OVARIAN FOLLICULAR DYNAMICS AND DURATION OF ESTRUS AND DIESTRUS IN ARABIAN VS QUARTER HORSE MARES

Arabian and Quarter Horse mares (n = 5 mares per breed in 1990 and 12 mares per breed in 1991), kept under identical conditions, were used to study breed differences in some aspects of ovarian function. Arabian mares had a smaller ovulatory follicle diameter on the first day of estrus (Day 0, 29.5 +/- 1.1 mm, mean +/- SE) compared with Quarter Horse mares (33.3 +/- 1.2 mm; P < 0.05). Although Arabian mares had a longer mean interval from Day 0 to ovulation (110.7 +/- 8.4 h vs. 84.6 +/- 6.1 h; P < 0.05), Quarter Horse mares had a larger follicle prior to ovulation (43.3 +/- 1.1 mm vs. 40.3 +/- 0.7 mm; P < 0.05). Additionally, estrus was longer in Arabian (6.2 +/- 0.4 days) vs. Quarter Horse mares (4.9 +/- 0.38 days, P < 0.05). This preliminary study showed differences in some aspects of ovarian function between mares of two breeds, Arabian and Quarter Horse.