diet in which half the corn silage was treated with urea levels of about 1 percent at the time of ensiling. In one trial with un- treated and unsupplemented corn silage, steer calves gained moderately well while consuming the basal diet containing 10.2 percent protein and an estimated 73 percent TON on a dry matter basis. Steers fed a similar diet supplemented with urea or soybean meal gained 7 and 16 percent greater, respectively. In another trial, 190-kg steers fed the untreated silage and hay diet supplemented with soybean meal outgained by 13 percent cattle receiving a similar diet supplemented with urea. When the urea added to the silage at ensiling time and the urea added at the time of feeding were not considered, this diet contained 10.7 percent protein and an estimated 71 percent TON on a dry matter basis. These authors in a third trial obtained rather similar gains in steers fed urea- untreated or treated silage and hay diets when each diet was supple- mented with urea. The untreated silage diet, before the urea supple- mentation, contained 9.8 percent protein and an estimated 73 percent TON on a dry matter basis. Davis et al. (1944) found that sorghum silages containing no urea and silages with 4.5 kg of urea added per 908 kg were consumed similarly by cattle. However, the cattle refused silages containing 13.6 kg of urea per 908 kg. Woodward and Shepherd (1944) reported that adding 0.5 percent urea to corn silage slightly lowered the acceptability of the diet

SO Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition for mature cows. In a review of eight Ohio experiments, Klosterman et al. (1963) found that, in half the experiments, cattle receiving lime- stone-urea-treated corn silage gained at a faster rate than those receiving untreated silage; in all experiments, feed requirements per unit of gain were lessened by the treatment. Essig (1968), in a review of 26 experi- ments, stated that there seemed only a slight benefit from adding urea at the time of ensiling as compared with adding urea as a supplement to feeds. Brown and Jacobson (1966) fed young heifers a diet composed of equal parts orchardgrass hay and one of three concentrate mixtures containing either 1.06 percent urea, 7.88 percent soybean meal, or 2.09 percent urea. The diets, when urea and soybean meal were not considered, contained an estimated 10 percent protein and 67 percent TON on a dry matter basis. Live-weight gains were similar, as were digestibility data collected on the three diets. Urea supplementation of a basal diet fed to 2-year-old steers by Dinning et al. (1949) resulted in no additional nitrogen retention in steers. On a dry matter basis, the diet contained an estimated 11.4 percent protein and 60 percent TON. Briggs et al. (1947) fed a diet estimated to contain 12.4 percent protein and 65 percent TON on a dry matter basis before supplementation. A urea supplement gave equal performance to a protein supplement. In summary, all medium-energy diets reviewed in this section except two were improved by urea supplementation provided they contained less than 10 percent protein on a dry matter basis or less protein than 12.0-14.5 percent of diet TDN. The two exceptions may have been due to ensiling methods, since the urea was added in each case prior to en- siling. Medium-energy diets, when benefitted by urea, were always bene- fitted more by protein when they were isonitrogenous and approxi- mately isocaloric. HIGH-ENERGY BEEF CATTLE DIETS High-energy beef cattle diets, for the purposes of this discussion, are those diets containing 75 percent or more TON in their dry matter. These diets have ample energy for supporting maximum live-weight gains or milk production in beef cattle. They contain considerable concentrate feeds. Many experiments conducted with these high-productivity diets have not included a negative control basal diet and have only compared the effects of urea supplementation versus those of protein. Such experi- ments are valuable in measuring differences in supplements, but they

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 51 may fail to measure whether a supplement had a negative influence, a positive influence, or no influence at all. In the case of growth or lacta- tion trials, only an experimental design containing a negative control diet without urea can be relied upon to measure whether or not bene- fits accrue from urea supplementation. The purified diets of Virtanen (1966) illustrate the positive benefits obtained from urea when added to a high-productivity lactation diet. In the many experiments con- ducted by Virtanen (1966) with diets essentially devoid of protein, but containing supplemental urea and energy equivalent to 95 percent TON on a dry matter basis, daily milk production over a 12-month period commonly averaged about 11 kg per animal. This amount of milk would be sufficient for beef cows rearing one to two calves and could not have been produced for a long period of time had not the urea been effectively utilized. However, this level of production was only about two-thirds that obtained in dairy cows (Virtanen, 1967) when they were placed on natural diets containing protein, thus suggesting that urea was not used as completely as was protein. Rys (1967) pointed out that: "Cows with a low or average milk production (from 2,000 to 3,000 kg of milk annually) are best fitted for the full utiliza- tion of urea. . . ." Benefits from urea were obtained by Oltjen and Putnam (1966) with young growing Aberdeen Angus steers fed a semipurified diet essentially devoid of protein and supplemented with urea containing about 84 per- cent TON on a dry matter basis. Nitrogen balance data indicated sub- stantial benefits from the urea, but animal performance was about two- thirds of that obtained when protein was fed as the nitrogen source. Oltjen and Bond (1967) reported that two beef cows reproduced suc- cessfully when raised on a purified diet containing urea as the only source of dietary nitrogen. In at least 26 experimental trials reported in the literature, cattle were fed high-productivity diets in which negative control diets devoid of urea were fed. Benefits from urea resulted in all experiments when the basal diet contained less than 11 or 12 percent protein on a dry matter basis or less than 13.5-14.5 percent on a TON basis (Table 1). Two such ex- periments representing the earliest experiments conducted with cattle in the United States were reported by Hart et al. (1939). In one experi- ment, a basal diet containing 5-6 percent protein and an estimated 76 percent TON in the dry matter was fed to young calves. The animals receiving the basal diet made a small live-weight gain, while the calves receiving urea gained 25 percent faster and calves receiving casein gained still more rapidly. In the second experiment, a similar basal diet com- posed of corn grain, cornstarch, cane molasses, and timothy hay with

52 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition an estimated 5 percent protein and 77 percent TON in its dry matter was fed. Calves fed the basal diet did not gain, while urea-supplemented calves gained moderately and the casein-supplemented calves gained more rapidly. Loosli and McCay (1943) fed a corn grain, cornstarch, cane molasses, and timothy hay basal diet containing an estimated 4 percent protein and 82 percent TON on a dry matter basis to young calves. Calves re- ceiving the basal diet barely maintained live weight, while the calves receiving the basal diet plus urea gained moderately and the calves re- ceiving the basal diet plus protein gained most rapidly. Brown et al. (1956) fed a basal diet composed of corn, oats, com- starch, cane molasses and timothy hay plus liquid milk to young calves. The dry diet contained 7.3 percent protein and 82 percent TON. Calves receiving this basal diet gained moderately; calves receiving urea in addi- tion to the basal diet gained 6.7 percent faster; and calves receiving lin- seed meal added to the basal diet gained most rapidly. Burroughs et al. (1971a,c), in two trials with steers, fed corn basal finishing diets containing an estimated 8.3 and 9.2 percent protein and 82 and 86 percent TON, respectively, on a dry matter basis. The cattle receiving the basal diets gained medium well, while the cattle fed the basal diet supplemented with urea gained faster and those supplemented with protein gained still more rapidly. An experiment reported by Putnam et al. (1969) made use of a basal diet composed of ground corn, cane molasses, and soybean oil that con- tained an estimated 100 percent TON and 8.8 percent protein on a dry matter basis. Urea supplementation failed to improve live-weight gains during the last 63 days of the finishing period, when 6.3 kg of corn dry matter were consumed daily per animal. Feed requirements per unit of gain, however, were improved 3 percent, even though the urea-supple- mented diet contained a smaller percentage of TON in the dry matter than that present in the unsupplemented diet. Thurbon and Winks (1968) fed cattle two diets composed of varying amounts of corn forage and corn grain with and without urea supple- mentation. The diets on a dry basis were estimated to contain 8.8 and 9.3 percent protein and 76 and 81 percent TON, respectively. Urea proved beneficial to each of the diets. Dowe et al. (1950) fed a corn basal diet containing an estimated 8.9 percent protein and 77 percent TON in the dry matter to yearling cattle. When this diet was supplemented with urea, the cattle gained 17 per- cent faster; and when it was supplemented with cottonseed meal, the cattle gained 27 percent more rapidly than the basal cattle. Baker et al. (1949) fed unsupplemented, supplemented-with-urea,

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 53 and supplemented-with-soybean-meal diets composed of corn grain and whole-plant corn silage to steer calves. The unsupplemented cattle gained the least, and the supplemented cattle gained 8 and 10 percent more rapidly. The unsupplemented diet contained an estimated 9.5 per- cent protein and 83 percent TON on a dry matter basis. Dinning et al. (1949) fed two basal diets containing an estimated 10 percent and 10.7 percent protein and 77 percent TON on a dry matter basis. Nitro- gen balance was increased by feeding supplemental urea in each of two metabolism trials. Clark et al. (1970) fed cattle a corn-orchardgrass-hay finishing diet estimated to contain 10.5 percent protein and 86 percent TON on a dry basis. Urea supplementation in one trial was not beneficial, but in an- other it was. Similarly, in one trial soybean meal supplementation was superior to urea, but not in the other. Newland and Henderson (1965) compared corn silage treated with 0.5 percent urea and 0.5 percent limestone with untreated silage fed to heifers in a diet containing corn grain and protein supplement. The un- treated silage diet contained an estimated 11.1 percent protein and 77 percent TON in the dry matter. The cattle receiving the urea-treated silage diet gained 7 percent more rapidly than those receiving the un- treated silage. The degree to which limestone contributed to this response was not measured. Greathouse et al. (1974) reported two trials with sorghum grain basal diets containing 8.4 and 9.6 percent protein, respectively, and estimated TON contents of 77 percent. Urea supplementation was beneficial in both trials, but also less beneficial than soybean meal supplementation. In six additional trials in two experiments reported by Burroughs et al. (1973, 1974b), basal diets containing 6.2, 7.8, and 10.5 percent pro- tein and 78 percent TDN were compared with similar diets supple- mented with equal amounts of urea or protein nitrogen. Each basal diet was benefitted by protein supplementation and by a larger amount than that observed with urea supplementation. No benefits occurred from urea supplementation when the diet before supplementation contained 10.5 percent protein on a dry matter basis, but benefits from urea were observed in all lower protein diets. Three additional trials have been conducted with higher protein diets in which no benefits from urea supplementation occurred. Harvey et al. (1963) compared whole-plant corn silage with and without urea addi- tions at the time of ensiling in steer calves fed a corn grain, alfalfa- brome hay, and linseed meal diet in addition to the silage. The diet without urea in the silage contained an estimated 13 percent protein and 76 percent TON in the dry matter and slightly outperformed the

