Delayed effect of heat stress on steroid production in medium-sized and preovulatory bovine follicles

During the autumn, the conception rate of dairy cattle in warm countries is low although ambient temperatures have decreased and cows are no longer ex posed to summer thermal stress, indicating that there may be a delayed effe ct of heat stress on cattle fertility. Two experiments were conducted to ex amine possible delayed effects of heat stress on follicular characteristics and steroid production at two distinct stages of follicular growth: medium -sized and preovulatory follicles, 20 and 26 days after heat exposure, resp ectively. Lactating cows were subjected to heat stress for 12 h a day in an environmental chamber, during days 2-6 of a synchronized oestrous cycle. I n Expt 1, ovaries were collected on day 3 of the subsequent cycle, before s election of the dominant follicle, and medium-sized follicles were classifi ed as atretic or healthy. In Expt 2, on day 7 of the subsequent cycle, PGF( 2 alpha) was administered and preovulatory follicles were collected 40 h la ter. In both cattle in the autumn. experiments, follicular fluid was aspira ted, granulosa and thecal cells were incubated, and steroid production was determined. In healthy medium-sized follicles (Expt 1), oestradiol producti on by granulosa cells and androstenedione production by thecal cells were l ower (P < 0.05) and the concentration of progesterone in the follicular flu id was higher in cows that had been previously heat-stressed than in contro l cows (P < 0.05). In preovulatory follicles (Expt 2), the viability of gra nulosa cells was lower (P < 0.05) and the concentration of androstenedione in the follicular fluid and its production by thecal cells were lower (P < 0.05) in cows that had been previously heat-stressed than in control cows. In both experiments, the oestradiol concentrations in the follicular fluids were not altered by heat stress. These results demonstrate a delayed effec t of heat stress on steroid production and follicular characteristics in bo th medium-sized and preovulatory follicles; this effect could be related to the low fertility of cattle in the autumn.