If a barn full of people were asked their opinion about breeding a mare during foal heat, a barn full of opinions would result. This topic has been controversial for a long time. There is debate about whether it is harmful to a mare’s health, whether it negatively impacts fertility, and whether it causes an increase in fetal loss during pregnancy. Unfortunately, there have only been a handful of good-sized scientific studies published that thoroughly evaluate the practice.

So, is there any reason to breed during foal heat?

What is Foal Heat?

“Foal heat” is the term used to describe the first estrus or heat that occurs after a mare delivers a foal. During this time the mare ovulates and can become pregnant again. Typically, estrus occurs every 21 to 22 days in the mare and lasts four to seven days. A large follicle develops on an ovary, growing to a size of 35-55 mm or more in diameter. Most mares will show behavioral signs of heat throughout estrus, including signs of interest in a stallion, squatting to urinate, and “winking” to show the clitoris. It is not until the last 24-48 hours of estrus, however, that the follicle ruptures and ovulation occurs. Only then is the mare able to conceive.

The process of foaling stimulates the mare to begin cycling within a matter of days. The typical interval from the time of foaling to the beginning of foal heat is eight to 12 days. Some mares enter foal heat sooner. Interestingly, season of the year plays a role here. Mares that foal early in the year, from January to March, tend to experience a longer interval; they reach foal heat about 15 days after fo