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The Broodmare’s Last Trimester

By Justin High, DVM

The New Year brings new expectations for what will be produced from last year’s breeding season. After coasting through the last few months of your mare’s pregnancy with no problems, we are all struck with the fact of how quickly foaling season is upon us.

The last few months of a mare’s pregnancy are, by far, the most important in terms of fetal growth and development. Establishing a foal’s immune system and ensuring that as many potential problems have been planned for are top priorities as the foaling date approaches. Knowing the landscape of the last trimester, where hope turns into reality, is well worth the effort to make sure the pregnancy ends well and your new foal has a great start.

I am convinced that God created horses with a veterinarian in mind. However, when it comes to broodmares and foaling, the fact that horses flourished long before vets and breeding farms came around proves the point that the process of reproduction tends to go well with little interference from us. But, as we have changed the way horses are raised, how they are handled, and what we expect of them, unique problems arise from our variation from “normal.” People tend to think horses have low reproductive performance, but in nature they do well when compared to other domestic animals. Most performance problems are due to management, and horses have the ability to magnify management problems through foal crops.

So, once your mare is pregnant, getting through mid-gestation is relatively easy. The short version of pregnancy from the end of the second month through the eighth month is not much different than caring for your mare if she were not pregnant. Maintaining good body condition (BCS 6 to 6.5) without getting mares fat is step one. Continuing with a strategic deworming program based off fecal egg-per-gram counts like you normally would is step two. Check with your vet just to be sure, but most all dewormers are safe to give mares throughout their pregnancy. Step three is to vaccinate for equine herpes virus (EHV-4) that can cause late-term abortions. Vaccinate at 3, 5, 7 and 9 months of gestation so your mare will be well protected when the time comes. “Pregnant mare rhino” shots are safe, effective, inexpensive and the closest thing to ensuring a pregnancy as you can get without filling out the paperwork.

Now that we are in the home stretch of pregnancy, our focus shifts from the general to the specific. Within the last few months of pregnancy, the nutritional demands of the mare go up by about 30 percent due to the rapid growth of the fetus in the latter stages of development. Be cautious when increasing concentrated feeds in pregnant mares based off that statement though. Most broodmares I see come into their last trimester overweight anyway, and when an additional calorie load is supplied to these mares, people seem shocked by the small size of foal such a big mare will have. Broodmares need nutrients, vitamins and mineral content with balance much more than they need straight calories. Feed to a specific body condition score, not to a label on a bag, and your mare will have less fat in her abdomen and more room for the foal to grow.

The most important set of vaccinations a mare will receive during her pregnancy are the ones prior to foaling. “Pre-foaling” shots are given approximately 30 days before foaling and include influenza, rhino, eastern/western encephalitis, tetanus, West Nile, and rabies.   Some veterinarians prefer to give strangles at this time also. Certainly, the mare benefits from being vaccinated, but this is primarily for establishing the foal’s immune system through the colostrum.  By presenting the vaccines at this time the mare will produce much higher levels of antibodies to these antigens, and will basically “vaccinate” the foal through her milk. We typically give newborn foals a liter of hyper-immune plasma at birth, but it does not replace prefoaling vaccines.

For all intents and purposes, foaling dates are merely a suggestion. Mares typically carry a foal 338 to 343 days, but few read the book on gestational length, and I don’t tend to worry when mares go as much as two weeks early of late. So, as the time approaches, keeping an eye on udder development, laxity in the tail head, abdominal swelling and comfort level are important factors. Signs of trouble late in gestation are typically seen by mares “waxing up” 30 days or more prior to their foaling date.

Sometimes abnormal vulvar discharge will be seen as well, and these often point to placental infections or problems causing early separation of the placenta from the uterine wall that precede a late term abortion. These signs must be taken seriously, and quick action needs to be instituted by your veterinarian in hopes of saving the pregnancy.

As the fetus develops, the last few weeks can be a bit unnerving with odd leg swellings, plaques of edema on her ever expanding belly, and periodic bouts of colic. But, I’m sure if I had a 120 lb. fetus laying on my colon those would be the least of my issues. Nothing is wrong with a little exercise, and maybe a little Banamine™ to help get through the day.  Those of you waiting out the last days may need something a little stronger than that!

drjustinhigh eh Justin High, DVM Dr. Justin High is a veterinarian and partner in Reata Equine Hospital in Weatherford, Texas. He graduated vet school from Texas A&M University and completed an internship at The Littleton Equine Medical Center in Denver, Colo. High’s years of practice focuses on the Western performances horse. Send any comments or questions to [email protected].

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