If you have open brood mares at this time of year, then something occurred that prevented them from becoming pregnant. The most common cause of infertility mares is endometritis or inflammation of the uterus. When most people think of endometritis, they think of infection, and certainly infection can cause endometritis. However, a condition called breeding-induced endometritis can also lead to infertility in mares. The uterus has a normal inflammatory response to breeding that is necessary to remove contaminating bacteria and excess semen and seminal fluid from the uterus. In a healthy uterus, this inflammation subsides in 48 hours. However, in some mares the inflammation does not subside but continues through days 5-6 when the embryo drops from the fallopian tubes into the uterus. This persistent mating-induced endometritis is fairly common in older mares. If inflammation is in the uterus when the embryo arrives on days 5-6, the embryo will not be able to implant and pregnancy will not occur.
Diagnosis of the condition is by exam of cells from the inside of the uterus, the lack of infection, and increased fluid in the uterus. As far as treatment, many vets use antibiotics but antibiotics are really not indicated since there is no infection in most cases. Flushing the uterus and using medications to clear the uterine fluid usually clears any bacteria introduced into the uterus during breeding. Some mares with persistent mating-induced metritis have responded to treatment with cortisone to reduce inflammation. Also, some mares have responded to medication to stimulate immunity but the most important thing to realize is that if a mare does not have an infection but is still infertile, persistent mating induced metritis could be the problem.