54 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition diet containing urea added to the silage. Harvey et al. (1964) repeated the experiment with identical results, except that the cattle receiving the control diet with 13 percent protein outperformed by a greater extent the cattle receiving additional urea. In a third high-protein diet, Stobo et al. (1967) fed a high concentrate-to-hay basal diet to young calves. The diet contained 13.8 percent protein and an estimated 80 percent TON on a dry matter basis. The calves receiving urea gained no more than calves receiving the basal diet. However, the basal diet was deficient since the young calves were substantially benefitted by protein supplementation. Turning to the beef finishing trials with high-energy diets where urea supplements were compared with protein supplements in which no unsupplemented control diet was fed reveals that in 10 trials (Baker, 1944; Briggs et al., 1947; Culbertson et al, 1950; Gallup et al, 1953; Klosterman et al, 1964; Kolari et al, 1963; Perry et al, 1967; Lowrey and McCormick, 1969; Muller et al, 1971; and Thompson, et al, 1972) urea proved to be less effective than protein supplements. There was, however, no difference in cattle performance between urea and protein supplementation in nine other trials (Briggs et al, 1947; Culbertson et al, 1950; Johnson et al, 1955; Harvey et al, 1962,1963, 1964; Jordon et al, 1965; Oltjen et al, 1974; Schmidt et al, 1974) where no negative control diets were fed. However, it is observed that in eight of these nine trials, the level of protein in the diets compared was rather high, ranging from 12 to 15 percent of the dry matter fed, suggesting the probability that in many of these trials, if a negative con- trol diet had been included, no need for supplemental protein would have been demonstrated. Where negative control diets were fed, all but two of the tested high- productivity diets containing 75 percent or more TON in the dry matter were benefitted by urea supplementation, provided the protein content of the dry matter did not exceed 11-12 percent or 14.5 percent TON. No high-productivity diet containing 13 percent or more protein was im- proved by urea supplementation in these more critical trials. Protein sup- plementation of nearly all high-productivity diets was superior to urea supplementation when the unsupplemented basal diet contained less that 11-12 percent protein on a dry matter basis. One obvious reason why a unit of supplemental urea nitrogen should be somewhat inferior to a unit of supplemental protein nitrogen is that only about 80 percent of the total urea nitrogen converted into rumen microbial nitrogen is alpha amino protein (Hungate, 1966). Therefore, any supplemental alpha amino protein that escapes rumen degradation will yield more alpha amino protein postruminally than an equivalent

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 55 amount of supplemental urea nitrogen converted to microbial proto- plasmic nitrogen. A second, less-obvious, but more important reason why urea nitrogen has sometimes proved inferior to supplemental pro- tein in the past has been a failure to recognize the more important characteristics (natural protein and fermentable energy) within each cattle diet that govern urea utilization as described by Satter and Roffler (1973) and Burroughs et al. (1974a). When these more impor- tant considerations are recognized, then it is possible to largely over- come small differences between the value of a unit of urea nitrogen compared with a unit of supplemental protein nitrogen. This can be done by feeding a slightly larger amount of the former, as compared with an equivalent amount of the latter type of supplement or by over- supplementing with each type with respect to animal protein needs. The results of Varner and Woods (1970) illustrate many of these princi- ples, in which a feeding system was employed in transforming a urea supplement giving inferior performance into one as beneficial as a pro- tein supplement. They used a starter protein supplement for several weeks until TDN consumption was sufficiently high for adequate rumi- nal urea conversion into microbial protein in satisfying body protein re- quirements. At this time, they abruptly switched to the urea supplement for the major part of the total feeding period. Without the use of the starter supplement, cattle performance with urea was inferior; but with the starter supplement, subsequent urea usage was as beneficial as pro- tein supplementation. Less-important diet considerations, such as sulfur deficiency, rumen microbial adaptability, and poor palatability characteristics of urea, doubtlessly exert some influence upon utilization; but the amount ap- pears to be relatively minor in the case of beef cattle. SUMMARY AND CONCLUSIONS This review of more than 100 beef cattle reports revealed an orderly picture of benefits from urea supplementation when the experiments were grouped in accordance with the energy and protein levels present in the unsupplemented negative control basal diets. The types of basal diets with respect to their energy content were as follows: (1) Sub- maintenance and low-energy beef cattle diets; (2) medium-energy beef cattle diets; and (3) high-energy beef cattle diets. Submaintenance and low-energy diets are defined as those containing less than 60 percent TON. Such diets are composed largely of forages such as hays and silages, range grasses and pastures, straws, corncobs, and stovers. These diets were benefitted by urea with or without readily

56 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition available carbohydrate being in the supplement, provided less than 7-8 percent protein was in the diet dry matter and provided the diet before supplementation had insufficient protein to meet body needs. On the contrary, low-energy diets were not benefitted by supplemental urea if their dry matter contained more than 7-8 percent protein or their pro- tein level exceeded about 14 percent of diet TON. Medium-energy diets are defined as those containing between 60 and 75 percent TON in the dry matter, and often they contain mixtures of forages and grain. Supplemental urea regularly improved most of these diets with or without readily available carbohydrates being present in the supplement, provided less than 10 percent protein was in the diet dry matter and provided the diet before supplementation had insuffi- cient protein to meet the cattle's body needs. On the contrary, medium- energy diets were not regularly benefitted by supplemental urea if their dry matter contained more than 10 percent protein or the protein level exceeded 12.0-14.5 percent of the TON. High-energy diets are defined as those containing more than 75 per- cent TON in the dry matter and large amounts of concentrate feeds. Urea supplementation was helpful in such diets, provided the protein level did not exceed 11-12 percent in the dry matter or 14.5 percent TON and provided the protein level in the diet before supplementation was sufficiently low to require supplementation in meeting body re- quirements. Contrarily, high-energy diets with 13 percent or more pro- tein in the dry matter were not benefitted by urea supplementation. Despite the many beef cattle trials reviewed where supplemental urea proved beneficial, the overall conclusion is that in the past a unit of urea nitrogen most often has not been equal or equivalent in feeding value to a unit of supplemental protein nitrogen. This conclusion is based upon 41 out of 44 observations reviewed in Table 1 in which a unit of urea nitrogen failed to yield benefits equivalent to a unit of protein nitrogen. The degree of failure varied most often from 10 to 30 percent, but in six trials it amounted to 100 percent, or total failure. Recognition and understanding of this lack of nitrogen equivalent between supplemental urea and protein become important in future beef cattle feeding stan- dards to better appraise NPN usefulness in meeting body amino acid requirements. They also call attention to further needed research to im- prove urea or other NPN supplements that can contribute even more significantly in the future than in the past to alleviating short world supplies of protein feedstuffs.

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 57 DAIRY CATTLE Reviews that have included some discussion of urea in dairy cattle feed- ing are those by Reid (1953), Tomme (1963), Rys (1967), Becker (1967), Loosli and McDonald (1968), Huber et al. (1968a), Chalupa (1968, 1970), Conrad et al (1969), Helmer and Hartley (1971), and guidelines given in the National Research Council (1971b) publication on dairy cattle. Reid (1953) concluded that for maintenance and production of lac- tating cows, diets containing up to 27 percent of the total nitrogen as urea were equal to diets containing protein as supplementary nitrogen. Urea nitrogen below this concentration had no adverse effects on re- productive performance, milk composition, or general health. Reid (1953) proposed that safe concentrations of urea equaled up to 3 per- cent of the concentrate mixture or 1 percent of the total diet for lac- tating cows. In a European review, Rys (1967) points out that utilization of urea nitrogen occurs when basal diets are deficient in protein. If a sufficient supply of amino acids, polypeptides, and protein to meet the require- ments of the rumen microflora is provided, urea utilization is depressed. Rys (1967) concluded that cows with a production level of 2,000- 3,000 kg of milk annually are best suited for the utilization of urea and that high producers fail to utilize urea efficiently; therefore, he did not recommend urea for their diets. Zein, a protein of corn, is less exten- sively degraded in the rumen than some other plant proteins; and Rys (1967) feels that the success with urea feeding in the United States and USSR stems from the large amounts of corn fed in these countries. The average annual milk production in the United States now exceeds 4,000 kg per cow, and some herds average more than 10,000 kg. There is also a well-established positive relationship between production per cow and income for labor. Therefore, urea in lactating dairy cow diets is acceptable only if high production rates are maintained for extended periods of time. The feeding of urea is justified only if it will reduce feed costs. Under present price conditions, which often prevail in the United States, least- cost diet formulation usually includes urea in the diet unless it is spe- cifically excluded or if protein supplementation is unnecessary. UREA IN DIETS FOR YOUNG DAIRY CALVES The National Research Council (1971b) recommendation for total pro- tein in calf starters is 16 percent of the total diet dry matter, with a

58 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition daily gain of 750 g per day. Brown et al. (1956) compared the following pelleted calf starter diets: (1) basal negative control, 7.4 percent pro- tein;* (2) basal plus 3 percent urea; and (3) basal plus linseed meal fed to calves from 2 to 86 days of age with medium-quality timothy hay. Milk was phased out from 21 to 49 days of age. The negative control diet depressed consumption and daily gain, but calves fed the urea or linseed meal diets gained equally and significantly faster. Brown et al (1960b) also compared pelleted starter diets containing 1.1, 2.2, and 3.3 percent urea added to a negative control of 7.1 percent protein. Daily gains were highest in calves fed the diet containing 2.2 percent urea. Nitrogen retention was higher in all urea-fed calves than in the controls. Results indicate that urea was being used by 5 weeks of age. Stobo et al. (1967) fed calves a low-protein hay plus various starter diets for 9 weeks, starting at 3 weeks of age. The diets were: (1) basal 13.3 percent protein, (2) basal plus skim milk, and (3) basal plus 2.8 percent urea. Intakes of the starter diets and hay did not differ among the treatment groups. Daily gains averaged 0.59 kg for the skim milk diet, which was significantly greater than the 0.45 and 0.48 kg for the low-protein and urea diets, respectively. Fish meal was compared to urea (1.6 percent) plus oats, urea (3 percent) plus oats, and mixed salts (primarily ammonium acetate) of volatile fatty acids (Kay et al., 1967a). The salts were given in the drinking water. Bull calves weaned at 28 days were the experimental animals. The diets contained 19 percent crude protein, and dicalcium phosphate was included in the urea-containing diets to equalize calcium and phosphorus contents. All diets were offered when the calves were 10 days of age. Intake and gain were recorded from weaning when weights averaged 50 kg until the average was 110 kg. Daily gain and feed efficiency were reduced when urea and ammonium acetate re- placed fish meal. The possibility that the branched-chain fatty acids (BCFA), isobutyric and isovaleric (plus valeric), might be limiting growth in calves fed urea- containing diets was examined by Miron et al. (1968). Thirty-two Hoi- stein calves were assigned to four starter diets of 20 percent crude pro- tein in which comparisons were: (1) soybean meal (SBM), (2) SBM +BCFA, (3) urea (1.9 percent), and (4) urea (1.9 percent + BCFA). The diets were fed free-choice, starting when the calves were 2 weeks old and ending when they were 12 weeks old. All diets were eaten readily by the calves, and the gains of calves fed SBM alone or SBM +BCFA diets were greater than those fed the urea-containing diets. As both the "Composition data expressed on an air-dry basis was converted to a dry matter basis through- out, assuming a 90 percent dry matter content if no dry matter data were given.

