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3 Use of Urea as a Protein Replacement for Ruminants Urea is the NPN compound most widely used in ruminant diets. The use of approximately 800,000 tons in 1973 (Allen, 1974), with appropriate supplemental sources and levels of energy, represents 4.5 million tons of 50 percent protein supplement. Although the chapter is limited pri- marily to urea as the NPN source, the general principles of ammonia utilization presented earlier apply. HOST NEEDS FOR PROTEIN There are many similarities among species of the animal's need for pro- tein or, specifically, amino acids at the tissue level. Physiological growth, function (maintenance, growth, production, or reproduction), levels of complementing and supplementing nutrients or additives, and environ- mental conditions (temperature, humidity, confinement) are among the factors that affect the animal's needs for amino acids. These influ- encing factors have been considered in the available feeding standard recommendations. Special consideration should be given for estimating the protein needs for the biological system of the ruminant. In addition to the amino acid needs of the host, there are nitrogen requirements for the microbiota if the ruminant is expected to use forage and other cellulose-containing energy sources, if the biological system is to syn- thesize nutrients not ordinarily added to the diets (such as B vitamins), and if the microbiota are to be used to detoxify dietary ingredients that may contain or be contaminated by chemical residues. Examples of pos- 17
18 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition sible rumen nitrogen deficiencies were observed in cattle and sheep fed limited protein, soybean meal-supplemented, high-concentrate diets; but they were not apparent in isonitrogenous, limited-protein, urea-supple- mented, high-concentrate diets (Braman et al., 1973). After the protein needs of the ruminant have been established for a particular performance or function, then the supplemental need can be determined after allow- ing for the protein contributions from the energy-furnishing ingredients in the diet. Because of the limitation in quantity and quality (patterns of essential amino acids) of protein synthesized by the microbiota of the ruminant, the decision to provide all of the supplemental needs is contingent upon the expected performance or function of the animal (total dietary protein requirement), protein contributions of other in- gredients, and levels or concentration of other components necessary for microbial synthesis. For example, a rapidly growing, young rumi- nant may respond significantly more to diets supplemented in part with some preformed protein, such as soybean meal, than it would respond to diets supplemented entirely with urea. However, it has been demonstrated many times that urea is a satisfactory source of supple- mentary protein in diets for ruminants that have low total dietary pro- tein requirements (maintaining a mature ruminant). An experimental diet to test both the animal's protein needs and the efficiency of the source of nitrogen in fulfilling those needs should con- tain both a "negative control" (test diet known to be protein deficient) and a "positive control" (test diet with preformed protein furnishing the supplemental protein and eliciting a positive animal response rela- tive to the negative control). The diet must be deficient in nitrogen or natural protein if supplementary urea is to be beneficial to the animal. This need, although well recognized, has not always been adhered to in some urea utilization experiments with cattle and sheep. Sound conclu- sions are not possible under the assumption that a diet, before urea sup- plementation, is protein-deficient when, in fact, it may contain adequate protein. Also, without a positive control diet, one might erroneously conclude that the negative control diet has adequate protein when, in fact, the animals consuming such a diet may be protein-deficient. This situation was demonstrated in cattle-feeding trials by Burroughs et al. (1972, 1973) in which two types of diets were identified that did not respond to urea supplementation. One type was protein-deficient and readily responded to natural protein supplementation, even though no response to urea was noted. The second type did not respond to either natural protein or urea supplementation and, therefore, could be ac- curately judged as being protein-adequate for the cattle being fed. This leads to the conclusion that, although the ruminant diet must be de-
