Maternal Age
Mares older than 14 yr begin to experience fertility problems. These mares have a 33% lower pregnancy rate and 26% lower foaling rate than younger mares. In one study, oocytes recovered from the oviduct of old mares (>14) were 90% fertilized and similar to that observered in young mares at 88% fertilized. Howerver higher embryonic death rates have been reported to occur in older mares. It was reported that the pregnancy rate at day 60 was 65% in young mares and only 30% in aged mares (>21 yr).
The above observations however do not resolve if the problem in older mares is due to oocyte, embryo, or the female reproductive tract. An abnormal uterine environment does not appear to be the problem in old mares as embryos transfered from young mares into the uterus of old mares have no problem in maintaining pregnancy. While severe endometrial fibrosis has been related to decreased fertility in some old mares this is not likely to be the major cause of reduce fertility in old mares.
To determine if the fertility difference between old and young mares was with the embryo, embryos were collected from oviducts and transferred to the oviduct of young mares. Embryos collected from young mares had a 50% pregnancy rate while it was only 23% for embryos from old mares. This experiment did not determine if the problem was with the oocyte/early embryo or the oviduct, since the embryos were recovered from the oviduct.
To determine if the problem is with the oocyte, GIFT (Gamete Intrafallopian Transfer) was used. Oocytes were aspirated from ovaries just before ovulation and transfered to the oviduct of a recipient mare. The recipient mare was then inseminated. Oocytes from young mares, when transferred into young mares, had a 92% fertility rate (embryos developed). When oocytes from old mares were transferred into young mares, there was only a 31 % fertility rate. Oocytes from the older mares are therefore abnormal and the major cause of reduced fertility in older mares. There is thus no way to solve the reduced fertility problem of most older mares.
