Modelling lipid metabolism in the rumen.

Abstract

AbstractNutritional manipulation through fat supplementation of diets is of interest because of the large energy requirements of high-producing ruminants, and because of human health concerns about saturated long-chain fatty acids (LCFA) in ruminant products. High dietary fat levels may adversely affect microbial activity and nutrient degradation in the rumen. Mathematical models of the rumen fermentation process have been developed and applied to quantify the profile of nutrients available for absorption. However, the representation of lipid dynamics is limited in these rumen models. Therefore, an extant rumen model was modified to represent the effects of LCFA on microbial metabolism and nutrient digestion. The primary objective of the model currently being developed is to integrate LCFA metabolism with microbial metabolism of other substrates in the rumen, and to predict the effect of amount and composition of dietary fat upon the profile of nutrients available for absorption. In the modified model, there are three rumen lipid pools: unhydrolysed lipid (Li), saturated free LCFA (Fs) and unsaturated free LCFA (Fu). Inputs to these pools are from the feed and outputs are flows to the duodenum. The rate of biohydrogenation is dependent on the amount of fibre and of unsaturated LCFA in the rumen. The unsaturated LCFA are considered to inhibit fibrolytic bacterial metabolism, but not amylolytic bacterial metabolism, whilst saturated LCFA are assumed not to affect bacterial metabolism. Protozoal metabolism is assumed to be inhibited by both saturated and unsaturated LCFA. The rates of biohydrogenation and inhibition of bacterial metabolism are represented by sigmoidal Michaelis-Menten equations, with parameters derived from in vitro and in vivo data. Preliminary results of sensitivity analyses suggest that the model responds appropriately to perturbations in dietary inputs. The degradation of fibre in the rumen was reduced when the amount of unsaturated LCFA in the diet was raised, but the reduction in fibre degradation was far more pronounced with a low forage diet in comparison to a high forage diet. In all cases, the efficiency of microbial growth was increased upon an increase in dietary LCFA content because of a reduction in protozoal numbers and consequently in microbial recycling within the rumen. Decreasing the forage proportion of the diet decreases the degree of saturation of LCFA flowing to the duodenum. A decrease in acetate to propionate ratio in rumen fluid was only apparent when a diet high in unsaturated LCFA was fed. Such responses are related to the effects of unsaturated LCFA and saturated LCFA on metabolism of amylolytic and fibrolytic bacteria and of protozoa, and consequently on nutrient degradation in the rumen.