EFFECTS OF CONTROLLED HEAT-STRESS ON OVARIAN-FUNCTION OF DAIRY-CATTLE- 1 - LACTATING COWS

The objective of this experiment was to determine the effects of contr olled heat stress on ovarian function of lactating dairy cows. Estrus was synchronized (estrus = d 0), and cows were randomly assigned to ei ther heat stress (n = 11; 29 degrees C, 60% relative humidity) or ther moneutral (n = 11; 19 degrees C, 60% relative humidity) treatment. For cows undergoing heat stress, ambient temperature (19 degrees C) was i ncreased from d 11 to 13 of the estrous cycle (3.3 degrees C/d increas e) and remained at 29 degrees C until d 21. Beginning on d II, the gro wth and regression of ovarian follicles and corpora lutea were measure d by using ultrasonography. Blood was collected daily by coccygeal ven ipuncture for measurement of serum concentrations of progesterone and estradiol. The second wave dominant follicle was more likely to ovulat e in cows in the thermoneutral treatment than in cows undergoing heat stress (91 vs. 18% ovulation, respectively). Patterns of follicular gr owth in cows under-going heat stress were associated with decreased se rum estradiol from d 11 to 21 and on the day of luteolysis. The averag e day of luteolysis was delayed by 9 d in heat-stressed cows. Conclusi ons were that follicular growth and development and luteolytic mechani sms were compromised in heat-stressed cows; as a result, luteolysis wa s delayed, and second wave dominant follicles did not ovulate.