Reproductive performance of a cohort of Standardbred mares under a commercial breeding system

Abstract

Background: Despite being a large commercial breeding industry, there is little published data on the reproductive success of Standardbred mares. Objectives: To quantify the reproductive performance of Standardbred mares under artificial breeding systems in a commercial setting and determine the incidence of early embryonic and other pre-partum losses. Study design: Retrospective cohort study. Methods: Data from four commercial farms were collected across four breeding years, and all mares were bred via artificial insemination. A total of 3995 mares contributed 7229 mare years. First-cycle pregnancy rate (FCPR) and end of season pregnancy rate (SPR) were analysed in mixed-effects logistic regression models. Time-to-conception interval was analysed in a Cox regression model. Results: The mean FCPR was 61.4% (confidence interval [CI] 60.3%–62.6%), the mean end of SPR was 84.7% (CI 83.8–85.5%), the mean live foal rate (FR) was 73.1% (CI 72.1%–74.2%). Mares located on-farm were more probable to be pregnant in terms of both FCPR (odds ratio [OR] 1.168, CI 1.018–1.340, p = 0.026) and SPR (OR 2.026, CI 1.673–2.454, p < 0.001), mares inseminated with thawed–frozen semen were less probable to be pregnant in terms of FCPR (OR 0.598, CI 0.457–0.783, p < 0.001) and SPR (OR 0.479, CI 0.354–0.647, p < 0.001) compared with insemination with fresh-extended semen. Older mares (14 years and older) were less probable to be pregnant in terms of FCPR (OR 0.795, CI 0.688–0.919, p = 0.002) and SPR (0.435, CI 0.352–0.538, p < 0.001) compared with young mares (3- to 8-year old). Main limitations: Retrospective data relied on accurate record keeping of stud farms and no mare-treatment or ovulation induction records were available. Live FRs relied mostly on annual foaling returns so fetal/foal deaths may be underrepresented. Conclusion: This study provides substantial baseline data on reproductive performance for Standardbred mares managed under a commercial artificial breeding system.