STEROID HORMONE PROFILES IN PERIPARTURIENT MARES AT DIFFERENT LEVELS OF BODY CONDITION (ESTROGENS, EQUINE, PROGESTERONE, FAT

Serum steroid hormone profiles were investigated in mares foaling at different levels of body condition. Six mares (Thin) were managed to lose weight during late gestation and foal in thin condition (condition score (LESSTHEQ) 4.5; scale 1-9) and six mares (Control) were managed such that they foaled in good condition ((GREATERTHEQ)6). Weight, body condition and rump fat thickness were measured weekly. Serum samples collected twice weekly prepartum and daily postpartum were analyzed for progesterone (P4), estrone (E1) and estradiol-17(beta) (E2) by RIA. Peak serum estrogen levels were correlated with peak serum bio (BLH) and immunoactive luteinizing hormone (ILH) levels in periestrous mares. Postpartum reproductive efficiency was determined by monitoring intervals to estrus and ovulation and by conception rates.;Gestation length was longer in Thin compared to Control mares (352 (+OR-) 1.6 d and 343 (+OR-) 2.3 d, respectively). Serum P4 was higher in Thin than in Control mares (P < .02) during the 4th and 3rd weeks before foaling. However, P4 in Thin mares declined during the last 2 weeks before foaling, then increased prior to foaling while serum P4 increased steadily in Control mares. Serum E1 and E2 decreased in both groups during the final 5 weeks of gestation except that E2 increased a few days before foaling in Control mares. Serum E1 during weeks 5, 3 and 2 prepartum, and serum E2 during weeks 5, 4, 3 and 2 prepartum were greater in Thin than in Control mares (P < .03).;Interval between foaling and the end of the first luteal phase was increased (P < .05) in Thin compared to Control mares. Serum P4 concentrations were lower in Thin than in Control mares (P < .01) during the first and second luteal phase. Serum E1 levels between parturition and the first luteal phase were higher (P < .001) in Thin than in Control mares as were serum E2 levels.;Based on these data, mares foaling in thin condition have longer gestation intervals, altered P4 profiles before foaling, elevated serum E1 and E2 concentrations prepartum and during the first postpartum estrous cycle and reduced circulating levels of P4 during the luteal phase. Significant reduction in stored body fat may alter steroid metabolism and/or alter the hypothalamic-pituitary-gonadal axis such that reproductive functions are impaired. (Abstract shortened with permission of author.).