[Purine and pyrimidine metabolites for the estimation of rumen metabolism: HPLC analysis in milk and blood plasma]

Metabolism of microbial nitrogen in ruminants with special reference to nucleic acids

Characteristically the metabolism of microbial nitrogen (N) compounds in ruminants involves the degradation of dietary N and synthesis of microbial protein (MP), compounds including a small amount of peptides and free amino acids, which may account for 75-85% of total N and the remainder are nucleic acids (NA: DNA and RNA). Generally rumen microbes contain 10-25% NA-N of the total N while 70-80% is in the form of RNA. This paper describes the degradation and synthesis of NA in the rumen and their fate in the lower digestive tracts. Their physiological and nutritional significance in different types of ruminant animals is also discussed. The research works on NA metabolism in ruminants has been mainly on metabolism of purines after rumen microbial digestion and absorption in the lower gut. Subsequently, the fate of absorbed purines has been intensively investigated to assess the extent of MP synthesis in the rumen. The method for predicting ruminal synthesized MP and subsequently digested MP has been proposed using urinary purine derivative (PD) excretion in sheep and cattle fed on ordinary feed. The latter approach has now been adopted for calculation of protein supply in some feeding standards, although there are still difficulties in predicting representative samples of rumen microbes, and also uncertainties in variations of non-renal and endogenous purine losses.

Lactational and metabolic effects in cows of lysine and methionine added to a ration deficient according to the I.N.R.A. method

The effects of adding lysine and/or methionine to a ration of calculated deficiency in these amino acids of 10% and 20%, respectively, were studied in 24 Brown Swiss cows. The mixed rations (27% grass silage, 19% maize silage, 5% hay and 49% concentrate on DM basis) contained 14.5% CP on average. Lysine supply was selectively elevated by adding fish meal in exchange for other concentrate ingredients. Methionine was supplied in a rumen-protected form. Milk protein content was elevated whereas fat amount decreased by adding both amino acids. Lactose content increased without additional lysine from fish meal. Live weight, milk yield, milk fat content and protein amount remained unaffected by any variation of amino acids supply. Also nutrient digestibility and nitrogen balance were not changed by the treatments. Blood plasma concentrations confirmed the assumed variation in metabolic lysine and, less clear, methionine supply. Effects on plasma concentrations of other amino acids were relatively small. Most plasma hormones and enzymes, and metabolites in plasma, urine and milk did not respond to the variation in amino acid supply. Lysine addition via fish meal increased aspartate amino transferase and decreased urinary allantoin concentration. Additional methionine elevated plasma ornithine. Overall lysine and methionine appear to have been only marginally deficient in the unsupplemented ration fed for 3 weeks despite the deficiency of 10% to 20% as calculated by the I.N.R.A. method.