Breeding a mare can represent a large economic and even emotional investment. Once the mare is pregnant our goal is to provide the best care for her so that eleven months later, she can deliver a healthy foal.
A pregnant mare is considered normal when there is no medical reason to think that she is prone to complications during pregnancy or delivery. If a mare is reasonably young, that is less than 20 year of age, and healthy, or there is no history of problems in previous gestations, this is a pretty good assumption. Therefore, with few exceptions, a pregnant mare should be managed and treated as any other horse in the farm:
A pregnant mare SHOULD NOT:
It is important that the mare is in good body condition prior to breeding. Body condition condition is usually scored in a scale from one (most thin) to nine (most overconditioned), and should also be monitored throughout pregnancy. Ideally, mares should go into a breeding program with a body condition score of five. A rule of thumb is that the ribs cannot be seen while the mare is breathing but they can be felt easily when the hand is run smoothly over the rib cage, which means that there is no excessive fat covering the spaces between the ribs. The general body conformation should also be taken into account when evaluating body condition.
Good quality roughage (grass or legume hay) should be the main nutritional source for any horse. In general, horses consume around 1.5 to 2 percent of their body weight in feed or dry matter. Roughage that includes hay and pasture should comprise at least 1 to 1.5 percent of this intake. The remainder may be supplemented with grain depending on the body condition of the horse and metabolic demands.
As pregnancy advances the mare will have increased metabolic needs due to marked growth of the foal in the last three months of gestation. Additionally, the foal will occupy a great deal of space in the abdomen, making consumption of large quantities of forage needed to meet metabolic demands difficult. Therefore, mares in the last trimester of pregnancy may need as much as 1 to 1.5 percent of their feed requirement in the form of grain or concentrate.
Lactation also places a great deal of strain in some mares, requiring further increases in feed and concentrate during the first couple of months of lactation. This is an important consideration and you should plan ahead to avoid sudden feed increases once lactation starts.
Many mares are often expected to become pregnant again right after foaling, and conception may be difficult or impaired if the mare loses excessive body condition while nursing the newborn foal. In any case, remember that each horse is an individual and generalizations cannot be made.
For any horse, sudden changes in diet should be avoided. This may predispose to gastrointestinal upset and colic. Any changes or upgrades in diet to meet requirements for late pregnancy or lactation should be done gradually, over a period of two to four weeks.
A balanced intake of minerals is important, especially in pregnant mares. For example, the calcium and phosphorous ratio should be strictly maintained between 1.5 to 2:1. The ratio of these minerals is more important than the individual amount of each one of them. To ensure an appropriate Ca:P ratio, feed your mare a balanced diet of grass hay and grain. Commercial concentrates are usually balanced for these minerals, but consult your veterinarian or feed supplier if unsure, since items such as corn and rolled oats are poor sources of calcium.
A diet consisting of alfalfa hay alone is not recommended. Alfalfa hay not only offers an unbalanced Ca:P ratio, but it is also extremely rich and can predispose some horses to gas colic or the development of enteroliths or gut stones. A feed of grass hay with concentrate or of a grass/alfalfa hay mix plus concentrate, provides a much better balanced diet than when alfalfa hay is fed alone. In any case, recently published surveys have suggested that changes in the type of hay are highly correlated with the incidence of colic. Therefore, if you are going to switch from one hay type to another, do it gradually.
It is not uncommon to overfeed protein to pregnant mares. During pregnancy, protein demands are not excessively increased, and pregnant mares can do well with a concentrate containing 10 to 11 percent protein. During the last two months of pregnancy the mare can gradually begin a concentrate containing 12 to 13 percent protein in preparation for lactation.
Vitamin E and Selenium may be supplemented in the last trimester of pregnancy, but be aware that some grain formulations for pregnant/lactating mares already take this into account.
Water and a salt or mineral block should always be available. If your mare is not using the salt block, it is advisable to supplement the feed with loose salt, up to 100 grams (5 tablespoons) per day.
Mares in Fescue Pasture
Care should be taken when pregnant mares graze in fescue pasture. This type of pasture is often infected with a fungus or endophyte: Acremonium coenophialum. There is a symbiotic relationship between the fungus and the grass – the grass protects the fungus and vice versa. This fungus does not induce problems in non-pregnant grazing horses.
However, if consumed by mares in late pregnancy, endophyte-infected fescue will result in prolonged gestation, dysmature abnormal foals, placentitis, problems during delivery, retained fetal membranes (afterbirth) and agalactia (lack of milk production). Removing mares from the infected pasture at least one month before the due date can easily prevent this disastrous situation.
