Estimating ruminal crude protein degradation with in situ and chemical fractionation procedures

The objective of this study was to utilize the fractionation of feed crude protein (CP) of the Cornell net carbohydrate and protein system (CNCPS) as a basis for estimating undegraded dietary protein (UDP) values of feedstuffs obtained from in situ trials. In addition, the experiments comprised a comparison between in situ UDP values of feedstuffs and CP solubility estimated from the protein dispersibility index. Eleven dairy compound feeds and 21 feedstuffs were inserted in polyester bags and incubated in the rumen of three steers. Values for in situ UDP at assumed ruminal passage rates of 2, 5, and 8% h−1, respectively, ranged from 63 to 616, 129 to 785, and 167 to 842 g kg−1 of CP. When fish meal data (n=2) were excluded from the data set, multiple regression equations that were based on concentrations of CP and cell wall, and on the A, B, and C fractions of the CNCPS fractionation schedule, explained 87, 93, and 94%, respectively, of the variation in UDP values at assumed ruminal passage rates of 2, 5, and 8% h−1. We conclude that in situ UDP values, which serve as one key variable in many protein evaluation systems for dairy cattle, may be reliably and accurately predicted from chemical fractionation of feed CP according to the CNCPS. The coefficients of determination of estimating UDP values at assumed ruminal passage rates of 2, 5, and 8% h−1, respectively, from the protein dispersibility index were only 0.30, 0.29, and 0.33. Hence, the protein dispersibility index was not suitable as a predictor of UDP values for the feedstuffs used in the present study.