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6 Considerations on the Use of NPN Compounds by Nonruminant Species The problems of formulating a diet for poultry or swine that supplies the needs of the animal for the essential amino acids yet which is de- ficient in the intact nonessential amino acids or nitrogen for nonessen- tial amino acid synthesis have been discussed (Featherston, 1967; Moran et al., 1967; Manoukas and Young, 1969; Lewis, 1972). The following factors influence the response of the chicken to NPN sources and to nonessential amino acids: (1) intake of total protein, (2) an adequate and balanced supply of essential amino acids, (3) a deficiency in dietary nonessential amino acids, (4) a single source of dietary non- essential amino acids that may not be as efficiently utilized as a mix- ture, (5) a nonutilizable source of nitrogen or a toxic effect from the NPN source, and (6) the dietary mineral mixture (D. Miller, 1973). The use of dietary protein to supply the required essential amino acids usually provides an adequate dietary supply of intact nonessential amino acids or nitrogen for nonessential amino acid synthesis. The avail- ability of one or a few essential amino acids or their corresponding alpha-keto acids at a sufficiently low price for practical animal diets does not alter the situation, since intact protein would be necessary to supply the remaining essential amino acids. The anatomical and meta- bolic limitations to the efficient utilization of NPN by nonruminant species have recently been reviewed by Lewis (1972). Considerations other than economics may also be associated with the utilization of alpha-keto acids in place of essential amino acids in poul- try or swine diets. Lysine and threonine would need to be supplied as 88
Considerations on the Use of NPN Compounds by Nonruminant Species 89 such, since their carbon skeletons are apparently not easily aminated. With the exception of extensive findings showing good utilization of the hydroxy analog of methionine, little is known about the effi- ciency of utilization of alpha-keto acids where a large portion of the essential amino acids might be supplied by alpha-keto acids plus a non- specific nitrogen source. Studies with rats have shown equal growth from the alpha-keto acid as with the amino acid for leucine, isoleucine, valine, and phenylalanine when tested individually in the diet (Meister and White, 1951) or for the alpha-keto acids of leucine, isoleucine, valine, phenylalanine, and methionine when tested together (Wood and Cooley, 1954). However, in both studies growth of only approximately 1 g/day was observed in any of the rats. Cruz et al. (1969) observed equal growth of rats fed equimolar amounts of L-valine or sodium alpha-ketoisovaleric acid; however, the levels fed were approximately double the requirement of the rat. Three times the amount of alpha- ketoisovaleric acid as compared to L-valine was necessary for mainte- nance of nitrogen balance in man (Gallinae/ a/., 1971). POULTRY The results of many studies on the utilization of NPN by the young chick have recently been reviewed (Featherston, 1967). Attempts to obtain satisfactory utilization of various sources of NPN by chicks fed diets containing natural feedstuffs have, for the most part, been unsuc- cessful. The utilization of various sources of NPN, including urea, diam- monium citrate, and triammonium phosphate, has been demonstrated by several workers in chicks fed crystalline amino acid diets devoid of nonessential amino acids. The chick, however, lacks the ability to syn- thesize glycine and proline at a rate commensurate with the needs for rapid growth. Allen and Baker (1972) recently reported the results of studies concerned with the utilization for weight gain and protein reten- tion of various sources of NPN when added to crystalline amino acid diets for chicks. In comparison to L-glutamic acid, diammonium citrate was utilized about 70-90 percent as efficiently. Diammonium phosphate and urea were utilized with an efficiency of less than 40 percent of L- glutamic acid. Studies on the utilization of NPN by the laying hen indicate greater promise; however, the results have often been conflicting. Johnson and Fisher (1956) observed that the replacement of alanine, aspartic acid, cystine, glycine, proline, and serine by ammonium citrate in synthetic- type diets was found to have no adverse effect on egg production. Im- provements of varying degrees in the egg production of hens fed well-
90 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition balanced, low-protein diets composed of natural feedstuffs and supplemented with NPN have been reported (Young et al, 1965; Chavez etal, 1966; Reid etal., 1972; Fernandez et al., 1973). Young et al. (1965) reported that the addition of 3 percent protein equivalent of nitrogen from diammonium citrate or glutamic acid to a 13 percent protein corn, soybean meal, fish meal diet resulted in im- proved egg production of hens up to that obtained with a 16 percent protein diet composed of the same natural protein sources. A response in egg production from the addition of diammonium citrate, but not glutamic acid, was noted when a 13 percent protein corn and soybean meal diet was used. Chavez et al. (1966) reported that adding 2 or 3 percent protein equivalents of nitrogen from diammonium phosphate or diammonium citrate, respectively, to a 12.75 percent protein control diet containing adequate amounts of essential amino acids resulted in 4-5 percent in- creases in egg production. The addition of 3 