Environmental factors, especially temperature and light exposure, influence the health and productivity of dairy cows during lactation, possibly via similar physiological mechanisms. For example, heat stress is a critical component of decreased milk yield during summer.

However, less is known about the effect of heat stress during the dry period. The objective of this study was to evaluate the effects of heat stress prepartum under a controlled photoperiod on lactation performance and hepatic metabolic gene expression of periparturient multiparous Holstein cows (n=16).

Cows were dried off approximately 46 days before expected calving date and assigned to treatment randomly after blocking by mature equivalent milk production and parity. Treatments consisted of either heat stress (HT) or cooling (CL) with fans and sprinklers, both under a photoperiod of 14L:10D.

Rectal temperature was measured twice daily during the dry period. After calving, cows were housed in a freestall barn with cooling devices, and milk yield was recorded daily up to 210 days in milk. Blood samples were taken from dry off until +42 days relative to calving for metabolites and from −2 until +2 days relative to calving for hormone analysis. Daily dry matter intake was measured from −35 to +42 days relative to calving. Liver biopsies were collected at dry off, −20, +2, and +20 days relative to calving for cows on HT (n=5) and CL (n=4) to measure mRNA expression of suppressors of cytokine signaling-2 (SOCS-2), insulin-like growth factor binding protein-5 (IGFBP-5), a key transcription factor in lipid biosynthesis (SREBP-1c), and enzymes of lipid metabolism (FASN, ACACA, and ACADVL) by real-time quantitative PCR. Heat stress increased rectal temperatures (39.2 vs. 38.8°C), plasma prolactin concentrations at −1 (171 vs. 79 ng/mL) and 0 days (210 vs. 115 ng/mL) relative to calving, and decreased dry matter intake at 0 and +14 days relative to calving and 3.5 percent fat-corrected milk postpartum (26.1 vs. 35.4kg/d) compared with CL cows.

Relative to CL cows, hepatic mRNA expression of SOCS-2 and IGFBP-5 was downregulated in HT cows. Expression of ACADVL was upregulated in CL cows at d +2 but downregulated at d +20 relative to HT cows. Concentrations of C16:0 and cis C18:1 were greater in the milk and liver of CL cows compared with HT cows, which reflects greater lipid mobilization. These results suggest that heat-stress abatement in the dry period improves subsequent lactation, possibly via suppression of plasma prolactin surge around calving, SOCS-2 expression, and regulation of hepatic lipid metabolism.

Source: Journal of Dairy Science