Differences of hormones involved in adipose metabolism and lactation between high and low producing Holstein cows during heat stress

The experiment was conducted to evaluate hormonal involvement in the adipose metabolism and lactation between high and low producing dairy cows in a hot environment. Forty Holstein healthy cows with a similar parity were used and assigned into high producing group (average production 41.44 ± 2.25 kg/d) and low producing group (average production 29.92 ± 1.02 kg/d) with 20 cows in each group. Blood samples were collected from caudal vein to determine the difference of hormones related to adipose metabolism and lactation. The highest, lowest, and average temperature humidity index (THI), recorded as 84.02, 79.35 and 81.89, respectively, indicated that cows were at the state of high heat stress. No significant differences between high and low producing groups were observed in the levels of nonestesterified fatty acid (NEFA), β-hydroxybutyrate (β-OHB), total cholesterol (TCHO), and insulin (INS) (P > 0.05). However, the very low density lipoprotein (VLDL), apolipoprotein B100 (apoB-100), high-density lipoprotein (HDL-C) and estrogen (E₂) concentrations in high producing group were significantly higher than those of low producing group (P < 0.05). No significant differences between high and low producing groups were observed in the levels of prolactin (PRL) and progesterone (PROG) (P > 0.05), whereas high producing group had a rise in the insulin-like growth factor-1 (IGF-1) level compared with low producing group (P < 0.05). These results indicated that, during summer, high and low producing dairy cows have similar levels of lipid catabolism, but high producing dairy cows have advantages in outputting hepatic triglyceride (TG).