Effects of Cooling and Recombinant Bovine Somatotropin Supplementation on Body Fluids, Mammary Blood Flow, and Nutrients Uptake by the Mammary Gland in Different Stages of Lactation of Crossbred Holstein Cattle

Two groups of five crossbred 87.5% Holstein cattle were housed in normal shade only (NS) as non-cooled cows and in shaded with misty-fan cooling (MFC) as cooled cows. The cows were treated with recombinant bovine somatotropin (rbST) in early, mid and late lactation with three consecutive injections of rbST 500 mg of rbST (POSILAC) in every 14 days. During the study, ambient temperature at the hottest period daily (1400 hr) in the MFC barn was significantly lower, while relative humidity was higher than that of the NS barn. The temperature humidity index (THI) in both barns ranged from 80.7-85.5 throughout the periods of study. Cows in the MFC barn showed a lower rectal temperature and respiration rate as compared with cows in the NS barn. Milk yield significantly increased in both cooled and non-cooled cows treated with rbST in each stage of lactation. The high milk yield in both groups of animals declined as lactation advanced to late lactation. Increases in mammary blood flow (MBF) accompanied with increases in total body water (TBW), extracellular fluid (ECF), blood volume (BV) and plasma volume (PV) in both cooled and non-cooled cows receiving rbST in each stage of lactation. The mean arterial plasma concentrations for glucose, acetate, β-hydroxybutyrate and triacylglycerol were unchanged but an increase in plasma free fatty acid concentrations in both cooled and non-cooled cows supplemental rbST. The net mammary glucose and triacylglycerol uptakes of cows in both groups markedly increased in mid and late stages of lactation, while no significant changes of the arteriovenous differences (A-V differences) and mammary extraction across the mammary gland were apparent in both cooled and non-cooled cows supplemental rbST. No significant changes in the A-V differences, mammary extraction and mammary uptake for acetate, β-hydroxybutyrate were apparent during rbST supplementation in both cooled and non-cooled cows. These results suggest that the effect of rbST supplementation on milk yield in each stage of lactation of either cooled or non-cooled cows is due to changes in the relative rates of delivery and uptake of nutrients by the mammary gland. The rate of decline in milk yield as lactation advances would be consequences in local changes for biosynthetic capacity within the mammary gland in the utilization of substrates occurring in both cooled and non-cooled cows whether supplemental rbST or not.