Use of Urea as a Protein Replacement for Ruminants 19 ficient in protein in order for urea supplementation to be beneficial, the reverse of this statement is not necessarily true; namely, urea will always be beneficial if added to a protein-deficient ruminant diet. The reason why this may not be true is that the negative control diet must not only be protein-deficient, but it must also have fermentation char- acteristics that permit it to be benefited by urea supplementation. The degree of protein deficiency in a ruminant diet also exerts a strong influence upon the relative effectiveness of urea supplementation, as compared with natural protein supplementation. It has long been known that the relative effectiveness of urea is greatest when fed in a high-energy, mildly protein-deficient diet in which only a small quantity of urea or natural protein is needed in correcting the deficiency. It is less well known that when urea is fed in a similar high-energy diet that is much more protein-deficient, urea is very beneficial but may be somewhat less beneficial in satisfying the total deficiency compared with natural protein supplementation. This situation was also demon- strated and confirmed in recent cattle experiments (Burroughs et al, 1972, 1973) in which urea was as effective as natural protein supple- mentation in the first type of diet and only about half as effective as natural protein supplementation in the second type of diet. These same experiments demonstrated that the degree to which cattle performance was poorer (30-40 percent) from urea compared with protein supplementation of a protein-deficient diet fed to small cattle (160 kg) entering the feedlot decreased progressively until no differences existed during the last part of a 10-month feeding period. During this period, the supplemented cattle approximately tripled their body weight, increased TON consumption, and thus conversion of urea to rumen microbial protein. Thus, this urea-supplemented diet that was deficient with limited TON intake in smaller cattle progres- sively became a protein-sufficient diet for the same cattle with increased TON intake. By contrast, the protein-supplemented cattle were not similarly influenced with respect to TON intake. In the feedlot, those cattle receiving urea supplements often have lowered performance during the initial weeks than those receiving protein supplements; however, subsequent performance is improved. With larger-size cattle, weighing 300 kg or more, the period of inferior performance observed is considerably shortened and may be of only 3-4 weeks' duration. Sometimes inferior performance for this short duration is attributed to the unpalatable nature of urea-containing feeds (Goodrich and Meiske, 1969). Such theories, however, were not sus- tained when tested by Varner and Woods (1970), who demonstrated that larger-sized cattle required no adaptation to urea provided TON
20 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition consumption was rapidly increased for a 3-week period prior to their initial consumption of urea supplements. Such cattle, when abruptly shifted in a single day from protein to urea feeds, suffered no setback and performed equally as well as cattle continuously maintained on nonurea protein feeds. Experimental and industry observations with high-corn, grain-corn silage diets tend to support the consensus that urea is a satisfactory source of supplementary protein for beef cattle and sheep when limited to approximately 20-30 percent of the total protein requirement, pro- vided the animal has attained full feeding conditions and the require- ment does not exceed 12-13 percent of the diet. This consensus would assume that other factors necessary and favoring microbial synthesis are adequate. Because the effectiveness of urea supplementation is de- pendent upon other dietary factors, urea is a satisfactory source of all supplemental protein needs in high-grain fattening diets properly forti- fied with minerals for animals having attained full feeding conditions and having no more than 12 percent total dietary protein requirements. This would assume that feeding management is appropriate for maxi- mizing urea supplementation. There may be production situations where it would be economically feasible to accept a lower rate and efficiency of performance and use urea for all of the supplemental needs in diets of animals having high protein requirements. DIETARY FACTORS AFFECTING MICROBIAL PROTEIN SYNTHESIS An optimal level or concentration of the following factors will maxi- mize rumen microbial synthesis: ammonia, readily available energy, carbon skeletons, minerals, vitamins, growth stimulators or inhibitors (antibiotics, hormones, anabolics), and factors that influence the chem- ical and physical environment (pH, temperature, diet particle size and density, presence or absence of oxygen, etc.). NITROGEN The influence of exogenous nitrogen compounds on net microbial syn- thesis has been measured and estimated in many experiments. As indi- cated earlier in this report, ammonia is a vital ingredient in micro- bial synthesis. Relevant points to be considered by formulators of ruminant diets include the sources and levels of ammonia precursors. The level of urea required to furnish supplemental ammonia to meet
