Breeding Mares on Foal Heat

April 20, 2012

Breeding mares on their foal heat is a strategy used to maximize reproductive efficiency. Since income is generated from selling offspring, yearly foal production is critical to offset maintenance and breeding expenses incurred by the mare owner. With an average gestational length of 333 to 345 days, mares must become pregnant within one month post partum to continue producing foals each year. Mating mares on the first postpartum estrus is one method used to improve the chance of maintaining yearly foal production. Reviewing this topic for us is guest writer, Dr. Margo Macpherson with an excerpt from the chapter Breeding Mares on Foal Heat co-authored by Dr. Margo Macpherson and Dr. Terry Blanchard in the 2nd Edition of Equine Reproduction.

The first post partum estrus generally begins 5-12 days after foaling; hence, the terminology foal-heat or “9 day heat.” In a large study involving Thoroughbred mares, ovulation during foal heat was reported on average at 10 days post partum, with most mares ovulating within 20 days post partum (Loy, 1980). Mares that foaled early in the year were less likely to ovulate by 10 days post partum than mares foaling later in the spring. Also, early foaling mares (January to March) were more likely to have a prolonged period of postpartum anestrus (> 30 days to first ovulation postpartum) than late foaling mares. While not proven, it is suspected that the postpartum anestrus seen in many foaling mares is thus related to length of days when parturition occurs. In this regard, supplementing artificial light to late pregnant mares has been shown to reduce the incidence of postpartum anestrus (Palmer and Driancourt 1983).

Advantages for mating on foal heat include:

  1. Lowering the odds that mares foaling early in the year will re-enter anestrus after the foal heat ovulation
  2. Avoiding a delay (3 weeks or more) in the foaling date the next year compared to mares mated for the first time on their second postpartum estrus
  3. Cumulative season pregnancy rates that are comparable between mares mated for the first time on their foal heat and mares mated for the first time on later postpartum estrous periods.

Disadvantages of breeding mares on foal heat are:

  1. Lower pregnancy rates achieved for mares mated on foal heat compared to mares mated for the first time on later postpartum estrous periods
  2. Potential for greater pregnancy losses in mares conceiving on foal heat matings compared to mares conceiving on later postpartum estrous periods.

Uterine Repair
In the normal foaling mare, several events occur to the uterus in the immediate post-foaling period. This is termed uterine involution. Inflammation helps rid the uterus of debris and contaminants that are present in the uterus for the week after foaling. This process results in the formation of fluid, called lochia, which is often seen coming from the mare’s vulva. This fluid is usually reddish-brown and does not have a foul odor. The uterus, which is a strong muscle, actively contracts after foaling to help evacuate the fluid and debris. Contractions are also important for returning the uterus to its pre-foaling size. Suckling by the foal and exercise are important stimulants for uterine contractions and evacuation. Tissue repair and uterine “clean up” are essential for re-establishing pregnancy. Involution occurs rapidly after a normal parturition. Within 30 days of foaling, the uterus should have returned to its’ pre-foaling state in both tissue health and size.

Major factors thought to be related to fertility achieved on foal heat breeding include an uncomplicated birth free from genital tract trauma, prompt passage of the placenta, rapid uterine repair/involution, and early return to regular estrous cycles. Selection of mares for foal heat breeding should be based on meeting these minimum criteria to optimize success.

Strategies For Breeding Mares on Foal Heat
Breeding mares on foal heat is not a recipe-driven event. Each mare must be treated as an individual and the conditions of her foaling considered. In general, young mares with uncomplicated foaling are better candidates for breeding on foal heat than older mares that have previously delivered several foals.

A general approach to breeding mares on foal heat starts in the week after foaling. First, all mares should be examined no later than 6-8 days after foaling. A visual examination of the mare’s reproductive tract will reveal the presence of urine pooling, pneumovagina or unresolved trauma to the vagina, vestibule or vulva. Although these conditions usually will improve over time, injured mares are not good candidates for breeding on the first postpartum estrus. Examination of the internal reproductive tract using an ultrasound should be performed to reveal the presence of intrauterine fluid accumulation and size of any developing follicle(s). These factors are monitored by repeating examinations at 1-2 day intervals. If fluid is present in the uterus at the time when foal heat breeding is anticipated, it is better to treat the mare to remove the fluid than to breed the mare. Mares judged to be involuting normally, with no significant intrauterine fluid accumulation, are good candidates for foal heat breeding. If the mare ovulates prior to day 10 post partum, breeding on foal heat can be bypassed and prostaglandin can be administered 5-6 days after ovulation to induce an earlier return to estrus for breeding. It is thought that this protocol allows the uterine environment more time for repair, as well as provides a shorter interval (1-2 weeks) to breeding than that achieved by waiting for the second postpartum estrus to spontaneously occur (typically around day 30).

Some mares will accumulate fluid after the foal heat breeding. Such mares should be treated to remove fluid and resolve any infection that may have been established. Performing uterine lavage (3-5L lactated ringers or saline solution) in these mares is important, and is thought to not interfere with fertility if the lavage is performed at least 4 hours after breeding (Brinsko et al 1991). Use of drugs to promote uterine contraction, such as oxytocin, can also be helpful when treating mares with uterine fluid. Often, oxytocin is combined with uterine lavage for fluid removal in the breeding period. While the inclusion of antibiotic infusions after breeding postpartum mares remains controversial, some individuals (Pycock, 1994) have reported that postpartum mares treated with antibiotics plus oxytocin had higher pregnancy rates than either mares treated only with oxytocin or mares that were left untreated.

In conclusion, not all mares are suitable candidates for breeding during the first postpartum estrus. However, using careful selection of mares, breeding during foal heat can result in favorable pregnancy rates in a highly efficient manner, and can reduce parturition to conception interval to help to maintain yearly foal production in mares.

Dr. Margo Macpherson received her DVM degree in 1990 from Michigan State University after which she completed a residency and Master’s Degree in Equine Theriogenology at Texas A&M University. After leaving Texas, Dr. Macpherson spent time at the University of Pennsylvania and in private practice in Central Kentucky. Currently an Associate Professor and Service Chief in the section of Reproduction at the University of Florida, Dr. Macpherson is primarily interested in conditions that affect pregnancy including twin pregnancy and placentitis.

In this video Margo reviews the reproduction topics that were presented at the 2011 AAEP Convention.

Blanchard, T.L. and Macperson M.L. 2011. Breeding Mares on Foal heat. Equine Reproduction, 2nd Edition, Editors: A.O. McKinnon, E.L. Squires, W.E. Vaala and D.D. Varner, Wiley-Blackwell, West Sussex, UK.
Brinsko, S.P., Varner, D.D., Blanchard, T.L. 1991. The effect of uterine lavage performed four hours post insemination on pregnancy rates in mares. Theriogenology 35:1111-1119.
Loy, R.G. 1980. Characteristics of post partum reproduction in the mare. Vet Clin N Amer: Large Anim Prac 2:345-359.
Palmer, E., Driancourt, M.A. 1983. Some interactions of season of foaling, photoperiod and ovarian activity in the equine. Livest Prod Sci 110:197-210.
Pycock, J. 1994. Assessment of oxytocin and intrauterine antibiotics on intrauterine fluid and pregnancy rates in mares. Proc Amer Assoc Eq Pract 40:19-20.