Abstract:
Two groups of five crossbred 87.5% Holstein cattle each, 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 supplemented 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 (1400h) 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 77.8-85.5 throughout the periods of study. Cows under the MFC barn showed a lower rectal temperature and respiration rate as compared with cows in the NS barn. Milk yield significantly increased in cows treated with rbST in each stage of lactation. Increases in mammary blood flow and plasma level of IGF-I 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 stages of lactation. The mean arterial plasma concentrations for glucose, acetate, β-hydroxybutyrate and triacylglycerol were unchanged, while the mean arterial plasma concentrations of free fatty acid increased 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. Glucose turnover rates were not significant different between cooled and non-cooled cows whether supplemental rbST or not. The glucose taken up by the mammary gland of both non-cooled and cooled cows increased flux through the lactose synthesis and the pentose cycle pathway with significant increases in NADPH formation for fatty acid synthesis during rbST supplementation. The utilization of glucose carbon incorporation into milk appeared to increase in milk lactose and milk triacylglycerol of both cooled and non-cooled cows supplemental rbST during early and mid lactation but not for milk citrate as lactation advances. Milk yield of both cooled and non-cooled cows without rbST decreased as lactation advanced to late lactation. Local changes for biosynthetic capacity within the mammary gland would be a factor in identification of the utilization of substrates in the rate of decline in milk yield with advancing lactation in both cooled and non-cooled cows. The proportion of glucose would be metabolized less for lactose synthesis, but metabolized more via the Embden-Meyerhof pathway and the tricarboxylic acid cycle as lactation advances whether supplemental rbST or not.