Use of Urea as a Protein Replacement for Ruminants 21 optimal ammonia concentrations in the rumen is dependent on (a) the amount of ammonia from degraded nitrogenous compounds contained in the other components of the diet, such as forages, grains, and other ingredients; (b) the amount of recycled endogenous urea; and (c) the levels of other necessary components (energy, minerals, etc.)- The ef- ficiency of nitrogen or ammonia utilization will be greatest whenever ammonia is the first-limiting factor necessary for synthesis. For exam- ple, urea utilization will be high in low-available nitrogen diets that con- tain abundant levels of available energy, carbon skeletons, minerals, vitamins, and other components that enhance microbial activity. SOURCE OF ENERGY Carbohydrates are normally the main source of energy and carbon skele- tons for microbial synthesis. The availability of energy from different sources is the key to evaluating its dietary effects in the ruminant. The necessity of adequate available energy is essential for evaluating the utilization potential of any nutrient. Pigden (1971) reported that the lignocellulose complex accounts for most of the energy in mature forages. He related the total dietary nitro- gen levels to the total digestible energy of forages and indicated that 1 percent dietary nitrogen was sufficient for the utilization of forages having no more than 50 percent digestible energy. However, he sug- gested 1.5 percent dietary nitrogen for forages having higher levels of digestible energy and 2 percent dietary nitrogen for diets containing 20 percent or more starch. Pigden and Heaney (1969) reported that each roughage has a digestion "ceiling . . . where rate of utilization by rumen microflora is reduced to a point where for practical purposes the energy is no longer available to the animal." They have established digestion ceilings on many low-protein roughages and found that each roughage has its own rate of digestion, which affects and is interrelated with necessary nitrogen requirements. However, several investigators (Nelson and Waller, 1962; Donefere/ al, 1969; Williams etal, 1969) reported that supplementary nitrogen did not improve the digestion of some roughages. Donefer et al. (1969) did observe that when the di- gestibility of straw was improved by treating it with sodium hydroxide, supplementary nitrogen was necessary to prevent depressed feed intake. These observations agree with those of Campling et al. (1962), who found that oat straw intake and digestibility were improved by infusing urea into the rumen of cows. Two recent attempts have been made in both beef and dairy cattle diets to predict by mathematical formulae (Satter and Roffler, 1973;
22 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition Burroughs et al, 1974a) the usefulness of urea supplementation based upon the total protein present in the diet as an index to rumen protein breakdown and in relation to the quantity of TON in the diet as an index of fermentable energy. These attempts each predict that in high- concentrate diets with TON values in excess of 75 percent on a dry matter basis, some ammonia will be synthesized into additional micro- bial protein at protein levels below 12 or 13 percent. At higher protein levels in these high-concentrate diets, the formulae predict that am- monia from protein breakdown will adequately support maximum mi- crobial growth and that ammonia from added NPN will not further enhance microbial synthesis and therefore will not be useful. The formulae also predict that the breaking point between urea-ammonia synthesis and no synthesis into microbial protein is about 7 percent total protein in diets with less than 60 percent TON on a dry matter basis. Intermediate breaking points are predicted for diets with inter- mediate TON values. Further work is needed to clarify and confirm these values. Starch was reported by Gallup et al. (1953) to be a superior source of energy for utilization of urea. This conclusion is not rejected by the widely successful use of urea as the supplementary source of nitrogen in high-grain diets. Feed molasses (most of which is sugarcane molasses) is the foundation ingredient for liquid supplements. Klett (1971) re- viewed the use of urea-molasses liquid supplements in winter diets of cows maintained in the plains area of the United States. He indicated that economics of feeding the liquid supplement was often an over- riding factor favoring its selection. In experiments in which animal performance appeared to be superior on preformed protein-supplemented diets over performance on NPN- supplemented diets and when protein needs were low, the differences were likely due to the energy contributions of the preformed protein in low-protein diets containing liberal levels of lignocellulose-complex compounds. In addition to furnishing energy, dietary preformed pro- teins are sources of the branched-chain carbon skeletons. Although lipids are important sources of energy in many nonrumi- nant diets, they are not extensively used as a source of energy in rumi- nant diets at the present time. Small levels of supplemental lipids are included in the ruminant diets for energy and factors other than en- ergy (dust control, lubricant for feeding equipment, and pelleting, etc.). The long-chain fatty acids per se, arising from hydrolysis of lipids, are not used by rumen microbes. Supplemental levels of dietary lipids (> 5 percent) often depress voluntary diet intake, performance,