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 59 starter diet and the hay contained 20 percent protein, it is doubtful whether the urea nitrogen was needed. Naylor and Leibholz (1970) used sorghum grain plus urea to replace meat meal in four calf diets. Levels of urea were 0, 1.33, 2.67, and 3.9 percent, which supplied urea as a percent of dietary nitrogen of 0, 20.1, 39.2, and 55.6 percent, respectively. Sodium sulfate was added to ob- tain a nitrogen:sulfur ratio of 11:1. The concentrate portions averaged 21.9 percent protein and were offered free-choice, while cottonseed hulls were offered separately up to a level of 17 percent of total feed intake. A milk replacer was fed for 5 weeks. Between 5 and 11 weeks of age, highest gains were made on the 2.67 percent urea diet averaging 0.81 kg per day for the 6-week trial. Total feed intake was lower in calves fed no urea than in those fed the two lowest levels of urea. Low- est nitrogen balance occurred in the diet containing 3.9 percent urea. The authors suggested that their excellent response to urea could have been due to the higher sulfur content of their diets. D. K. Nelson (1970) compared three calf starters in which SBM, urea (2.7 percent), and SBM plus urea (1.35 percent) served as the nitrogen supplements in isonitrogenous diets, which contained about 18 percent protein. Calves were assigned to these diets from 4 through 84 days of age. A limited whole-milk feeding program was used, but no forage was fed. Daily gains were greater for calves fed the diets containing SBM alone and SBM plus urea than for those fed the diet with 2.7 percent urea. D. K. Nelson (1970) also assignee, calves to three diets as follows: (1) SBM as the only supplemental nitrogen, (2) SBM plus urea (1.35 per- cent), and (3) SBM in a diet having only 14.3 percent protein to serve as a negative control. Diets 1 and 2 contained about 18 percent protein. Calves fed the urea and 18 percent SBM diet gained 0.60 kg/day; nega- tive controls gained 0.52 kg/day (P< 0.07). Studies on the use of urea in calf starters can be divided into those that obtained results comparable to those found on conventional pro- tein supplements (Brown et al, 1956, 1960b; Naylor and Leibholz, 1970; L. F. Nelson, 1970) and those that found a lower response when urea was fed (Kay et al, 1967a; Stobo etal, 1967; Miron etal, 1968). Brown etal. (1960b) and D. K. Nelson (1970) fed no separate forage, although the pellet fed by the former contained 30 percent ground timothy hay. In these studies, milk feeding varied from 28 days (Kay etal, 1967a) to 49 days (Brown etal, 1956). Urea concentration in the diets is another variable that may explain some of the differences. Stobo etal. (1967) fed 2 percent urea, while Kay etal. (1967a) fed 1.6 and 3 percent and Miron et al (1968) 1.9 percent urea. Although Naylor and Leibholz (1970) obtained the greatest response with 2.6 per-

60 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition cent urea, D. K. Nelson (1970) observed the greatest response at 1.35 percent urea. Naylor and Leibholz (1970) felt that their successful results with urea diets were due to supplemental sulfur. Use of negative controls and positive responses from urea additions by several authors (Brown et al., 1956; Naylor and Leibholz, 1970; D. K. Nelson, 1970) demonstrates the use of urea by young calves. It is suggested that calf starters fed with limited milk feeding pro- grams can contain from 1 to 1.5 percent urea and give satisfactory results if a period of adjustment is provided while milk is being with- drawn from the diets. UREA IN DIETS FOR HEIFERS The minimum total protein required by growing dairy heifers is given (National Research Council, 1971b) as 10 percent of the total diet dry matter at a daily gain of 750 g per day. Hart et al. (1939) at Wisconsin conclusively showed that young dairy males and females used urea for growth during two 40-week studies. Neg- ative control diets containing 6 and 7 percent protein and supplemental diets of urea, ammonium bicarbonate, and casein at 20 percent protein in one trial and graded levels of urea from 1.4-4.3 percent in a second trial raised total diet protein levels to 19.4. Although some overfeeding of protein was apparent, growth response and carcass analysis clearly demonstrated that urea was used by these cattle, although not as effi- ciently as casein. The widely used guidelines of 1 percent of the total diet or 3 percent of the grain mixture appears to stem from this work. Work and Henke (1939) confirmed the Wisconsin work with a 52-week study, and Mills et al. (1944) showed that addition of casein or starch to a molasses-urea-timothy diet sharply increased daily gains. Bohman et al. (1954) determined the value of urea and corn for re- placing soybean meal in growing heifers fed cane molasses and poor- quality timothy hay. The supplements were: (1) molasses and urea; (2) corn and soybean meal; (3) molasses and soybean meal; and (4) molasses, corn, and urea. The supplements were fed once per day, and those fed the soybean meal diets gained more than those fed the urea diets. Little use was made of urea in a diet of poor forage and molasses. Substitution of corn for molasses gave only a small increase in gains. Merrill and co-workers (1959) compared cane molasses and ear corn meal as energy supplements to early and late-cut hays fed with urea and soybean meal as sources of protein for growing dairy heifers. Sixty-six heifers were used in each of 2 years in continuous trials of 24 weeks to

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 61 measure growth response. Corn silage and the dry concentrates were fed at the morning feeding, while hay with molasses poured over it was fed in the afternoon. Substitution of cane molasses for corncob meal on an equal TON basis resulted in equal gains by dairy heifers, although the group fed molasses ate more hay. Those fed the molasses- and urea- containing diets made lower gains than those receiving molasses and soybean meal or those fed corncob meal and soybean meal. Bates et al. (1960) fed two mixtures that contained urea, molasses, water, and minerals with and without ethyl alcohol as essentially the sole source of digestible protein for dairy heifers. Ground corncobs fed ad libitum and separately were the only roughage. During a 140-day trial, gains were low and were not affected by ethyl alcohol in the diet. Two Jersey heifers were continued on the diet for an additional 438 days, and these animals grew, reproduced, and lactated. Employing a concentrate diet of 3 percent urea (0.129 percent S) and a low-protein hay (0.065 percent S), Jones and Haag (1946) observed an improvement in growth response of dairy heifers when the grain mix- ture was supplemented with 1 percent sodium sulfate. Brown et al. (1960a) also obtained increased gains in dairy heifers fed urea-containing supplements when sodium sulfate was used to reduce the N:S ratio from 22:1 to 16:1. As in other studies, gains by groups fed urea supplements were lower than those fed supplements containing soybean meal as the supplemental nitrogen source. A 118-day continuous feeding trial was used by Martz et al. (1964) to measure the growth response of 40 Guernsey heifers that were fed corn silage ad libitum and the following daily supplements: (1) 1,362 g of soybean meal, (2) 681 g of soybean meal, (3) 1,362 g of urea (8 per- cent) mixture, and (4) 681 g of a urea (8 percent) mixture. Groups fed diets 2 and 4 received 681 g of a standard grain to equalize energy intake from supplements, and the supplements were spread twice daily on top of the silage. Dry matter consumption was not affected by the level of the supplements. However, the soybean meal supplement promoted faster gains than urea. J. R. Campbell et al. (1963) fed dairy heifers soybean and urea sup- plements twice versus six times daily. Excellent and similar gains were made by the heifers fed the soybean meal supplement either twice or six times daily and when the urea supplement was fed six times per day. Gains of heifers fed the urea supplement twice daily were lower. Flet- cher et al. (1968) compared the effects of feeding urea-corn silage (0.5 percent urea) and a low-protein grain mixture two versus four times per day. Forty Guernsey heifers were assigned to the four treatments in a 74-day continuous trial. Feeding urea-corn silage four times daily in-

62 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition creased daily gains, and the animals required 14 percent less TON to produce a unit of gain. Some of the protein needed by growing heifers can be supplied by urea, and better results seem to be obtained when complete diets or urea forages are fed ad libitum-in which consump- tion can take place several times during the day. Webb and colleagues (1963) found that dairy cows exposed to hay and silage in a loose hous- ing system ate hay 8 times and silage 11 times during a 24-h period. Corn silage provides a high-energy forage base for the effective use of urea. Only a modest level of urea (0.5-1.0 percent in the total diet) is needed with corn silage or other forage of similar protein content to meet protein requirements. UREA IN DIETS FOR LACTATING COWS Experiments reported with lactating cows are frequently difficult to interpret. Adaptation and adjustment seem to be important when urea- containing diets are fed, yet short-term changeover designs have been used by many workers. Also, negative controls have not been used in many studies. These factors are further complicated by the small num- ber of animals employed. Furthermore, some researchers have used cows that were in low production. Because of the capacity of high- producing dairy cattle to mobilize both energy and nitrogen from body reserves for milk production, nitrogen balance trials are helpful to establish the nutritional status of lactating cows fed urea-containing diets. It is also important to describe the specific conditions of the study, such as the number of times per day animals are fed and char- acterization of the diets with respect to their content of major and trace minerals and other factors. For these reasons, primary emphasis will be given to those studies of longer duration and to some with ni- trogen balance data. Total-diet protein-requirement levels for lactating cows is given by the National Research Council (1971b) as 14, 15, and 16 percent for milk production levels of < 20 kg, 20-30 kg, and > 30 kg per day, respectively. A high-producing cow (600 kg of body weight) consuming dry matter at a level of 3.5 percent of body weight with 1 percent urea in the diet would ingest 210 g of urea in a 24-h period. This compares to a 400-kg steer fed a finishing diet of primarily corn plus an NPN supplement that provides about 60 g of urea/day. The early experiment of Archibald (1943) is one of the more conclu- sive studies on urea utilization by dairy cows, because the study ex- tended over 3 years. Urea at 3 percent of the grain mixture supplied 25 percent of the diet nitrogen and was compared to soybean and cotton-

