Understanding Physiological Responses of Lactating Crossbred Holstein Cattle to Heat Stress: Mitigation Strategies Using Exogenous rbST and Mist-fan Cooling to Sustain Milk Production

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The study aimed to evaluate the effects of cooling and rbST supplementation on milk production in crossbred Holstein Friesians and to elucidate the physiological mechanisms regulating mammary (intra-mammary) and extra-mammary functions across different stages of lactation. Ambient temperature and THI were higher in normal shade (NS) barns, while relative humidity was higher in mist-fan cooling (MFC) barns. Non-cooled cows showed higher respiration rates and rectal temperatures regardless of rbST. The rbST supplementation increased milk yield, feed intake, and efficiency, with peak responses in early lactation followed by a decline. It also enhanced mammary blood flow, expanded body fluid compartments, and increased water turnover without altering osmolality or hematological indices. The hormonal profile was characterized by increased IGF-I and decreased leptin concentrations, minor variations in insulin and thyroxine, and unchanged cortisol levels. It elevated free fatty acids as key metabolic markers. Renal hemodynamics remained stable; however, increased electrolyte retention contributed to fluid expansion via enhanced tubular reabsorption mediated by aldosterone and IGF-I. Cooling and rbST improved feed intake and digesta passage without affecting digestibility, while rumen fermentation products increased. Glucose metabolism and metabolic pathway studies using radiolabeled glucose showed that rbST did not alter glucose turnover or plasma glucose levels but enhanced mammary glucose uptake and partitioning toward lactose synthesis and the pentose phosphate pathway, increasing NADPH for fatty acid synthesis, particularly in early–mid lactation. As lactation advanced, glucose utilization shifted toward glycolysis and the TCA cycle, contributing to declining milk yield, while milk electrolytes and plasma osmolality remained unchanged.

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