Metabolic constraints on voluntary intake in ruminants.

The weak point in all current methods or models of diet formulation is the prediction of intake. The major uncertainty is not in the cases in which physical constraints apply, but in those in which voluntary intake is limited by feedback from metabolic factors. Voluntary intake is, ultimately, a psychological phenomenon, involving the integration of many signals, and reflects the flexibility of biological systems evolved to cope with variability in food supply, composition and animal state. Conditions giving rise to regulatory signals may provide a framework for modeling metabolic constraints on intake. The empirical evidence for metabolic feedback shows that the animal's productive potential, which affects its ability to utilize nutrients, interacts with the balance of absorbed nutrients to regulate intake. The relative importance of the sites where nutrient imbalance occurs (microbial or host animal metabolism) is unclear, as is the relevant time scale (minutes or days) of response. A model of the effects of asynchrony of nutrient supply to ruminal microbes suggests that ammonia and microbial recycling and the contribution of hind-gut fermentation reduce the asynchrony in the balance of nutrients absorbed into the bloodstream. Hitherto, rather little progress has been made in mathematical modeling of the metabolic processes controlling intake. Models that describe the phenomenon in terms of global variables, such as total energy intake, protein supply, and protein synthetic capacity, can simulate the way constraints may operate without requiring or providing a deeper understanding of the metabolic processes involved. Models describing the flux of energy and materials down established metabolic pathways have the potential to explore constraints on intake, but until the problem of parameterizing such models can be overcome, that potential will remain untapped.