Effects of Cooling and Supplemental Bovine Somatotropin on Milk Production relating to Body Glucose Metabolism and Utilization of Glucose by the Mammary Gland in Crossbred Holstein Cattle

Problem statement: The low milk yield and shorter persistency of lactation of dairy cattle is the major problem for the dairy practices in the tropics. High environmental temperatures and rapid decline of plasma growth hormone level can influence milk production. Regulation of the milk yield of animals is mainly based on the mechanisms governing the quantity of glucose extracted by the mammary gland for lactose biosynthetic pathways. The mechanism(s) underlying the effects of cooling and supplemental bovine somatotropin on milk production relating to body glucose metabolism and intracellular metabolism of glucose in the mammary gland of crossbred Holstein cattle in the tropics have not been investigated to date. Approach: Ten crossbred 87.5% Holstein cows were divided into two groups of five animals each. Animals were housed in Normal Shade barn (NS) as non-cooled cows and cows in the second group were housed in barn which was equipped with a two Misty-Fan cooling system (MF) as cooled cows. Supplementation of recombinant bovine Somatotropin (rbST) (POSILAC, 500 mg per cow) were performed in both groups to study body glucose metabolism and the utilization of glucose in the mammary gland using a continuous infusion of [3-³H] glucose and [U- ₁₄C] glucose as markers in early, mid and late stages of lactation. Results: Milk yield significantly increased in both groups during supplemental rbST with a high level of mammary blood flow. Body glucose turnover rates were not significant different between cooled and non-cooled cows whether supplemental rbST or not. The glucose taken up by the mammary gland of both non-cooled and cooled cows increased flux through the lactose synthesis and the pentose cycle pathway with significant increases in NADPH formation for fatty acid synthesis during rbST supplementation. The utilization of glucose carbon incorporation into milk appeared to increase in milk lactose and milk triacylglycerol but not for milk citrate during supplemental rbST in both non-cooled and cooled cows in early and mid lactation. Conclusion: The present study demonstrated that local changes for biosynthetic capacity within the mammary gland would be a factor in identification of the utilization of substrates in the rate of decline in milk yield. The proportion of glucose was metabolized less for lactose synthesis, but metabolized more via the Embden-Meyerhof pathway and the tricarboxylic acid cycle as lactation advanced to late lactation in both cooled and non-cooled cows whether supplemental rbST or not.