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 63 seed meals as supplementary nitrogen sources in both changeover and continuous-type trials. The urea-fed cows ate grain less readily, ex- hibited similar changes in body weight, similar lactation length, simi- lar dry periods, and nearly identical reproductive performance to those fed protein. During two lactations, the cows fed urea continuously averaged 18.1 kg/milk/day and the control cows 18 kg/day. An ad- justment interval was apparent in double reversal trials. Distillers' grains, brewers' grains, and urea were compared as pro- tein supplements during two lactation studies by Loosli and Warner (1958). Urea was fed at a level of 3 percent in the concentrate mixture. The first week of the 6-week experimental period was considered tran- sitional; data were used from the last 5 weeks. The urea-containing diet appeared to be less acceptable, as the cows ate it more slowly. However, total concentrate intakes did not differ among the treatments. Milk production averaged 19.4, 19.1, 18.9, and 18.2 kg/day, respectively, during both years for the cows fed diets containing distillers' grains, brewers' grains, urea, and the low-protein control. When the concen- trate mixtures contained 1.5-2.0 percent urea, plus either distillers' or brewers' grains, the diets were completely acceptable. Using a 4 X 4 Latin square design, Colovos et al. (1967) compared high-quality concentrate mixtures that contained 0, 1.25, 2, and 2.5 percent urea in diets of Holstein cows that were in early lactation. All diets contained a good-quality timothy hay. Diet effects were nonsig- nificant on dry matter intake, dry matter digestibility, milk production, milk composition, and molar proportions of rumen acetic, propionic, and butyric acids. Milk production was relatively high (ca. 27 kg/day). However, digestible and metabolizable energy levels were lower in the two highest urea diets. An 18-month split herd comparison was made by Holter et al. (1968b) on the production and reproductive performance of high- producing Holstein cows fed concentrate mixtures containing 0 or 1.5 percent urea. Soybean meal, corn gluten feed, and corn distillers' dried grains in the control mixture were replaced by hominy and urea to equate the two mixtures in energy and nitrogen. A maximum of 11.3 kg of concentrate was fed with excellent hay, hay crop silage, and corn silage plus some seasonal pasture. Although there was little difference in the acceptance of the two grain mixtures, several cows were slow in starting to eat and then ate the urea-containing concentrate more slowly. Milk production averaged 7,319 and 7,306 kg per 305-day lactation for the urea and control groups, respectively. Conception rates averaged 1.6 and 1.9 services per conception in the urea and control groups, respec- tively. The total diet protein level appears to have been about 15 percent

64 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition during early lactation. A comparison of the difference between the ac- tual and predicted milk production of both groups during the first and second months of the study revealed a significant interaction between treatments and milk production; the urea-fed cows produced less than predicted during the first month and more later, thereby lending further support to the idea that some adaptation or adjustment is necessary when cows are first fed a urea-containing feed. This is one of the more useful field studies on the use of urea in practical diets of dairy cows. Complete diets containing 25 percent bagasse and 75 percent concen- trates were used to compare urea with urea plus tuna fish meal (TFM) as nitrogen supplements for lactating Holstein cows (Randel, 1970). Diet 1 contained 9 percent TFM and 1.5 percent urea, diet 2 contained 4.75 percent TFM and 2.25 percent urea, and diet 3 contained 3 per- cent urea. The diets contained 15.9, 15.9, and 16.8 percent protein, respectively. The animals were fed the diets during a 35-day preliminary period, a 7-day adjustment period, and a 105-day comparison period. Feed consumption and milk production were both depressed nearly 4 kg/day by increasing levels of urea. Ramage and Woolf (1970) fed three complete rations containing 70 percent grain and 30 percent hay with urea at levels of 0, 0.8, and 1.6 percent to Holstein cows in a 29- week continuous trial. Time for adaptation was provided by including a low level of urea in diets fed during a 6-week postpartum preliminary period. The feeds were offered ad libitum during the first 6 weeks, and thereafter the amount of feed was reduced weekly at the rate of 0.18 kg/day. Milk production was 22.8, 26.5, and 25.1 kg/day with the cor- responding additions of urea. UREA AND CORN SILAGE Because corn silage is high in digestible energy but relatively low in pro- tein, NPN addition is indicated. Brigl and Windheuser (1931) added urea to corn at ensiling. They felt that this procedure might avoid toxicity and show whether silage bacteria would use urea nitrogen to form mi- crobial protein. Their results indicated that about 60 percent of the urea remained unaltered and that the remaining appeared to be present as ammonium salts. Interest in adding urea to corn at ensiling in the United States devel- oped during World War II (Wise et al, 1944; Woodward and Shepherd, 1944). Both added 0.5 percent urea to the green forage at ensiling. Woodward and Shepherd (1944) fed the urea-corn silage with low- protein concentrate and hay to cows for 100 days in a single reversal experiment. Another group of cows received the same level of urea in

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 65 the concentrate mixture. Method of feeding urea had no effect on pro- duction. Wise et al. (1944) reported that urea-corn silage was slightly less acceptable than untreated corn silage as the sole forage for lactating cows. Davis et al. (1944) and Cullison (1943) added urea to sorghum silage at levels of 0.5, 1.5, and 2.5 percent of the green forage weight at en- siling. Free ammonia was observed in the silage that contained 2.5 per- cent urea, and cattle refused to eat the silage until the free ammonia had disappeared; however, there was no problem of acceptance when the silage contained 0.5 percent urea. Most studies since 1944 have dealt with urea additions to corn at en- siling. Conrad and Hibbs (1961) fed cows that received only urea-corn silage (0.7 percent urea) and found that they only used 8.1 percent of their nitrogen intake for milk secretion and retention in body tissue, compared to 22 percent for a group that received alfalfa hay and grain. The poor use of urea nitrogen may have been caused by a low energy intake, and the addition of grain to provide additional readily ferment- able carbohydrate in the urea-containing diets might have given im- provement. Huberetal. (1967) reported three experiments conducted with 91 lactating cows fed diets containing corn silage in which urea provided from 0 to 48 percent of total nitrogen. A 3-week preliminary period was followed by 12-week periods. In experiment 1, equal levels of ni- trogen were provided by: (1) a grain mixture that provided 16 percent protein, (2) soybean or cottonseed meal, (3) equal nitrogen from the oilseed meals and urea, and (4) urea. Milk production was depressed from about 18.5 to 13.5 kg/day by treatment 4, but it was felt that the effect of urea was confounded by the diet energy levels. In experiment 2, three levels of energy and three levels of urea were fed; urea provided 21 and 38 percent of the dietary nitrogen, and the nitrogen supplements were mixed with the corn silage immediately be- fore feeding. Total milk production ranging from 16 to 24 kg/day and persistency were increased by additional energy and depressed by in- creasing urea level. In the third experiment, concentrate mixtures (18 percent protein) containing 0, 1.1, and 2.2 percent urea that provided 0, 10, and 20 percent dietary nitrogen, respectively, were compared. One kg of concentrate was fed per 3 kg of milk. A 7-day nitrogen bal- ance trial was conducted near the end of the experiment. The 2.2 per- cent urea level depressed milk production from 22.9 to 21 kg/day, but did not reduce silage or concentrate intake, although the concen- trate portion was eaten slowly. As in the two previous studies, the cows were unable to use a high level of urea when fed corn silage as

66 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition the only forage. Urinary nitrogen increased, and total milk nitrogen decreased as a percentage of intake with increasing levels of urea. It is significant that in all three experiments, milk yields were lower when- ever urea provided more than 20 percent of the total dietary nitrogen. Holter et al. (1968a) found no depressing effect on feed intake and milk production (average 26 kg/day) when high-quality multi-ingredient concentrate mixtures containing up to 2.5 percent urea were fed with corn-silage forage. Their highest level of urea provided about 300 g of urea per day. Nitrogen balance was positive for all treatments but lower in the urea-fed cows. Urea increased rumen ammonia nitrogen from about 15 mg to 30 mg/100 ml fluid during the first hour after feeding. The concentrate mixture contained seven major energy sources, which were believed to contribute to the high feed intake and excellent per- formance. In contrast, the mixtures used by Huber et al. (1967) con- tained only three energy sources. Also, the calcium and phosphorus levels in the concentrate mixes used by Holter et al. (1968a) were higher than those fed by Huber et al. (1967). These factors may offer an explanation for the differences in results. Dietary levels of other mineral elements should also be equated when urea and grain replace an oilseed meal in dairy cow diets. Polan et al. (1968) used a 70-day continuous trial to compare corn silages containing 0, 0.5, and 0.75 percent* urea as the only forage for lactating cows fed concentrates at the rate of 1 kg/3 kg of milk. The diets were isocaloric and isonitrogenous. Level of urea did not influ- ence silage intake, total feed intake, or milk production of about 19 kg/day. In a second trial, whole-plant corn forage that had been en- siled with 0, 0.6, or 0.85 percent urea was fed ad libitum as the only forage to lactating cows in a 63-day trial. Again, no significant differ- ences in intake or milk production were noted. Blood urea nitrogen levels tended to increase as the level of dietary urea increased. Nitrogen balance data revealed that the cows fed the 0.85 percent urea-corn silage were in negative nitrogen balance because of low protein digesti- bility and high urinary nitrogen losses. These results emphasize the need for production trials of long duration, negative controls, and balance data for evaluating the effect of urea on milk production. Corn ensiled with no additive contains a variable amount of NPN that may account for 40-50 percent of the total nitrogen (Johnson et al, 1967; Huber et al, 1973). Corn silage with 0.5 percent urea added at ensiling contained 30-40 percent of the total nitrogen as ammonia and urea (Johnson et al, 1967; Huber et al., 1968a; Huber and Thomas, 1971). Corn forage that contained 30, 36, and 44 percent dry matter was *Urea additions are expressed on a wet forage basis.

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 67 ensiled with and without 0.5 percent urea (Huber et al, 1968b). Con- centrate mixtures containing 16.2 and 22 percent protein (dry basis) were fed with the control silages; the urea-containing silages were fed with a 16.2 percent protein concentrate. The silages were fed ad libitum, and the concentrate portion was fed at the rate of 1:2.5 kg of milk for 80 days. Increasing silage dry matter depressed milk yields from 23.8 to 23 kg/day. Urea additions apparently increased consumption of silage dry matter, and milk persistency appeared to be better in cows fed urea- treated silages. Feed intake was lower for cows fed the urea-treated 44 percent dry matter silage. Since milk production of the cows fed un- treated silage plus 16.2 percent protein concentrate was similar to those receiving the 22 percent protein concentrate or urea-treated silage, it was impossible to determine the extent of urea nitrogen use because the negative control cows may have been depleting body protein reserves during the trial. Urea (0.5 percent) was also added to whole-plant corn that contained 32 or 48 percent dry matter (Van Horn et al, 1969a) at ensiling. The silages were fed ad libitum to 18 Holstein cows, which received 2.3 kg of hay/day plus a concentrate mixture (1 percent urea) for 63 days. Milk production was lower in cows fed the high dry matter silage, even though feed consumption was not affected. These results and others support the idea that urea should not be added to whole-plant corn containing more than 40 percent dry matter. The well-matured corn plant may contain more than 50 percent grain. Consequently, corn silage made from whole corn is in reality a mixture of forage and grain. This fact led Boman et al. (1969) to study restricted concentrate sup- plementation with corn silage ad libitum. Diets compared were: (1) corn silage plus a 20 percent protein grain supplement that was fed at a ratio of 1:3, (2) corn silage plus 45 percent cottonseed meal (CSM) fed at a ratio of 1:9, and (3) urea-corn silage (0.5 percent) plus CSM and shelled corn at 1:9. Twenty-four cows were fed the silages (36 per- cent dry matter) ad libitum during the 16-week continuous trial. Dif- ferences among treatment groups were small in total dry matter intake, but there was a trend in favor of groups fed restricted concentrate and more urea with respect to milk production (ca. 19 kg/day) and weight gain. Consumption of the urea-corn silage was depressed during the first month of the study. This subject deserves much more attention in high- producing cows early in lactation. To obtain further information on levels of NPN that can be used by lactating cows, Van Horn et al. (1969b) employed a 4 X 3 factorial de- sign with four levels of urea (0, 82, 160, and 232 g/day) and three levels of protein for lactating cows. Four urea levels were obtained by using

