Estrus Synchronization in Cycling Mares
Synchronizing the estrous cycle in mares that are cycling is much easier than in transitional or anestrus mares. The treatments we describe can begin on any day of the estrous cycle. One approach is to mimic diestrus hormone levels by giving progesterone. This inhibits follicular growth. When progesterone supplementation is stopped, follicles grow, the mare comes into estrus and ovulation occurs. The second approach is to trigger the early regression of the CL with PGF2a. The natural progesterone from the CL has been inhibiting follicular growth and when progesterone decreases, follicles grow, the mare comes into estrus and ovulation occurs. In both of the approaches, the mare will exhibit estrus behavior 3-6 days after stopping progesterone supplementation or PGF2a treatment and on average ovulates day 8-9.
The time until the mare comes into estrus depends on the size of the follicles when the inhibition of follicle growth is released (stop giving progesterone or give PGF2a). If the follicle is only 20 mm then it will take the mare 5-6 days to come into heat. If the follicle is 30 mm then the mare will be in heat in 1-4 days.
Potential successful estrus synchronization schemes are:
- Progesterone for 14 days (Regumate or IM injection of progesterone)
- Progesterone (or P+E) for 7 days or long acting progesterone (1 injection), PGF2a on day 7
- PGF2a: single injection (mares are responsive only when there is a functioning CL present: day 5-12 of cycle, 80% of mares respond)
- PGF2a: two injections 12 days apart (80% of treated animals in estrus on day 14-20 after start of treatment)
The horse owner should be aware that giving PGF2a results in extensive sweating, abdominal cramps and in rare occasions, colic. There are several different preparations of PGF2a and it should only be used after consulting with your veterinarian on the dose appropriate for your horse.
