Steroid hormone profiles in periparturient mares at different levels of body condition

Abstract

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 [less than or equal to] 4.5; scale 1-9) and six mares (Control) were managed such that they foaled in good condition ([greater than or equal to] 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-17β (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 [plus or minus] 1.6 d and 343 [plus or minus] 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 (31.8 [plus or minus] 1.8 d) compared to Control mares (27.1 [less than or equal to] 1.0 d). Serum P4 concentrations were lower in Thin than in Control mares (P < .01) during the first (5.30 [plus or minus] .18 and 6.10 [plus or minus] .19 ng/ml, respectively) and second luteal phase (3.62 [plus or minus] .34 and 5.10 [plus or minus] .34 ng/ml, respectively)...