Body composition and nutrient metabolism in juvenile athletic horses treated with exogenous equine somatotropin

Abstract

Twenty-nine long-yearling horses completed 128 days of conventional race training. Horses were assigned by age to treatment groups; fifteen were injected daily with exogenous equine somatotropin, while fourteen received saline injections. They were fed a balanced diet that met or exceeded NRC (1989) nutrient requirements for long-yearlings in training. Treatment with eST elevated plasma IGF-I concentrations by d 14 of treatment, which remained elevated until the end of the treatment period, then returned to pretreatment values in a 28-day post-eST injection period. Physical measurements indicated that eST treated horses outgrew the control group as the trial progressed. Increases in weight gain and lean muscle tissue in the eST treated group were likely due to anabolic effects of eST. The eST treated horses consumed more feed and energy per kg body weight than the control horses as the trial advanced. Normalized feed intake data revealed an increase in the eST treated horses above that of the control horses with a significant difference at d128 (p<.05). Blood urea nitrogen (BUN) was lowered by d 14 in the eST treated horses, but seemed to be affected by confinement during total collections of feces and urine. Urinary nitrogen was not different between treatments, but increased to d 32 then declined to d 128; the increase at d 32 corresponded with elevated BUN in both treatment groups. Although nitrogen retention did not differ between groups, the pattern of retention was very similar to the pattern of bone remodeling found in horses introduced to race training (Nielsen et al., 1995). Mineral balances and serum concentrations were not different between treatments. There was a decrease in urinary Ca (p<.05) in the eST treated horses between d 64 and d 96, when bone remodeling was apparently taking place. The resorption phase of bone remodeling was easily seen at d 64 in the Ca and Mg data. At d 64, there were increases in fecal and urinary mineral excretion, increases in serum mineral concentrations, and decreases in mineral absorption. Overall, nitrogen and mineral metabolism appeared to be affected more by exercise and confinement than by administration of eST.