percent protein equivalent from urea did not increase egg production. Feed conversion was im- proved by the first two nonprotein sources studied. More recent studies (Reid et al., 1972) have shown increases in egg production in three of four experiments when diammonium citrate, ammonium sulfate, and diammonium phosphate were used as sources of NPN. Results indicate that the nonessential nitrogen requirement appears to be 1.4-1.9 g/day, of which 0.4 g could be furnished by NPN sources. Fernandez et al. (1973) showed some improvement in egg production from supplementa- tion of low-protein laying diets with urea but little or no effect from additions of monosodium glutamate, diammonium citrate, or diammo- nium phosphate. Other workers, including Moran et al. (1967), Akintunde et al. (1968), Bornstein and Lipstein (1968), and Kazemi and Balloun (1973), ob- served no improvement in egg production from dietary additions of urea, diammonium citrate, or diammonium phosphate. Moran and co-workers (1967) found that additions of 5 percent protein equivalent from urea or diammonium citrate to a 10 percent protein basal proved ineffective for both the growing chick and laying hen and that this level of diam- monium citrate proved toxic. Reducing the level of diammonium citrate to 2 percent protein equivalent relieved the depression in chick perfor- mance. In a third experiment, in which a semipurified diet containing 8 percent protein from soybean meal plus additions of 11 essential amino acids was used, diammonium citrate additions again failed to result in improved laying hen performance. These workers concluded that a dietary inadequacy of nonessential nitrogen for the laying hen was improbable when normal feedstuffs were used. Kazemi and Balloun
Considerations on the Use of NPN Compounds by Nonruminant Species 91 (1973) observed that additions of 2 or 4 percent equivalents from urea or diammonium citrate to a 10.14 percent protein basal diet did not sup- port equal performance of laying hens as that noted when equal protein equivalents were added from soybean meal. It was not possible to deter- mine whether any utilization of the NPN sources occurred, since an un- supplemented basal diet was not fed. The reason for the differences noted in utilization of NPN by the lay- ing hen remains unsettled. It would appear to date, however, that the extent of utilization of NPN by poultry would be the addition of 2-3 percent protein equivalent to a 12-13 percent laying diet that is well balanced in the essential amino acids and that the results from such a substitution might well be quite variable and of questionable value from a practical standpoint. SWINE Hoefer (1967), in an extensive review of the literature involving studies on the addition of urea to swine diets, concluded that the effect of urea on daily gain was either negative or neutral. Even in instances where the diet was deficient in total protein (Hanson and Ferrin, 1955; Hays et al., 1957), the addition of urea to the basal diet had little beneficial effect. However, no clinical evidence of toxicity was noted when pigs were fed urea at 1.5 percent of the diet (Hanson and Ferrin, 1955). Recent studies by Grimson et al. (1971) using urea labeled with 15N agree with the previous findings of Liu et al. (1955) in showing that a small but definite amount of the administered urea was incorporated into body protein. Liver, plasma, and intestinal scrappings had mark- edly higher levels of 15N label in the protein than existed in skeletal muscle. In further studies, Grimson and Rowland (1971) substituted 2 percent urea for herring meal on an isonitrogenous basis and noted depressed feed intake, daily gain, and feed conversion in pigs from 4 to 11 weeks of age when they were limit-fed, but not when they were fed ad libitum. Growing pigs that were fed a urea-containing diet from 11 weeks of age to 90 kg live weight gained less weight and required more feed per kg gain than nonurea fed controls. Supplementation of the urea-containing diet with L-lysine or L-lysine plus DL-methionine re- sulted in improved performance of pigs as compared to those fed the nonsupplemented diet. The performance of pigs fed the urea plus lysine diet was similar to that of pigs fed the control diet, which contained an equivalent amount of nitrogen from soybean and herring meal rather than from 2 percent urea. These studies indicated some utilization of
92 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition dietary urea nitrogen by pigs in the presence of supplemental L-lysine or L-lysine and DL-methionine; however, Crimson and Bowland (1971) state that the use of urea at a level of 2 percent in swine diets cannot be recommended as having practical value, even when the limiting essential amino acids are supplemented. Kornegay et al. (1970) demonstrated a limited degree of urea utiliza- tion in some experiments by growing pigs fed corn-soybean meal diets in which the urea was added at the expense of either corn or soybean meal protein. They noted that the basal diet must supply a proper bal- ance of essential amino acids before a positive response was obtained. In more recent studies, however, Kornegay (1972) observed that urea and ammonium polyphosphate, either singly or in combination, were ineffective in improving weight gain and gain:feed ratios when added to a low-protein diet. Wehrbein et al. (1970) conducted studies to evaluate the utilization of diammonium citrate or diammonium phosphate as sources of dietary nitrogen for growing-finishing pigs. Efforts were made to replace 