Commercial horse breeding farms operate under the same economic rules as other forms of intensive livestock production. Use of stallions must be efficient, and mares must produce the greatest number of foals possible. This requires intensive management. The primary reason for foal heat-breeding is to increase the efficiency of the production unit.
The performance of a herd of mares should be measured not only by the final conception rate but also by how efficiently that pregnancy rate was achieved. This can be measured by the number of breedings per conception and by noting when in the breeding season each mare becomes pregnant. A set of criteria can be established such that the performance of individual mares and the herd can be measured against the ideal. The criteria can be flexible and should be tailored to meet the economic objectives of the operation. For example, the time at which the farm would like the earliest foal to be born is determined by the management practices of that farm and by the requirements of a particular breed registry. Once these criteria are established, a management goal can be set. In central Kentucky the main commercial business is the production and sale of Thoroughbred yearlings. Thus a workable criterion is that no mare is bred before the fifteenth of February. Optimal fertility dictates that no mare is bred before the tenth day after foaling. That means that all maiden, barren, and January foaling mares should be bred as close to the fifteenth of February as possible. All other foaling mares are bred on or soon after the tenth day from foaling. Of course, this schedule is not always possible, nor in many instances is it advisable. However, if management strives to meet these goals, the net result will be that the farm can push foaling dates to the front of the season. This shift will result in increased production from the mare herd. It can be readily seen that foal heat-breeding plays an important part in such a strategy because it keeps the interval between foaling dates to a minimum.
Determining An Appropriate Time To Breed The Mare
Mares that are normal and eligible to be bred on the foal heat can then be monitored for follicular development and bred at the optimal time. Several studies have shown that time of ovulation is critical to the success of foal heat-breeding. Mares that ovulate before ten days after foaling have a much lower pregnancy rate than do mares that ovulate at 10 days or later. Therefore if ovulation occurs too early, it is better not to breed the mare even if she is deemed suitable for mating. The performance of the herd will be better if these mares are not bred at the foal heat and then are managed so that they can be bred at the earliest time possible after this first ovulation.
If a decision has been made to skip the foal heat, these mares may be bred earlier by using prostaglandin to cause regression of the corpus luteum. The prostaglandin can be given at day six or seven postovulation, and the mare will often be ready to breed six or seven days later. This will shorten the time from foaling to breeding by about a week compared to allowing the mare to return to heat naturally. Mares that have a uterine infection, have fluid in their uteri, or have poor uterine involution after foaling often will also benefit from prostaglandin therapy. The early return to estrus appears to have a cleansing effect on the reproductive tract of these mares. It also gives the veterinarian a chance to continue therapy if necessary.
Some managers use hormonal therapy to delay the onset of the first estrus and thus ensure that the ovulation will occur ten days or more after foaling. Two methods are used to achieve this delay. One is with the use of oral altrenogest, and the other with the use of injectable progesterone and estradiol. Although the altrenogest (0.044 mg/kg) is the simplest and most readily available product, it is also the least precise. The progesterone (150 mg) estradiol-17β (10 mg) in oil combination provides more precise control of follicular development but must be given by daily injections and thus can cause some muscle soreness. Irrespective of the method chosen, beginning treatment on the first day postfoaling is important. If treatment is begun later — after follicular development has commenced — it may be difficult to suppress this growth, and some mares will continue through the therapy and ovulate. Mares that are successfully managed in this manner can be made to ovulate at day ten, twelve, or even later from foaling. Some investigators believe that this can be helpful. However, a controlled study in which this author participated showed very little benefit over the intensive management of foal heat alone.
Care Of The Mare After Breeding
Mares bred on foal heat should be examined the day after breeding — not only to confirm ovulation but also to ensure that the uterus has not retained a significant amount of fluid. If an ultrasound examination reveals an echogenic fluid accumulation, the mare should be treated with oxytocin to help promote elimination of this fluid. If a large volume of fluid is present, it may be advisable to lavage the uterus. Postbreeding antibiotic infusion may also be indicated in foal heat mares — more so than at other times. If the mare requires a Caslick operation, it should be performed at this time. The mare is less forgiving at the foal heat than during later estrous periods. Thus attention to detail is especially important.
Foal heat-breeding is a useful management tool, but it must be done carefully and with thought. Indiscriminate foal heat-breeding can often be detrimental to the mare and thus make the overall management program less efficient.