Effects of pituitary-derived bovine growth hormone on production parameters and biokinetics of key metabolites in lactating dairy cows at peak and mid-lactation.

Changes in production parameters and metabolite biokinetics induced by treatment with pituitary-derived bovine growth hormone (bGH) were monitored at peak (c. 40 days) and mid-lactation (c. 130 days) in dairy cows. During treatment with bGH milk production increased by 6 and 14% at peak and mid-lactation respectively. At peak lactation the content of milk fat tended to increase, whereas milk protein tended to decrease and milk lactose decreased significantly. Yield of milk fat increased, but there was no change in the yield of milk protein and lactose. The content of milk fat tended to increase at mid-lactation. Milk protein decreased and there was no change in milk lactose. Yields of milk fat and lactose but not protein increased. Growth hormone exerted metabolic effects which differed with stage of lactation. At peak lactation plasma glucose concentration and its irreversible loss increased, plasma urea and acetate were unchanged and their irreversible losses tended to increase. No change was measured for plasma non-esterified fatty acids (NEFA) and the irreversible loss of NEFA decreased. At mid-lactation plasma concentrations of glucose and NEFA were increased, plasma urea decreased and acetate and 3-hydroxybutyrate tended to increase. Irreversible losses of NEFA increased, urea tended to decrease and acetate and glucose remained essentially constant. The results show that exogenous pituitary bGH exerts metabolic effects which result in the supply of increased nutrients to support milk synthesis. The metabolic effects differ with the stage of lactation, reflecting differences in physiological and/or nutritional state.