Ovulation time in mares is often manipulated to synchronize shipments of semen or availability of a stallion for breeding. Several drugs are useful to hasten ovulation to a more fixed time to coordinate a schedule that works with breeding farms for both the stallion and the mare. A Thai study looked at methods for possible improvement of time to ovulation [Phetudomsinsuk, K. Investigation into the effect of prostaglandin F2a, GnRH analogue and hCG on induction of ovulation in mares. Thai J Vet Med, 2017. 47(4): 493-499].
All mares in the study received 5 mg of prostaglandin-F2alpha on Day 6 after ovulation to hasten return to estrus within 14 days rather than 21 days. Optimal breeding occurs when a follicle reaches its largest diameter and uterine edema is greatest.
Four ovulation protocols were implemented in different groups of mares once a follicle reached 35 mm along with a uterine edema score of 2 based on ultrasound exam:
- Spontaneous ovulation without any drug influence
- 40 ug of buserelin (GnRH analogue) as a single IM dose
- 1500 IU hCG (human chorionic gonadotropin) as a single IV dose
- 3000 IU hCG as a single IV dose
As expected, the non-treated mares had the slowest follicular growth. The mares treated with 1500 IU hCG experienced the fastest follicular growth of 6.3 mm, with ovulation in 1.53 days. Buserelin-treated mares ovulated in 1.83 days; mares given 3000 IU hCG ovulated in 2.1 days; and the non-treated group ovulated in 3.2 days. The higher dose of hCG might down-regulate the hypothalamic-pituitary-ovary pathway; in this study, the lower dose of hCG achieved 100% ovulation rate.
The mares were inseminated once 24 hours following their treatment. In general, the mares treated once with 1500 IU hCG ovulated within 48 hours with a 93% conception rate at Day 15, giving the best overall outcome.
