Animal Reproduction (AR)
Animal Reproduction (AR)
Original Article

Follicle and gonadotropin relationships during the beginning of the anovulatory season in mares

M.M. Nunes, E.L. Gastal, M.O. Gastal, A.N. Rocha Filho, A.P. Mellagi, O.J. Ginther

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The initial portion of the anovulatory season of mares consists of a follicle-receding phase followed by a mid-anovulatory phase (follicles ≤ 20 mm). Follicle dynamics were studied in a control group (n = 6) and a follicle-ablation group (n = 8), using the last ovulatory period of the season and three 20-day anovulatory periods (Days 11–30, 31–50, and 61–80; last ovulation of the season = Day 0). In the ablation group, all follicles ≥ 6 mm were ablated 10 days after the second last ovulation of the year and on Days 10, 30, and 60. The diversity in follicle dynamics within each group was considerable, including a follicle-receding phase that terminated in a mid-anovulatory phase (10 mares), a major anovulatory wave (dominant follicle ≥ 30 mm) during Days 11-30 (eight mares), multiple major anovulatory waves (two mares), and a short anovulatory season (< 80 days; two mares). Four additional mares were excluded from the study, owing to continuous ovulatory intervals throughout the 80 days. In each of 11 control and follicle-ablated mares that did not have a short anovulatory season or multiple major anovulatory waves, the diameter of the largest follicle was greater during Days 11–30 than during Days 31–50. Growth rate of the largest follicle for 2–10 days after ablation was greater (P < 0.05) during the last ovulatory period (2.3 ± 0.2 mm/day) or during Days 11–30 (1.9 ± 0.2 mm/day) than during Days 31–50 (0.6 ± 0.2 mm/day) or 61–80 (0.8 ± 0.2 mm/day). Concentrations of FSH and LH increased (P <0.05) following ablation before the ovulatory period and on Day 10. The LH concentrations further increased during the ovulatory period a few days before ovulation, but decreased during a corresponding time of the first anovulatory period (Days 11–30). Concentrations of gonadotropins did not increase following ablation on Days 30 or 60. The number of periods with a post-ablation statistically identified FSH surge was different (P < 0.02) among the ovulatory period (five of five), Days 11–30 (eight of eight), Days 31–50 (three of eight) and Days 61–80 (three of seven). Results indicated: 1) diversity in follicle dynamics among mares was considerable during the first 80 days of the anovulatory season in association with diversity in FSH and LH concentrations and 2) an increase in mean diameter of the largest follicle occurred following of the largest follicle occurred following ablation on Days 30 and 60, despite the absence of a detected FSH increase. The hypothesis that concentrations of LH during the follicle-receding phase are greater in mares with major anovulatory waves than with minor waves was supported.


Anovulatory season, follicles, gonadotropins, mares
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