S. W. Wilkinson, J. W. Fuquay, B. L. Clark¹, R. B. Moore, and A. B. Moore

ABSTRACT

In earlier research, heat stress altered follicular chemistry and suppressed subsequent luteal progesterone concentrations, whereas fan cooling resulted in increased luteal progesterone. The objective of this study was to determine the effect of fan and sprinkler cooling on steroid hormones and IGF-I in the follicular fluid of lactating Holsteins after the onset of estrus. Twenty cows were randomly sorted into two groups, with one group placed in a shaded pen with fan and sprinkler cooling while the other group was assigned to a similar pen with no supplemental cooling. After 7 d of acclimation, cows were synchronized with two injections of PGF2, 10 d apart. Starting 36 h after the second injection, observations for estrus, rectal temperatures, and blood samples were obtained every 6 h for 72 h. Serum was separated and stored for luteinizing hormone and progesterone radioimmunoassays (RIA). Approximately 8 h after observation of estrus, the ovary with the ovulatory follicle was removed surgically via standing laparotomy. Follicular diameter was determined and follicular fluid was aspirated and frozen for RIA of estradiol, progesterone, testosterone, and IGF-I. Average daily temperature ranged from 21.7 to 31.1^o C and relative humidity from 57.5 to 94.4%. Average rectal temperatures were 39.1 and 39.8^o C for cooled and control cows, respectively (P<0.0001). Serum progesterone was 0.45 0.14 and 0.24 0.12 ng/ml for cooled and control cows, respectively, indicating complete luteolysis. Fan cooling lowered (P<0.1) testosterone in follicular fluid (3.1 6.9 vs. 20.7 7.6 ng/ml), but did not alter (P>0.1) estradiol, progesterone or IGF-I, or the ratio of estradiol:testosterone and estradiol:IGF-I. Within treatment, variation for these variables was high. Further study is needed to elucidate the cause of reduced luteal function during heat stress.

¹College of Veterinary Medicine, Mississippi State University