Hormonal alterations in the lactating dairy cow in response to thermal stress.

The influence of acute exposure to thermal stress on the secretion of progesterone, estradiol, cortisol, and LH was monitored in 14 lactating Holstein cows. Eight cows were maintained throughout the summer in a refrigerated air-conditioned tie stall barn. An additional eight cows were maintained in outdoor corrals with access only to shade. Rectal temperatures and respiration rates of cows under heat stress conditions were elevated above the cows maintained under air conditioning. Cows in both environments exhibited similar serum concentrations of progesterone and estradiol throughout the estrous cycle. Serum cortisol concentrations were higher in heat-stressed cows compared to cows maintained under cooling. The number of LH pulses on d 5 of the estrous cycle was greatly reduced in the heat-stressed cows compared to the cows under cooling. No differences in the number of pulses of LH were observed on d 12 of the estrous cycle between cows in the two environments. Data suggest that heat stress can suppress anterior pituitary release of LH without having a measurable influence on ovarian steroid hormone secretion.