10/04/2022
Nutrition of the Broodmare and Osteochondrosis
Brian S. Burks, DVM
Diplomate, ABVP
Board Certified Equine Specialist
Developmental orthopedic diseases are conditions that occur in young, growing horses. Included are osteochondrosis, osteochondritis dissecans (OCD), angular limb deformity, physitis, subchondral bone cysts, flexural deformities, and cuboidal bone malformations. Some cervical lesions are also included.
Most horses are diagnosed between birth and two years of age. Most purebred breeds can be affected.
The etiology of DODs is complex, including the following factors: Genetic predisposition, rapid growth, nutrition, biomechanics and conformation, trauma, and metabolic function. Likely, DODs are multifactorial and not a single cause.
The nutritional requirements of a broodmare can be divided into three stages based on phase of production:
STAGE I (Early Pregnancy): 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 also fit into this category.
STAGE II (Late Pregnancy): The second stage encompasses the last trimester of pregnancy, which is from around seven months of gestation through parturition (foaling).
STAGE III (Lactation): 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.
The fetus does not grow at the same rate throughout gestation. During the first five months, the fetus is small, although organogenesis is complete by 45 days. In fact, the fetus only weighs approximately 20% of its birthweight by seven months of gestation. This is less than 2% of the mare’s weight, making its nutritional requirements miniscule compared to that of the mare. The mare can be fed as if she were not in foal during this stage.
Often, owners increase the mare’s food ration when pronounced in foal, but this increase is unnecessary and may lead to obesity and dystocia.
After seven months of gestation, the fetus begins to grow rapidly, and the demand for nutrition exceeds the mare’s requirements. Adjustments to the mare’s diet only should be made in the last four months of pregnancy.
Digestible energy requirements only increase about 15%, but protein and mineral requirements increase dramatically during the last few months because the new fetal tissue is high in protein, calcium, and phosphorus. Trace minerals are critical during late gestation because iron, zinc, copper, and manganese are stored in the liver for use after foaling, as these are minimal in mare’s milk.
Copper supplementation of mares is associated with a significant reduction of DOD in foals. Foals born to mares that do not receive extra copper are more likely to suffer from orthopedic diseases. Copper supplementation of foals does not change the incidence or severity of OCD lesions.
Many mares are overfed energy in late term pregnancy to attempt to supply adequate protein and minerals to the fetus. This leads to excessive weight gain and potential problems for the mare, such as laminitis and dystocia. Feeding in late gestation should be concentrated in protein and minerals, and fewer carbohydrates, which will keep the mare at a healthier weight.
When energy, primarily carbohydrates, is fed in excess, endocrine aberrations occur. Thyroid levels will decrease. Thyroid hormone (T4) is necessary for the proper formation of cartilage of the joint surface. There can also be imbalances in vitamins and minerals, particularly copper and zinc, along with calcium and phosphorus. Excessive zinc levels can decrease copper absorption, which is necessary for several enzymes that cross link collagen and help to form cartilaginous matrix. Similarly, excessive phosphorus decreases calcium absorption, leading to poorly calcified bone which is easily broken. This is secondary nutritional hyperparathyroidism that caused, in the early to mid-1900s, “Big head or bran disease”. Many horses were on too much concentrate and bran, both of which are very high in phosphorus.
Calcium and phosphorus are not stored in high quantities in the foal's liver, instead being obtained from ingesting mare's milk. It is important to provide adequate supplementation of these minerals to the mare during lactation. Mare’s milk is rich in energy, protein, calcium, phosphorus, and vitamins, but contains low levels of other trace minerals (such as iron, zinc, copper, and manganese). Improper micronutrient levels may affect the balance of antioxidants and free radical formation; therefore, copper, zinc, iron, selenium, vitamin E and beta-carotene may have significant roles to play in fetal growth.
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