EFFECTS OF CONTROLLED HEAT-STRESS ON OVARIAN-FUNCTION OF DAIRY-CATTLE- 2 - HEIFERS

The objective of this experiment was to determine the effects of contr olled heat stress on ovarian function of dairy heifers. Estrus was syn chronized in Holstein heifers (estrus = d 0), and heifers then were ra ndomly assigned to either heat stress (n = 10; 33 degrees C, 60% relat ive humidity) or thermoneutral (n = II; 21 degrees C, 60% relative hum idity) treatment. For heat-stressed heifers, ambient temperature was i ncreased from thermoneutrality to heat stress (33 degrees C) between d 9 and 14 (2.4 degrees C/d increase) after the synchronized estrus and remained between 31 and 33.5 degrees C: until d 22. From d II to 21, the growth and regression of ovarian follicles and corpora lutea were measured by using ultrasonography, and blood was collected daily for s erum progesterone and estradiol analyses. The second wave dominant fol licle was larger for the heifers in the thermoneutral environment than for heat-stressed heifers, and ovulation of the second wave dominant follicle occurred in 9 of 11 thermoneutral heifers. For 6 of 10 heat-s tressed heifers, the second wave dominant follicle regressed and was r eplaced by an ovulatory third wave dominant follicle. Smaller follicul ar size in heat stressed heifers was associated with decreased serum e stradiol concentrations between d II and 21. Serum concentrations of p rogesterone during the luteal phase were similar, but luteolysis was d elayed in heat-stressed heifers compared with onset in heifers in the thermoneutral treatment. Conclusions were that heat stress inhibited t he growth and function of the dominant follicle so that most of the he at-stressed heifers had three follicular waves and a delay in corpus l uteum regression.