Follicular growth, ovulation, and pregnancy responses to PGF-indued luteolysis and spontaneous return to estrus in Standardbred mares with large diestrous follicles.

Follicular growth, ovulation, and pregnancy outcomes were compared between Standardbred mares with large diestrous follicles that were treated with prostaglandin F2α (PGF) to induce luteolysis and those that were left untreated to return to estrus spontaneously.

Study Design and Groups

• Population: 46 Standardbred mares with a corpus luteum (CL) of unknown age and a follicle ≥ 30 mm in diameter.
• Random assignment to two groups:
  • PGF group (n=23): received prostaglandin F2α treatment on Day 0.
  • SP group (n=23): untreated and monitored for spontaneous return to estrus over 7 days.

Outcomes in the PGF Group

• Most frequent response (74%) was estrus followed by ovulation of the dominant follicle within 7 days after PGF treatment.
• Mares in this group were bred quickly after treatment, with 83% bred within 7 days.

Outcomes in the SP (Spontaneous) Group

• Diverse outcomes observed 7 days after Day 0:
  • 30% exhibited estrus but follicle regression without ovulation.
  • 22% experienced diestrous ovulation.
  • 26% remained in diestrus with follicle regression.
• Only 9% of these mares were bred within 7 days.

Pregnancy Rates and Artificial Insemination (AI) Usage

• 14-day pregnancy rate was higher (though not statistically significant) in the SP group (65%) compared to the PGF group (39%).
• More AIs were needed per mare to achieve pregnancy in the PGF group (average 1.7 inseminations) than in the SP group (average 1.3 inseminations), a statistically significant difference.
• The time interval from Day 0 to confirmed 14-day pregnancy was not significantly different between groups (around 37-40 days).
• End-of-season pregnancy rates were similar between groups (SP: 87%, PGF: 96%).

Conclusions and Implications

• Administering PGF to induce luteolysis and prompt estrus and ovulation did not provide a clear advantage over waiting for spontaneous estrus in mares with large diestrous follicles.
• Although PGF treatment allowed earlier breeding, it resulted in lower early pregnancy rates and required more inseminations per pregnancy.
• Long-term reproductive success by the end of the breeding season was comparable regardless of management strategy.
• These findings suggest that while PGF treatment can hasten the breeding timeline, spontaneous return to estrus may be more efficient in achieving early pregnancy with fewer inseminations.