Effect of food restriction and pharmacological repartitioning of energy intake on reproductive activity in the mare

In a variety of species, the lack of sufficient metabolizable energy causes a suppression of the reproductive axis such as decreased secretion of luteinizing hormone, suppression of estrous cycles, delayed puberty, long breeding intervals, and decreased fertility. In the horse, inadequate dietary intake is associated with the suppression of estrous cycles in mares undergoing intensive training regimens while the presence of large stores of body fat is associated with the expression of estrous cycles during the nonbreeding season. Recently, an association between high levels of body fat and serum leptin concentrations and the expression of estrous cycles during the nonbreeding season was found in mature mares. This association indicated that the presence of large reserves of metabolic fuel in body fat stores may override the inhibitory or stimulatory effects of photoperiod in the mare.; In this dissertation, four experiments were performed to examine the relationship between nutrition and the activity of the reproductive axis in the mare. In the first two experiments, the hypothesis that short periods of food deprivation would cause decreases in serum gonadotropins, leptin, and metabolites was examined in mature and young mares, respectively. Despite decreases in serum leptin and increases in free fatty acids, no suppression of the gonadotropins or glucose concentrations was found. Because glucose concentrations remained within physiological ranges, sufficient energy may have been present to permit the continued functioning of the reproductive axis. The third and fourth experiments examined the effects of pharmacological repartitioning of energetic intake on serum leptin, body fat percentages, body weight and the expression of seasonal anestrus in mares. The administration of the repartitioning agent clenbuterol to mares was associated with decreased percentages of body fat and serum leptin concentrations, but a significant increase in the proportion of mares exhibiting seasonal anestrus was not found in either experiment. This lack of a significant increase may be due to the fact that energy balance was not different between groups. Recent evidence from other species indicates that the availability and oxidation of metabolic fuels may be the primary signal that alters the functioning of the reproductive axis.