Recent advances in understanding of microbial transformation in ruminants

Abstract

In this review some recent advances in understanding of microbial transformations of nutrients in ruminants is discussed. It is argued that a greater understanding of the importance of an optimal rumen environment for cellulolysis has been achieved in recent years. It is recognised that not only deficient nutrients e.g. N and S, and low pH can limit degradation rate but also that the number of microbes in solution can be a limiting factor particularly for very poor quality roughages. The general conclusion that microbial biomass is a constant relative to fermented carbohydrate is now challenged largely due to development of a new non-invasive technique of estimating microbial protein by urinary purine derivative. Turnover rate of microbes within the rumen and rate of outflow has been identified as a very important factor affecting the ratio of volatile fatty acids to microbial biomass. While the proportion of volatile fatty acids has generally no effect on their utilization, a high propionic acid proportion can adversely affect milk production. High propionic acid proportion is generally associated with production of large amounts of B 12 analogues which can compete with true B 12 for transport systems and thus indirectly cause B 12 deficiency in sheep. It is concluded that optimal feed utilization and health of the animals can be achieved by ensuring optimal condition for cellulolysis in the rumen and that except for protein and fat there is little or no advantage in encouragement of postruminal digestion of carbohydrates.