Abstract:
The objective of the present study was to determine the effect of exogenous bovine somatotropin on mammary function in late lactating Holstein cows whether its effect involved changes of extra-mammary factors or intra-mammary factors. Twelve, 87.5% late lactating crossbred Holstein cows, approximately 30 weeks postpartum, were divided into two groups of 6 animals each. Animals in the control group were given sodium bicarbonate buffer while animals in the treated group were given recombinant bovine somatotropin (bST) by subcutaneous injection at the tailhead depression with 500 mg of bST (14-day prolonged-release bST). After bST injection, milk yield increased 5.51% to 30.95% and mammary blood flow increase significantly (P<0.01). The increase in mammary blood flow in response to bST treatment was proportionally greater than an increase in milk production. An increased plasma concentration of IGF-1 was associated with an increase in mammary blood flow. The concentration of arterial plasma glucose decreased, while the arterio-venous concentration difference and the mammary glucose uptake increased when compared to the control group. The concentration of arterial plasma triglyceride decreased throughout the experimental period. The plasma concentration of acetate, the arterio-venous concentration difference and the mammary acetate uptake increased during bST treatment. However, bST did not affect parameters studied the plasma concentration of β-hydroxybutyrate. Milk composition of fat and lactose tended to increase during bST treatment. Milk protein concentration increased a few days while was followed by slight decreases after bST injection when compared to the pretreated period. The present results indicated that bST affected the mammary function to increase milk yield in late lactating cows by involving both extra-mammary factors and intra-mammary factors. The action of IGF-1 may cause an increase in blood flow to mammary gland and the presentation of milk precursors to the gland.
