PREDICTING AMINO-ACID ADEQUACY OF DIETS FED TO HOLSTEIN STEERS

The Cornell Net Carbohydrate and Protein System (CNCPS) and NRC (1985) models were evaluated for accuracy in predicting metabolizable protei n (MP) and essential amino acid (EAA) allowable ADG, using chemical bo dy and feed composition data from feeding trials with Holstein steers. Nine Holstein steers (113 to 200 kg) were slaughtered and determined to have the following whole-body essential amino acid composition of ( grams/100 grams of protein): arginine, 5.94; histidine, 2.07; isoleuci ne, 2.28; leucine, 5.72; lysine, 5.81; methionine, 1.99; phenylalanine , 3.04; threonine, 3.52; tryptophan, .57; and valine, 3.32. The NRC an d CNCPS were then tested against data from 25 feeding periods, each re presenting the 56-d growth of 10 Holstein steers (mean BW of 162 kg), to determine their ability to predict the gain allowed by the supply o f MP and the first-limiting EAA. The NRC (1985) system accounted for 4 6% of the variation in MP allowable gain, with an average bias of -30% . The CNCPS accounted for 87 and 73% of the variation in MP and EAA al lowable gain, with a bias of 8 and 5%, respectively. The bias was redu ced to 3% (R2 of .82) when ADG was predicted by the factor (ME, MP, or EAA) first-limiting ADG.