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Preparing Your Mare for Breeding

Preparing Your Mare for Breeding, Nebraska Extension Acreage Insights for March 1, 2018, http://communityenvironment.unl/mare-breeding

In the world of equine reproduction, there are two groups of mares to prepare to for breeding: open/barren mares and pregnant/lactating mares. Because gestation lasts about 11 and a half months, mares are generally bred for the next year while nursing the current year’s foal. Management of these two groups of mares has both some similarities and differences.

Nutrition for Open Mares
Let’s first address the nutritional needs and body condition of each group of mares when entering the breeding season. If your mare is not pregnant or is “open”, her nutritional needs are similar to a horse at maintenance or light work, depending on her activity level. A mare at this stage consumes from 1 1/2  to 1 percent of her body weight in feed. Her diet can consist of 75 to 100 percent forage (pasture or hay) and 0 to 25 percent grain. All horses need to eat a minimum of 1 percent of their body weight in forage which is generally pasture or hay.

For example, a 1,000 pound mare would consume 10 to 15 pounds of hay/pasture and 0 to four pounds of grain per day. These rations will be adequate for open mares until the last three months of gestation, when nutritional requirements increase significantly.

Preparing Your Mare for Breeding, Nebraska Extension Acreage Insights for March 1, 2018, http://communityenvironment.unl/mare-breedingA mare produces as much as 3% of her body weight a day in milk - for a 1,000 lb. mare that comes to around 30 pounds of milk daily.

Nutrition for Pregnant Mares
In contrast, if your mare is pregnant, she will most certainly be in peak lactation by the time you breed her. A mare in early lactation has the highest energy requirement of all horses. She will consume between 2 and 2.5 percent or even up to 3 percent of her body weight in feed per day. These mares should be supplemented with grain, because hay/pasture alone may not meet all their energy needs.

A mare produces as much at 3 percent of her body weight a day in milk—for the 1,000 mare that comes to around 30 pounds of milk daily! Early lactating mares consume 60 to 40 percent of their diet in forage (pasture/hay) and 40 to 60 percent of their diet in grain.

So a 1,000 pound mare would consume around 25 to 30 total pounds of feed a day of which 10 to 15 pounds may be grain and 10 to 15 pounds may be hay. These values will naturally vary somewhat depending on the nutrient composition of both grain and hay being fed.

For more information of feeding broodmares check out: Basics of Feeding Horses: What to Feed and Why, G1875.

Preparing Your Mare for Breeding, Nebraska Extension Acreage Insights for March 1, 2018, http://communityenvironment.unl/mare-breedingIt is very difficult to put weight on lactating mares that foal at a low body condition score. 

Body Weight and Conception Rates
Research has shown that body condition has an effect on conception rates. Body condition scores, which basically estimate the amount of fat on a horse’s body, range from 1 (emaciated) to 9 (obese). It has been well documented that mares with a condition score of 4.5 or below take longer to cycle, take more cycles to “settle” (conceive) and have greater chance for early embryonic loss.

Mares at a condition score of 5 or above, cycle earlier in the year, take fewer cycles to be bred, and have improved pregnancy maintenance than the thin mares. Additionally, pregnant mares foaling at a condition score of 4.5 or lower have a slightly longer gestation length. Furthermore, it is very difficult to put weight on lactating mares that foal at a lower condition score.

It is recommended that all mares entering the breeding season have a condition score of at least 5 to 6 and lactating mares may even need to be at a condition score of 7 at foaling. Often times, very heavy milking mares will lose weight in early lactation. Thus foaling at a condition score of 7 provides a bit of a “buffer” for them to lose some weight yet not affect their re-breeding potential.

Body condition scores at the higher end of the scale, 8 or 9, have no reported benefits for breeding. Obese mares did not have any improved reproductive efficiency, no difference in foal growth, and no difference in milk production. However, mares at these high condition scores were more susceptible to laminitis, and other metabolic problems such as insulin resistance. Therefore it is not recommended mares be at condition scores above an 8.

For more information on condition scoring horses see: Basics of Feeding Horses: Feeding Management, G1781.

Health Care
Minimizing stress for broodmares, particularly prior to breeding, should be a primary concern. Most veterinarians recommend that vaccinations, deworming, dental procedures and other elective care be done at least three to four weeks before breeding or after the mare is “safe in foal,” which means past 40 days of gestation.

Consult your veterinarian for recommended vaccinations for your area and the suggested parasite control program. Once a mare is pregnant, it is recommended that she be vaccinated against the abortive form of rhinopheumonitis (EHV-1) at 5, 7 and 9 months of gestation.

