Mares with no apparent problems do not settle everytime they are bred through a cycle. How many cycles it takes, on average for a healthy mare, to settle depends on a number of factors. The age of the mare, fertility of the stallion, when breeding starts in the cycle, and how often she is bred all affect conception rates. So how do your decide if your mare has a fertility problem and how do you go about investigating, diagnosing, then treating the cause. This article discusses these issues and is presented as a step wise diagnostic guide which then links to specific articles on the causes and treatment of infertility. Also discussed is the use of embryo transfer and oocyte transfer in subfertile mares. Problems with mares cycling poorly are discussed in the article Abnormal Cycling of the Mare...more.

Healthy mares under 16 years of age, bred starting on the second day of heat and covered every other day, under 16 years of age have a 70% chance of having a healthy conceptus by day 40 after the last breeding. For example:

• If you had 100 healthy young mares and you bred them all for one cycle, you would expect 70 to be pregnant by day 40 post breeding.
• If you bred the 30 not pregnant on the second cycle, 21 would have settled when checked day 30 post breeding.
• 9 healthy mares would still not be settled.

I pick day 40 for measuring pregnancy rates because conception rates are in reality higher than the 70% found pregnant at 40 days, but the incidence of embryonic loss between day 13 (the first day you can easily determine pregnancy) and day 40 is as high as 25%. Mares with earlier pregnancy diagnosis should be rechecked at day 40 to be sure they are still pregnant.

Older mares generally have lower conception rates and increased rates of early embryonic loss. Therefore they may require more covered cycles to get in foal. The reasons are not completely understood but have come to center around the uterus as the problem. Older mares have more chronic degenerative changes in the endometrium and impaired immune response to the normal infection and inflammation that occurs following breeding have been documented. It has been conjectured that there may be a problem with the mares older eggs being less healthy for which there is some support. Embryo transfer from older mares to younger mare uteri does not substantially decrease the rate of early embryonic loss.

So when your mare does not settle, the first question is deciding whether you were just unlucky or does the mare have a a fertility problem. Most people will start looking for problems when a mare has not settled by the second or third breeding because 90% of the healthy mares should have settled by then.