The first ovulation of the breeding season in the mare: the effect of progesterone priming on pregnancy rate and breeding management (hCG response rate and number of services per cycle and mare).

The mare is a seasonally polyestrus breeder. In early spring, the mare enters a "transition period" between the anovulatory season and the first ovulation of the year. This period is characterized by irregular estrus cycles and high incidence of regressing dominant follicles. There is a belief that pregnancy rates resulting from the first ovulation of the season is lower than in subsequent ovulations, however this has never been studied critically. Progestagens are often used as an aid to manage the transition period. The objective of this study was to compare pregnancy rates of mares from the first ovulation of the year with: (a) mares on their second or subsequent ovulations and (b) mares with progesterone-primed first ovulations. A total of 136 thoroughbred mares were used in the study. The mares were classified into four groups: (1) mares mated at the first ovulation of the year (n=46); (2) mares mated in the first ovulation of the year after removal of a previously inserted progesterone device (CIDR) (n=29); (3) mares mated at the second or more ovulations of the year after prostaglandin-induced estrus (n=50); and (4) mares mated after spontaneous return to estrus (n=11). Pregnancy rates were not different in any of the groups studied: 65.2, 75.9, 76 and 72.7 for groups 1-4 respectively (P>0.05). Group 1 mares had the lowest response to hCG treatment which resulted in the highest number of services per cycle (P<0.05). In conclusion, although the use of progesterone priming did not affect pregnancy rates, it did improve the breeding management of transitional mares by enhancing the hCG response rate and consequently decreasing the number of services per cycle.