Heat tolerance in lactating Holstein dairy cows is associated with oxygen transport and mammary aerobic glucose metabolism under heat stress

This study explored physiological and metabolic differences and the underlying mechanisms in dairy cows with distinct heat resistance. Forty-five mid-lactation Holstein cows were evaluated for milk yield, respiratory rate and rectal temperature under non-heat stress (THI < 68) and moderate heat stress (79 < THI ≤ 88). Based on principal component analysis and cluster analysis of milk yield, respiratory rate and rectal temperature, 10 heat-sensitive (HS) and 10 heat-tolerant (HT) cows were screened for in-depth analysis. Compared with HS cows, HT cows had lower rectal temperature (38.8 vs. 39.6°C; P < 0.001) and greater milk yield (34.3 vs. 27.0 kg; P < 0.001). The HT cows showed higher peripheral red blood cells count ( P = 0.006) and hemoglobin concentration ( P = 0.001), accompanied by lower serum total bilirubin ( P = 0.045), improved blood acid-base status ( P = 0.021) and higher arterial and venous blood oxygen concentrations ( P = 0.030, P = 0.021). Additionally, HT cows exhibited higher mammary plasma flow ( P = 0.011), glucose uptake ( P = 0.002) and clearance rate ( P = 0.001). Concentrations of milk lactate, glucose-6-phosphate and pyruvate were lower ( P = 0.009), whereas citrate was higher ( P = 0.020) in HT cows. Collectively, these findings indicate that superior heat tolerance in HT cows is associated with enhanced systemic oxygen transport capacity and mammary plasma flow, supporting efficient aerobic glucose metabolism alongside preserved physiological homeostasis and sustained lactation performance under heat stress.