Ovulation rate and distribution in the thoroughbred mare, as determined byultrasonic scanning: the effect of age

Ovarian activity was monitored in 2181 oestrous cycles from 1136 thoroughbr ed brood mares to accurately determine overall ovulation rate, the relative distribution of ovulations between left and right ovary, whether mare age had an effect on these parameters and whether ovulation pattern in one cycl e affected the pattern within the next. Ultrasonic scanning was used in pre ference to rectal palpation and slaughterhouse material due to its greater accuracy. Mare's ages ranged from 3 to 22 years. The results obtained indic ated a similar incidence of ovulation occurring on the right (49.5% of all ovulations) or left ovary (50.5%). Multiple ovulations (MO) occurred in 22. 4% of oestrous cycles. Significantly (P < 0.01), more double ovulations (DO ) were bilateral (57.2%) than unilateral(42.8%): 20.7% were unilateral righ t and 22.1% were unilateral left. There were 828 mares grouped into six age groups: A (3-5 years), B (6-8 years), C (9-11 years), D (12-14 years), E ( 15-18 years), F (18-22 years). The distribution of ovulation between the le ft and right ovary was not affected by age group, but significant associati on(P < 0.01) was evident between age and MO. Group A had 15% of cycles resu lting in MO compared to 19.4, 24.6, 25.8, 28.6 and 35.1% in groups B-F, res pectively. A significant(P < 0.01) association between MO in successive cyc les was also demonstrated, with a greater chance (P < 0.05) of successive s ingle ovulations (SO) being on alternate ovaries. There was no significant repeatability of the classification of DO in one cycle compared to the next . We conclude that: (i) the mare is a symmetrical ovulator; (ii) 22.1% of c ycles yield MO (ovulation rate 1.23); (iii) this MO incidence increased (P < 0.01) with age; (iv) the exhibition of MO in one cycle was linked to an i ncreased probability (P < 0.01) of a MO occurring at the next one and (v) S O in one ovary was more likely (P < 0.05) to be followed by a SO on the alt ernate ovary during the next cycle. (C) 2001 Elsevier Science B.V. All righ ts reserved.