Comparison of intramuscular vehicles, with or without progesterone, on serum progesterone concentrations in noncycling mares

Abstract

Progesterone (P4) supplementation, daily or weekly, is common in clinical broodmare practice. The objective was to evaluate serum concentrations of progesterone following a single intramuscular injection of 3 vehicles (BioRelease, BET labs, Lexington, KY; and cotton seed and sesame seed oils, Rood and Riddle Veterinary Pharmacy, Lexington, KY), with or without P4. Anestrus light horse mares (n = 48) were used; they had no luteal tissue, ovarian follicles d 20 mm and serum P4 concentrations < 0.5 ng/ml over a 2 week interval. Mares were randomly allocated into 6 treatment groups: 1500 mg P4 in 10 ml BioRelease (BR + P4; n = 12); 1500 mg P4 in 10 ml sesame seed oil (SSO + P4; n = 12); 1500 mg P4 in 15 ml cotton seed oil (CSO + P4; n = 11); 10 ml BioRelease without P4 (BR only; n = 5); 10 ml sesame seed oil without P4 (SSO only; n = 4); and 15 ml cotton seed oil without P4 (CSO only; n = 4). Day 0 was designated as the day on which mares were given P4. Mares were monitored by transrectal ultrasonography every 3 days and serum collected on days 1 - 14, 17, and 20. Serum P4 concentrations were determined using the clinical enzyme immunoassay analyzer, TOSOH A1A 900. One-way ANOVA was used to compare differences in mean P4 concentrations among treatment groups, with significance set at p < 0.05. Serum P4 concentrations were higher (p < 0.003) by Day 7 in the BR + P4 group compared to all other groups (Table). By Day 14 no difference (p = 0.127) in serum P4 was detected among treatment groups. Serum P4 concentrations were > 1.0 ng/ml in non-cycling mares for at least 7 days after a single intramuscular injection of 1500 mg P4 when delivered in BioRelease, sesame seed oil or cotton seed oil.