Perspectives on ruminant nutrition and metabolism I. Metabolism in the rumen.

Advances in knowledge of ruminant nutrition and metabolism during the second half of the twentieth century have been reviewed. Part I is concerned with metabolism in the rumen: Part II discusses utilization of nutrients absorbed from the rumen and lower tract to support growth and reproduction. The time frame was prompted by the crucial advances in ruminant physiology which arose from the work of Sir Jospeh Barcroft and his colleagues at Cambridge in the 1940s and 50s, and by the brilliant studies of Robert Hungate on rumen microbiology at much the same time. In reviewing the growth of knowledge of the role of bacteria, protozoa, fungi and bacteriophages in the rumen, outstanding developments have included the identification and characterization of fungi and the recognition that the utilization of polysaccharides in the rumen is accomplished by the sequential activities of consortia of rumen microorganisms. The role of protozoa is discussed in relation to the long standing debate on whether or not the removal of protozoa (defaunation) improves the efficiency of ruminant production. In relation to nitrogen (N) metabolism, the predation of bacteria by protozoa increases protein turnover in the rumen and reduces the efficiency of microbial protein production. This may account for the beneficial effects of defaunation where dietary N intakes are low and possibly rate limiting for growth and production. Current approaches to the measurement of rates of production of short chain fatty acids (SCFA) in the rumen based on the mathematical modelling of isotope dilution data are outlined. The absorption of SCFA from the rumen and hindgut is primarily a passive permeation process. The role of microorganisms in N metabolism in the rumen has been discussed in relation to ammonia and urea interrelationships and to current inadequacies in the measurement of both protein degradation in the rumen and microbial protein synthesis. The growth of knowledge of digestion and absorption of dietary lipids has been reviewed with emphasis on the antimicrobial activity of lipids and the biohydrogenation of unsaturated fatty acids. The protection of unsaturated dietary fats from ruminal biohydrogenation is an approach to the manipulation of the fatty acid composition of meat and dairy products. Discussion of the production of toxins in the rumen and the role of microorganisms in detoxification has focused on the metabolism of oxalate, nitrate, mycotoxins, saponins and the amino acid mimosine. Mimosine occurs in the tropical shrub leucaena, which is toxic to cattle in Australia but not in Hawaii. Tolerance to leucaena stems from the presence of a bacterium found in the rumen of Hawaiian cattle, which when transferred to Australian cattle survives and confers protection from mimosine. The genetic modification of rumen microorganisms to improve their capacity to ultilize nutrients or to detoxify antinutritive factors is an attractive strategy which has been pursued with outstanding success in the case of fluoroacetate. A common rumen bacterium has been genetically modified to express the enzyme fluoroacetate dehalogenase. The modified organism has been shown to survive in the rumen at metabolically significant levels and to confer substantial protection from fluoroacetate poisoning.