How does the reproduction and mating of the mare work? In the wild, horse populations proliferate to the point that, in certain regions, the problem of overpopulation and environmental degradation has arisen. Thus, the reputation of low fertility of the equine species is only justified when man wants to lead it in artificial conditions without sufficient knowledge of the reproduction phenomena he intends to control. In this article, we will try to follow the path of the progressive sophistication of horse breeding methods developed by zootechnicians and veterinarians, without highlighting all breeding techniques. How many mares can serve a stallion in an open herd? How and when to introduce the mare in the stallion’s harem? When can we take her back being sure that she is in foal? When will she foal? What is in-hand riding? Find all the necessary information concerning the question of reproduction in horses.
The reproduction in wild horses
They live in herds of a few mares (3 to 8) with a stallion. The other males, called bachelors, live in groups, and do not have access to mares.
The adult mares foal each year in early spring. They are covered quickly after foaling (10 to 15 days), during one or two other heats, but are very quickly fertilized. Thus, the normal physiological situation of the mare is gestation. Cyclicity is occasional and lasts a short time.
In the case of an empty mare and in the case of prepubescent youngsters, the winter season is accompanied by sterility and the onset of fertility is synchronous in all mares in early spring. When a mare is in heat, the stallion covers her two or three times a day during the heat. If more than one mare is in heat, each mare is serviced one or more times a day, so the stallion may do as many as seven or eight servings in a day. The refusal as well as the acceptance of the mating is done without violent demonstration, the stallion going to sniff each other; the mares in heat stop or even approach and present themselves to be mated while the mares which are not in heat do not show interest or simply move away of three or four steps.
It is possible to obtain horses produced under such conditions by capturing wild horses from free-ranging herds, but this is not strictly speaking a production method and no control of crossbreeding can be achieved.
First steps towards artificial breeding: horses in open herds
With domestication, it quickly becomes apparent that certain individuals are better (more adapted to the use one makes of them, more beautiful…) and, even if one does not know the genetics, one would like to have descendants of these chosen animals. The individual ownership of the animals means that, in general, except for the rich breeder who owns a herd, the owner of a mare will have to put his mare in the presence of the stallion which does not belong to him for a time to be determined.
The work done on open herd breeding dates back to the 70’s and shows that a limit of 20, at most 25, mares per year for a stallion is necessary to ensure the maintenance of its fertility. The risk is overwork and also that the stallion, exhausted, stops covering all the mares in heat. The introduction of mares into the herd can be done before foaling, provided that the grass surface is sufficient for the mare to move away to foal. The stallion is never aggressive towards small foals. The introduction of several mares at the same time is preferable to the introduction of an individual who risks being surrounded and chased by the whole herd. Establishing a new hierarchy does not always happen without kicking.
To know when to separate the mare from the stallion, one must first detect if she is mated. Since it is not possible to permanently monitor the stallion’s activity, a system of marking by the stallion wearing a harness and carrying a colored marker resin block has been invented. It allows each ride of the stallion to leave a colored mark on the croup of the mare. The first condition to separate the mare from the stallion is that the heat is over. As the duration of the heat is rather unpredictable (see table I), it is wiser to wait until it is over, the mare not being marked for 2 or 3 successive days. However, this is far from sufficient to be sure that she will be in foal. The first approach to a pregnancy diagnosis is that the mare should not come back into heat (i.e., be bred by the stallion in the herd) after a normal interval (see Table I). As this interval is also quite variable, it is advised to wait 3 weeks without mating.
How to know if a mare is in foal?
In 1769, M. de la Guérinière proposed to pour cold water in the ears and concluded that “if the mare shook roughly, she was not in foal”. Since then, more reliable techniques have been established: ultrasound, rectal palpation, progesterone dosage, E.C.G. (Equine Chorionic Gondadotro- pin, formerly called P.M.S.G., Pregnant Mare Serum Gonadotropin) dosage, estrogen dosage in blood or urine.
Riding in hand
Although free-range breeding allows high fertility to be achieved without great technical skill, it has the disadvantage of presenting unacceptable risks of accident for stallions and mares of great value. Moreover, the productivity of the stallions is limited to 20 or 25 mares, which is insufficient for an improving stallion.
