Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


The effect of heat stress on plasma profiles of progesterone, luteinizing hormone (LH) and cortisol was evaluated following cloprostenol (500 (mu)g) induced luteolysis in lactating and non-lactating cows in a controlled environment. Another experiment was conducted to evaluate the effect of heat stress on the superovulatory response in lactating cows. The plasma progesterone, LH and total estrogen concentrations were not altered by heat stress (P > 0.05) when the average rectal temperatures of both lactating and non-lactating cows were raised to 39.83(DEGREES)C (103.7(DEGREES)F). However, plasma cortisol was significantly suppressed (P < 0.05) during heat stress. Luteolysis was followed by a preovulatory LH surge in all cows following cloprostenol and thus heat stress did not appear to alter the responsiveness of the corpus luteum to cloprostenol and the factors which regulate the hypothalamic control of LH release. Preovulatory LH peaks were preceded by total estrogen peaks and therefore heat stress did not appear to alter the positive feedback of estrogens from the preovulatory follicles to release LH from the pituitary gland. The magnitude of the diurnal variation of cortisol was higher during lower stress. LH peaks were seen irrespective of cortisol concentrations during both higher and lower stress treatments. However, LH peaks coincided with those of cortisol and total estrogens during lower stress treatments in both lactating and non-lactating cows. An altered diurnal variation in rectal temperatures was seen following LH peaks during lower stress. Fifteen transferable embryos were recovered during lower stress (15/34 or 44%) compared to one transferable embryo (1/4 or 25%) during higher stress (P > 0.05) in the superovulation study. Thus it appears that heat stress reduced the responsiveness of the ovary to follicle stimulating hormone (FSH). Plasma cortisol concentrations were similar during lower and higher stress treatments in the superovulation study. However, they were below the concentrations observed in the lower stress treatments in non-lactating and lactating cows. The higher stress treatment in the superovulation study did not impair the preovulatory LH surge, ovulation or fertilization following cloprostenol induced luteolysis. It appears that the reduced responsiveness of the ovaries to FSH during higher stress treatment in the superovulation study may contribute to the overall decrease in fertility in addition to the delayed LH peaks and suppressed cortisol concentrations seen during higher stress treatments in non-lactating and lactating cows.