Studies in Ruminant Metabolism

Part I. Experiments on the Value of vitro Studies, and on the Decomposition of Carbohydrate by Rumen Liquid Most of the work described in the present thesis involved incubating rumen contents in the laboratory under conditions which were designed to resemble those existing in the normal rumen. Before adopting this technique, however, it was important to determine whether the changes which took place in the incubating material in vitro were similar to those which are known to occur in vivo. To obtain information on this matter the decomposition of carbohydrate during in vitro incubation was studied. It was shown that the amounts of volatile fatty acids, lactic acid, carbon dioxide and methane produced from maltose in the laboratory were comparable to the amounts produced in vivo. It may be concluded, therefore, that the nitrogen changes which invariably accompany these carbohydrate changes in vitro also occur in vivo. Of the maltose decomposed during incubation, 81-90% was accounted for by the production of bacterial protein, bacterial polysaccharide, carbon dioxide, methane, lactic acid and volatile fatty acids The remaining 10-19% may be accounted for partly by the fact that volatile fatty acids were calculated as acetic acid whereas almost certainly propionic and butyric acids also occurred, and partly by the production in small amounts of substances not so far identified. Part II. The Preparation of a dried Sample of Rumen Bacteria and the Biological Value of its Protein The results of many feeding trials and in vitro studies described in the literature have suggested that non-protein nitrogen (NPN) maybe converted to microbial protein in the rumen and that this protein becomes available to the ruminating animal. To determine the biological value of this microbial protein it was decided to prepare a sample of dried rumen bacteria sufficient in amount to enable feeding tests to be made with rats. Approximately 460 g. of dried rumen bacteria were prepared and analysed. The preparation contained 44% protein, of which about two-fifths was synthesised from NPN during incubation. Microscopical and chemical examination of the material suggested that contamination by fibre and protein from feedingstuffs was very slight. When fed to rats at a level of 8% of the diet the biological value of the protein was 88.2% and the digestibility 73.2%. Work similar to that described for rumen bacteria has been begun with rumen protozoa. Part III. The Effect of Animal Management Factors on the Synthesising Power of Rumen Liquid in vitro The synthesising power of rumen liquid as judged by the decrease in NPN , which occurs on incubating the liquid with added carbohydrate under controlled conditions in the laboratory varies considerably from time to time. Attempts have been made to relate these variations to differences in protein or NPN content of the rumen liquid when the animal was fed different diets. However, apart from the finding that the protein and NPN levels and the amount of synthesis were all higher when the animals were at pasture than when they were stall-fed there appeared to be no definite relationship between the diet, the naturally occurring level of NPN or protein in the rumen liquid and the amount of synthesis. An apparent increase in protein synthesis was detected when urea was added to rumen liquid, but further work is required to determine whether the increase was real. The effect of the time of sampling on synthesis was also investigated, and it was shown conclusively that maximum synthesis occurred in vitro, in samples which had been removed from the rumen approximately one hour after the animal had been fed. (Abstract shortened by ProQuest.)