Evaluation of supplements to enhance nutrient supply and milk production of cattle fed diets based on elephant grass ( Pennisetum purpureum) using a mathematical model

Abstract

A previously developed dynamic model of digestion and absorption of nutrients in cattle fed sugarcane-based diets ( Dijkstra et al., 1996a) was modified to evaluate the effect of various supplements on the supply of nutrients and milk production of dairy cows fed diets based on elephant grass ( Pennisetum purpureum). The predicted nutrient supply was examined by varying the fractional passage rates of fluid and solid material, and by varying the microbial biomass composition. Besides, model predictions were compared with observations of faecal NDF flow, rumen ammonia and VFA concentrations, and milk production. The milk production is predicted from the simulated availability of glucogenic, lipogenic and aminogenic nutrients and the total amount of energy available, to indicate which type of nutrient is most limiting. The behaviour of the model with respect to overall stability and sensitivity to parameter perturbation was generally considered to be satisfactory. The simulations showed the relevance of careful estimation of fractional passage rates. In comparison with experimental observations, the model behaviour is unsatisfactory with respect to the prediction of rumen Am and VFA concentrations. Model predictions appeared adequate to predict faecal NDF flow. However, model predictions on milk production were not always adequate. The simulations suggest that the low availability of glucose and glucose precursors is the factor most limiting milk production on unsupplemented elephant grass-based diets. The paucity of firm data on elephant grass-based diets prevents the use of this model for prediction purposes in practice.