After foaling, most mares will have a foal heat and breeding at this time can be successful. However, there are some factors to consider when making the decision to breed during foal heat. If there is excessive fluid in the uterus and the uterus has not involuted well, then the chances of pregnancy are slim and breeding should be delayed. It has been shown that in general if a mare ovulates before day 10 after foaling, the chances of pregnancy are decreased. If she ovulates after day 10, breeding should be considered. Many vets in the past have routinely performed uterine lavage on all mares after foaling but this was not shown to increase pregnancy rate unless a mare has a specific problem, so routine flushing after foaling is not recommended at this time.
Over the last several years it has been determined that the semen itself can cause inflammation of the mare's uterus; this is called persistent mating induced endometritis. Dr. John Hurtgen indicates this syndrome can increase as the mare ages, with bacterial contamination at mating, or with abnormalities of the reproductive tract. Uterine fluid should not be present 6 to 12 hours after mating in the normal mare. If there is fluid, oxytocin can be used multiple times to aid in clearing the fluid. If an infection is suspected after breeding, the uterus can be flushed with sterile solution and then antibiotics can be used even after breeding. Prostaglandins can also be used to clear the uterus of fluid, but a recent report indicated the use of prostaglandins after ovulation can have an effect on the production of progesterone, so consult with your vet about the use of prostaglandin after ovulation in mares.
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