Effect of Heat Stress on Follicular Development during the Estrous Cycle in Lactating Dairy Cattle1

In this study we examined, in two experiments, patterns of follicular development and dominance under conditions of heat stress. Estrous cycles were programmed to include two follicular waves (wave 1 and 2). On Day 1 of the estrous cycle (Day 0 = estrus), cows were assigned randomly to cooled (C; n = 6) or heat-stressed (H; n = 6) groups. In experiment 1, on Day 12 prostaglandin (PG) F2α, was injected and a controlled intravaginal drug release device (1.9 g progesterone) was inserted (this was removed on Day 17). In experiment 2, PGF2α, 0 was injected on Day 14. Ovarian structures were examined daily by ultrasonography, and blood samples were collected at each scanning. Cycle lengths were 20 and 17 days in experiments 1 and 2, respectively. Mean maximal body temperatures were higher (p < 0.01) in H (40.3°C) than in C (38.8°C) cows. In experiment 1, the rate of increase in number of large follicles (≥ 10 mm) was greater in H than in C cows (p < 0.01), resulting in 53% more large follicles in H cows during wave 1; this was associated with a lower (p < 0.05) number of mediumsized (6−9 mm) follicles between Days 7 and 10 of the cycle. Heat stress hastened (p < 0.02) the decrease in size of the firstwave dominant follicle and hastened (p < 0.01) the emergence of the second dominant (preovulatory) follicle by 2 days. In H cows, lack of decline (p < 0.02) was noted in the number of medium-sized follicles during wave 2. In experiment 2, as in experiment 1, the rate of increase in the number of large follicles was greater in H than in C cows (p < 0.05), resulting in 50% more large follicles in H cows during wave 1. The preovulatory follicle emerged earlier (p < 0.05) in H cows, and greater variance (p < 0.05) indicated that in half of H cows the emergence of dominance occurred 4 days earlier. Heat stress suppressed (p < 0.01) plasma concentrations of estradiol during the second half of wave 1 and tended to reduce (p < 0.09) plasma concentration of inhibin. Results indicate that heat stress appears to impair follicular development and to alter the dominance of the first-wave dominant follicle and the preovulatory follicle in cattle.