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Understanding How the Time of Year Affects a Mare’s Breeding


Mike Thompson, DVM, DACT
Raising a foal of your own can be an incredibly rewarding experience. A special relationship can develop when a person knows their horse from birth. In addition, it is very enjoyable watching a foal just be a foal. Unfortunately, any of us involved with mare reproduction are well aware of how frustrating it can be to get a mare in foal. Confusion can occur when mares do not cycle and breed as we expect them. Knowing how the seasons affect reproduction can help us formulate the best plan to lead to newborn foals arriving on our farms.
Mares are seasonal breeders, with natural, fertile cycles occurring during the long days of the year. The seasonal reproductive patterns of mares can be separated into 4 seasons, Vernal (or springtime) Transition, Breeding Season, Autumnal Transition, and Anestrus. These four seasons will be discussed further to help mare owners better understand the importance of timing in breeding.
Springtime transition can be a confusing time for many mare owners, and most likely for the mares as well. Beginning shortly after the winter solstice, production of important reproductive hormones begins to increase from the low levels maintained during the short day, winter months. These hormones slowly begin to increase toward the levels maintained during the breeding season, leading to the mare exhibiting behavior suggesting she is in heat. Many mares are bred during this time based solely on behavior. Unfortunately, during the early spring, the mare can go through several “heat cycles” before she is actually ready to ovulate a fertile egg. The first several follicles produced by these mares’ ovaries generally do not mature to ovulation, thus the signs of heat are not accompanied by a fertile cycle. I have, as well as many of you have, seen mares bred successfully during the early spring, but without manipulation of their cycle through artificial lighting and possible hormone therapy many early spring mares are not consistent. The springtime transition phase will begin around January 1, and can continue until the first week in April.
The Breeding, or Ovulatory, season begins after the first complete ovulation occurs, usually beginning around the first of April. It is during this season that the mare cycles most regularly, ovulating approximately every 21 days. The normal length of heat during this time is usually from 5-7 days, with ovulation occurring toward the end of heat. If a mare does not become pregnant during a heat cycle, the hormone patterns resume, bringing her back into heat. A non-pregnant mare will have multiple cycles during the breeding season, giving her multiple opportunities to become pregnant. This season will usually continue until sometime after the first of October, though it can end as early as September or as late as November or December.
The Autumnal Transition season begins to occur as the daylight hours begin to shorten, and the hormone levels responsible for normal cycling begin to diminish. The beginning and end of this season is quite variable, starting at the end of the breeding season, which can be from September until as late as December. As the hormone levels decrease, the regularity of the mare’s cycle is interrupted, causing unfavorable conditions for breeding and conception. . Although the summer, ovulatory season is the optimal time for conception, we still have many mares conceive in the fall during the Autumnal Transition.
Anestrus occurs after the Autumnal season, and continues until the springtime transition begins. Anestrus is a time of complete reproductive incompetence. Almost no follicles are produced on the ovaries, and none will mature and ovulate. During this time, the mare often will show indifference to a stallion, and usually will not allow breeding. As with the other seasons, the length of anestrous varies from mare to mare and can vary from year to year. Your veterinarian can help you shorten the length of the anestrous season with the use of artificial lighting.
So, now what do we do with this information? It is important to remember that every mare is different and sometimes the same mare will have variations in her individual cycles. Just because something worked with one mare does not mean it will work in another. Simply applying what proved to be an effective method in one mare to another does not guarantee success. You may have noticed that these seasons can overlap quite a bit, with some mares having barely experienced one season or another. Understanding that signs of heat during early spring are not likely to be accompanied by ovulation is of paramount importance. If there is no ovulation, there will be no pregnancy. Equally as important is to know that in the fall, a cycle may or may not be an ovulatory cycle. There are several things that can be done to affect the cycle of a mare such as the use of artificial lighting and hormone therapy. Contact a veterinarian experienced in equine reproduction (theriogenology) as early as possible to establish a plan for breeding. Your veterinarian is the best equipped individual to help you with your mare’s cycle, as well as all of her individual tendencies. I have often heard the statement that, "it doesn’t matter what the books say, because the mare won’t read the books." The more you breed mares, the more this statement becomes true. Understanding why mares can be so variable in their cycles can help you make decisions to decrease costs and increase reproductive efficiency.
Seasonal Reproductive Patterns of Mares

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