Reproductive efficiency and endocrine status of postpartum mares at different levels of body condition

Abstract

To further study the endocrine environment of foaling mares at different levels of body condition, 12 mares were divided into two groups, one designed to foal at a condition score of 4.5 or less and the other to maintain their body condition of 6 or greater. Serum samples were collected twice weekly before foaling, and daily thereafter until 7 days following the second estrous cycle after parturition. In addition the mares were weighed, condition scored, and body fat content measured weekly. Serum levels of luteinizing hormone (LH) were measured using two assay systems, radioimmunoassay (RIA) and rat interstitial cell testosterone (RICT) assay. One of the consequences of low body condition at foaling of the thin group of mares was longer gestation length than the control mares. Also, the pattern of LH from cycle one to cycle two was different between the thin mare group and the control mare group. The total amount of LH released (release curve area) and LH peak height in cycle two was greater than that found in cycle one for the thin mares. Also, the thin mares had irregular initiation of estrus following parturition. In contrast, for the control mares, both total LH and peak height during cycle one and cycle two were not different. Also, these mares exhibited a regular and predictable initiation of estrous following parturition. A change in the biological to immunological activity (B:I) ratio of LH potency during the estrous LH surge was not found during this series of experiments. There was no difference in total LH released during the estrous surge nor during diestrus as measured by either RIA or RICT. Although there was an equal amount of LH measured by either RIA or RICT after foaling, the B:I ratio of LH before foaling was significantly lowered. In conclusion, this study demonstrates some of the effects of low body condition and body fat on the reproductive cyclicity of mares, as reflected by LH release. The LH patterns of the thin mare group are similar to those observed in the transitional mare, whereas the LH pattern of the control mares is similar to what is observed in the middle of the breeding season. Irrespective of a mare's body condition at foaling, the B:I ratio of LH was depressed before parturition, immediately rising to unity following parturition.