EFFECT OF SEASON ON OVARIAN FOLLICULAR DYNAMICS IN BRAHMAN COWS

The effect of season on ovarian follicular dynamics was studied using 32 Brahman (Bos indicus) cows exhibiting normal estrous cycles. Observ ations were made during September and October(fall: 10 cows) and durin g April and May (spring: 22 cows). The cows were observed for estrous behavior twice daily, and ultrasound scans of the ovaries were perform ed per rectum daily throughout 1 complete estrous cycle. Daily blood s amples were collected via puncture of a coccygeal vessel and processed to yield serum; serum progesterone (P4) was determined by RIA. Length of the estrous cycle was not influenced by season (P>0.10); however, serum P4 remained above 1 ng/mL 1.7 +/- 0.4 d longer (P<0.05) in the s pring (May) than in the fail (October). Serum P4 concentrations were h igher (P<0.05) in the spring than in the fall. On the day that serum P 4 declined to < 1 ng/mL, there was a tendency (P<0.10) for the cows to have a larger dominant follicle in the spring (9.6 +/- 0.4 mm) than i n the fall (8.2 +/- 0.6 mm). The number of follicular waves did not di ffer (P>0.10) between seasons, and 53% (n=17) of the cows had 3 follic ular waves, 386 (n=12) had 2 follicular waves and 9% (n=3) had 4 folli cular waves. Estrous cycle lengths were similar in cows with 2 (20.3 /- 0.4 d) and 3 (20.8 +/- 0.3 d) follicular waves, but cows with 4 fol licular waves had longer (P<0.05) estrous cycles (24.3 +/- 0.8 d).