Follicular growth, ovulation, and pregnancy responses to PGF-indued luteolysis and spontaneous return to estrus in Standardbred mares with large diestrous follicles.
Authors: Cuervo-Arango J
Journal: Journal of equine veterinary science
Summary
When large follicles (≥30 mm) are identified during diestrus, practitioners face a clinical decision: administer prostaglandin F2α to induce luteolysis and predictable estrus, or allow spontaneous return to estrus over the following week. Cuervo-Arango's study of 46 Standardbred mares randomly allocated to either PGF treatment or a seven-day waiting period (spontaneous, SP) reveals that whilst PGF reliably triggered estrus and ovulation in 74% of mares—enabling 83% to be bred within a week—the SP group achieved a higher 14-day pregnancy rate (65% versus 39%, P = 0.092) despite only 9% being bred in that window. The SP group required fewer inseminations to achieve pregnancy (1.3 versus 1.7, P = 0.02), suggesting that the quality of oestrous cycles developing spontaneously may confer reproductive advantages despite their delayed timing. By the end of the breeding season, both strategies yielded comparable outcomes: similar intervals from presentation to pregnancy (approximately 37–40 days) and notably high pregnancy rates (87% and 96% respectively), indicating that whilst PGF accelerates breeding opportunities, it does not improve overall fertility and may necessitate additional inseminations. For practitioners, this challenges the routine use of PGF in this scenario; withholding treatment to allow natural oestrous development may optimise pregnancy achievement rates, particularly valuable when mare numbers or breeding slots are limited.
Read the full abstract on PubMed
Practical Takeaways
- •PGF treatment predictably induces estrus in diestrous mares with large follicles (74% success), enabling earlier breeding but does not improve ultimate pregnancy outcomes compared to waiting for spontaneous estrus
- •While more mares in the PGF group were bred within 7 days (83% vs 9%), spontaneously cycling mares required fewer inseminations per conception, suggesting better embryo quality or timing
- •End-of-season pregnancy rates are equivalent regardless of treatment choice, so management strategy should be based on operational factors (timing needs, breeding season length) rather than fertility advantage
Key Findings
- •PGF treatment resulted in estrus and ovulation in 74% of mares compared to variable outcomes in untreated controls over 7 days
- •14-day pregnancy rate was 65% in spontaneous (SP) group versus 39% in PGF group (P=0.092)
- •PGF-treated mares required more artificial inseminations to achieve pregnancy (1.7±0.6 vs 1.3±0.5; P=0.02)
- •End-of-season pregnancy rates were similar between groups (SP: 87% vs PGF: 96%; P>0.1) with comparable intervals to pregnancy (39.6±23.2 and 37.4±16.3 days respectively)