This type of fungus is not usually associated with other types of grass. If you are moving your pregnant mare to a different location, it is wise to inquire about the type of pasture available and take appropriate precautions.
Diagnosis of pregnancy is usually performed by ultrasound examination of the uterus at 12 to 14 days after ovulation. The particular situation and vet preferences dictate the timing for further examinations.
Most veterinarians recommend rechecking the pregnancy at 21 to 25, 30 and 60 days post-ovulation, since most early embryonic losses occur prior to 45 days in mares. Late pregnancy checks by palpation or ultrasound can be performed when the mare is vaccinated or in case of health problems or complications.
At each pregnancy check, your veterinarian may choose to perform a complete physical examination to ensure that your mare is in good health and body condition. The udder should be checked periodically for abnormalities or for timely development towards lactation. Mild increases in udder size may be noticed 4 to 6 weeks prior to the due date, mostly due to fluid accumulation or edema; however, glandular development and marked increases in udder size are not usually noted until 2 to 3 weeks prior to the foaling date. Premature udder development and/or lactation are usually a sign of impending abortion.
Regular vaccinations and deworming programs should be continued throughout pregnancy, with some exceptions, following the routine schedule for the particular farm. These include the following:
Boostered vaccinations. Some vaccinations should be boostered specifically in pregnant mares. These include: Rhinopneumonitis (EHV-1), which should be given at 5, 7 and 9 months of pregnancy to prevent abortion; equine viral arteritis, prior to breeding, in farms or for breeds where the disease is a problem, or when breeding to a stallion that carries the virus; tetanus, encephalomyelitis (Eastern/Western) and sometimes also influenza, which should be given around 30 days prior to foaling; and, in certain geographic areas of the US (i.e. Maryland), a booster for botulism should also be given at 60 and 30 days prior to the due date. Vaccines given 30 days prior to foaling enhance concentrations of antibodies against these diseases in the colostrum (first milk), promoting adequate protection of the foal after nursing.
In addition to the regular deworming schedule, pregnant mares are usually dewormed one month prior to the foaling date, and sometimes right after foaling. There is anecdotal evidence that deworming mares right after foaling can prevent foal heat diarrhea by killing certain parasites in milk called Strongyloides westerii, but this has not been scientifically proven.
In any case, you should always consult your veterinarian in regards to optimal vaccination and deworming treatments for your pregnant mare.
Loss of Pregnancy
Embryonic loss is loss of the embryo before 45 days of pregnancy. Early loss of the pregnancy usually goes undetected, unless the mare is checked regularly by ultrasound during the pregnancy. The cause of embryonic loss in mares is not always easy to ascertain but it is often associated with endometrial changes in older mares (periglandular fibrosis, cystic glandular distension) or endometritis in mares of all ages, or problems specific to older mares, such as uterine diverticula, or aged oocytes (such as it happens in women).
Fetal loss or abortion is defined as that pregnancy loss occurring after 45 days of pregnancy. It is usually undetected, however, until after 5 to 6 months of pregnancy, when the fetus has acquired considerable size. Common causes of abortion in mares include twinning, rhinopneumonitis and other viruses, like equine viral arteritis. Adequate management and vaccination for rhinopneumonitis may prevent abortion in your mare.
Twinning is undesirable in mares. If mares become pregnant with twins it usually results in late term abortion of both fetuses before they are mature and developed enough to survive due to lack of sufficient room and nutrition for both foals in the uterus. Twins in horses are always the result of double ovulations, or fraternal twins. Therefore, it is important to record double ovulations if mares are being monitored by an internal exam during breeding management. Whenever double ovulations are suspected or recorded, a thorough pregnancy exam should be performed at 12 and 14 days after ovulation.
Early detection of twins allows elimination of one of the embryos very easily, by pinching it through the rectal and uterine walls during ultrasound exam. If twins are detected later in pregnancy then the only alternative may be to induce abortion, or wait/hope for spontaneous reduction/death of one of the embryos and survival of the other one.
Several illnesses can affect the mare as a result of pregnancy. These include:
Most pregnancies are problem-free, and the incidence of complications decreases dramatically in mares that receive appropriate preventative care. It is essential that you inform your veterinarian of your intention to breed a mare so he or she can design a preventative health program and schedule you for regular examinations. A good working relationship with a veterinarian experienced in reproduction can make the difference between a healthy and a high-risk pregnancy, and assure you that you've done everything possible to usher on the birth of a healthy happy foal, the ultimate goal.