Make Sure She Is Cycling
Mares are considered a “seasonal polyestrous breeder” meaning they have several cycles during their breeding season that are dictated by the length of day, or hours of sunlight. Left to nature, mares cycle when the day length is long, which would have them foal and rebreed in the spring of the year. Therefore, as day length increases, mares begin to cycle, and when the days become shorter, they cycle less or become “anestrous”. In addition, mares go through “transitional” phases in which they slowly convert from not cycling (winter anestrous) to cycling (estrous phase). The transitional phase can be frustrating because it is typically characterized by long, erratic heat periods in which the mare may or may not truly ovulate. These heat periods can last as long as 7 to 10 days in some mares.

Preparing Your Mare for Breeding, Nebraska Extension Acreage Insights for March 1, 2018, http://communityenvironment.unl/mare-breeding

Most horses in the Northern Hemisphere go through the transitional phase around late February, March or early April and are ready to be bred in April or May. Once a mare moves into her estrous season, or true, fertile breeding season, she typically cycles about every 21 days. During this period, she will normally have three- to five-day heat periods and will ovulate near the end of her heat cycle.

Mares are different from other species in that they ovulate while still showing heat or estrus. Typically mares ovulate 24 to 48 hours before they stop showing the behavioral signs of estrus. Therefore, many farms breed mares every other day until they go out of heat. However, due to the increased use of transported, processed semen some breeding programs use palpation and/or ultrasound examination to determine the most optimum time to inseminate.

Some mare owners have a desire to have foals earlier in the year and thus place the mares under artificial lights, which can be used to mimic spring day length earlier in the year. However, it takes nearly three months of stimulation by the artificial lights before the mare will have fertile estrous cycles. Therefore, most lighting programs are initiated in mid-November and place mares under 16 hours of light daily. Mares generally go through the transitional phase within 60 days and begin cycling and could be bred 30 days later, or a total of 90 days after the start of the lighting program.

Mares can be housed in any type of facility that allows them to be “under” lights. These include stalls, indoor arenas, and outdoor pens with over head lights. The general guideline is if a person stands in the darkest corner and can easily read a newspaper, there is sufficient light. One other factor to consider is that most mares will shed their winter hair coat as they begin to cycle. Therefore, mares under lights during the winter months in some climates need protection from the cold.

Manipulating the Cycle
Various compounds can be used to manipulate a mare’s estrous cycle. To ensure safe and effective administration of these products, consulting with either a veterinarian experienced in equine reproduction or a well-trained breeding manager is recommended.

Most injections/medications must be given at specific stages of the cycle for optimum results. One of the most common compounds used is prostaglandin F2 alpha. This compound can be given to shorten a mare’s cycle and bring her back in to heat sooner than normal. However, prostaglandins must be given when the mare is at the correct stage of her estrous cycle. It should be given between the 4th or 5th day after she has gone out of heat and through 10 or 11 days after she has gone out of heat, then the mare should return to estrus within approximately 3 to 4 days and have her typical cycle. If given at the wrong time, she will not respond as expected.

Another frequently used compound, Human Chorionic Gonadotrophin (HcG), induces ovulation in mares. Again, it must be given at the correct stage of the cycle in order to stimulate the mare to ovulate. Mares should have follicles of 30 to 35 mm in size when the injection is administered and ovulation will typically occur within 48 hours. Other hormone therapies are also available which aid in inducing ovulation and manipulating the mares’ estrous cycle. However these hormonal therapies should be used with consultation with a veterinarian which specializes in equine reproduction.

Various tests and examinations can be done by a veterinarian to evaluate the reproductive soundness of a mare or to determine why she may not be getting in foal. Rectal palpations can help to determine the overall health of the reproductive tract and detect abnormalities in the ovaries, uterus and cervix. Ultrasounds of the mare’s reproductive tract, including the uterus, ovaries, and cervix are often done to detect any abnormalities.

Further diagnostics can be performed through a uterine culture in which a long pipette is used to swab the uterine lining. An endometrial biopsy, which involves the removal and analysis of small sections of uterine lining, may also be done.

Breeding mares and raising foals can be a very rewarding experience, but it’s wise to educate yourself about the process beforehand. The more background knowledge one has, the more educated decisions can be made. Remember to begin early in the year, if considering breeding a mare for the first time. If the foal is desired to be born early in the year, the lighting program must be started in the late fall. However, remember, as these mares begin to cycle, they will have “summer type” hair coats during the cold winter months. Also foals will be born early in the year and need to be properly managed, if located in cold climates. Many times letting mares cycle naturally and having foals in the spring is much simpler, requires less management and may even result in healthier mares and foals.

Kathleen Anderson
Kathleen Anderson
Nebraska Extension Horse Specialist
Contact Kathy at:
C203 Animal Sciences
Lincoln, NE