Although much knowledge has been gained regarding the nutrient requirements of the young growing horse, there are still many unanswered questions concerning optimum nutrition for growth and the avoidance of developmental orthopedic disease (DOD). What degree, if any, of DOD can be considered normal? If any DOD is abnormal, then what degree can be considered acceptable or recoverable, such that future athletic performance will not be compromised? Is there a perceived increase in the incidence due to a greater ability to detect DOD or due to changes in management practices that predispose horses to the disease, or both? Our ability to recognize and detect DOD has most likely increased, which leads back to the question of how much of what we detect is normal or inconsequential regarding the development and maturity of the tissue. Whether changes in management practices have affected the incidence of DOD is difficult to ascertain. Confounding factors of genetics, environment, non-nutritional management, and their interactions with nutrition make the establishment of accurate and precise optimal nutrient requirements for growth complex and difficult. The end goal is to grow individuals that will be more durable during their athletic career. So we will continue to work toward nutritional, management and medical practices that will help to develop earlier maturing horses and horses with improved skeletal strength and integrity.
We must define to what degree our nutritional management affects the horse during different stages of its growth. It would seem logical that nutrition would have the most profound effects during the most rapid periods of growth. Therefore, concerns have been directed toward the last third of gestation and the possible indirect effect broodmare nutrition has on fetal development. The early postnatal period is another period of rapid growth; thus, the effect of broodmare nutrition on lactation may affect the growth of the foal. Nutrition also will have a direct effect on growth and development from the initiation of creep feeding programs through maturity. Thus, the nutritional and management concerns regarding growth should start at conception and continue through maturity.
Genetics is the primary determinant in the expression of DOD, and there is some degree of heritability in the horse. Heritability estimates could vary with specific joints and breeds since most of the studies only have addressed the hock in Standardbreds. Selection against DOD could be feasible; however, it would have to be based on progeny testing since stallions free of DOD have produced foals with a significant incidence of DOD lesions. As a population, feral horses have significantly less radiographic incidence of DOD than the domestic population. This is probably primarily due to natural selection pressures, although nutrient intake differences and activity levels between feral and domestic horses also may play a role. Although genetics is the primary determinant, knowing the possible genetic predisposition of a particular horse may allow adjustment to nutrition and management programs in order to reduce the risk of DOD.
Concerns regarding the maternal effects on foal growth and development should include the prenatal period, particularly during the last two thirds of gestation. Some evidence suggest neuroendocrine control of growth hormone secretion in the horse may occur before parturition, which is earlier than in other species and may emphasize the importance of broodmare nutrition during gestation and lactation.
Increasing the amount of nutrients available to the broodmare during the last third of gestation and during lactation is well accepted. Such increases help insure proper fetal growth and development. In the mare, it provides for adequate milk production and maintains a desired broodmare body condition score of 5 to 7, which enhances rebreeding efficiencies. Currently we do not understand how nutritional supplementation in the mare may affect DOD in the foal. There is still much to learn regarding broodmare nutrition and possible fetal imprinting effects.
The effects of supplemental feeding programs for young, growing horses may be most profound during the transition period when the young horse begins receiving most of its nutrients from feed sources rather than mare’s milk. This transition period is usually associated with weaning (3 to 5 months of age), a period of rapid growth that may compound the effects of feeding programs on growth. Creep fed foals (at approximately 1.5% of body weight) generally show a greater growth response than non-creep fed foals. However, differences in weight and height between creep and non- creep fed horses that may be evident at the weaning to yearling stage are not maintained provided the non-creep fed horses receive adequate nutrition for growth until maturity. Growth in the foal has been expressed in a curvilinear fashion as it relates to age. However, the growth pattern may be more reflective of a stair step pattern with a negative rate of gain for 2-3 days post weaning. Where on the growth curve a horse should be at a given point in time is dependent on breed, the growth potential of the individual horse, and the desired growth rate. It is generally accepted that moderate or less rates of growth contribute less to the incidence of DOD than rapid rates of growth. Horses that may be predisposed to DOD (i.e., large framed individuals with an apparent potential for rapid growth) may benefit from diets of lower caloric density. This is sometimes confused with nutrient deprivation (starvation), which is counter-productive to reducing the potential for DOD. Protein, vitamins and minerals are needed to insure sound tissue development, but do not accelerate growth rate as an increase in calories. Managing the growth rate of predisposed horses is best achieved by reducing calorie intake while still providing adequate protein, vitamins and minerals.
The other aspect of the growing horse’s diet that is often neglected is the forage component, which can vary from 30% of the diet or less to 70% or more. Depending on the quality of the hay or pasture, this aspect of the diet will have a greater impact on the amount of calories consumed than the concentrate portion of the diet and thus should be considered in the overall calorie reduction and management of the growth rate of predisposed horses.
The possible effects of exercise / activity in young, growing horses and DOD are not often discussed. Although exercise contributes positively to bone density, exercise seems to have a dual role regarding DOD depending on the circumstance. One role is as a contributing factor to DOD. Researchers have suggested that non-clinical lesions present at an early age manifest into clinical signs with the increased trauma of exercise. More recent research suggests another role serves as a potential prophylactic to the development and severity of DOD.
In summary, genetics is the primary determinant of DOD in horses. Nutritional management may help diminish the severity and frequency of DOD, particularly in those identified as “high risk”. Such nutritional management should consist of a reduction in daily calorie intake, but not a reduction in the daily protein, vitamin and mineral intake. Increased exercise / activity levels in young, growing horses not expressing signs of DOD may be beneficial to long term joint health and well-being.