Use of Urea as a Protein Replacement for Ruminants 23 and/or digestibility (Bradley et al, 1966; Thompson et al, 1967; John- son and McClure, 1973). A possible explanation of lipid-depressing effects on rumen digestibility of dietary ingredients is attributable to the physical property of coating particles. SOURCES OF CARBON SKELETONS The main sources of carbon chains for microbial synthesis are from fer- mented carbohydrates and preformed dietary amino acids. The degraded proteins are the main source of the branched-chain carbon skeletons. DIETARY SULFUR AND OTHER MINERALS Substitution of urea for natural protein sharply changes the quality and quantity of minerals available for ruminal bacteria and the host animal, but the presence of urea does not change the requirements for any mineral for either the ruminal microorganisms or the host animal. Avail- ability of minerals may be altered by substitution; for example, added sulfur may be less available than the sulfur source found in the natural diet. Because major diet changes are often made when urea is included, other dietary ingredients will dictate which mineral elements need to be supplemented when urea replaces intact protein (Ovejero and Hogue. 1970). Albert et al. (1956) identified the relative utilization of sulfur by growing lambs as elemental sulfur < sulfate < methionine when these were incorporated in a diet in which 92 percent of the nitrogen was supplied by urea. Garrigus (1970) comprehensively reviewed the need for sulfur in the diet of ruminants, and Moir (1970) reported a dietary requirement for sulfur by sheep as a nitrogen:sulfur ratio of 10:1. Hatfield (1972) indicated that an N:S ratio of 15:1 was superior to 10:1 for growing-fattening cattle. Presumably this reflects a species difference in relative production of keratin. Other research results indicate that either organic sulfur, supplied as the sulfur-containing amino acids, or the inorganic sources such as sulfur and sulfate (Chalupa et al, 1973) can be used as sulfur sources in ruminant diets. The trace-mineralized salt formulations sold for ruminants usually meet the needs for trace minerals required for cellulose digestion (Mar- tinez and Church, 1970) and for animal metabolism and tissue depo- sition. Concentrate finishing diets for cattle and sheep may require supplementation with potassium and sulfur. Additional sulfur is needed with low-sulfur forages such as corn silage.
24 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition OTHER FACTORS Lipids The influence of dietary lipids as a source of energy has been indicated earlier. There has been no consistent indication that dietary lipids improve urea utilization. Although many experiments suggest some depressing effect of supplementary dietary lipids on voluntary diet intake, digestibility, and/or general feedlot performance, there is little, if any, conclusive evidence to show that dietary lipids have a direct depressing effect on microbial synthesis. Antibiotics Theoretically, high concentrations of true antibiotics would be expected to have some direct or indirect inhibitory effect on the rumen microbiota. However, at low concentrations there may be little, if any, inhibitory effect. Prescott (1953) was one of the first investigators to report that many antibiotics are nonspecific inhibitors of urease in rumen fluid. Visek et al. (1959) have also shown that oral antibiotics decrease the gastrointestinal urease activity of chicks and rats. Since urea utilization is influenced by the rate of urea hydrolysis, the effect of antibiotic supplementation may be due to a reduced urea hydrolysis. Brown et al. (1960b) fed four starter diets containing dif- ferent protein levels to 42-day-old calves. After 3 weeks, calves receiving the antibiotic-fortified diets had the greatest gains, but feed consumption was not affected. Calves receiving the antibiotic made satisfactory gains on diets containing 3 percent less protein than levels required to produce similar gains without antibiotics. Cahill and McAleese (1964) reported beneficial effects for chlorotetracycline in urea-supplemented diets for growing-fattening lambs. Although a low level of antibiotic supplemen- tation in ruminant diets is often reported to have a favorable influence, it is difficult to indicate specifically the mode of action of the beneficial effect-if there is one. Urease Inhibitors Ruminal hydrolysis of urea usually occurs at a faster rate than subsequent microbial utilization of the liberated ammonia (Bloomfield et al., 1960). Maximum urea utilization for microbial pro- tein synthesis occurs when there is the simultaneous appearance of ammonia from urea and carbon skeletons from other dietary constit- uents. Some researchers have treated dietary carbohydrates to increase the rate of hydrolysis, while others have attempted to inhibit ureolytic activity in the rumen fluid. It has been shown that high levels of dietary urea inhibit urease activity (Caffrey et al., 1967a; Chalupa et al, 1970). However, there appears to be enough activity to completely hydrolyze urea under a variety of conditions. Clifford et al. (1968) found no bene-
