Interrelationships among follicles during the common-growth phase of a follicular wave and capacity of individual follicles for dominance in mares.

Abstract

The changing diameter interrelationships among follicles during the interval from emergence to deviation (common-growth phase) were studied in 59 mares. All follicles of > or =6.0 mm were ablated 10 days after ovulation. The four largest follicles of the postablation wave were ranked D1, D2, D3 and D4 at the expected beginning of deviation (D1 > or = 20.0 mm), according to descending diameter. The four follicles were also ranked independently, according to order of emergence at 6.0-6.9 mm as E1 (first to emerge), E2, E3 and E4. The follicles emerged during 1.3 +/- 0.1 to 3.1 +/- 0.1 days, and expected deviation began 6.5 +/- 0.1 days after ablation. The frequency of emerging follicles becoming the largest follicle at the beginning of deviation was different (P < 0.0001; chi-square test) among follicles E1 (61%), E2 (25%), E3 (9%) and E4 (5%). There were no differences in growth rates among the four follicles throughout the common-growth phase (overall, 2.8 +/- 0.04 mm/day). The differences in diameters between follicles E1 and E2 were similar between 3 days (2.7 +/- 0.2 mm) and 6 days (2.9 +/- 0.4 mm) after ablation. In controls and after ablation of D1; D1 and D2; or D1, D2 and D3 at the expected beginning of deviation, the largest remaining follicle became dominant in 26 of 34 mares (76%). In 10 of 15 mares (67%), the second-largest follicle became dominant when the largest follicle was ablated 1 or 2 days after the expected beginning of deviation. Results indicated the following: 1) the first follicle to emerge maintained its diameter advantage in most mares and average diameter growth rates were similar among the four follicles throughout the common-growth phase; 2) the hypothesis was supported that the capacity for dominance is similar among the four largest follicles at the beginning of deviation, but dominance by a smaller follicle is blocked when a larger follicle is present; and 3) the second-largest follicle retained the capacity for dominance in most mares for as long as 2 days after the beginning of deviation.