Immediate and delayed effects of heat stress on follicular development andits association with plasma FSH and inhibin concentration in cows

The aim of this study was to characterize the immediate effects of heat str ess on plasma FSH and inhibin concentrations, and its involvement in follic ular dynamics during a complete oestrous cycle, and to examine a possible d elayed effect of heat stress on follicular development. Holstein dairy cows were oestrous synchronized and randomly assigned to either cooled (n = 7) or heat-stressed (n = 6) treatment groups. During a complete oestrous cycle , control cows, which were cooled, maintained normothermia, whereas heat-st ressed cows, which were exposed to direct solar radiation, developed hypert hermia. At the end of this oestrous cycle (treated cycle), both groups were cooled and maintained normothermia for the first 10 days of the subsequent oestrous cycle. Throughout this period, follicular development was examine d by ultrasonography, and plasma samples were collected. During the second follicular wave of the treated oestrous cycle, a significantly larger cohor t of medium sized follicles (6-9 mm) was found in heat-stressed cows than i n cooled cows (P < 0.05). The enhanced growth of follicles in this wave in heat-stressed cows was associated with a higher plasma FSH increase which l asted 4 more days (days 8-13 of the oestrous cycle; P < 0.05), and coincide d with a decrease in the plasma concentration of immunoreactive inhibin (da ys 5-18 of the oestrous cycle; P < 0.05). During the follicular phase (days 17-20 of the treated cycle), heat-stressed cows showed an increase in the number of large follicles (greater than or equal to 10 mm), and the preovul atory plasma FSH surge was significantly higher in heat-stressed cows than in cooled cows (P < 0.01). The effect of heat stress was also observed duri ng the first follicular wave of the subsequent cycle: the postovulatory pla sma FSH concentration was higher (P < 0.01), but fewer medium follicles dev eloped, and the first follicular wave decreased at a slower rate in previou sly heat-stressed cows than in cooled cows (0.40 and 0.71 follicles per day , respectively). This study shows both immediate and delayed effects of hea t stress on follicular dynamics, which were associated with high FSH and lo w inhibin concentrations in plasma. These alterations may have physiologica l significance that could be associated with low fertility of cattle during the summer and autumn.