EFFECT OF HEAT-STRESS ON FOLLICULAR DEVELOPMENT DURING THE ESTROUS-CYCLE IN LACTATING DAIRY-CATTLE

In this study we examined, in two experiments, patterns of follicular development and dominance under conditions of heat stress. Estrous cyc les were programmed to include two follicular waves (wave 1 and 2). On Day 1 of the estrous cycle (Day 0 = estrus), cows were assigned rando mly to cooled (C; n = 6) or heat-stressed (H; n = 6) groups. In experi ment 1, on Day 12 prostaglandin (PG) F-2 alpha was injected and a cont rolled intravaginal drug release device (1.9 g progesterone) was inser ted (this was removed on Day 17). In experiment 2, PGF(2 alpha) was in jected on Day 14. Ovarian structures were examined daily by ultrasonog raphy, and blood samples were collected at each scanning. Cycle length s were 20 and 17 days in experiments 1 and 2, respectively. Mean maxim al body temperatures were higher (P < 0.01) in H (40.3 degrees C) than in C (38.8 degrees C) cows. In experiment 1, the rate of increase in number of large follicles (greater than or equal to 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 medium-sized (6-9 mm) follicles between Days 7 and 10 of the cycle. Heat stress hastened (p < 0.02) the decrease in size of the fi rst-wave dominant follicle and hastened (p < 0.01) the emergence of th e second dominant (preovulatory) follicle by 2 days. In H cows, lack o f 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 incre ase 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, an d greater variance (p < 0.05) indicated that in half of H cows the eme rgence 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 devel opment and to alter the dominance of the first-wave dominant follicle and the preovulatory follicle in cattle.