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)

Abstract

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.

Comparison of Different Treatments for Oestrous Induction in Seasonally Anovulatory Mares

“…However, this value was similar to that observed in non‐treated mares during the first ovulation of the breeding season (46.7%). This percentage was slightly lower than described by Hanlon and Firth (), but strongly lower than that reported by Cuervo‐Arango and Clark (). Both studies conclude that pregnancy rate was not affected by progesterone priming treatment, in agreement with data here obtained.…”

“…As previously reported by Cuervo‐Arango and Clark (), treatment of transitional mares with intravaginal progesterone‐releasing devices has significant advantages in improving the breeding management during a difficult time of the year for both the veterinarian and the stud manager. In group T2, the progesterone levels drop below 1 ng/ml on day 11, showing that the progesterone release is much reduced at this time, and suggesting that shorter treatments could exert a similar effect.…”

“…In the present study, 1.38 g progesterone‐releasing devices were used, that is, lower doses than the used in other recent studies (Handler et al. ; Cuervo‐Arango and Clark ; Hanlon and Firth ) (Table ). Results show that devices with lower progesterone dosage offer similar results than reported in other studies with higher doses, and the reduction in the time of treatment from 11 to 8 days does not affect negatively the reproductive performance in seasonally anovulatory mares.…”

Growth rate of ovulatory follicles during the first ovulatory oestrus (after seasonal anoestrus) and subsequent oestrous period in Irish Draught mares

“…Thus both the means and ranges of growth rates, and pre-ovulatory diameters were almost identical for both groups. Previous studies found non-significant differences in pregnancy rate between ovulations from first and subsequent cycles in thoroughbred mares, 65.2% v. 76.0% [8] and 67.2% v.73.7% (Newcombe and Cuervo-Arango: unpublished). In the latter study, the multiple pregnancy rate was 50% higher at second cycles (23.8% v. 15.7%).…”

The use of progestins in equine medicine: A review

“…The use of an intravaginal progesterone‐releasing devices (PRID/CIDR) have been investigated for hastening the first ovulation, but with mixed results. CIDRs impregnated with progesterone were found ineffective in improving pregnancy rates on the first cycle (Cuervo‐Arango and Clark 2010), while Newcombe reported 89% of anoestrus mares to ovulate within 10 days following the removal of a progesterone‐releasing intravaginal devices (PRIDs) (Newcombe 2002). It should be noted that mild‐to‐moderate vaginitis is reported in a subset of mares following implantation of the progesterone‐releasing devices, and the devices are not approved for equine use in most countries (Crabtree et al 2018).…”