0, 5, 10, or 20 percent of the protein in a corn-soybean diet for growing swine with nonprotein nitrogen from an equimolar mixture of diam- monium citrate and diammonium phosphate. With this experimental design, dietary essential amino acids as well as nonessential amino acids were being reduced. Average daily gain decreased as the increments of supplemented nonprotein nitrogen increased in the diets. Nitrogen re- tention was also decreased in pigs fed the diet containing NPN equivalent to 20 percent of the protein in the control diet. Supplementation of this diet with 0.05 percent DL-methionine, 0.20 percent L-lysine, and 0.05 percent DL-tryptophan partially alleviated the depressed growth rate and nitrogen retention. The results of recent studies with the pig therefore give little basis for changing the views of numerous investigators, as summarized by Hoefer (1967), that nonprotein nitrogen cannot be used to any great extent in practical swine diets. HORSES Prospects for utilization of nonprotein nitrogen by horses would appear to be better than that observed in poultry and swine as a result of the large functional cecum and possibility of some protein digestion and absorption of amino acids in the colon. Studies by Reitnour et al. (1969) with ponies with cecal fistula that were fed diets containing corn, oats, or barley as the protein sources indicated that the apparent protein di- gestion anterior and posterior to the fistula averaged 11 and 40 percent,
Considerations on the Use of NPN Compounds by Nonruminant Species 93 respectively. Subsequent studies by these workers, however, showed closer relationships between the amino acid patterns of dietary protein and serum than between those of cecal bacteria or cecal contents and serum (Reitnourera/., 1970). These workers note the problem of de- fining the precise location of sampling from cecal fistula because of mixing in the cecum. Hintz and Schryver (1972) observed equal nitro- gen retention from urea that was either fed or administered by cecal fistula, indicating that absorption of urea, ammonia, and/or amino acids produced in the cecum does occur. Whether the retention is due to nonessential amino acids synthesized by the liver or the absorption of amino acids synthesized by the microflora of the intestine remains unsettled. Studies measuring NPN utilization by the horse have given conflicting results in some cases. Reitnour and Treece (1971) observed that urea was not utilized. Slade et al. (1970) observed that nitrogen balance and digestibility were improved as a result of urea additions to diets in which fish meal or corn gluten meal were the major sources of protein. They suggested that the improved nitrogen retention resulted from microbial synthesis of protein or free amino acids from urea nitrogen and their subsequent digestion and absorption. Hintz and Schryver (1972) ob- served equal increases in nitrogen retention by mature ponies from iso- nitrogenous additions of either urea, soybean meal, or linseed meal when added to a low-protein basal diet. These workers concluded that equines can utilize urea to increase nitrogen retention when fed low- protein diets, but that in general the efficiency of the utilization of the absorbed nitrogen from urea is considerably less than that of nitrogen from intact protein. The horse appears to have the capacity to tolerate considerable quan- tities of urea. Ratliff et al. (1963) and Rusoff et al. (1965) fed from 0.5 to 0.55 Ib of urea per day and did not notice any adverse effects. In the studies by Rusoff et al. (1965), horses fed levels of urea up to 0.5 Ib/day for a period of 4 weeks exhibited weight gains, glossy coats, and good physical condition. Blood urea values increased from initial levels of 16 mg/100 ml to 90 mg/100 ml in horses fed the highest level of urea. SUMMARY AND CONCLUSIONS The following factors influence the response of nonruminants to NPN sources and nonessential amino acids: 1. Intake of total protein. 2. An adequate and balanced supply of essential amino acids.
94 Urea and Other Nonprotein Nitrogen Compounds in Animal Nutrition 3. A deficiency in dietary nonessential amino acids. 4. A single source of dietary nonessential amino acid that may not be as efficiently utilized as a mixture. 5. A nonutilizable source of nitrogen or a toxic effect from NPN. 6. The dietary mineral mixture. Attempts to obtain satisfactory utilization of various sources of NPN by chicks fed diets containing natural feedstuffs have not been success- ful. More success has been obtained when urea, diammonium citrate, or triammonium phosphate were added to crystalline amino acid diets de- void of nonessential amino acids. However, the practical use of NPN in chick diets awaits future research results. The inclusion of NPN in diets of laying hens has yielded controversial results. However, present results indicate that the extent of NPN utiliza- tion in diets of laying hens would be the substitution of 2-3 percent pro- tein equivalent in diets containing 12-13 percent crude protein in which the balance of dietary essential amino acids would have to be well balanced. The results from such a substitution may be variable and of questionable value from a practical standpoint. The results of studies in feeding NPN to swine indicate that it cannot be used to any appreciable extent in swine diets. The horse appears to have the capacity for greater utilization of NPN than chickens or swine; however, the efficiency of nitrogen utilization from NPN is considerably less than that of nitrogen from intact protein.
