A simulation model to predict body weight gain in growing steers grazing tropical pastures

A simulation model was developed to predict performance of growing steers grazing tropical pastures. The model is deterministic and integrates the effects of protein and energy intake from forages and supplements. Protein deposition was predicted by the PDI system. Energy balance was based on the California Net Energy System with modifications for animal activity and heat stress. Three output results of average daily gain (ADG) were obtained with the model: from energy (ADGe), from PDIN (ADGpdin) and from PDIE (ADGpdie). The model was validated using data from five experiments (29 treatments) conducted in the humid tropics of Mexico. The model did not accurately predict weight loss in steers based on energy or protein intake. Estimation of ADG from energy intake was successful in only six treatments, under-estimating gain by 30% with ADGe. Gain was predicted by ADGpdin in only one treatment, showing a general under-estimation of 40%. Values of ADGpdie were similar to the observed values in 10 treatments, under-estimating gain in 70% of the observed values. Prediction of performance in grazing steers in tropical conditions requires more information about environmental factors, as well as precise and accurate determinations of intake.