Mare and Foal• Photo courtesy of SDP Buffalo Ranch.

Health Matters: Foal Heat

For most of us, February is the start of breeding season. With a good number of foals already on the ground, mare owners are eager to get their mares back in foal. To my knowledge, mares are the only domesticated animal that have a fertile ovulation so close to giving birth. If we say the average gestational length of an equine pregnancy is around 330 to 345 days, that means you have about one month to get your mare back in foal to maintain a 12-month cycle per foal. 

As time goes by, you can get further behind on the breeding calendar as missed cycles and lost pregnancies add up over the years. When your mare foals later into the summer, your ability to catch up is relegated to basically two options. One, you skip breeding a season and lose a year of production to start early the next breeding season. Two, you pay the several thousand dollars it takes to do an embryo transfer so your mare will be open and ready to breed early next year. 

Mare Cleaning Foal
Mare Cleaning Foal

This is why so many breeders choose to breed on the “foal heat” cycle in their programs. Foal heat is the first estrous cycle the mare has after delivering her foal. It typically starts five to 12 days post foaling, with the majority of mares ovulating by day 20, thereby keeping you on a 12-month cycle per foal. 

Now, whether folks should breed on foal heat or not can be a widely debated issue. Like anything else, there are advantages and disadvantages to consider. If your mare gets pregnant on her foal heat, it removes the possibility of her going anestrus, or shutting down cycling, later in the breeding season. If she is already pregnant, it is no longer an issue. 

Some breeds and disciplines in the equine world are quite obsessed with having foals as close to Jan. 1 as possible. Foal heat breeding allows for the shortest foaling intervals and the amazing ability for some people to always have a Jan. 1 foal. (Sarcasm implied.) 

On the other hand, there is a greater chance of pregnancy loss when a mare is bred on her foal heat. Depending on the statistics you read, where the study was performed and how the analysis was calculated, you can also make an argument for or against pregnancy rates with foal heat breeding or those cycles used later in the year. Former British Prime Minister Benjamin Disraeli had a great quote to remember regarding statistics and interpreting scientific data. He said, “There are three kinds of lies: lies, damned lies and statistics.” 

Statistics aside, here are three basic concepts to understand when deciding on a foal heat breeding. The first is uterine involution. This is the term for the mare’s uterus returning to its normal, pre-pregnant shape and size. It takes somewhere around 23 days for this to happen. That seems fairly quick, in my opinion, to have your uterus go from the diameter of a large duffle bag to a roll of cookie dough in less than a month. 

Many mares complete involution within 14 days of foaling or less. In this time, normal uterine discharge, called locha, is expelled and sequential ultrasound exams will confirm the stage of involution, as well as the presence or absence of abnormal uterine fluid that is a good clinical indicator involution is not complete. 

The second factor to consider is foaling complications. By that I mean the various ways foaling can go wrong and negatively impact the mare’s reproductive status. 

Dystocia or a difficult birth is more common than you may think in mares. A true dystocia that requires a C-section is rare, but more often than not, the trauma induced by large foals or unassisted deliveries causes significant bruising and tissue trauma to the mare’s caudal reproductive tract. Just as a kick wound takes time to heal from injury and repair, the soft tissues of the mare are no different. 

Retained placentas are an issue worth mentioning, as well. If your mare does not pass her placenta within three hours of foaling, it is considered retained and needs attention. The longer it stays in, and the more physical and chemical manipulation that is required to extract it, the higher the chance of delayed uterine involution. 

Mare and Foal• Photo courtesy of SDP Buffalo Ranch.

Time is the third factor. Time matters in fertility. To be specific, day 10 matters. If your mare ovulates before day 10 post foaling, the chances of her getting pregnant go down significantly because of the concept of uterine involution. If her uterus is not ready to accept the embryo in a fluid-free environment with a competently closed cervix, the chances of her hanging onto that pregnancy are very low. 

If, however, your mare ovulated on day 10 or later after foaling, the chances for a healthy pregnancy are much higher. After the mare ovulates and the egg is fertilized, it takes five to six days for the embryo to make its way into the uterus, where we can see it on the ultrasound as a 15- to 16-day pregnancy. Buying a few extra days in the happy confines of the oviduct allows the uterus to complete involution and expel any remaining fluid that is detrimental to embryonic health. 

So, with all that being said, are you going to breed your mare on foal heat this year? Good question. If you asked me, I would say to take it one day at a time and one mare at a time. Some mares breed on foal heat like clockwork, but not very many. There are so many factors to take into account when making the decision to breed a foal heat cycle that can vary from day to day. 

Be flexible and understanding with your mare, and remember the two most important things you need to have a successful breeding season – patience and money. Hopefully one doesn’t run out before the other! 

Dr. Justin High is a veterinarian and partner in Reata Equine Hospital in Weatherford, Texas. Send your questions and comments to  [email protected].