Oregon Equine Reproduction Center: Preparing Your Mare for the Breeding Season

Ron Friedman, MS, DVM, Dipl. A.C.T.
It's January and breeding season seems like it's a long way off. "No problem," you tell yourself However, if you tried to breed your mare last year and she didn't settle (become pregnant) then you've already gotten off to a slow start this breeding season. If your mare is easy to settle or you have a maiden mare (not bred before), you are in a better position. If your mare is pregnant now and you are waiting for her to foal, there are still some things you can do to prepare for her next breeding. In all cases, it's not too soon to get started.

In this article we'll learn about the mare's reproductive cycle and its response to natural and artificial light. Then we'll discuss why it is important to have mares cycling and ready to be bred at the beginning of the breeding season. Next month, we'll detail some important points about hard to settle mares and breeding with shipped semen.

Mares are seasonal breeders. Their reproductive patterns are called estrous cycles. The estrous cycle includes the time when they are receptive to the stallion, also known as being in heat, or more precisely estrus (note the difference in spelling) and the time between two consecutive estruses when they are out of heat (called diestrus). Estrus lasts five to six days; diestrus is 14 to 16 days long. The total length of the estrous cycle in most mares is 21 to 22 days.

Mares are seasonal because they do not have estrous cycles all year long.

Instead, most mares cycle from Spring to Fall. When mares don't cycle that stage is known as anestrus. As mares progress from anestrus to estrous cycles they go through a period of transition when they are adjusting to the changes springtime brings about. This phase, from anestrus to estrous, is known as transition. When mares are in transition, their cycles are irregular, ovulations highly unpredictable, and trying to breed them is generally frustrating. Transition ends when the mare has her first ovulation of the season and the cycle starts ... diestrus, estrus, diestrus and so on.

The seasonal nature of estrous and anestrus seen in the mare is caused by the changes in daylight throughout the year. As mentioned, the mare starts to cycle in spring and goes into anestrus in the fall. During springtime, the days lengthen until the summer solstice on June 21st, after which they get shorter again. (It is interesting to note that although most horses are long day breeders, some mares never go into anestrus and continue to cycle through the year.) If allowed to respond to natural daylight, the horse's breeding cycle actually doesn't depend on the length of daylight but on the change from short to long days. Put another way, increasing daylight won't start a mare cycling unless it follows a time of shorter daylight.

Have you ever wondered why the breeding sheds open in February, before the mare naturally starts cycling in March, April, or May? In a nutshell, it comes down to competing youngsters. Foals born close to January 1^st (the "birthday" for horses of any given age), have more time to develop and mature than horses born later in the year. When a two year old born on January 1^st competes against a two year old born on August 1^st, it has an extra eight months of maturity, with related attributes of strength, balance, muscle development, and speed, in its favor. A mare who settles in February will foal very near January 1^st - a real bonus for the competitive breeder.

How do we prepare mares for the breeding shed? A mare is ready for the breeding shed when she is cycling regularly and free of uterine infection or inflammation. Many people say that preparation isn't necessary, you just take the mare to the stallion and bada-bing bada-boom, a foal is made. Owners who've had such an easy experience are blessed. Many, many, more people wish it could be so easy. Horses are not as fertile as we'd like and we often interfere with their natural behavior. As a result, horses frequently need help in getting in foal.

Most breeding sheds open around February 15th. You may avoid some headaches if you have your mare examined and ready to be bred before, not when, you take her to the stallion or have the semen shipped. Below we'll discuss in detail why it is a good idea to have your mare ready when the sheds open, and not just starting on her pre-breeding examinations. While it's best to start in November and December, it's still not too late to get the breeding season off to a good start.

One of the main goals of prepping mares for the breeding shed is to get them through transition and cycling regularly as early in the season as possible. What can we do to help mares progress from anestrus through transition and start cycling earlier? One of the easiest and cheapest broodmare preparations is to place them under lights. We can shorten anestrus and bring around transition and normal estrous cycles by bumping up their physiological 'clocks' a little. Exposing mares to 16 hours (continuous) of light per day during the winter makes for a "long day' and fools the brain into thinking it is springtime. Within 70 days half the mares will have ovulated, by 85 days 95% of mares will start cycling.

