any energy for ruminal microorganisms, and the energy obtained from protein fermentation is minimal (Nocek and Russell, 1988). Further, ensiled (fermented) forages may provide less energy for microorganisms than comparable fresh or dry feeds (Agricultural and Food Research Council, 1992), but this has not been documented for silages and high-moisture grains in the United States.
Carbohydrate digestion in the rumen is likely the most accurate predictor of BCP synthesis, and this mechanism is used in model level 2. However, for feedstuffs used for beef cattle few good data are available for rates of digestion and of passage of the different carbohydrates potentially digested in the rumen. More accurate values are available for TDN, and laboratory predictors of TDN can be used to estimate BCP synthesis. Therefore TDN is used as the indicator of energy availability in the rumen for level 1. The Agricultural and Food Research Council (1992) found that total tract digestible organic matter intake was the most precise indicator of BCP synthesis when nitrogen intake was adequate. Digestible organic matter and TDN are roughly equivalent in feedstuffs and diets.
The requirement for rumen degradable protein (including nonprotein nitrogen [NPN]) is considered equal to BCP synthesis. This assumes that the loss of ammonia from the rumen as a result of flushing to the duodenum and absorption through the rumen wall is equal to the amount of recycled nitrogen. A number of factors affect each of these fluxes of nitrogen (National Research Council, 1985) but rather complex modeling is needed (Russell et al., 1992) to account for them. Simply put, a deficiency of ruminal ammonia encourages recycling and an excess encourages absorption from the rumen. Therefore, a balance (rumen degradable protein in diet equal to BCP synthesis) minimizes both recycling and absorption. Few studies have attempted to titrate the need for rumen degradable protein. Karges (1990) found 10.9 percent of TDN as rumen degradable protein was needed to maximize gain in beef cows, presumably to maximize BCP synthesis; Hollingsworth-Jenkins (1994) found only 7.1 percent DIP was needed to maximize gain. These values are smaller than the value of 13 percent used in this publication to calculate BCP synthesis.
Optimum use of rumen degraded protein (including nonprotein nitrogen) would logically occur if protein and carbohydrate degradation in the rumen were occurring simultaneously. This is not the case in many diets. Protein degradation of many of the forages, for example, is rapid and degradation of energy-yielding components of NDF is much slower. With grains (for example, corn and sorghum) the opposite is true—slow protein degradation and rapid starch degradation. This results in low ruminal ammonia levels from high-grain diets postfeeding and high levels from forage diets, which is influenced by CP levels.
The ruminant compensates by recycling nitrogen. An excellent example of this is how cow performance is similar with protein supplementation either three times per week or once per day (Beaty et al., 1994). More basic studies with animals (Henning et al., 1993; Rihani et al., 1993) suggest little or no advantage to synchrony of energy availability and protein breakdown. Cattle also compensate by eating numerous meals per day such as in the feedlot.
Use of NPN is appropriate in high-grain diets (National Research Council, 1984, 1985; Sindt et al., 1993) because of the rapid rumen degradation of starch. The value of NPN in low-protein, high-forage diets is less clear (Rush and Totusek, 1975; Clanton, 1979). Reduced gains when using urea as opposed to a “natural” protein may be the result of insufficient UIP rather than the faster rate of ammonia release in the rumen. Until more information is available, it is advisable to use caution when using urea in low-protein, high-forage diets.
Russell et al. (1992) have demonstrated the need for amino acids and peptides for optimum BCP synthesis, and this concept is used in model level 2. A lack of amino acids or peptides is unlikely to be a problem in typical diets for beef cattle. Adequate MP in finishing diets can be accomplished by adding urea (Sindt et al., 1993). Fiber-digesting bacteria use primarily ammonia for BCP synthesis (Russell et al., 1992), so amino acids/peptides should not be limiting in the rumen. However, these fiber-digesting bacteria may require branched-chain volatile fatty acids (National Research Council, 1985), which would be supplied by amino acid degradation. A need for rumen degradable protein (other than NPN) might occur in diets containing mixtures of forage and grain such as “step-up” rations for finishing cattle (Sindt et al., 1993).
Digestibility of protein is important—for both BCP and UIP. In this publication, the value of 80 percent digestibility of BTP (National Research Council, 1985) is used. UIP digestibility may vary with the source; however, it is assumed that UIP is 80 percent digestible. National Research Council (1985) used 0.8 BCP=BTP because BCP contains approximately 20 percent nucleic acids. This value has been challenged by other MP systems (Agricultural and Food Research Council, 1992). Logically, the important measure is amino acid content (true protein) of BCP. These measures (Agricultural and Food Research Council, 1992) suggest a value of 0.75 rather than 0.8. However, the net absorption of amino acids is the important coefficient. Systems using lower BCP to BTP values used higher (0.85) digestibility values for BTP; therefore, these values compensate. Until more definitive data are available in the United States on digestible amino acid content of rumen bacteria, use of the value of 0.64, calculated as 0.8 BCP=BTP * 0.8 digestibility of BTP is suggested.