Care of the Normal Pregnant Mare
Dr. Sylvia Bedford-Guaus
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.