Use of Urea as a Protein Replacement for Ruminants 25 ficial effects on urea utilization from adding urease inhibitors such as barbituric acid, copper, or nitrates. A high level of acetohydroxamic acid decreased ruminal ammonia production and increased nitrogen retention in ruminants (Streeter et al., 1969). Brent et al. (1971) have considered the kinetics of urease inhibition by acetohydroxamic acid. Oklahoma workers (Glimp and Tillman, 1965; Harbers et al., 1965; Sidhu et al., 1968, 1969) injected purified jackbean urease subcuta- neously into cattle and sheep to produce circulating antibodies to urease. Ureolytic activity was reduced throughout the intestinal tract, and animal performance was improved when urea was the dietary ni- trogen source. The practical application of this method is questionable. Chemical inhibitors do not presently appear to offer much promise for improving urea utilization. FEEDING PROCEDURES TO IMPROVE UREA USE ADAPTATION Retention of nitrogen by ruminants fed urea sometimes appears to in- crease with length of the feeding period until a plateau is attained. This period of increased efficiency of utilization is sometimes referred to as an "adaptation period." Some investigators (Repp et al., 1955a; Ander- son etal, 1959; McLaren etal, 1959; Smith et al, 1960; J. R. Campbell et al, 1963) have noted this adaptation, while others (Miller and Morri- son, 1942;Ewane/a/., 1958; Hirose et al, 1960; Karr, 1964; Schaadt et al, 1966;Caffreye/a/., 1967a;Oltjen etal, 1969) have not. Caffrey (1965) presented experimental evidence that the response with time noted by some workers is an adjustment to the nutritional regimen rather than an adjustment to urea. Even with infusion of urea intra- venously for 60 days, he noted no change in blood or ruminal am- monia. LEVEL AND FREQUENCY OF FEEDING Previous discussions have enumerated effects of total and/or supple- mental dietary protein levels. Although frequency-of-fceding effects are sometimes confounded with changes of total diet intake, most of the data support the hypothesis that a constant or continuous intake of urea will improve its utilization over abrupt or periodic intake. Most of the monitoring of ruminal ammonia concentrations show that, after a single administration of dietary urea, the ruminal ammonia increases rapidly, peaking in 60-90 min, and then declines, reaching
26 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition initial ruminal ammonia concentrations in 4-5 h after administration. Obviously, this type of erratic ammonia environment for the micro- biota can be improved by decreasing the ammonia concentrations, by feeding less urea, and by increasing the frequency of feeding. Several investigators have reported improved performances by in- creasing the frequency of feeding (Campbell and Merilan, 1961; J. R. Campbell e/a/., 1963; Simpson and Woods, 1965; Goodrich and Meiske, 1966; Deif et al., 1970). Prior (1974) found essentially the same per- formance from lambs fed twice or 12 times daily soybean meal-sup- plemented diets. However, he found that feeding urea-supplemented diets twice daily produced negative nitrogen balances that were positive when urea was fed 12 times daily. Contrary to the favorable effects of more frequent feeding, other workers have observed only small or no favorable responses (Schoenemann and Kilian, 1960; Bloomfield et al., 1961; Knight and Owens, 1973). THOROUGH MIXING Field and laboratory observations have shown that inadequate mixing can increase the incidence for acute ammonia toxicity by permitting overconsumption of urea. Diet acceptability is often reduced by insuf- ficient mixing of the dietary ingredients. Also, the utilization of the ingested urea will be less with an inadequately mixed diet. ADDITION AT ENSILING One of the most effective ways to ensure thorough mixing is to add urea to chopped, whole plant, corn forage at ensiling; and this method has been advocated and adopted in a number of states. A diluting effect oc- curs that will reduce the possibilities of urea toxicity when the urea- enriched silage is fed. The levels of urea supplementation to silages can be made to meet specific nitrogen requirements for a particular purpose (maintenance, growing, fattening, etc.) or just to upgrade the silage for later formulation (Hatfield and Garrigus, 1967). Fortifying the fresh material to be ensiled with approximately 0.5 percent urea has given good results. Assuming that the fresh material is 35-40 percent dry matter, the addition of 0.5 percent urea on an "as is" basis at ensiling will raise the protein equivalent level of the dry matter about four per- centage points. Adding higher levels of urea at ensiling time, although successful in some instances, may create problems by buffering the silage-preserving acids sufficiently to create storage problems. Both the acid-buffering property of urea and the acid production potential of the
