Factors affecting post-breeding endometritis, pregnancy rate and embryonic/fetal death in sport mares in two French commercial stud farms: special focus on age, parity and lactating status effects

Abstract

Fertility in mares depends on management and reproduction mode, but overall analyses have shown that there is an important and progressive decline in fertility in older mares. Age is often related to maternal parity, but the role of parity in the fertility decline in old mares is not known. Moreover, the positive or negative impacts of breeding mares yearly are a subject of controversy. The aim of this study was to identify whether these factors, as well as reproductive management, played a role on post-insemination endometritis and fertility in sports mares in commercial studs. Altogether, the breeding records of 277 mares (506 cycles) over one breeding season in two French commercial stud farms were analyzed. Age, parity, whether the mare was barren or suckling, follicular size before ovulation, the use of estrus and/or ovulation induction, artificial insemination (AI) protocol, post-breeding inflammation and treatment, Day 14 pregnancies and number of embryos, as well as subsequent foaling the next year were recorded. Data were analyzed using multivariate logistic regression models. The pregnancy rate by cycle was 41.9% and 76.5% of mares were pregnant at the end of the season. Mares older than 10 years, barren mares or mares inseminated in July or August were more likely to have inflammatory reaction after insemination (p<0.0001, OR=3.29, 5.389 and 3.329, respectively). Mares >10 years and mares inseminated with frozen semen were less likely to be pregnant on Day 14 vs younger mares and mares inseminated with fresh and refrigerated semen (p<0.05, OR=0.622 and 0.582, respectively). As expected, mares with multiple ovulations were more likely to be pregnant on Day 14 compared to mares that had only one ovulation (p<0.05, OR=1.791). Altogether, parity only tended to improve the likelihood of being pregnant on Day 14 (p=0.07, OR=1.434 in parous vs nulliparous) but in older mares (age >10), parity increased pregnancy rates (44.09% vs 30.89% in parous vs nulliparous, p<0.05). Post-breeding inflammation was not related to pregnancy rate on Day 14 (37.8% vs 44.2%, p=0.16) but all mares with post-breeding inflammation received a treatment. No effect of suckling at insemination was observed. None of the studied factor could be related to embryo/fetal mortality. In conclusion, maternal age seems to be more important than parity and lactating status for reproductive success in mare. Cumulative effects on parity and aging were observed on pregnancy rates.