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

Abstract

During the autumn, the conception rate of dairy cattle in warm countries is low although ambient temperatures have decreased and cows are no longer exposed to summer thermal stress, indicating that there may be a delayed effect of heat stress on cattle fertility. Two experiments were conducted to examine 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, respectively. 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. In Expt 1, ovaries were collected on day 3 of the subsequent cycle, before selection of the dominant follicle, and medium-sized follicles were classified as atretic or healthy. In Expt 2, on day 7 of the subsequent cycle, PGF(2a) was administered and preovulatory follicles were collected 40 h later. In both experiments, follicular fluid was aspirated, granulosa and thecal cells were incubated, and steroid production was determined. In healthy medium-sized follicles (Expt 1), oestradiol production by granulosa cells and androstenedione production by thecal cells were lower (P < 0.05) and the concentration of progesterone in the follicular fluid was higher in cows that had been previously heat-stressed than in control cows (P < 0.05). In preovulatory follicles (Expt 2), the viability of granulosa 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 effect of heat stress on steroid production and follicular characteristics in both medium-sized and preovulatory follicles; this effect could be related to the low fertility of cattle in the autumn.

The effect of some seasonal conditions on oestrus occurrence in cows

“…Some researchers reported that high environment temperature influences the endocrine activity and decreases the oestrus expression (LEE 1993, WOLFENSON et al 1997, ROTH et al 2001. In this study oestrus occurrence was positively correlated with environment temperature and the increase in the environment temperature up to 23 °C did not cause a suppressive effect on the ovarian activity.…”

Effect of prepartum somatotropin injection in late-pregnant Holstein heifers on metabolism, milk production and postpartum resumption of ovulation

“…Although this hypothesis was never tested with regard to the NEB challenge, it was found to be true for cows undergoing a short period of heat stress (Roth et al, 2001a and2001b) and can explain the effects of prepartum IGF-I on postpartum follicle development observed in this study.…”

Environmental Stress Effect on Animal Reproduction

“…In another study, Roth et al, [6], indicate that exposing domestic female mammals during heat stress is altered follicular dynamics and lowers the follicular dominance in the next cycle. In 1997, Wolfensohn et al, [10] argued that the preovulatory follicle is a key component in the reproductive system and the deterioration of their function during heat stress may trigger other reproductive effects, such as distortion in gonadotropin secretion, underdevelopment of the body luteum and embryo, resulting in a low fertility.…”

“…Recent studies conducted by Roth et al, [6], mentioned for the first time, evidence of the delayed effect of heat stress on follicular steroidogenesis revealed that exposure of female domestic heat stress led to a decrease in steroidogenesis, 20 and 26 days later, in mid-sized preovulatory follicles. Furthermore, the theca cells more susceptible found that granulose cells and they expressed low estrogen production in medium-sized follicles and low viability in preovulatory follicles.…”