Evaluation of the Tabular Feed Energy and Protein Undegradability Values of the National Research Council Nutrient Requirements of Beef Cattle

Evaluations with animal performance data indicated the tabular feed energy and undegraded intake protein (UIP) values used by the National Research Council's (NRC) Nutrient Requirements of Beef Cattle (2000) model Level 1 were less accurate than those predicted by their model Level 2 (the Cornell Net Carbohydrate and Protein System model; CNCPS; Fox et al., 2004) or by an alternative mechanistic equation developed by Weiss et al, (1992)(WEISS). The r2 of observed vs predicted ADG using tabular, WEISS, and CNCPS total digestible nutrients (TDN) were 0.61, 0.73, and 0.80 for ME-allowable ADG and were 0.80, 0.79, and 0.92 for metabolizable protein (MP)-allowable ADG, respectively. The tabular system overpredicted ME- and MP-allowable ADG by 11 and 4%, respectively, whereas WEISS and CNCPS biases were <3%. The variation accounted for ranged from 48% for grass forages to 82% for grain-type forages, suggesting that each feed's tabular TDN and feed carbohydrate and protein fractions did not come from the same sources. An evaluation of the tabular UIP values indicated inconsistencies between tabular- and CNCPS-predicted UIP values. The CNCPS was used to develop equations to adjust maintenance TDN and UIP values for levels of intake. We recommend the prediction of TDN by WEISS or CNCPS and further adjustment for level of DMI to predict NE values rather than tabular values of TDN to estimate diet ME, NEm, and NEg. The CNCPS model contains the NRC (2000) requirements and uses WEISS in Level 1. It can be downloaded at http://www.cncps.cornell.edu.