The easiest easy way to expose mares to this length of light is by bringing them inside at dusk, and leaving a light on until 1 1:00 p.m. These lights need to be bright enough so that you can easily read newsprint from any corner in the stall. Please be careful when you go crawling around that corner of your stall with a piece of newspaper flapping around-horses can get spooked by the most unlikely things. Note that if your mare can poke her head out a window or door into the darkness, her physiological clock will not be fooled.

It is particularly important to have "problem mares," or mares who have a difficult time settling, prepared at the beginning of breeding season. For our purposes, problem mares are mares who require three or more cycles to get in foal. If your mare is barren (bred several cycles in a season without pregnancy) she is also considered under the broad category of problem mares.

When breeding these mares it helps to have as many cycles as possible to work with in the season. If you can have your mare ready, cultured free of infection, and cycling normally by the time the sheds open you are ahead of the game.

If the stallion shed opens February 15 and closes June 30 we only have 137 days! When working with problem mares, we often lose three or more cycles in finding out she isn't pregnant, with exams, cultures, treatment and so on. Which is why time is of the essence. (While an inability to become pregnant could be a stallion fertility problem, we'll focus on the mare in this article.)

How can we prepare maiden and fertile mares? Many mares are fertile and their owners, if not breeding many years, wonder what all the commotion regarding lights, cycling and so on is about. Many mares go to the stallion for a few days and return pregnant! For those mares there is no reason, except peace of mind, to have them cycling early-unless you want an early foal.

Veterinarians and stallion managers tend to remember the problem mares and want to get as many mares pregnant, as soon as we can. This allows us to concentrate on fewer mares as the season closes, when those problem mares become a priority. If you know your mare is fertile, you may have only a little benefit from using lights. However, fertility will naturally decrease as your mare ages. Anytime fertility is decreased, prepping is a great idea to maximize your mare's chance at pregnancy.

When breeding maiden mares, remember that not all maidens have the same fertility. Generally, maidens under age six are more fertile than older maidens. If we rank mare fertility, the most fertile would be young maidens, followed by foaling mares without problems, followed by older maidens, and finally problem and barren mares. Once mares reach the age of l2, many will start to have lower fertility and by the time they are 16 most mares have declining fertility. Like problem mares, older maiden mares may need the benefit of early cycling.

The last type of mare we'll discuss are pregnant mares. How can we prepare these mares for the next breeding season? Once again, the answer is lights. If your mare is pregnant and due this winter you may want to put her under lights now. Doing so will increase the likelihood that she cycles after she foals. If she is due in late spring or summer, lights aren't usually necessary, as almost all mares will cycle after foaling in the long day season. If you do put her under lights now, her pregnancy won't be altered and it will help ensure that she will start estrous after foaling. One note of caution on these mares: if you used the lights before foaling, do not stop using the lights after delivery, turning the lights off may inadvertently encourage the mare to go into anestrus.

As simple as the use of lighting sounds, a few mares will not respond as expected. One reason is due to the way we house horses. If your mare is in a barn where lights are not turned off until late in the evening, she may not cycle when you expect. Many performance and show mares are kept under such circumstances throughout the year, and getting them to cycle regularly can be frustrating. The problem arises because mares exposed to long day light all year will often continue to cycle seasonally through anestrus and estrous. Unfortunately, the cycles of these mares can have no correlation to the calendar and they may decide to go into anestrus in February!

What sometimes works is to kick these mares out and allow them to experience darkness and cold weather (use a blanket if your mare isn't used to the cold). After exposure to as little as 30 days of short daylight and winter conditions, these mares often respond and start cycling when you put them on long daylight. If that doesn't work, your mare may be caught in a physiological loop which needs closer attention. If this is the case, it may be necessary to talk to your veterinarian (or feel free to call our office with questions) to help your mare start cycling.

Next month we'll discuss breeding methods, cultures, and additional options for increasing your chances of successfully breeding your mare.