S U M M A R YA previously described model of digestion in cattle fed sugarcane diets was applied to predict nutrient supply to the host animal from dietary intake, to indicate pre-experimentally the suitability of various supplements to enhance the milk production of dairy cattle in the tropics fed sugarcane-based diets. Potential milk production was calculated according to simulated energy and lipogenic, glucogenic and aminogenic nutrient availability. The addition of small amounts of urea (10 g/kg sugarcane fresh weight) improved microbial efficiency and degradation of substrates in the rumen, but further increases could only be achieved by supplements containing rumen degradable protein. The simulated level of absorbed amino acids was low in comparison with the level of absorbed energy and glucogenic substrates. This low amino acid availability was the factor most limiting milk production on sugarcane diets and supplements which increase the amino acid availability in the gastrointestinal tract are recommended. At increasing supplementation or intake levels, and once amino acid requirements were met, an increased absorption of energy and long chain fatty acids could enhance milk production. These suggestions should be evaluated experimentally in order to improve the model and ultimately, to result in an improved system to evaluate feeds, based on the supply and utilization of individual substrates.

A dynamic model of digestion and absorption of nutrients in cattle fed sugarcane-based diets is described. There are 11 rumen state variables, and four zero pools representing absorbed nutrients. The rumen state variables represent nitrogen, carbohydrate, long chain fatty acid, microbial and volatile fatty acid pools. The zero pools relate to absorbed amino acids, glucose, long chain fatty acids, and volatile fatty acids. The flux equations are described by mass-action and Michaelis-Menten forms. Wherever possible, data derived from trials with cattle fed sugarcane-based diets were used to parameterize the model. Sensitivity analyses revealed that stability and behaviour of the model was generally satisfactory. The model was most sensitive to changes in fractional passage and substrate hydrolysis rates and to the microbial maintenance requirement. Within the limited information available for comparison, the simulations agreed with observations of fibre flows and ammonia and volatile fatty acid concentrations in rumen fluid. Duodenal non-ammonia nitrogen flow was consistently underpredicted and reasons for this underprediction are suggested.

A model of sugarcane digestion was applied to indicate the suitability of various locally available supplements for enhancing milk production of Indian crossbred dairy cattle. Milk production was calculated according to simulated energy, lipogenic, glucogenic and aminogenic substrate availability. The model identified the most limiting substrate for milk production from different sugarcane-based diets. For sugarcane tops/urea fed alone, milk production was most limited by amino acid followed by long chain fatty acid availability. Among the protein-rich oil cake supplements at 100, 200 and 300 g supplement/kg total DM, cottonseed oil cake proved superior with a milk yield of 5·5, 7·3 and 8·3 kg/day, respectively. This was followed by mustard oil cake with 5·1, 6·5 and 7·6 kg/day, respectively. In the case of a protein-rich supplement (fish meal), milk yield was limited to 6·6 kg/day due to a shortage of long chain fatty acids. However, at 300 g of supplementation, energy became limiting, with a milk yield of 6·7 kg/day. Supplementation with rice bran and rice polishings at 100, 200 and 300 g restricted milk yield to 4·3, 4·9 and 5·5 and 4·5, 5·3 and 6·1 kg/day, respectively, and amino acids became the factor limiting milk production. The diet comprising basal sugarcane tops supplemented by leguminous fodder, dry fodder (e.g. rice or wheat straw) and concentrates at levels of 100, 200 and 300 g supplements/kg total diet DM proved to be the most balanced with a milk yield of 5·1, 6·7 and 9·0 kg/day, respectively.

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To eliminate unnecessary feeding trials, a mechanistic model of sugarcane digestion was used in the search for suitable supplements to improve milk production. Milk production simulated by the model was compared with data observed in four feeding trials published in the tropical literature where crossbred dairy cows were fed sugarcane/urea diets with different types of supplements. The predicted effects of the supplements on the ruminal microbial population, concentrations of ammonia and volatile fatty acids were also compared with the published results in one experiment. The model indicated the nutrient most limiting milk production for the different feeding situations. The addtion of Leucaena to the basal sugarcane/urea improved the availability of amino acids and long-chain fatty acids, with energy becoming the limiting factor. Supplementation with rice bran increased the availability of energy and long-chain fatty acids, but amino acids then became the limiting factor. Supplementation with both Leucaena and rice bran further improved the milk yield, but availability of energy now limited milk production. Supplementation with Leucaena increased milk production more than supplementation with king grass. The main reason for this increase was increased amino acid absorption due to increased microbial outflow. In all feeding situations, the average difference between the predicted milk production and that observed experimentally was 0.57 kg/d (ranging from 0.08 to 1 kg/d).