Horse Breeding Guide: Heat, Covering, Pregnancy to Foaling

Mares cycle with daylight. In the Northern Hemisphere the natural breeding season runs about April through September, with winter anestrus from roughly November to February and irregular transition heats on either side. Commercial farms put mares under lights starting in late fall to push cycles earlier; nature's version is a spring cover and a spring foal 11 months later.

Both produce the same pregnancies, so logistics decide. Live cover in hand or pasture is simple and suits registries that require it, like the Jockey Club. AI with fresh or chilled semen opens the stallion market nationwide, and frozen semen ships worldwide but must be timed to the hour of ovulation, so it costs more in vet work.

Foal heat is the mare's first heat after foaling, arriving about 7 to 9 days postpartum. It is fertile, and breeding it keeps foals coming yearly, but conception rates run a bit lower and the uterus may not be fully recovered. Many managers skip it and breed the next cycle around day 30, sometimes short-cycling with hormones; decide mare by mare at the foal heat check.

Work backward from the foal you want: gestation averages 340 days (range 320 to 370), so a May 1 cover foals around mid-April next year. To hit early-year foals, like Thoroughbreds aiming close to January 1, mares must be cycling in winter, which means lights or hormonal programs. Otherwise breed in the natural season and foal onto spring grass.

Most mares foal at night, quickly and alone, so vigil matters. Udder development, waxing on the teats, softening at the tailhead, and milk calcium changes narrow the window from weeks to hours. A stall camera is the standard setup, and halter or tail transmitters that alarm in the foaling position catch the sneaky ones; plan to check hourly once she looks close.