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Decision to Breed

What is the best time of year to breed?

Mares have a natural breeding season. Increasing daylight stimulates receptor centers in the brain, which trigger the production of reproductive hormones. These hormones initiate the pattern of regular periods of heat that characterize the breeding season each spring. These periods continue throughout the summer, and stop during the fall. By artificially increasing the amount of light, perhaps by using electric lights in a stable, it is possible to begin the breeding season earlier. This practice occurs frequently on Thoroughbred farms, which try to produce foals as close as possible to January 1st (this is because January 1st is the official birthday of all Thoroughbred racehorses). The ideal time for a foal to be born is between May and July, when the most grass is available, helping the mare’s milk supply. Because pregnancy in horses lasts 11 months, the best time to have the mare covered is June through August. This time period is the natural breeding time of horses and ponies in the wild.

During the breeding season, from March to October, mares show regular heats lasting four to six days. These recur 14 to 16 days after the end of the previous heat. Mares have a breeding cycle of around three weeks in duration. At the beginning and end of the stud session, mares may show irregular heat cycles. At any time, individual mares may return into heat early, due to uterine infection. A foaling heat occurs four to eight days after birth. Mares can be covered then, if they are “clean” or free from uterine infection.

Should your mare be bred?

Although horses do not have many problems in breeding, it is advisable for inexperienced owners to think twice before putting their mares in foal. Rearing a foal means extra work, and requires special facilities, including separate accommodations for the foal when it is weaned. If the mare is purebred, there could be financial benefit from breeding. Breeding crossbred animals will most likely fail in financial success, meaning that the extra outlay required will outweigh any potential profit. Sometimes, mares that are retired due to injury are sometimes put in foal. Whereas this may help pay for the mare’s keep, an objective view of her suitability for breeding must always be taken into consideration.

Mares often go on breeding until late in life, and suffer no ill effects from it. In general, this is certainly true of animals that have bred regularly; it is more difficult to get an old mare to foal for the first time. However, the only reason not to breed from an old mare would be a medical condition that could be made worse by pregnancy, such as severe lung problems (heaves), or possible foot problems. Fillies become sexually mature around 18 months old, and can foal as two-year-olds. However, they are still growing at this stage, and pregnancy may hinder their growth. Ideally, mares should not begin breeding until four years of age.

When will a mare accept the stallion?

Mares normally accept the stallion throughout the heat period. It is sometimes difficult to determine if a mare is truly on heat. At breeding farms, the willingness of a mare to stand for a stallion is often tested by using a stallion as a “teaser.” When a mare has been sent to a breeding farm, she is frequently presented with a substitute stallion, usually of lesser quality than the chosen sire, in order to test whether she is ready for mating. If she is not ready, and kicks or strikes out as a result, it will matter less if she injures a substitute. When on heat and confronted by a stallion, a mare normally stands, raises her tail, urinates, and contracts her vulva. If she is not, she moves away and may kick out at the teaser. Mares normally ovulate 24 hours before the end of the heat cycle. For maximum fertility, they should be mated just before this time, not afterwards. At breeding farms, veterinary exams of the ovaries, via the rectum, gives a good idea of when the mare is likely to ovulate. When such tests are not an available option, it is best to cover the mare on the second day of heat, and should be repeated every other day until she goes out of heat. At the start of the breeding season, mares may occasionally remain on heat for long periods without ovulating. This is particularly true of maidens, or mares which have not previously bred; there is little point in covering these horses, and veterinary advice should be sought.

Is your colt good enough to use as a stallion?

Very few homebred colts are good enough to use as stallions. There are many first-rate stallions available commercially, and they are far better choices for use in breeding. In any case, young colts are difficult to handle and are probably better gelded, unless there is a specific reason for not doing so. Both colts and stallions need expert handling, with the type of experienced skill that is usually only available at breeding farms. It is difficult and possibly even dangerous for amateurs, so it is not to be recommended without the supervision and involvement of individuals with a high level of experience in the matter. Stallions that only occasionally cover a mare can also be difficult to manage, and it is better to use one that is covering regularly.


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