68 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition a combination of corn silage, with and without 0.5 percent urea, and a concentrate with and without 1.5 percent urea. A 5-day nitrogen bal- ance and digestion trial was conducted at the end of the 70-day trial with one-half the 24 cows. Treatments did not affect milk production, milk composition, or body weight change. The average milk production during the 70-day trial was 19.5 kg/day at the start, but declined to 12.5 kg/day during the nitrogen balance trial. All cows were in positive nitro- gen balance during the metabolism phase. However, negative nitrogen balance may have occurred during the early feeding period, and the animals adjusted production accordingly. Some nitrogen reserve exists, and part of this reserve can apparently be mobilized for milk production without depressing production. Protein levels of 9, 13.7, and 20.4 percent were used in concentrate mixtures based upon shelled corn and soybean meal (Huber and Thomas, 1971). Whole-plant corn was ensiled with (1) no additive, (2) 0.5 per- cent urea, (3) 0.75 percent urea, and (4) 0.75 percent urea plus 0.17 percent CaSO4 and fed as the only forage. The diet combinations and some of the results of the 70-day continuous trial are shown in Table 2. The five highest-producing cows from each group were used in a 7-day nitrogen balance trial following the feeding period. Increased produc- tion on diets B and C compared to the negative control again shows that cows use urea in corn silage for milk synthesis when protein is limiting. High yields were obtained on diet E, which contained corn silage (0.5 percent urea) plus a 13.7 percent crude protein concentrate, and diet F, which contained control silage plus 20.4 percent crude protein con- centrate. These results indicate that under some conditions cows are able to use urea when it is included in corn silage as well as they use nitrogen from diets without added urea. Knott et al. (1972) used two intensive, continuous experiments to examine the ability of lactating cows to use urea nitrogen. Cows were adjusted to urea previous to the study. In the first experiment, urea was not well used in either 12, 17, or 22 percent protein concentrate mixes (1.5 percent urea) fed with corn silage as the only forage. Ni- trogen balance confirmed the milk production indication that urea was not being used to an appreciable extent. In the second experiment, higher production throughout, negative controls, and nitrogen partition evidence confirmed that cows were using urea nitrogen when some of it was carried in the corn silage and some in the concentrate. Urea in corn silage stimulated both dry matter consumption and milk produc- tion over that of the negative controls. The reason(s) for difference in responses between these two experiments is not apparent, but higher production in the second experiment demanded more protein.

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70 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition Van Horn et al. (1967) assigned three groups of eight cows each to the following diets: (1) control com silage plus concentrate (19.3 per- cent protein, no urea), (2) urea-corn silage (0.5 percent) plus concen- trate (13.2 percent protein, no urea), and (3) urea-corn silage (0.5 percent) plus concentrate (14 percent protein, 1 percent urea). The corn silages were offered at levels up to 27.2 kg/day, and hay was fed at 2.3 kg/day. Concentrates were fed to meet energy needs of the cows. The diets were essentially isonitrogenous. The forages were fed twice and concentrate three times per day for 80 days. Feed consumption, milk production of ca. 25 kg/day, and body weight changes did not differ among treatments. It appears that when urea is included in both the silage and grain more dietary urea can be used without decreasing the acceptability of the concentrate portion of the diet. The feeding of 2.3 kg of legume hay daily and frequent feedings of the concentrate mixture may have contributed to the successful use of urea in this study. Continuing a dual approach, Van Horn et al. (1969a) used a 28-day adaptation period to precede an 84-day experimental period in feeding cows. A urea-containing silage (0.5 percent) and a concentrate mixture (1 percent urea) or a control diet (no urea) were fed. All cows received 2.3 kg of legume hay/day, as in the previous trial. Milk production was depressed by urea, even though total feed intakes were similar. How- ever, the cows ate more of the urea-containing silage while reducing their concentrate consumption. Differences between this and the pre- vious study were in the dry matter level of the silage. In this trial, it was about 44 percent vs. 32 percent in the previous trial. These results further support the recommendation to avoid adding urea to corn forage of more than 40 percent dry matter. OTHER ADDITIVES COMPARED TO UREA FOR CORN AT ENSILING Schmutz et al. (1969) compared corn silages that contained (1) no ad- ditive, (2) 0.57 percent urea, (3) 1 percent diammonium phosphate (DAP), (4) 0.5 percent CaCO3, (5) 0.5 percent urea plus 0.5 percent CaCO3, and (6) 0.5 percent CaCO3 plus 1 percent DAP as the forage source for Holstein cows in a 90-day continuous trial. Grain was fed to equalize energy and protein intakes when silage consumption was 35 kg/day. In a second experiment, corn silage was fed with (1) no addi- tive, (2) 0.5 percent urea, (3) 0.75 percent urea, (4) 0.5 percent CaCO3, (5) urea plus 0.5 percent CaCO3, and (6) 0.75 percent urea and 0.5 per- cent CaCO3. In the first experiment, cows fed corn silage containing DAP ate less silage and produced less milk (ranging from ca. 14-18

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 71 kg/day) than cows on other treatments. In the second experiment, cows fed the 0.75 percent urea silage ate less silage. So either DAP at 1 per- cent or urea at 0.75 percent may depress silage intake. In some geographic regions, the least costly form of NPN is anhy- drous ammonia. Michigan workers (Huber and Santana, 1972; Huber et al, 1973) have pioneered in using ammonia solutions as NPN addi- tives to whole-plant corn at ensiling for dairy cattle. Huber and Santana (1972) compared urea (0.5 percent) and aqueous ammonia (0.28 per- cent ammonia and 3 percent water) as additives for 35 percent dry matter whole-plant corn at ensiling to a control silage as the sole forage. Four groups of seven lactating cows each were fed the silages, and con- centrate mixtures were formulated to provide both negative and positive controls. Feed intakes and milk yields of cows fed the urea silage, am- monia silage, and positive control diets were all similar and significantly greater than for the cows fed the negative control diet. Milk production averaged 25.7 kg/day for the cows on the experimental diets compared to 19.4 kg/day for the negative controls. More water-insoluble nitrogen and higher lactic acid was present in ammonia-treated silage than in urea-treated silage. In further extensive work, Huber et al. (1973) compared no additive, urea at 0.5 and 0.75 percent, urea plus minerals at 2 percent and several ammonia solutions at 2-4 percent as NPN fortification for corn silage of low dry matter (ca. 30 percent) and high dry matter (42-52 percent). Addition of ammonia solutions to 52 percent dry matter silages or at 4 percent depressed silage lactic acid content and milk production from 23 to 18.4 kg/day. Similarly, urea added to 42 per- cent dry matter forage depressed milk production as shown previously. Highest milk production of about 27 kg/day was obtained with the am- monia silages. The ammoniated silages again contained more lactic acid and water-insoluble nitrogen than the urea-treated silage, but the latter contained more water-insoluble nitrogen than the control. In view of the price advantage of ammonia over urea, this form of NPN warrants more research. UREA ADDITIONS TO BARLEY AT ENSILING Barley silages that had been ensiled with 0 and 0.55 percent urea were fed with three concentrates (no urea) that contained protein levels of 10.2, 14.1, and 18.8 percent to 30 lactating Holstein cows in a con- tinuous trial (Polan et al., 1970). Nitrogen balances were determined midway through the trial. The concentrate portion of the diet was de- signed to provide 85, 100, and 115 percent of the nitrogen requirements

72 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition of cows fed the control (no urea) silage and 100, 115, and 130 percent of the nitrogen requirements of cows fed the urea-barley silage. Neither urea addition nor concentrate protein level affected the ad libitum con- sumption of silage dry matter. Differences in milk production, analyzed as a percentage of pretreatment production, were significantly less for cows fed urea-barley silage than the two higher protein concentrates. However, urea met some of the nitrogen requirements; and dry matter, protein, and acid detergent fiber digestibilities were increased by urea. Nitrogen balance data revealed that urea in the silage increased nitrogen retention, but only when fed with low-protein concentrates. These re- sults lend emphasis to the concept that only if nitrogen is limiting can urea be used for a productive function. UREA IN LIQUID SUPPLEMENTS FOR DAIRY CATTLE In earlier reviews Loosli and McDonald (1968), Wornick (1969), Cop- pock (1969), and Huber(1972) discussed the use of liquid supplements in cattle feeding. However, there are few studies that relate specifically to dairy cattle. Owen et al. (1943) replaced the nitrogen of blood meal with a urea- plus starch mixture in diets for lactating Ayrshire cows. The mixture supplied 25 percent of the total dietary nitrogen requirements. The total mixed ration was fed twice daily, and no palatability problems were encountered. Though production was low (ca. 13 kg/day), nitrogen balance data indicated use of urea. Balch and Campling (1961) com- pared: (1) a low basal control, (2) urea in molasses plus ethyl alcohol plus phosphoric acid, (3) urea plus molasses plus phosphoric acid, and (4) groundnut meal as nitrogen supplements. They concluded that if the basal diet was low in protein and contained large amounts of starch, the nitrogen of urea could be used almost completely by dairy cows. Dietary alcohol did not increase the utilization of urea, which agrees with other results. A liquid supplement containing both urea and ammonium polyphos- phate was compared to soybean meal in complete feeds that contained 70 percent corn silage and 30 percent concentrate (wet basis) by Van Horn et al. (1969a). Liquid-supplement-fed cows produced less milk and lost more weight. In further work Van Horn and Mudd (1971) compared soybean meal to two levels of ammonium polyphosphate plus urea dissolved in cane molasses. The liquid supplement was fed at two levels to supplement a basal diet composed of pelleted concen- trate (1 percent urea) and corn silage ad libitum. The liquids were spread over the corn silage, and it was found that the higher level de-

