Why we feed what we feed –

Feeding the Rapidly Growing Foal

Sarah L. Ralston, VMD, Ph.D., dACVN, Department of Animal Science, School of Environmental and Biological Sciences, Rutgers University

Fact Sheet #895 – Reviewed 2004

Large foals that are growing rapidly are often considered to be at increased risk of developmental orthopedic disease (DOD). A multifactorial problem, DOD includes problems such as osteochondrosis dessicans (defective bone and cartilage at the joint surface), epiphysitis (enlarged, painful growth plates), flexure and angular limb deformities, and perhaps wobblers syndrome. Genetics, nutrition and exercise all play a role in the incidence of DOD in horses.

Hereditary predisposition to at least OCD is well documented in Standardbred and Swedish warmblood horses, with the incidence as high as 45% in some bloodlines. However, the genetic defect that causes the growth associated problems in the horses has not been identified. Breeds selected for rapid growth are at increased risk of developing problems, but it is not growth rate alone that causes the problem. It is not always the most rapidly growing foal that develops DOD, but often the one with the most erratic growth rate.

Trauma due to excessive concussion, due either to severe obesity or forced exercise, may increase the incidence of DOD. Other reports, however, revealed that restriction of exercise adversely impacts bone growth and development in young horses. Turning the foals out in as large an area (either pasture or paddock) as possible for as long as possible is highly recommended. Ideally they should get 24-hour turnout. However, strenuous forced exercise, especially lunging in circles, should be avoided. Foals should not be allowed to become obese.

Mineral imbalances have been well documented to cause DOD. Deficiencies of calcium, phosphorus and/or copper all result in defective bone maturation. Zinc toxicity and perhaps deficiency also have resulted in lesions, though the effects of simple zinc deficiency are not well documented. The optimal intakes of copper and zinc for young horses have not been well defined. Current recommendations for mineral content of rations for foals less than 1 year of age are given in Table 1.

Excessive protein (greater than 16%) was incriminated as a cause of DOD in the 1970s but subsequent studies have not revealed a direct relationship between high protein rations and DOD. Weanlings fed rations deficient in protein (less than 12%) had reduced growth rates and poor bone mineralization compared to weanlings fed rations which were higher in protein. Restricting protein in a rapidly growing foal’s ration will not result in improved bone growth and may actually be detrimental to the animal.

Rations providing over 100% of the National Research Council’s recommended amounts of energy for rapid growth in foals may cause an increased incidence of DOD, especially if the ration contains more than 50% sweet feed (grain mix plus molasses) or other high sugar concentrate by weight. High carbohydrate rations such as sweet feeds may contribute to the appearance of DOD, possibly related to the high blood glucose and insulin and low blood pH they cause for up to 4 hours after feeding. Pelleted and extruded feeds tend to have lower molasses contents and higher fiber concentrations have a lesser effect on glucose and insulin. Pelleting appears to affect the availability of carbohydrates and pelleted concentrates may cause lower glucose and insulin changes than textured feeds with the same basic formulation (Ralston, 1992). There may be a correlation between OCD and glucose intolerance (abnormally high blood glucose and insulin after a meal of sweet feed) in foals that are genetically predisposed to the problem.

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