Horse Breeding

Older Maidens

October 14, 2011

Cervical issues might limit fertility.

Ultrasound of a uterus filled with fluid. Courtesy of Dr. McCue.

By Dr. Patrick McCue in The American Quarter Horse Journal

A “maiden mare” is one who has never carried her own foal to term; “maiden” can also be used to describe a mare who has never been bred.

The inherent fertility of mares remains high until approximately 12 to 15 years of age, after which there is a slow progressive decline in pregnancy and foaling rates until mares are in their mid- to late 20s. A majority of mares are bred to carry a foal well before they reach their mid-teens. However, occasionally a mare is bred for the first time at a relatively advanced age.

Mares older than 15 typically exhibit one or more uterine problems, including scar tissue in their uterine lining or endometrium, development of uterine cysts and a decrease in the ability of the uterine musculature to contract, which leads to fluid accumulation in the uterine lumen. These factors result in a lower pregnancy rate in older mares.

In a reproductively normal mare, the morphology of the cervix changes dynamically during the course of the estrous cycle. A mare in heat should have a relaxed cervix that allows semen access into the uterus at breeding. The presence of sperm in the uterus results in an inflammatory response in all mares. In the normal mare, the inflammatory cells, uterine fluid and dead sperm are passed out through the relaxed cervix by uterine contractions. In most mares, the inflammation peaks at eight to 12 hours and is resolved 24 hours after breeding.

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In contrast, an older maiden mare might have a cervix that fails to relax during estrus. The presence of sperm in the uterus after mating will result in the expected inflammatory response.

However, uterine contractions won’t evacuate the inflammatory fluid through the closed cervix of the older maiden mare, which results in an accumulation of a large volume of fluid within the uterine lumen and failure of the mare to become pregnant.

A vaginal speculum exam and manual examination of the cervix when the mare is in heat will determine the degree of cervical relaxation and the probability of uterine fluid accumulation after breeding. If the cervix is not relaxed sufficiently, the mare should be bred or inseminated only once, just prior to ovulation.

Second, the uterus should be rinsed out or lavaged four to six hours after breeding. The sperm that will eventually fertilize the egg will be safe within the oviduct within four hours after breeding, so uterine lavage can be performed without affecting pregnancy.

The goal of uterine lavage is to remove residual sperm, inflammatory cells and fluid from the uterine lumen and prevent or limit the post-mating inflammatory response. An ultrasound exam should be performed a day or two after insemination to confirm that the mare ovulated and that the uterus is free of fluid. If all is well, an ultrasound exam for pregnancy may be performed 14 to 16 days after ovulation.

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Stretching of the cervix during foaling will prevent formation of the tight “fibrotic” cervix that is prevalent in older maiden mares. Mares used exclusively as embryo donors are also at risk of developing a tight fibrotic cervix and subsequent uterine fluid accumulation.

Embryo donor mares should carry a foal to term by the time they are 10 or 12 and should also be allowed to carry their own foals every three to five years to optimize long-term reproductive health.