Age‐associated subfertility before entry of embryos into the uterus in mares

Abstract

Summary Age‐associated changes in the fertility of pony mares were studied in 3 experiments. On Days 1.5 and 3 (Day 0=day of ovulation), respectively, the collection rate of ova (cleaved or uncleaved) per ovulation was significantly lower in old mares (>20 years; 11/20 and 6/9) than in young mares (2–10 years; 17/18 and 9/9). The percentage of cleaved ova was significantly lower for old mares (5/11) than for young mares (15/17) on Day 1.5 but was not different on Day 3. Embryos from old mares had significantly more morphological abnormalities and fewer embryonic cells on Day 3. The transfer of embryos from young or old donors into the oviducts of young or old recipients did not significantly alter embryo development between Days 1.5 and 3.5. On Day 11, significantly fewer pregnancies and smaller embryonic vesicles were present in old mares than in young mares. The pregnancy rate on Day 3 was not different from the pregnancy rate on Day 11 within old mares (6/9 versus 4/9) or within young mares (9/9 versus 8/9). The studies indicated that a high incidence of age‐associated defects occurred before the day that an embryo would be expected to enter the uterus.