Feeding to Prevent Developmental Orthopedic Disease

Developmental orthopedic disease can rob a young horse of its future as a sound athlete. Each year the consequences of diseases such as osteochondritis dissecans and physitis render hundreds, maybe thousands, of young horses structurally inadequate for the work they were bred to do. Over the last several years, researchers have found that appropriate nutrition of the gestating mare as well as her young offspring lays the foundation for proper bone development, thus sidestepping developmental problems and possible unsoundness.

Broodmares

The nutritional requirements of a broodmare can be divided into three stages based on stage of production. The first stage begins at conception and lasts through approximately the first seven months of gestation. Barren mares and pregnant mares without sucklings by their sides fit into this category. The second stage encompasses the last trimester of pregnancy, which is from around seven months of pregnancy through foaling. The final stage is lactation, a period that lasts four to six months after foaling. The most common mistakes made in feeding broodmares are overfeeding during early pregnancy and underfeeding during lactation.

Early pregnancy. Proper feeding during pregnancy requires an understanding of how the fetus develops during gestation. Contrary to popular belief, the fetus does not grow at a constant rate throughout the entire 11 months of pregnancy. The fetus is small during the first five months of pregnancy. Even at seven months of pregnancy, the fetus is only about 20% of its weight at birth. Because this equals less than 2% of the mare’s weight, the fetus’s nutrient requirements are minuscule compared with the mare’s own maintenance requirements. Therefore, the mare can be fed essentially the same as if she were not pregnant. Mare owners often increase feed intake after the mare is pronounced in foal, reasoning that she is now “eating for two.” Increased feeding is unnecessary and may lead to obesity and foaling difficulties, especially if the mare has access to high quality pasture during early pregnancy.

Late pregnancy. The fetus begins to develop rapidly after seven months of pregnancy, and its nutrient requirements become significantly greater than the mare’s maintenance requirements; therefore, adjustments should be made to the mare’s diet. Digestible energy requirements only increase about 15% over early pregnancy. Protein and mineral requirements increase to a greater extent. This is because the fetal tissue being synthesized during this time is quite high in protein, calcium, and phosphorus.

Trace mineral supplementation is also critical during this period because the fetus stores iron, zinc, copper, and manganese in its liver for use after it is born. The fetus has developed this nutritional strategy of storing trace minerals during pregnancy because mare’s milk is quite low in these elements.

New Zealand researchers studied the effect of copper supplementation on the incidence of developmental orthopedic disease in Thoroughbred foals. Pregnant Thoroughbred mares were divided into either copper-supplemented or control groups. Live foals born to each group of mares were divided into copper-supplemented or control groups. Copper supplementation of mares was associated with a significant reduction in the occurrence of physitis (inflammation of the bone growth plates) of foals at 150 days of age. Foals from mares that received no supplementation had significantly more incidence of physitis than foals out of supplemented mares. A significantly lower incidence of articular cartilage lesions occurred in foals from supplemented mares.

However, copper supplementation of the foals had no significant effect on the severity of developmental orthopedic disease. Mares in late pregnancy are often overfed energy in an attempt to supply adequate protein and minerals to the developing foal. If the pregnant mare becomes fat during late gestation, she should be switched to a feed that is more concentrated in protein and minerals so that less can be fed per day. This will restrict her energy intake while ensuring that she receives adequate quantities of other key nutrients.

Lactation. A mare’s nutrient requirements increase significantly after foaling. During the first three months of lactation, mares produce milk at a rate equal to about 3% of their body weight per day. This milk is rich in energy, protein, calcium, phosphorus, and vitamins. Therefore, the mare should be fed enough grain to meet her greatly increased nutrient requirements. Mares in early lactation usually require from 10 to 14 pounds (4.5 to 6.5 kg) of grain per day depending on the type and quality of forage they are consuming. This grain mix should be fortified with additional nutrients to meet the lactating mare’s needs. Trace mineral fortification is not extremely important for lactating mares because milk contains low levels of these nutrients and research has shown that adding more to the lactating mare’s diet does not increase the trace mineral content of the milk. Calcium and phosphorus are the minerals that should be of primary concern during lactation.

Grain intake should be increased gradually during the last few weeks of pregnancy so that the mare is consuming nearly the amount that she will require for milk production at the time she foals. A rapid increase in grain should be avoided at foaling because this may lead to colic or founder. Milk production begins to decline after about three months of lactation, and grain intake can be reduced to keep the mare in a desirable body condition.

Sucklings

If the broodmare has been fed properly during late pregnancy, it is unnecessary to supplement the suckling with minerals until it reaches 90 days of age. At this time, moderate amounts of a well fortified foal feed can be introduced and gradually increased until the suckling is consuming around one pound (0.5 kg) per month of age. It is critical that the suckling be accustomed to eating grain before it is weaned. If it is not, there is a very good chance that there will be a dramatic decrease in growth rate at weaning. This is sometimes referred to as a “growth slump” or “weaning slump.” When the weanling finally starts eating grain, a compensatory growth spurt will occur that may result in developmental orthopedic disease.

Weanlings

The most critical stage of growth for preventing developmental orthopedic disease is from weaning to 12 months of age, when the skeleton is most vulnerable to disease and nutrient intake and balance is most important. Weanlings should be grown at a moderate rate with adequate mineral supplementation. In temperate regions, the contribution of pasture is often underestimated, leading to excessive growth and developmental orthopedic disease.

Yearlings

Once a horse reaches 12 months of age, it is much less likely than a younger horse to be affected by developmental disease. Many of the lesions that become clinically relevant after this age are typically formed at a younger age. Still, proper nutrient balance remains important for the yearling. It is best to delay the increased energy intakes that are required for show conditioning or sales prepping as long as possible because the skeleton is less vulnerable to developmental orthopedic disease as the yearling ages. Normally, increasing energy intake 90 days before a sale is enough time to add the extra body condition that is often expected in a sales yearling.

Bone-related problems of the knee, namely physitis, are often a major concern with yearlings that have been pushed for growth. To reduce the incidence of physitis in these horses, the level of trace mineral supplementation should remain high and a significant portion of the energy normally supplied from grain should be replaced with fat and fermentable fiber. Feed mixes for these horses can contain as much as 10% fat. Sources of fermentable fiber include beet pulp and soy hulls.

Proper nutrition of the mare throughout pregnancy and lactation and of the young horse during its first two years of life will help prevent developmental orthopedic problems.