Abstract
The study was carried out to evaluate the effect of exogenous bovine somatotropin on water metabolism in relation to mammary function in early lactation of crossbred Holstein cattle. Ten, 87.5% crossbred Holstein cattle were divided into two groups of 5 animals each. At day 60 of lactation, the control group was given placebo while animals in the experimental group were given recombinant bovine somatotropin (rbST) by subcutaneous injection with 500 mg of rbST (14-days prolonged-release rbST). In rbST- treated animals, milk yield increased 19.8%, which coincided with a significant increase in water intake (p<0.01), while DM daily intake was not different when compared to the control animals. Water turnover rate as absolute values significantly increased (p<0.05), while the biological half-life of water did not change in rbST-treated animals. Total body water (TBW) and total body water space (TOH) as absolute values significantly increased (p<0.01) in rbST-treated animals, while it was decreased in the control animals. Absolute values of empty body water (EBW) markedly increased (p<0.05), which was associated with an increase in the extracellular fluid (ECF) volume. Absolute values of plasma volume and blood volume were also significantly increased (p<0.05) in rbST-treated animals. The increase in mammary blood flow in rbST-treated animals was proportionally higher than an increase in milk production. The plasma IGF-1 concentration was significantly increased (p<0.01) in rbST-treated animals when compared with those of control animals during the treatment period. Milk fat concentration increased during rbST treatment, while the concentrations of both protein and lactose in milk were not affected. The present results indicate that rbST exerts its effect on an increase in both TBW and EBW. An increased ECF compartment in rbST-treated animals might partly result from the decrease in fat mass during early lactation. The action of rbST on mammary blood flow might not be mediated solely by the action of IGF-1 for increase in blood flow to mammary gland. An elevation of body fluid during rbST treatment in early lactation may be partly a result of an increase in mammary blood flow in distribution of milk precursors to the gland. (Asian-Aust. J. Anim. Sci. 2005. Vol 18, No. 11 : 1600-1608)
Highlights
It is known that lactating dairy cows metabolize large amounts of water and are affected rapidly by water deprivation
No significant differences in the total daily dry matter intake (DM) or DM as a percent body weight were apparent between control and recombinant bovine somatotropin (rbST)-treated animals
In contrast to rbST-treated animals, milk yield of the control animals were not significantly different between pretreatment and treatment periods the peak of milk yield occurred at week 10 in both groups
Summary
It is known that lactating dairy cows metabolize large amounts of water and are affected rapidly by water deprivation. An alteration in bodily function during lactation is apparent; for example, blood volume (Chaiyabutr et al, 1997) and cardiac output (Hanwell and Peaker, 1977), are increased. These changes may effectively alter body fluid and circulatory distribution including the blood supply to the mammary gland. The lactating mammary gland receives signals from the rest of body in forms of nutrient and hormones from blood. Mammary blood flow is a major parameter controlling milk production in a way to carry milk precursors to the mammary gland at the process of milk synthesis
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