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 73 pressed milk production from 22.2 kg/day to 20.8 kg compared to 22.5 kg for soybean meal (SBM). Liquid supplements increased plasma urea nitrogen and reduced gains compared to soybean meal. In a second trial, complete diets containing 77 percent corn silage and 23 percent concentrate were compared (wet basis). The concentrate por- tion included urea, soybean meal, or liquid supplement as the primary nitrogen additive. The latter provided about 135 g of urea equivalent per cow per day. Cows fed the diet containing soybean meal ate more total feed and produced about 1.5 kg/day milk more than cows fed either added NPN. Cows fed soybean meal also had significantly lower plasma urea nitrogen levels and greater body weight gain than those fed NPN. Huber (1972) compared liquid and dry NPN supplements added to corn forage at ensiling, or the same supplements added at feeding, and found no detectable milk production differences due to NPN form or place of addition. These results and others indicate that NPN supple- ments fed in liquid form are comparable to similar supplements in dry form for milk production. SULFUR AND OTHER MINERAL SUPPLEMENTATION OF UREA DIETS Jones and Haag (1946) observed a growth response in dairy heifers fed low sulfur hay plus grain with 3 percent urea and 1 percent sodium sulfate. Lassiter et al. (1958a,b) and Brown et al. (1960a) also observed a growth response to a sulfur supplement with dairy heifers fed high levels of urea and low-sulfur forage. Other studies with sulfur have given inconsistent results. This probably indicates that the level of sul- fur in the basal diets was sufficient for the production levels achieved. Davis et al. (1954) obtained no increase in milk production when sodium sulfate (0.25 percent) was added to a urea-containing concen- trate (2.3 percent urea). However, the animals' diets were relatively high in sulfur, giving a nitrogen to sulfur (N:S) ratio of about 5-8:1. Based upon published values (National Research Council, 197la) for nitrogen and sulfur, it can be shown that the use of a 0.5 percent urea- corn silage and 1 percent urea in simple concentrate mixes of practical diets for dairy cattle can result in N:S ratios of 18-20:1. The National Research Council (1971b) requirement for sulfur by lactating cows is given at 0.2 percent of the total diet, which implies an N:S ratio of 12:1 for medium-producing cows (15 percent protein in the total diet dry matter). Moreover, Allaway (1969) pointed out that a downward trend in sulfur concentration in crops is likely because of increasing use of fertilizers that contain low levels of sulfur and emphasis on environ- mental quality.

74 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition Jacobson etal. (1967) obtained a milk production response from a sodium sulfate addition that increased the sulfur level to 0.18 percent in a complete diet, but no response was obtained by Huber and Thomas (1971) with a calcium sulfate addition that narrowed the N:S ratio from 20:1 to 14:1. A similar change in the N:S ratio with sodium sulfate did not produce a change in milk production or feed intake in the studies of Grieve et al. (1973). Recent balance trials with lactating cows (Bouchard and Conrad, 1973) indicate that the requirement for sulfur lies between 0.12 and 0.18 percent of the total diet dry matter and that sulfur is highly available from sodium sulfate, calcium sulfate, and magnesium sulfate. The broad area of mineral supplementation of urea diets needs much more research. For example, the addition of cobalt to urea-containing diets increased gains in steers (Bentley et al, 1954) and seemed to in- crease diet acceptance by dairy cows (Bowstead and Fredeen, 1948). It would be helpful if authors would more completely define the mineral composition of the diets fed. For example, calcium in the corn plant is accumulated rather early in its development, and about 85 percent is found in the stalk and leaf (Chandler, 1960). As the plant matures, starch fills the kernel and the calcium content as a percentage of the whole plant declines. Morrison (1956) lists 0.35 and the National Re- search Council (197la) lists 0.30 as the percentage of calcium in the dry matter of corn silage. Recent emphasis on harvesting a more mature plant for silage has resulted in corn silage (Johnson and McClure, 1968) that may have a calcium content of less than 0.20 percent. Conse- quently, it appears that some workers may have fed diets that were borderline in calcium and diets that contained less calcium than phos- phorus. Listing the mineral contents of diets used will make future in- terpretation of such experiments more useful. NEW PRODUCTS CONTAINING UREA Ohio workers (Conrad and Hibbs, 1968; Conrad et al., 1969) reported that a pelleted mixture containing 66 percent dehydrated alfalfa, 31.6 percent urea, 2 percent dicalcium phosphate, and 0.4 percent sodium metabisulfite or sodium propionate makes a very effective substitute for soybean meal in diets of lactating cows. When cornmeal and oats were combined with about 9 percent of this pellet, a concentrate mix- ture containing 19 percent protein and 2.84 percent urea was obtained. During a 305-day lactation test, cows fed the urea-dehy pellet concen- trate averaged 6,965 kg of milk compared to 7,013 kg by cows fed a soybean meal containing concentrate (Conrad et al, 1969). Also, cows

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 75 fed the urea-dehy pellet received up to 40 percent of the dietary ni- trogen from urea. Forage was primarily corn silage plus about 2 kg alfalfa hay/day, so some of the urea was needed. In a field trial, it was shown that an abrupt change from a soybean-meal-supplemented con- centrate to one containing the urea-dehy pellet had no effect on grain consumption or milk production. In an attempt to reduce the acceptance problem with urea-containing diets and to slow the release of ammonia from urea in the rumen, Kansas researchers (Helmer et al, 1970a) developed a product in which a cereal grain and urea were combined and processed through a cooker-extruder under conditions of moisture, temperature, and pressure that cause the starch to gelatinize. This extruded urea-grain product (EUGP) supple- ment made from ground corn and urea was compared to urea plus corn and soybean meal as protein supplement for lactating cows (Helmer et al, 1970b). Eighteen lactating dairy cows were assigned to a 3 X 3 Latin square design for 6-week periods, with the first 2 weeks being considered the changeover interval. The concentrate mixtures were fed twice daily to appetite. The preexperimental concentrate contained 1 percent urea. The experimental mixture and EUGP contained 2.8 percent urea. The cows fed EUGP and soybean supplement ate considerably more grain, produced more milk, and gained more weight than the cows fed the urea supplement. The urea-grain mixture was apparently unpalatable. Pre- vious in vitro work had shown that EUGP slowed urea hydrolysis (Helmer et al, 1970a) in the rumen and increased synthesis of micro- bial protein. Owen and Appleman (1970) also reported a significant increase in milk production when a gelatinized milo-urea mixture or a urea-dehydrated alfalfa combination was compared to a control diet containing 2.5 percent urea. These preliminary results indicate that a urea-dehy combination and an extruded urea-grain product may par- tially resolve one problem of feeding urea; namely, poor acceptance of the urea diet and perhaps a more constant release of ammonia in the rumen. More work is needed on these products, especially with high- producing cows in early lactation before general recommendations can be made. RELATIONSHIP OF RUMEN AMMONIA LEVEL TO NPN USE Roffler and Satter (1973) recently described studies that show the rela- tionship of rumen ammonia levels to microbial growth in vitro and to the total protein level in the diet. Using continuous culture fermentors, Satter and Slyter(1974) found that maximum microbial protein produc- tion occurred at a rumen ammonia level of 5 mg ammonia-nitrogen/100

76 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition ml rumen fluid. From analyses of 1,038 rumen ingesta samples from 207 dairy cows fed a large range of diet protein levels (but not containing urea), it was discovered that rumen ammonia nitrogen levels exceeded 5 mg ammonia nitrogen/100 ml rumen fluid whenever diet total pro- tein levels exceeded 13 percent of diet dry matter (Roffler and Satter, 1973); therefore, it was concluded that diets containing more than 13 percent natural protein could not be benefitted by NPN additions. If these results are applied to all lactating cow diets, then no urea or NPN should be added, because National Research Council (1971b, Table 3) requirement levels are greater than 13 percent for all lactating cows. Recent studies by Sparrow et al. (1973) and Gardner and Park (1973) with nonurea diets do not suggest that National Research Coun- cil protein levels are too high for high-producing cows. Although a nega- tive control group was not used by Conrad et al (1969), a urea-dehy- containing diet with a calculated protein level of 15.9 percent resulted in average lactation yields of 7,542 kg in nine Holsteins, which was al- most identical to a diet of SBM. One cow produced 9,343 kg of milk in 305 days. The calculated total protein in the diet without the urea- dehy supplement was 11 percent. It appears that these cows must have used a substantial amount of the urea from the urea-dehy supplement. A recent study by Ramage and Woolf (1973), which extends their previous work (1970) referred to above, supports the Ohio work that indicates that high-producing cows fed near National Research Council requirements may use urea nitrogen. Urea levels from 0 to 1.6 percent of the total diet were used in complete feeds of chopped hay and con- centrate. The primary protein source used in the basal diet was corn gluten, chosen because of its relative insolubility. Only high-producing cows (> 32 kg/day for a week during peak production) were used. The standard basal complete diet of 70 percent concentrate, 30 percent chopped hay, and 0.8 percent urea was fed the first 6 weeks postpartum; then experimental diets were fed for the subsequent 29 weeks. Over 1 percent urea in the complete feeds did not adversely affect intake or production. The protein solubility of feeds in the basal diet can have a marked effect on rumen ammonia levels and the usefulness of added urea to the diet. Wohlt and others (1973) showed that the protein solubility of en- ergy feeds ranged from 4 to 42 percent and the protein solubility of protein supplements from 3 to 93 percent. Moreover, the solubility of processed feedstuffs varies greatly from batch to batch (Sniffen, 1973). The amount of natural protein in the diet that escapes rumen degrada- tion is largely a function of solubility and the rate of passage through the rumen. The high-producing cow is characterized by a high feed in-

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 77 take relative to the steer or dry cow and must have a faster rate of pas- sage. Those feedstuffs with a low rumen solubility will be most useful in diets with added urea, and future studies will undoubtedly relate to this point. Total diet soluble nitrogen should also be considered, be- cause both corn silage (Johnson et al., 1967) and hay crop silage (Waldo, 1968) can have a high level of soluble nitrogen. Unfortunately, at this time solubility factors are not well enough understood to apply them in practical feeding situations, but differences in natural protein solu- bility have probably contributed to the great variation in response to added urea in many reported studies. Very few experiments have been reported on the use of urea-containing diets that were fed to cows pro- ducing 40-50 kg/day of milk in early lactation when such cows are usually in negative energy balance. As production levels continue to increase, it becomes necessary to conduct experiments on cows in this critical production period before the results can be extrapolated to the general population. EFFECT OF UREA ON THE HEALTH OF DAIRY CATTLE Rys (1967) stated that, although urea at a level of 2 percent has been routinely included as an ingredient in concentrates for cattle in Poland, veterinarians have not reported any symptoms of liver or kidney dam- age, except in cases of acute urea poisoning, which were caused by ex- cessive urea intake. Toxicity problems can arise if hungry cattle are allowed access to large amounts of a feed containing a high level of urea; sometimes a urea supplement not intended for direct feeding is fed or a supplement is top-dressed, and aggressive cows eat large amounts in a short time. Because of the potential toxicity of urea, some have implicated it in reproductive disorders, mastitis, milk fever, and nearly every other ill that afflicts dairy cattle. Archibald (1943), in one of the few long-term studies with urea at 3 percent of the concentrate mixture, found no detrimental effects on health and reproduction. Urea at a level of 2.8 percent of the concentrate (up to 250 g/day) fed to Hoi- stein steers for 7 months caused no differences in carcass quality and histopathology of liver, spleen, kidney, or adrenal and pancreas tissues (Muller et al, 1971). The effects of urea, providing 45 percent of dietary nitrogen, was compared to plant protein in diets for Holstein heifers be- ginning 2 months prior to breeding (Patton et al, 1970). Daily gains, blood urea, services per conception, and cycle lengths in repeat breeders were not affected by the feeding of urea. The effect of dietary urea on luteal function was examined by Garve- rick and others (1971), who fed complete mixed feeds of corn silage

