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UBC Theses and Dissertations

Relationships between plasma amino acid concentration and milk protein production Shelford, James Arthur


In ruminants the aerobic host animal is dependent on the anaerobic fermentation within the rumen to supply protein and energy. This anaerobic fermentation results in a rather constant ratio between protein and energy. In young growing animals and lactating animals the demand for protein is greater than the demand for energy. Plasma amino acid data indicate that protein could be limiting in these two situations. In the first phase of the study, plasma free amino acids, milk volume and milk components were determined at two week intervals throughout the lactations of two mature Ayrshire cows. The large amounts of protein that were secreted daily during the first third of lactation placed heavy demands on the animals for protein supplies. These demands were reflected in changes in plasma free amino acids during the lactation cycle. Lysine, leucine, isoleucine and methionine exhibited the greatest variation in response to the demands of lactation. A series of abomasal infusions of the above amino acids and others was undertaken to determine the importance of amino acids for the synthesis of milk protein. The infusions were grouped into three lots; those containing methionine, those containing isoleucine and those containing lysine. Effects observed during infusions containing methionine suggested that methionine itself had little effect on- protein production. Methionine did not alter the effects of other amino acids on milk protein synthesis. The responses noted when methionine was accompanied by branched chain amino acids or by lysine were typical of responses of branched chain amino acids or lysine alone. Infusions containing isoleucine and other branched chain amino acids caused an increase in milk protein concentration, a decrease in milk production and no increase in total protein production. Plasma amino acid data revealed that infusion of branched chain amino acids resulted in decreases in concentrations of the other plasma amino acids. The decrease in plasma amino acid concentration, the lack of response in milk protein production and the known effects of branched chain amino acids on insulin secretion suggest that insulin might be affecting the intake of amino acids by muscle tissue. When lysine-containing infusions were examined milk production was found to increase. Milk protein concentration remained constant resulting in an increase in total milk protein production. The most dramatic increase in milk and protein production occurred when lysine was infused alone. Infusions containing lysine did not result in decreases in plasma free amino acids. There was a general trend for all the essential amino acids to increase or remain at the same level during the infusion. Lysine appears to be the limiting amino acid for milk protein synthesis. It is possible that the increased supply of lysine from the abomasal infusion affected the amino acid metabolism in muscle in such a way as to free amino acids for milk protein synthesis and energy metabolism.

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