Preparing Your Mare for Breeding

David Schwinghamer, DVM

February 2005

There are many things to consider when breeding your mare. It’s a good idea to start early in the year as not all mares get pregnant the first time. Horses actually have the lowest fertility rate of all domestic animals.  The mare is pregnant 340 days or 11 months and 10 days.   Therefore, it’s advised to have your mare ready to breed at the end of April or early May.  This will assure that your foal has optimal opportunity to grow and thrive on the fresh green grass and warm weather of the spring and summer.  

When deciding who to breed your mare to, you should consider picking a stallion that you think will cross well with your mare, both in conformation and temperament.  You also need to decide if you want to ship your mare to the stallion or keep her at home and use artificial insemination.  Look at other offspring from the stallion to see if these foals have characteristics you like and if they have been successful in their chosen occupation. 

Your mare must be cycling to successfully breed her.  Most mares start cycling as early as March when the days start getting long and continue thru October. The use of lights in your mares stall extending the amount of daylight to 16 hours will help bring your mare into heat earlier in the year. Cycles occur every 21 days during these months with the mare being in heat or estrus during the last 4-7 days.  A mare ovulates at the end of estrus so it’s important to breed at the end of her heat cycle or until she no longer receptive to the stallion. 

There are certain steps you want to take before breeding.  We recommend a uterine culture for all mares.  The reproductive tract should also be examined both by rectal palpation and by ultrasound which may show us fluid in the uterus or other problems.   Your mare most likely will not become pregnant if the reproductive tract is infected.  We would need to treat her for the infection before we begin breeding.  This will give her the optimum opportunity to conceive. 

When you are planning to breed by artificial insemination, the mare’s cycle will need to be followed with an ultrasound so that we can order semen when needed and breed her at the time of ovulation. 

Pregnancy diagnosis can be made at 15-16 days after ovulation with an ultrasound.  This saves time as you will know whether or not to breed your mare again the following cycle.  Ultrasound can also identify twins which the mare may abort in late pregnancy or cause problems during the foaling as the twins can get tangled up with their long legs. 

When you are planning to breed an older mare that has never had a foal or a mare that has had trouble in the past becoming pregnant, a uterine biopsy should be considered.  A uterine biopsy will help determine the probability that your mare will be able to become pregnant and whether she will be able to carry the foal full term.  

Many of the above techniques are routine in breeding mares.   It is a good idea to read as much as you can on the subject.  A book we highly recommend to our clients is: Blessed are the Brood Mares, by M. Phyllis Lose, V.M.D.   Discuss your mares breeding plans with veterinarian, know your mare’s cycle, make sure she is free of infection, and plan for the arrival of your new healthy foal in the spring.