Effect of season on ovarian follicular dynamics in Brahman cows

Abstract

The effect of season on ovarian follicular dynamics was studied using 32 Brahman ( Bos indicus ) cows exhibiting normal estrous cycles. Observations were made during September and October (fall: 10 cows) and during April and May (spring: 22 cows). The cows were observed for estrous behavior twice daily, and ultrasound scans of the ovaries were performed per rectum daily throughout 1 complete estrous cycle. Daily blood samples 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 spring (May) than in the fall (October). Serum P4 concentrations were higher (P < 0.05) in the spring than in the fall. On the day that serum P4 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 in the fall (8.2 ± 0.6 mm). The number of follicular waves did not differ (P > 0.10) between seasons, and 53% (n = 17) of the cows had 3 follicular waves, 38% (n = 12) had 2 follicular waves and 9% (n = 3) had 4 follicular 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 follicular waves had longer (P < 0.05) estrous cycles (24.3 ± 0.8 d).