INVITED REVIEW: Applied protein nutrition of ruminants-Current status and future directions1

The metabolizable protein (MP) system initially outlined by Burroughs et al. (1975) and expanded in later NRC publications separates the need for N for ruminal microbes within the rumen from the postruminal need for amino acids for growth and maintenance of the host ruminant. Compared with the CP system, the MP system represents a clearer understanding of the complexity of the protein metabolism of ruminants. Using compiled data sets from recent publications, the effect of dietary CP concentration on performance of feedlot and dairy cattle was reevaluated. Maximum performance (rate of gain; milk production) required higher CP concentrations than routinely are being fed, with the added performance being due at least partially to greater DMI of diets containing more CP. Precision of the NRC (2000) MP model 1 was evaluated by comparing its predicted values with measurements of duodenal flow determined with growing-finishing cattle fed 118 different concentrate-rich diets. Though duodenal supply of microbial protein increased with intake of TDN, diet TDN concentration was not correlated (R2 = 0.00) with the extent of OM fermented in the rumen. Although undegraded intake protein and MP increased with DMI, precision of predicting degraded intake protein, undegraded intake protein, and MP per kilogram of diet was poor, reflecting imprecision of estimates or equations and failure of prediction equations to match in vivo measurements. The paucity of data supporting values suggested for individual feeds and inclusion of numerous theoretical but unverified equations within current MP models severely limits their precision and usefulness for field application.