Abstract:
The objective of the study was carried out to evaluate the effect of exogenous bovine somatotropin on mammary function in relation to water metabolism in early lactation of crossbred Holstein cattle. Ten, late pregnancy 87.5% crossbred Holstein cattle were divided into two groups of 5 animals each. At day 60 of lactation, animals in the control group were given placebo while animals in the treated group were given recombinant bovine somatotropin by subcutaneous injection at the postscapular with 500 mg of bST (14-day prolong-release bST). After bST injection, milk yield increased 19.8% and water intake increased significantly (P<0.01) while total dry matter intake was not different when compared to the control group. Plasma volume and blood volume absolute values were significantly increased (P<0.05) in animals given bST. Water turnover rate as absolute values significantly increased (P<0.05). The biological half-life of water did not change while, total body water space and total body water as absolute values significantly increased (P<0.01) during bST treatment. An increase the empty body water as absolute values was higher (P<0.05) which associated with an increase the extracellular fluid volume. The increase in mammary blood flow in response to bST treatment was proportionally greater than an increase in milk production. The concentration of arterial plasma glucose and mammary glucose uptake of animals treated with bST were not significant different, while the arterio-venous concentration difference and mammary extraction ratio significantly decreased (P<0.05) as compared the control animals. The mammary extraction ratio of acetate and mammary acetate uptake increased during bST treatment. However, bST did not affect the plasma concentrations of β-hydroxybutyrate and triglyceride. Milk fat concentration increased during bST treatment in early lactating period, while the concentration of both protein and lactose in milk were not affected. The present results indicate that bST affects the mammary function by increase in milk yield in early lactating crossbred Holstein cattle. It involves both extra-mammary factors and intra-mammary factors. The action of bST may mediate via IGF-1 causing an increase in blood flow to mammary gland and carrying milk precursors to the gland. The effect of bST also increases TBW to make up the largest portion of milk during milk synthesis.
