likelihood of ammonia toxicity (Bartley et al., 1976). If NPN is substituted for dietary protein, special emphasis should be given to the supplementation of potassium, phosphorus, and sulfur, which are absent in urea. Particular attention should be given to sulfur supplementation, because wool contains a high percentage of sulfur-containing amino acids.
In attempts to reduce the threat of ammonia toxicity and/or improve the utilization of ruminal ammonia, other forms of NPN (biuret, triuret, and complexes of urea with formaldehyde or molasses) have been developed (Nikolic et al., 1980). These compounds have slower ammonia release, which should more nearly parallel energy availability and increase bacterial protein synthesis (Johnson, 1976). However, these slow release forms of NPN have not consistently improved nitrogen utilization (Owens and Bergen, 1983).
Dietary protein is either digested in the rumen or escapes undigested to the omasum and abomasum. If it is not digested in the rumen, it is described as "bypass" or "escape" protein (Owens and Bergen, 1983). Bypass protein is either digested postruminally or excreted in the feces. Dietary protein degraded in the rumen yields ammonia (Chalupa, 1975), which can then be incorporated into microbial protein. Chalupa (1975), Satter and Roffler (1975), and ARC (1980) have classified protein sources on the basis of the extent to which they bypass ruminal degradation (percentage of dietary protein that reaches the small intestine undigested). Low-bypass sources (‹ 40 percent) include casein, soybean meal, sunflower meal, and peanut meal; medium-bypass sources (40 to 60 percent) include cottonseed meal, dehydrated alfalfa meal, corn grain, and brewers dried grains; and high-bypass sources (› 60 percent) include meat meal, corn gluten meal, blood meal, feather meal, fish meal, and formaldehyde-treated proteins. Feed processing conditions, animal variations, dietary alterations, and changes in microbial population affect extent of dietary protein bypass, but these effects have not been well quantitated. When high-bypass protein sources are fed, supplementation with NPN will be needed to maintain adequate ruminal ammonia levels for microbial protein synthesis.
Increased bypass of dietary protein does not always increase production, because bypassed protein may be poorly digested postruminally, the balance of amino acids available for absorption from the small intestine may be poor, or other nutrients may limit production (Young et al., 1981; Owens and Bergen, 1983). Conversely, if microbial protein is the only protein reaching the small intestine, animal production may not be maximal (Satter et al., 1977). Presentation to the small intestine of a mixture of microbial protein and complementary dietary protein is desired. Striving to optimize this mixture will undoubtedly be the subject of much research activity in the future, as it has been in the past.
Amino acids available for absorption from the small intestine are supplied by microbial and/or bypassed dietary protein. The tissues of sheep require the same amino acids as those of the nonruminant (Black et al., 1957; Downes, 1961). In sheep, however, the relationship of dietary amino acid supply with tissue requirements has been difficult to define because of the intervention of the protein digestive and synthetic functions in the rumen. Also, amino acid requirements are difficult to quantitate because of variability in requirements for various productive functions. For example, wool growth responds to sulfur amino acid supplementation (Reis and Schinckel, 1963), whereas other functions do not. Hogan (1975) concluded that the amino acid composition of protein deposited in the tissues and that secreted in milk, plus the maintenance requirement, should equal the total needed by the animal. Owens and Bergen (1983) further concluded that the quantity, as well as the ratios, of amino acids required by the animal varies with both the productive function and the level of production.
Dietary amino acids are normally rapidly degraded in the rumen. To increase bypass, Neudoerffer et al. (1971) and Digenis et al. (1974) coated dietary amino acids so they would be ruminally stable but available for absorption postruminally, suggesting that the combination of amino acid and NPN supplementation may be feasible in the future.
Ammonia deficiency in the rumen reduces the extent and efficiency of rumen function. Deficiencies or imbalances of amino acids at the tissue level result in decreased protein synthesis, as well as reduced feed intake and lower efficiency of feed utilization. Growth rate and milk and wool production all react to inadequate protein intake. Extreme deficiency results in severe digestive disturbances, loss of weight, anemia, edema, and reduced resistance to disease. Increased feed intake after that protein was deficient (NRC, 1984).
Excess protein becomes an expensive and inefficient source of energy, but rather large excesses can be fed without producing acute toxicity (Fenderson and Bergen, 1976). Excesses of NPN or highly soluble protein may