Fecal crude protein content as an estimate for the digestibility of forage in grazing sheep

Regression equations were developed to predict the organic matter (OM) digestibility of forage-based diets from contents of crude protein (CP) and acid detergent soluble CP (ADSCP) in feces, with data (721 observations of nine types of diets) obtained from in vivo digestibility trials (n = 159) with sheep conducted at the Federal Research Centre of Agriculture, Braunschweig, Germany and in Inner Mongolia, China. Using the non-linear mixed model procedure, the developed regression equation was: OM digestibility = 0.899 − 0.644 × exp (−0.5774 × fecal CP (g/kg OM)/100). The probability values of estimated parameters of the equation were higher than 0.05, and a random effect of diet was not observed (P=0.131). The mean prediction error (MPE) as proportion of observed mean was 0.071, indicating that the equation fits the data reasonably well. The equation slightly under-predicts the OM digestibility with a small mean bias of −0.014, representing 0.089 of the total mean square prediction error (MSPE). The difference between estimated and measured OM digestibility was mainly caused by random variation (0.899). This indicates that the developed equation is accurate and can be used to predict the OM digestibility of forage ingested by sheep grazing on heterogeneous pasture. Although inclusion of ADSCP instead of CP in the prediction equation results in a slightly lower under-prediction (mean bias of −0.011, representing 0.062 of MSPE), the prediction accuracy is not improved.