Few of us pay much attention to the species of grasses in our pastures. But if your mare is pregnant, it’s crucial to look out for one type of grass—tall fescue—found throughout the country.

The grass itself isn’t a problem; what makes tall fescue a hazard is that it is often infected with a microscopic endophytic fungus called Neotyphodium coenophialum, which produces a chemical called ergovaline that can cause fescue toxicosis in horses.

In the general equine population, fescue toxicosis causes fairly minor problems such as loose manure and profuse sweating. In pregnant mares, however, the condition can be catastrophic, leading to lack of milk production, prolonged gestation, difficult births, thick or retained placentas, and the death of unborn and newborn foals.

You cannot see or smell the fungus on tall fescue; the only way to detect it is through laboratory analysis, and the status may differ from year to year. Fields with as little at 5 to 10 percent infection have been known to cause problems for broodmares. It is possible to renovate pastures by killing all the problematic grass and replanting the pasture with endophyte-resistant species, but the field could be overrun again in a few years.

The easiest way to avoid fescue toxicosis is to remove broodmares from endophyte-infected pastures 90 days prior to foaling. If that’s not possible, or if you’re unsure of the status of your pasture, talk to your veterinarian about treating broodmares with the drug domperidone in the weeks leading up to foaling. The exact dosing schedule will vary with each situation, but the drug has been shown to reliably prevent and even reverse the signs of fescue toxicosis. 

This article was originally published in EQUUS 486, March 2018