A review of energy metabolism in producing ruminants. Part 1: Metabolism of energy substrates.

The efficiency of metabolisable energy utilisation, for growth and fattening, is dependent upon the relative VFA proportions produced in the rumen. Sufficient propionate is required to meet glucose demand for producing NADPH, glycerol and nucleic acid synthesis. Since diet has the greatest effect on the pattern of VFA fermentation, it will play a major role in controlling the supply of VFA to the animal. Magnitude of the acetate supply determines the proportion of acetate supplied to oxidation or to fatty acid synthesis, which is also dependent upon the extracellular supply of glucose, NADPH and ATP. Since the optimal levels of acetate and glucose for lipogenesis appear to vary with glucose concentration, a diet that decreases the supply of glucogenic precursors, but increases the acetate supply, may suppress fatty acid synthesis. An increased supply of propionate may suppress glucose synthesis from other sources. The isoenergetic replacement of roughage by concentrate, appears to increase the glucose entry rate, due to both an increase in propionate, and glucose absorbed from the small intestine. Dietary nitrogen source also affects the rate of gluconeogenesis. An optimum dietary energy-protein ratio exists for maximum efficiency of utilisation of both dietary energy and protein. In dairy cows, for example, the energy is most effectively metabolised when protein content of the diet is 15-25% of net energy.