78 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition concentrates and ground corncobs to 15 Holstein-Friesian heifers at least 60 days prior to estrus. One group received from 145 to 240 g of urea per animal per day; the other group received equivalent nitrogen as SBM. Plasma concentrations of progesterone, luteinizing hormone, corticosterone, and cortisol were nearly the same for the two groups. Twelve corpora lutea from urea-fed heifers were lighter, softer, and more fragile than 10 from SBM-fed heifers, but no histological differ- ences were detectable. However, greater progesterone synthesis by corpus luteum tissue incubated in vitro was observed from heifers fed SBM. It is not known whether this difference had any effect on repro- ductive performance. The Michigan Dairy Herd Improvement Association records of feeding practices were used by Ryderetal. (1972) to determine any relation- ship of urea feeding to milk production, calving interval, and cows sold for sterility. Amount of urea fed was divided into four levels to aid in the detection of any trends. Data from more than 600 herds during a 5- year interval provided 85,281 individual lactation observations for the study. Although urea was fed in over half the observations, no effect or trend could be seen in any of the parameters measured. A long-term study on the use of urea in diets for dairy cattle fed corn silage as the only forage has been conducted at the University of Illinois. The first phase related to heifer growth from 20 to 104 weeks of age (Clark, 1971). Fifteen Holstein heifers were assigned to three diets at 12-20 weeks of age. Both the corn-soybean meal concentrate and corn silage were used to carry urea, so that average daily consumption was 0, 70, and 114 g urea and the dry matter contained 0, 1.2, and 2.1 percent urea. Dry matter intake decreased, but not significantly, as the urea level increased. However, no effect was seen on wither height, heart girth, or body weight. A second trial employed five heifers, each on the same treatments, plus a fourth group fed the high level of urea and 0.45 kg of alfalfa meal per heifer per day. No significant differences were ob- served in intake or body measurements. The total diets contained from 11.2 to 14.6 percent protein, with the urea diets containing the higher level, so that some of the nitrogen was probably unneeded. The second phase of the Illinois work (Clark et al., 1973) described the effect of the urea diets on milk production, diet digestibility, and nitrogen balance by cows fed the diets for 1 year (Trial 1) or 2 years (Trial 2) before the study. Six-day digestibility and nitrogen balance trials were carried out with four cows per diet at early and mid-lactation and during the dry period. Corn silage contained 0, 0.4, or 0.75 percent urea; the concentrate mixtures 0, 1.6, and 2.4 percent urea. In Trial 1 the concentrate mixtures were nearly isonitrogenous (ca. 16.5 percent

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 79 protein), so the higher levels of urea in the silage caused the urea-silage groups to receive more protein than the control group. In Trial 2, the concentrate mixtures were adjusted to make the total diets isonitro- genous. Diets imposed had no measurable effects on dry matter intake, 4 percent fat correct milk (FCM), or milk fat content in either trial. However, in Trial 2 milk protein content was lower on both urea diets. Milk production (FCM) averaged 16.2 to 18.1 kg/day in early lactation and 12.4 to 16.2 kg/day in mid-lactation. Nitrogen balances revealed that productive nitrogen (milk plus retained nitrogen) was 69, 58, and 51 g/day for cows fed the control, low-urea, and high-urea diets in Trial 1. More nitrogen was excreted via urine by cows fed urea, but productive nitrogen was equal in early lactation and lower in mid- lactation and in the dry period. In Trial 2 with isonitrogenous diets, there was no detectable diet effects on nitrogen intake, nitrogen ab- sorbed, or productive nitrogen. Urinary nitrogen was greater, however, in cows fed the high-urea diet than those fed the control. Nitrogen from soybean meal was used more efficiently than from urea. No data on reproductive efficiency or health disorders have yet been reported on this study. Results of another long-term study on urea feeding have recently been reported from Purdue (Erb et al, 1975). In Trial 1, 81 Holstein heifers were randomly assigned to three isonitrogenous diets in which urea provided 0, 50, and 100 percent supplemental protein for diets 1, 2, and 3, respectively. Corn silage plus alfalfa grass silage was blended with the concentrate premix to provide a complete ration. The heifer diets were 12 percent protein and were changed 2 weeks before calving to lactation diets of 54 percent silage and 46 percent concentrate (dry basis), which contained urea at 0, 18, and 36 percent of the total diet nitrogen. The lower urea level during lactation was about 1 percent of diet dry matter and the higher level 2 percent. In Trial 2, 18 heifers were each assigned to diets 1 and 3 just prior to calving. No problems of diet acceptance were encountered. Milk production (FCM) was 3.2 percent and 4.5 percent lower for cows on diet 3 compared to those on diet 1 during lactation one in both trials (nonsignificant). No significant diet effects were seen for incidence of infectious disease, days post- partum to first estrus and breeding, or in services per conception. Num- ber of abortions for cows on diet 3 in Trial 1 was higher than for cows on diets 1 and 2, but this effect was not observed in Trial 2. In Trial 2, cows on diet 3 had more retained placentas compared to the other two groups, but this effect was not observed in Trial 1. Nothing in this study, carried on for four lactations in Trial 1 and two lactations in Trial 2, showed any cause for alarm at a urea feeding level of 1 percent

80 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition of the total diet, or about 200 g/day. These data do suggest caution in moving to higher levels for a sustained continuous time. UREA PLUS NITRATE Both urea and nitrate, which liberate NH3 upon hydrolysis, can be used by rumen microbes for protein synthesis. It has been postulated that adding urea to diets that have nitrate-containing forages may decrease the reduction of nitrate to ammonia and increase nitrite toxicity. How- ever, no interrelationship between sodium nitrate, included at 2.5 per- cent of the total diet, and urea at 1 percent was found (Hoar et al, 1968) with fattening lambs. Elliot et al. (1968) found that a combina- tion of urea plus nitrates did not affect feed intake, milk production, level of hemoglobin, or level of methemoglobin in dairy cows. The re- sults of Sebaugh et al. (1970) are in close agreement. SUMMARY AND CONCLUSIONS Two major factors are related to urea usage by ruminants: (1) accep- tance of urea-containing diets and (2) effective utilization of the con- sumed urea. Acceptance of the urea-containing diet is favored by: • Multi-ingredient concentrate mixtures that include industrial by- products such as distillers' grains (a protein supplement) and molasses (a carbohydrate supplement). • Feeding conditions that permit cows to eat small amounts through- out the day. • Thorough mixing of urea into the diet. • Diluting urea into the total diet, i.e., use of complete diets. • Avoidance of ingredients such as raw soybeans that contain urease capable of releasing ammonia from urea and wet mangers that favor fer- mentation of the feed. • The use of products that slow ammonia release. Consumption, however, does not necessarily mean effective use. Utilization of urea is improved by: • A level of protein in the basal diet that is below the requirement. • A continuous supply of soluble carbohydrates such as starch. Con- rad et al. (1969) suggest about 1 kg of readily fermentable carbohydrate is needed per 100 g of urea in the diet. • Ad libitum feeding that will result in small amounts of feed eaten

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 81 throughout the day. This approach should ensure that ammonia release and soluble carbohydrate presence more nearly coincide. • A minimum adjustment period of 2-4 weeks. In practice, a step- wise introduction of urea into the rations should be considered. • Formulation of diets to contain appropriate levels of all known re- quired minerals. • Nitrogen compounds in the basal diet that have low rumen solu- bility. It is now known that some feedstuffs contain high levels of nat- ural NPN and/or high levels of rumen soluble-true proteins; consequently, there may be dietary conditions (e.g., those with two or three fermented feeds) under which no added NPN would be used. In terms of actual amounts, 1.5-2.0 percent urea in the grain mixture under general conditions or 1 percent of the total diet dry matter are good general guidelines for lactating cows, or 200-250 g/day. When high- moisture hay crop silage is the primary forage, and/or when this forage is fed with fermented grain, these levels should be reduced by at least one-half. SHEEP Many of the principles of urea use discussed in earlier sections were ob- tained using the sheep as a model for ruminants. Therefore, much of the research and applications discussed in the beef and dairy cattle sections apply also to sheep. Specific studies that relate to sheep include those by Harris and Mitchell (1941 b), Johnson etal. (1942), Hamilton et al. (1948), StangeK 1967), Humeetal. (1970), and Braman etal (1973); general guidelines are found in the National Research Council (1975) publication on sheep. Because the following factors are specifically critical, they are repeated for emphasis: 1. Because urea is very soluble in aqueous solutions and rapidly hy- drolyzed to ammonia and carbon dioxide in the presence of urease, periodic feeding of high levels should be avoided to prevent acute am- monia toxicity and inefficient use. 2. Adequate levels of fermentable carbohydrates enriched with suf- ficient sources and levels of minerals enhance microbial use of urea. 3. Feeding management that insures frequent or continuous intake of urea-containing diets will tend to maximize microbial growth. The ultimate value of urea in sheep diets is fundamentally based upon the quality (levels and patterns of essential amino acids) and quantity of microbial protein. Hall (1972) demonstrated that rumen microbial pro-

