In 1996 the Subcommittee on Beef Cattle Nutrition of the National Research Council released the seventh revised edition of the 'Nutrient Requirements of Beef Cattle' (subsequently updated as NRC, 2000). Among the more dramatic changes with this revision was the enhanced characterization of feedstuffs with respect to composition and degradation rates of N and carbohydrate fractions, with the bold intent of factorializing them into estimates of metabolizable protein supply to the small intestine. It had been recognized for many years that net protein supply to the small intestine was a function of UIP content of the diet and ruminal microbial protein synthesis. However, at the time the sixth revised edition was released (NRC, 1984) the factors involved in predicting protein supply were poorly understood, and mathematical manipulations were sufficiently complicated to make calculations impractical for most nutritionist. Subsequent advances in ruminant protein nutrition and computer technology led to this first attempt (NRC, 2000) at expressing requirements on the basis of metabolizable protein and amino acid supplies. Estimates of supply are derived at two levels of abstraction. "Level 1" (L1) combines tabular measures of dietary undegradable intakeprotein (UIP) and with estimates of microbial N yield based on dietary TDN (20.8 g/kg TDN), adjusted for eNDF [microbial N = (0.0208TDN) * (1- ((20 - eNDF) * 0.025), where TDN is expressed in kg/d, and eNDF as a percentage of DM intake]. "Level 2" (L2),on the other hand, combines estimates of degradation rates and passage rates of dietary carbohydrate and protein fractions to calculate UIP and MP.
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