Growth and the Somatotropic Axis in Young Thoroughbreds

This group of experiments focused on relationships between diet, somatotropic axis, and growth. Growth hormone (GH) and insulin-like growth factor I (IGF-I) are factors in the somatotropic axis, and important to development of growth cartilage in the young animal. The entire study was divided into four main experiments. Characteristics of growth in 113 Thoroughbred foals born over a five year period were described with a series of empirical and physiological equations. Glycemic and insulinemic responses to different feed compositions were evaluated with glycemic response tests. The 24 hr pattern of plasma glucose, insulin, GH, and IGF-I was described in yearlings fed two meals a day. Finally, an association between ADG and IGF-I was described in Thoroughbreds from birth to 16 mo of age. Feeding diets to the foal that influence the somatotropic axis during growth may affect development of growth cartilage in unexpected or detrimental ways.

The pattern of weight in Thoroughbred foals from birth to 16 mo of age was most closely described by multiple regression with a combination of age, girth, body length, and physeal circumference (R2 = 0.99). Glycemic and insulinemic responses were significantly higher in yearlings fed a sugar and starch supplement when compared to those fed a fat and fiber supplement (P = 0.043 and 0.031; respectively). Glucose and insulin secretion was significantly affected by the feeding of two meals in a 24 hr period (P < 0.0001). Plasma IGF-I was positively correlated with ADG from birth to 16 mo of age in foals fed either a fat and fiber or sugar and starch supplement (r = 0.34, P < 0.0001).

The results from the series of experiments described here suggest a possible role of dietary mangement in reducing the risk of skeletal disorders that involve the influence of IGF-I on chondrocyte maturation.