Relationship between body condition and levels of serum luteinizing hormone in postpartum mares

Abstract

To study the endocrine environment of foaling mares at different levels of body condition, 12 mares were divided equally into two groups, one designed to foal at a condition score of 4.5 or less (thin) and the other to maintain a condition score of 6 or greater (control). Serum samples were collected twice weekly before foaling, and daily thereafter until 7 d following the second estrous cycle after parturition. Serum levels of luteinizing hormone (LH) were measured by Radioimmunoassay. In addition, mares were weighed, body condition was scored and body fat content was measured weekly. One consequence of low body condition at foaling in thin mares was a longer mean gestation length than in control mares. Also, the pattern of LH observed during the first and second estrous cycles after foaling was different for thin vs control mares. Serum LH concentrations for thin mares were greater during their second estrous cycle than during their first estrous cycle. Also, in thin mares, the interval from parturition to ovulation, as indicated by the day of maximum concentration of LH during estrus, was random. Control mares had equivalent concentrations of serum LH in both cycles, and they had predictable intervals from parturition to ovulation. LH patterns in thin mares were similar to those reported in mares during seasonal transition, whereas LH patterns of control mares were similar to those reported in mares during the middle of the breeding season.