Fixed-time insemination with frozen semen in mares: is it suitable for poorly fertile stallions?

The purpose of the present study was to compare two protocols for equine frozen semen programs using either postovulation insemination or fixed-time insemination (FT), evaluating both pregnancy rates and intrauterine fluid (IUF) accumulation after artificial insemination with semen obtained from either highly or poorly fertile stallions. Six ejaculates from two stallions (n = 12) were processed. After thawing, semen samples were evaluated by computerized semen analysis. Fifteen mares (30 cycles) were inseminated with frozen semen from highly fertile stallion A, and 14 mares (28 cycles) were inseminated with frozen semen from poorly fertile stallion B. Ovulations were induced with 1 mg (intramuscular) of deslorelin acetate after the observation of a greater than 35 mm follicle and uterine edema. In postovulation insemination group, mares were inseminated once with 800 × 106 total sperm in a maximum 6-hour interval after ovulation. In FT group, mares were inseminated twice with 400 × 106 total sperm, 24 and 40 hours after induction. Mares were ultrasonographically examined for IUF accumulation 24 hours and for pregnancy diagnosis 14 days after the last insemination. Although IUF accumulation was more evident in mares inseminated once postovulation, pregnancy rates were similar for both protocols, regardless of the stallion, although a significant effect of the stallion was observed. These results indicated that FTs may be used for both highly and poorly fertile stallions as a practical tool to help spreading the use of frozen semen in equine reproduction programs.