Effect of GnRH treatment during the anovulatory season on multiple ovulation rate and on follicular development during the ensuing pregnancy in mares

摘要

Summary. Seasonally anovulatory mares were injected, i.m., twice daily with a GnRH analogue (GnRH-A), and hCG was given when the largest follicle reached 35 mm in diameter. In Exp. 1, treatment was initiated on 23 December when the largest follicle per mare was ? 17 mm. An ovulatory response (ovulation within 21 days) occurred in 17 of 30 (57%) GnRH-A-treated mares on a mean of 15·8 days. The shortest interval to ovulation in control mares (N = 10) was 57 days. The diameter of the largest follicle first increased significantly 6 days after start of treatment. In Exp. 2, treatment was begun on 15 January and mares were categorized according to the largest follicle at start of treatment. The proportion of mares ovulating within 21 days increased significantly according to initial diameter of largest follicle (? 15 mm, 9/25 mares ovulated; 15–19 mm, 13/21; 20–24 mm, 20/24; ≥ 25 mm, 3/3). The multiple ovulation rate was greater (P < 0·01) for treated mares (27/86 mares had multiple ovulations) than for control mares (2/35). Treated mares in which the largest follicle at start of treatment was ≥ 25 mm had a higher (P < 0·01) multiple ovulation rate (9/14) than did mares in which the largest follicle was <25 mm (18/72). The pregnancy rate for single ovulators was not different between control mares (26/30 pregnant mares) and treated mares (43/54). There was a significant interaction for diameter of largest follicle during pregnancy between day and group (control group and 3 treated groups in which the largest follicle on first day of treatment was < 15, 15–19 or 20–24 mm). On all days from Days 17 to 38, the largest follicle was larger (P < 0·05) in the control group (largest mean, 27·9 mm) than in the groups first treated when the largest follicle was < 15 mm (largest mean, 15·3 mm), 15–19 mm (largest mean, 17·9 mm), or 20–24 mm (largest mean 20·2 mm).