82 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition tein was a relatively good protein compared to soybean meal, corn gluten meal, or crystalline amino acids in test diets for the nonruminant. Nevertheless, Schelling and Hatfield (1968) provided evidence that mi- crobial protein in sheep could be significantly improved by supplementa- tion (postruminally) with specific amino acids or casein. Subsequently, Nimrick et al. (1970a) showed that the first three limiting amino acids in microbial protein were methionine, lysine, and threonine as indicated by nitrogen-balance studies with lambs. The predominant influencing factors that determine the patterns and levels of amino acids at the absorption sites are: (a) the amount of di- etary protein that escapes rumen degradation, (b) the amount of micro- bial protein synthesized, and (c) the completeness of postruminal digestion of residual dietary protein and microbial protein. The comple- menting effect of one source of protein (residual dietary) to another source of protein (microbial) could partly or completely correct defi- ciencies from a single-source protein that could enhance the amino acid status of the animal. The application of feeding practices can influence these factors. For example, the selection of ingredients with high rumen solubility (permitting more rumen degradation) and physical characteris- tics favoring longer rumen retention time (long hay), and low in natural proteins supplemented with liberal levels of NPN, would increase the ratio of microbial protein to residual dietary protein postruminally. DIETARY LEVELS OF UREA The supplementary protein needs of sheep diets are influenced by age and function of the animal (animal's requirements) as well as by the contributions of protein by the energy-furnishing forage and grain in- gredients of the diet. Is urea an effective ingredient to furnish all or only part of the supplementary protein needs? Stangel (1967) reported that a series of experiments were conducted in Germany between 1907 and 1924 in which urea satisfactorily re- placed 30-40 percent of the nitrogen in sheep diets. Detailed evidence (Harris and Mitchell, 1941a,b) showed that nitrogen equilibrium in sheep could be maintained when 90 percent of the nitrogen was fur- nished by urea. Also near-normal growth of lambs was obtained in which urea furnished 50 percent of nitrogen in the 11 percent protein diets. Other investigators (Johnson et al, 1942; Hamilton et al, 1948; Braman et al, 1973) have shown that urea is a useful source of nitrogen in diets containing no more than 12 percent protein. With sheep fed purified urea-containing diets (Hume et al, 1970) with protein levels ranging from 3-18 percent, the maximum amount of protein synthe-

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 83 sized in the rumen was recorded on diets containing 10.4 percent pro- tein. Other experiments (Sharma et al., 1969) have shown that a maximum amount of alpha amino nitrogen was found in the duodenum of sheep when they were consuming diets containing from 5.8 to 8.3 percent protein. Data from Braman et al. (1973) indicated that lambs receiving urea- supplemented 10 percent protein diets performed better than lambs fed similar soybean meal-supplemented diets. A possible explanation was that the high-concentrate, soybean meal-supplemented, 10 percent pro- tein diet was not sufficiently degraded in the rumen to supply adequate nitrogen for rumen microbial synthesis of necessary nutrients such as B vitamins, while the high-concentrate, urea-supplemented, 10 percent protein diet may have provided sufficient rumen ammonia levels for adequate microbial synthesis. However, at higher protein levels, Bra- man et al. (1973) found that the soybean meal-supplemented, high- concentrate diets were superior to the urea-supplemented diets. Similar trends also favored urea supplementation over soybean sup- plementation in low-protein, high-concentrate diets for cattle (Braman et al., 1973). However, higher performance in cattle was observed for soybean supplementation over urea supplementation at higher protein levels in high-concentrate diets (Braman et al., 1973; Hatfield and Can- tner, 1973). High biological responses to soybean meal-supplemented diets over urea-supplemented diets of young growing ruminants that have rela- tively high protein requirements suggest that rumen microbial synthesis is inadequate (quantitatively or qualitatively) to meet the amino acid needs (Hungate, 1966; Sharma et al., 1969; Mathison and Milligan, 1971). Experimental data and field observations support the general state- ment that NPN compounds are satisfactory sources of supplementary nitrogen for sheep if they are limited from about one-fourth to one- third of the total dietary nitrogen and if the total dietary protein level does not exceed 12 percent of the diet. Since the effectiveness of urea nitrogen in replacing supplemental protein nitrogen is partially contin- gent upon other dietary ingredients, supplementary nitrogen can be furnished almost entirely by NPN sources in high-grain fattening diets. However, in maintenance or growing diets containing large amounts of low-quality forages that are low in protein and soluble carbohydrates, the amount of supplemental nitrogen required can exceed 50 percent of the total dietary nitrogen. For such diets the preformed protein should supply some of the supplemental nitrogen, and general nitrogen utilization would likely be improved if some soluble carbohydrates were

84 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition supplied with any of the NPN sources. At times it may be economically desirable to use NPN for all of the supplemental nitrogen and accept a lower rate and efficiency of gain. Whenever high concentrations of NPN compounds are used, safety precautions as outlined elsewhere in this volume should be observed. PROTEIN QUALITY Changing the composition of the diet and/or varying the level of feeding may alter the rate of microbial synthesis, but the amino acid composi- tion of the different strains of microorganisms is fairly constant. Al- though the biological value of microbial protein is relatively high (Chalupa, 1972), recent evidence is available to show that microbial protein or a combination of microbial protein and residual dietary pro- tein can be improved by exogenous amino acids administered in a man- ner to prevent rumen degradation and alteration (Schelling and Hatfield, 1968; Al-Rabbat et al, 1971a,b). Methionine, lysine, and threonine are the three first-limiting (and in the order of limitation) amino acids of microbial protein for supporting nitrogen balances in growing lambs. However, there have been conflicting reports regarding the value of free unprotected dietary amino acids. Reported results have varied from fav- orable responses to no responses or even depressed performances. The reported effects of dietary methionine hydroxy analog on performance have been as variable (Bishop, 1971; Wilson et al, 1971) as the dietary amino acids data. It is likely that any favorable responses in the func- tional ruminant to dietary amino acids can be attributed to nonspe- cific nitrogen and/or sulfur responses. At reasonable dietary levels, neither methionine nor its analog increased plasma methionine levels in lambs (Papas et al, 1974). The recent observations that ruminants do respond favorably to spe- cific protein precursors that reach the absorption sites is a challenge for the development of new practical methods and techniques that will pro- tect dietary constituents from rumen degradation and alteration. Treating dietary proteins with aldehydes or tannins have significantly improved rate of growth, feed efficiency, and nitrogen balances of grow- ing lambs (Ferguson et al, 1967; Nimrick e/a/., 1970a,b, 1972; Peter e/ al, 1971; Driedger and Hatfield, 1972). There are a number of practices presently employed that will reduce protein solubility that permits more rumen bypass of dietary proteins. Specific heat treatment of proteins, which will decrease rumen degradation, may increase dietary amino acid concentration at the absorption sites. However, overtreatment can easily occur that will result in resistance to digestion in the lower tract. In the light of these developments, more research should be conducted to de-

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 85 termine the effect of time and intensity of grain-drying temperatures on protein and starch solubility, residual moisture levels, and overall grain quality. Changing feeding methods of ruminants will affect the amount of dietary constituents reaching the absorption sites. High-concentrate diets move more rapidly through the digestive tract than high-forage diets. Both the level of feeding and composition of the diets have some effect on rumen retention time. For example, the favorable responses to high-grain, high-protein diets may be due, in part, to some rumen by- pass of the dietary protein and starch. As indicated earlier, the ideal practical feeding diet might be one containing sufficient NPN to sus- tain an active rumen microflora that (a) would supply the host with an optimum supply of microbial protein and (b) is fortified with enough "protected" amino acids or with proteins containing high levels of the limiting amino acids, which would elicit maximum performance in the animals. It is especially important to have nonspecific nitrogen avail- able to the microbiota if ruminants are expected to derive energy from forages and other cellulose-containing ingredients, synthesize nutrients not ordinarily added to rumen diets, such as B vitamins, and to detoxify compounds, such as chemical residues or other undesirable dietary con- taminants. EFFECT OF DIETARY UREA ON UTILIZATION OF OTHER INGREDIENTS Numerous studies have been designed to show the interaction between urea and other nutrients. Urea, in general, did not adversely affect the utilization of other nutrients by sheep. Some studies report favorable effects of certain additives to urea-containing diets. However, studies reporting favorable responses to additives are usually studies that are deficient in the additive. For example, favorable responses to sulfur in urea diets have frequently been observed in diets in which urea replaced preformed protein, which usually supplies most of the dietary sulfur. Whenever urea has been substituted for alfalfa meal, there has appeared to be some favorable response to macro and micro mineral additives. There appears to be no consistent adverse effect of urea on carotene or vitamin A utilization. Sheep producers may often be concerned about urea additions to diets containing forages produced under conditions that could possibly raise the forage nitrate content. Urea-containing diets do not increase chronic or acute nitrate (nitrite) toxicity. Dietary urea is not a precursor to nitrite, thus it will not substantially increase the amount of nitrite absorbed from the rumen. Smith and Hatfield (1967) found that urea tends to counteract adverse effects of dietary nitrate. Even though urea, added in excess to silages, may buffer acids suf-

86 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition ficiently to affect its spoilage rate, it is unlikely that dietary urea properly used will adversely affect the utilization of other dietary ad- ditives. ADAPTATION TO UREA-CONTAINING DIETS Total composition of the diet has some effect on adaptation responses in sheep: Nitrogen retention in lambs fed urea-containing diets was improved significantly at the rate of 2 percent per 10-day period for 50 days (Smith et al, 1960). In instances where there are abrupt and drastic changes in the chemical composition of the diet, there may be some adaptation time required for the biological system (microbiota and host). There have been some indications that the microbiota adapted within a few days (Barth et al, 1961; Caffrey et al, 1967a), while changes in animal responses may continue over several days (Ewanetal., 1958; Welch et al, 1957). Some data indicated that mea- surable responses may be attributed to adaptation to the feeding re- gime or total diet rather than to administered ammonia (Caffrey et al, 1967a,b). SUMMARY AND CONCLUSIONS Urea can serve as a useful source of nitrogen in practical sheep diets. The most favorable responses from urea supplementation have occur- red in diets containing relatively high concentration of readily ferment- able carbohydrates and relatively low dietary levels of total nitrogen- 12 percent protein equivalent or less after supplementation. High levels of urea and/or urea fortification of high-nitrogenous diets have often depressed performance by reducing diet acceptability or by reducing nitrogenous utilization. Fortifying forages at ensiling is a satisfactory and convenient method of nitrogen enrichment of materials to be used in silage-containing diets. Supplemental sulfur is needed also with most corn products. Levels of urea used and methods of feeding to avoid urea toxicity in sheep appear to be managerial rather than nutritional problems. GOATS Little research on the use of NPN compounds in the nutrition of goats is reported in the scientific literature; however, there is no apparent reason why the principles governing the use of nonprotein nitrogen by lactating cattle would not apply to lactating goats and why principles that apply to wooled sheep would not apply to Angora goats.

Feeding Urea-Containing Diets to Beef Cattle, Dairy Cattle, Sheep, and Goats 87 Two specific reports on the use of urea in goat diets were found. Lindahl (1954) reported no significant difference in utilization of NPN- containing diets from that reported for sheep and cattle. However, Carrera and Killian (1970) fed a sorghum grain supplement containing 4 percent urea once daily to goats grazing poor-quality pasture in Mexico and observed some decrease in milk production. This might have been predicted with such a readily soluble compound fed at that level once daily. Comprehensive publications on the nutrition of Angora goats (Huston etal., 1971) and on milking goats (Colby et al, 1969) are available.