Use of Urea as a Protein Replacement for Ruminants 27 material to be ensiled should be considered in determining the level of urea to fortify materials to be ensiled. For example, silage materials having little or no grain should have a lower level of urea supplementa- tion than high-grain or all-grain materials. LIQUID SUPPLEMENTS These products are primarily molasses-based materials with urea or other NPN compounds as the major nitrogen source plus an array of minerals. Their use has become increasingly widespread in recent years because of the availability and price of ingredients used and the con- venience of handling and feeding liquids. Some of the advantages of the molasses-based liquid supplements include: 1. Supplies available energy for the rumen microbiota that convert NPN to microbial nitrogen. 2. Provides a suitable delivery system for many soluble micronutri- ents and other dietary nonnutritive additives in properly formulated supplements. 3. Reduces dust hazards and wind erosion. 4. Provides a cohesive medium for combining the supplement with other ingredients of the diet that will improve diet uniformity-partic- ularly high-forage diets. 5. Improves acceptability of diets containing high levels of low- quality forages. 6. Fits well into certain mechanized feeding systems. Some problems reported with liquid supplements include: 1. Needs to be kept in solution or suspension over a period of time with different environmental temperatures. 2. Requires special equipment for convenient addition and mixing into the rest of the diet. 3. Possibly leads to overconsumption and great variation in indi- vidual intakes when self-fed. 4. Causes some corrosive effect on equipment. The choice of dry or liquid supplements is likely determined by the economics and/or convenience for a particular operation. Although the lick wheel feeder has significant advantages for group- fed cattle, especially those on pasture, variation in consumption from lick wheel feeders is large (Webb et al, 1973). Specific reviews on the
28 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition use of liquid supplements are those by Loosli and McDonald (1968), Wornick( 1969), and Huber(1972). ANABOLIC AGENTS A review of numerous experiments in the literature indicate that ana- bolic-like substances elicit some favorable biological response over com- parable controls. The responses to these compounds were not corre- lated with any specific dietary nitrogen source and occurred as often in animals receiving diets supplemented with natural proteins as with NPN. It appears that the effect of these compounds occurs at the tissue level; consequently, methods of administration have some influence on concentrations that get to the tissue level and appear as a residue in excreta. There appears to be no substantial effect of these substances on the microbiota in the rumen. SUMMARY AND CONCLUSIONS Since dietary urea is used as a source of nitrogen for microbial protein synthesis, which in turn furnishes amino acids for the host animal, it is apparent that nutritional management must be considered that will maximize microbial protein production. Because urea is hydrolyzed so rapidly in the rumen, which in rare cases could produce urea (ammonia) toxicity, feeding procedures that consider safety must have first pri- ority. Many of the recommended practices that reduce the chances of ammonia toxicity will, coincidentally, be practices that will improve urea utilization. For example, recommendations for continuous feeding of low levels (or avoiding periodic feeding of high levels that are haz- ardous) will provide a relatively continuous supply of ammonia in the rumen for microbial activity whenever other necessary ingredients are present. The microbiota of the rumen will adapt rather quickly to their chem- ical environment. Whenever urea or urea-containing supplements are first introduced in the diet, a few days adaptation appear to be neces- sary for maximum utilization. Part of the adaptation may be due to diet acceptability and adaptation to diet components other than urea. For maximum utilization, the level of urea supplementation should be adequate to meet the total nitrogen requirements of the animals, but frequency of feeding should be often. Urea cannot be recommended as a safe or satisfactory source of supplementary nitrogen if the manage- ment involves infrequent feedings. However, urea is an excellent source of supplementary nitrogen whenever the levels are low and the diet is
Use of Urea as a Protein Replacement for Ruminants 29 fed frequently or ad libitum. Most ruminants will eat 10-16 meals daily when fed ad libitum, which distributes the urea intake over the day. Because urea is very soluble and quickly hydrolyzed, care must be exercised to insure that it is completely and uniformly mixed with the remainder of the diet. Top-dressing other ingredients with urea or urea- containing supplements is not a recommended practice and may be hazardous. The addition of urea to forage at ensiling time is a convenient method of urea supplementation to silage diets. The use of liquid supplements as a method for supplementing the diet with urea has several advantages, such as furnishing available energy for the microbiota that are necessary in urea utilization, providing a de- livery system for micronutrients and other dietary additives, and im- proving nutrition management to insure diet uniformity and reducing losses due to dust and wind erosion. The response of ruminants to anabolic agents appears to be at the tissue level of the host with little or no direct effect on the microbiota; consequently, anabolic agents would generally improve the utilization of all nutrients, including either natural proteins or NPN compounds.
