Integration of Theories of Intake Regulation in Growing Ruminants

Abstract

A model to examine physical and metabolic limits to intake was derived. It used a metabolism submodel to investigate metabolic factors involved in intake regulation, these being genetic limit to protein deposition, heat dissipation and ATP degradation via substrate cycling. The metabolism submodel relied on a knowledge of the profile of absorbed nutrients and utilized kinetics of known metabolic transactions to predict protein and fat deposition, heat production and ATP balance. Intake may also be limited by physical means and three factors were investigated, namely rate of intake, rumen turnover of plant cell wall and faecal output. The calculation of rumen turnover utilized an approach based on fractional digestion rate, extent of digestion for escape to occur and maximum rumen fill. The inclusion of rate of intake would enable data from grazing behaviour studies to be incorporated and so widen application of the model. The approach was to identify pathways (factors) that could limit intake (six identified), calculate the maximum intake possible for each pathway and in doing so identify the pathway with the lowest intake, which became the predicted intake. Sensitivity analysis was done on key parameters. Seven diets were chosen for examination, ranging from low digestibility grass to a predominantly cereal diet. In general, the model behaved satisfactorily predicting intakes in line with those reported in the literature. Two major points arose. First, for some diets, more than one pathway may be approaching its maximum intake simultaneously. This means that removal of one of the constraining pathways by some experimental means may not lead to the expected increase in intake, and hence result in incorrect rejection of the hypothesis that this pathway was involved in intake control. Second, ATP degradation featured as a limiting pathway for most diets and reflects the importance that balance of nutrients has on intake regulation. It was concluded that the model provided a way to integrate a number of pathways, both physical and metabolic, involved in intake control and indicated that metabolic regulation may be more encompassing than previously realized.