The next step is mating in hand, which consists of the stallion detecting the heat of the mare, deciding on the mating(s) during the heat and proceeding with the mating under high supervision (the mare is tied up, sometimes held by a twister, and the stallion is held in hand. The expected benefits are a reduction in the risk of accidents and, to a certain extent, in the health risks due to the disinfection of the stallion’s penis at each jump and to the increase in the number of mares covered by the stallion. Depending on the accuracy of the breeding decision, it is possible to limit the number of mares bred to one per heat and per mare, and thus to breed up to 100 mares during a breeding season (from February 15 to July 1). Some Thoroughbred stallions even perform a second breeding season in the year by traveling to the Southern Hemisphere from August to December.
How to decide when to breed by detecting estrus and predicting the time of ovulation?
Detecting estrus (or heat, the period of acceptance of mating by the mare) is traditionally done by the “bar test”.
How to detect heat in the mare
The mare is placed in a stall along a wall and a stallion called “blower” is brought in hand nearby and allowed to sniff, sometimes to bite the mare who, maintained in her stall, can neither escape nor defend herself. We then observe her reactions which can go from the typical situation of acceptance of the mating (immobile mare, tail in the air, blinking of the vulva) to the typical situation of refusal (squeaking, tail clamped on the vulva, ears lying down, kicking in the lower flank, trying to escape). However, there are situations that are difficult to interpret in mares that are particularly skittish, nervous, or worried about being separated from their foal, and that respond to this test in a way that is biased by stress. Conversely, some lymphatic mares are indifferent to the test, and it is also difficult to detect their heat.
It is not enough to know that a mare is in heat, but it is also necessary to determine when she is most fertile. This time is close to ovulation, which should be detected, even predicted, or induced. To do this, we must remember the components of the mare’s cycle according to several aspects.
During the breeding season, if a mare is not in heat, it is because she is under the influence of progesterone secreted by her ovaries carrying a corpus luteum. This hormone effectively inhibits heat behavior and ovulation but does not prevent follicles from appearing in waves about ten days apart. If progesterone is present, the follicles stop growing and begin to degenerate; they are said to become atre-tic. If it is not pregnant, the uterus secretes the prostaglandin F2 a after H days of progesterone influence. This prostaglandin has the effect of destroying the corpus luteum and consequently allowing progesterone to disappear.
The fall of the progesterone level is accompanied by the appearance of the heat behavior and allows the continuation of the growth of the follicles towards ovulation. However, the size of the follicle can vary greatly at this stage; if it is close to the pre-ovulatory size (35 mm), then ovulation can take place very quickly, after 1 or 2 days of heat; if the largest healthy follicle is very small (<10 mm), then it will be necessary to wait for it to grow at a rate of 3 mm per day, i.e. approximately 8 to 10 days until ovulation. This explains why the interval between the onset of heat and ovulation is so variable. Detection of the follicle and its distance from ovulation is a key factor in managing the mare during heat. Traditionally this was done by rectal palpation: this technique involves the practitioner inserting a gloved arm into the rectum, which has been emptied of dung, to palpate the ovaries, which are just adjacent to the rectum. With practice, the practitioner can detect with some accuracy the presence of a follicle and its consistency.
Palpation of the mare
Palpation was the only means available until 1980, when ultrasound was introduced. This medical imaging technique based on the return (echo) of ultrasound emitted and received by a piezoelectric probe represents dense, echogenic tissues in clear, whereas liquids, such as follicular fluid, are represented by a black image (without echo). Compared to palpation, the contribution of ultrasound is greater precision in measuring the follicles (anechoic spheres), the correction of errors in interpretation of palpation by distinguishing, for example, two follicles next to each other that could give rise to twins, where palpation detected only one. Some cases where a young corpus luteum retains the consistency of a soft follicle are also resolved by ultrasound.
Thus, by palpation and/or ultrasound, it is possible to predict ovulation approximately, but above all to observe it precisely afterwards.
Predicting the ovulation period in the mare
The fertile period is shortly before ovulation. For mating or insemination of fresh semen, the mating is considered fertile if it is 60 hours to 6 hours after ovulation. To protect against poor semen survival, it is recommended that a mating be performed less than 48 hours before ovulation. This is achieved simply by servicing every 48 hours until the end of heat. Since the end of heat is usually 36 hours after ovulation, this strategy results in one more service than necessary after ovulation.
If the stallion is overloaded, the stud farmer may have to limit the number of services offered to one during heat. To do this, he will postpone the first service until the largest follicle is pre-ovulatory (>35 mm and soft consistency). Moreover, by checking the ovulation, he will be able to avoid post-ovulation mating which are not useful. Such a strategy allows for acceptable fertility, although several jumps on a heat are necessary to achieve maximum fertility.
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