In situ digestibility method as an input-source in the context of the Pampa Corte model: predicting average daily gain by free-ranging cattle

ABSTRACT: This study evaluated the adequacy of using digestibility values obtained by the in situ (ISOMD) method for predicting the average daily gain (ADG, g day-1) of free-ranging cattle through the Pampa Corte model. The evaluation was based on a data set compiled from two independent trials conducted with growing beef cattle grazing native grasslands of southern Brazil. Values of digestibility obtained through the N fecal method (FNOMD) were used as comparative reference in the present study. The predictions of ADG were performed at both individual (n= 41) and group (i.e. diet treatment within trials, n= 10) levels. Parameters generated from regression analyses between observed versus estimated values of ADG (i.e. determination coefficient (R2), root mean square error (RMSE), root mean square error of prediction (RMSEP), concordance correlation coefficient (CCC) and mean bias) were used to evaluate the predictive adequacy. The observed values of ADG were linearly related to those predicted by the Pampa Corte at both individual (R2= 0.52; RMSE= 148.9, CCC = 0.72, P < 0.05) and group levels (R2= 0.67; RMSE= 108.8, CCC = 0.81, P < 0.05), without effect of the method used to estimate herbage digestibility. However, compared to the FNOMD, the ISOMD resulted in higher mean bias of predictions (i.e. - 4.3 vs 61.6 g day-1). The ADG of free-ranging cattle grazing native grasslands of Southern Brazil can be adequately predicted by the Pampa Corte model using herbage digestibility values obtained through the ISOMD method. However, constraints associated with herbage sampling in heterogenous swards should be considered.