Uterine involution, day and variance of first postpartum ovulation in mares treated with progesterone and estradiol-17beta for 1 or 2 days postpartum.

The study explores the effects of progesterone and estradiol-17beta hormones on ovulation in postpartum mares. While the hormones did not significantly alter the timing of first ovulation or pregnancy rates when compared to a control group, the variance in ovulation days was significantly lower in hormone-treated mares.

Hormone Administration and Experimental Design

• The study examined 50 postparturient mares, which were divided between two trials based on their hormone treatment. In each trial, the mares were given either progesterone and estradiol-17beta or a saline solution as a control.
• In Trial 1, seven mares were administered 150 mg of progesterone and 10 mg of estradiol-17beta, while 13 mares were given saline solution. This occurred once within the first 24 hours after giving birth.
• In Trial 2, the same regimen was followed but with double the progesterone and estradiol-17beta dosage, and 13 mares received the hormones while 17 were given the control solution.
• The mares were examined daily to determine ovulation day. On day 15 postpartum, ultrasonographic measurements of the uterine body and horn diameters were taken.

Impact on Uterine Diameters, Pregnancy Rates and Ovulation

• There was no significant difference in mean uterine diameters between treatment and control groups in either trial, indicating that the hormone treatment did not affect uterine involution.
• Pregnancy rates in mares bred on the first postpartum estrus did not significantly differ between treatment groups (16/18, 89%) and controls (22/30, 81%), suggesting that receiving hormone treatment postpartum neither increased nor decreased the likelihood of conception.
• There also was no significant difference in the mean day to first postpartum ovulation between treated and control groups in the trials, indicating that progesterone and estradiol-17beta administration did not accelerate or delay initial postpartum ovulation.

Variance in Ovulation

• Despite the lack of significant differences in mean ovulation day or pregnancy rates, the study found a lower variance in ovulation days in hormonally treated mares.
• The hormone treatment seemed to create a more predictable ovulation pattern, with mares less likely to ovulate prior to Day 10 postpartum compared to control mares.
• This outcome suggests that while the hormones may not affect when ovulation begins, they may provide a more standardized ovulation window, a finding potentially useful for breeding management.