181 Age-related changes in oocyte yield and embryonic development in light horse mares

Reproductive aging significantly influences ovarian function and oocyte quality across species, including horses. This retrospective study aimed to assess the association of reproductive age with oocyte retrieval, maturation, cleavage, and blastocyst production in cycling mares. A total of 119 light horse mares were categorized into three age groups: young (n = 26, 5.9 ± 0.5 years), mature (n = 60, 14.5 ± 0.3 years), and senior (n = 33, 21.9 ± 0.4 years). Ovum pickup (OPU) was conducted via transvaginal ultrasound-guided follicular aspirations of antral follicles during April and May 2024, aligning with the peak seasonality period when mares typically experience heightened follicular activity in response to the northern hemisphere’s photoperiod. In total, 2151 oocytes underwent proprietary ICSI protocols. Only MII oocytes were injected, cleavage-stage embryos were identified by cellular division, and failed cleavage was noted when no division occurred. Blastocysts were identified by blastomere presence. Analysis used mixed-effects modeling in PROC MIXED in SAS, with statistical significance set at P < 0.05 and trends noted at P < 0.1. Degrees of freedom for fixed effects were adjusted using the Kenward-Roger or Satterthwaite method, accounting for random mare effects and considering age group as a fixed effect. All data are presented as mean ± standard error of the mean (s.e.m.). Overall, no difference was observed in the number of OPU aspirations among mature (1.58 ± 0.10), senior (1.27 ± 0.14), and young mares (1.26 ± 0.16; P = 0.11). Mature mares yielded significantly more oocytes (21.38 ± 1.37) compared with senior mares (12.76 ± 1.85; P = 0.0012). However, no significant differences in oocyte yield were observed between young (17.19 ± 2.08) and either mature (P = 0.99) or senior mares (P = 0.96). No difference (P = 0.65) was observed in the proportions of injected oocytes among the age groups: mature (12.65 ± 4.5), senior (7.97 ± 3.9), and young mares (9.77 ± 1.1). Mature mares exhibited a higher cleavage rate (7.08 ± 0.55) compared with senior mares (4.15 ± 0.73; P = 0.0048). No difference was observed in cleavage rates between mature and young mares (6.12 ± 0.82; P = 0.59), or between senior and young mares (P = 0.18). With regard to blastocyst production, mature mares produced more blastocysts (2.93 ± 0.33) than senior mares (1.73 ± 0.46), with this difference approaching significance (P = 0.09). However, no significant differences were found when comparing mature mares to young mares (2.77 ± 0.50; P = 0.96) or senior mares to young mares (P = 0.13). These findings suggest that age is associated with certain reproductive parameters in mares, with mature mares showing superior oocyte yield and cleavage rates compared with senior mares. Although age was not significantly associated with proportions of injected oocytes or blastocyst production, these aspects of the reproductive process may be affected differently. Additionally, although semen quality variability was not accounted for, the trend in blastocyst production underscores its clinical and economic relevance, especially for senior mares.