12 PROGESTERONE SUPPLEMENTATION DAY 3 TO 5 POST-OVULATION IN AI-BRED MARES

Abstract

The purpose of this study was to determine the effectiveness of increasing progesterone (P4) concentrations during early pregnancy, by supplementing a low dose of Regu-Mate® (Intervet, Millsboro, DE, USA) on Days 3, 4, and 5, post-ovulation to AI mares. Sixty-three AI-bred mares were randomly allocated to 1 of 2 treatment groups. All mares underwent ovulation induction using 1.5 mg mL–1 of deslorelin acetate (IM) following observation of ≥30-mm follicles and were inseminated with either fresh (n = 57) or cooled (n = 6) semen from fertile stallions approximately 24 h later. Ovulation was verified via ultrasonography in all mares. Mares allocated to the treatment group (n = 29) received altrenogest (Regu-Mate®) orally at a rate of 15 mg per day on Days 3, 4, and 5 following ovulation (d = 0), whereas control mares (n = 34) received no treatment. Blood samples were collected from all mares from Day 0 (ovulation) through Day 6 post-ovulation via jugular venipuncture. Pregnancies were determined via ultrasonography at 18 days post-ovulation. The pregnancy rate for untreated mares (18/34, 53%) was not different (P > 0.05) from altrenogest-treated mares (13/29, 45%). There was no difference (P > 0.50) in pregnancy rates when mares were AI bred 1 (19/39, 49%) or 2 (10/20, 50%) days from ovulation. The mean plasma P4 concentration on Day 5 was higher (P < 0.06) for pregnant control mares (4.75 ± 0.59 ng mL–1) compared with open control mares (3.50 ± 0.25 ng mL–1). There was a greater increase (P < 0.02) in plasma P4 concentrations (Day 5 – Day 1 concentration) in untreated pregnant mares (3.87 ± 0.56 ng mL–1) compared with untreated open mares (2.38 ± 0.22 ng mL–1). In conclusion, it was determined that altrenogest supplementation from Day 3 to 5 post-ovulation was ineffective in improving pregnancy rates. However, it is unknown if this was the result of the dose used or the length of supplementation. Additionally, the timing of altrenogest coincided with a significant increase in P4 in pregnant mares compared with open mares, indicating that the timing may be correct or there was action of an unknown embryonic luteotropin(s), which would describe the differences between P4 concentrations in pregnant versus open mares.