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

The objective of this study was to compare the fertility of diestrous mares with a follicle ≥ 30 mm in diameter treated with prostaglandin F2α (PGF) (PGF group) or left untreated for 7 days (SP group). Standardbred mares (n = 46) were assigned randomly to the SP (n = 23) and PGF groups (n = 23) when they were identified with a corpus luteum (CL) of unknown age and a follicle ≥ 30 mm in diameter (Day 0). The most frequent outcome in the PGF group was estrus and ovulation of the dominant follicle (74 %), while the outcomes of the SP mares 7 days after Day 0 were more variable: estrus and follicle regression (30 %), diestrous ovulation (22 %) and diestrus along with follicle regression (26 %) of the dominant follicles. Therefore, more mares from the PGF group were bred within 7 days after treatment (83 %) than SP mares (9 %; P < 0.001). The 14-day pregnancy rate tended to be higher in SP mares (65 %; P = 0.092; OR = 2.716) than in PGF mares (39 %), and more artificial inseminations (AI) were required in the PGF than in the SP group to reach a 14-day pregnancy (1.7 ± 0.6 and 1.3 ± 0.5, respectively; P = 0.02). However, the interval from Day 0 to 14-day pregnancy and the end of season pregnancy rates were similar (P > 0.1) in both groups (SP: 39.6 ± 23.2 days and 87 %; PGF: 37.4 ± 16.3 days and 96 %, respectively). In conclusion, there is no advantage of waiting for diestrous mares with a large follicle to enter estrus spontaneously over administering a PGF treatment. However, more AIs were needed to obtain a pregnancy in the PGF than in the SP group.