Grazing may seem natural for horses, but that doesn’t mean it’s without danger. For some horses, fresh grass itself can cause problems.

“Horses with founder potential are hard to manage,” said Shea Porr, Ph.D, and extension agent with the Virginia Cooperative Extension. “There’s no solid rule for how to keep your horse from foundering on grass other than to not have him on grass. In general, the plants tend to accumulate more sugars throughout the day, so the highest sugars in some of our grasses are in the afternoon and evening hours, and then after sundown they begin to decrease. If you wanted to turn out a founder-prone horse on pasture and wanted the lowest sugars, probably midnight to 6 a.m. is the best time to turn him out. However, there are some grasses that don’t adhere to that schedule. It depends on environment and species.”

Porr recommended six hours or fewer on grass for these horses or a grazing muzzle.
Poisonous plants and trees are another grazing danger. “Pasture management is the biggest thing that will help keep poisonous plants down,” said Porr. “Most of the time if horses have an option, they won’t eat the toxic plants. It’s usually more of a problem in hay, because they don’t discriminate as easily in hay. However, if you have a horse on an overgrazed field, and the only thing in there are weeds, chances are higher that he’s going to eat something that’s not good for him.”

It’s important to know which poisonous plants are native to your area and patrol your pastures for them.

And then there are some grasses that work well as forage under normal circumstances but not without exception. Fescue, one of the most common pasture grasses because it’s hardy and aggressive, can be a death sentence for foals.

Fescue contains an endophytic fungus that forms a symbiotic relationship with the plant and helps it thrive. However, the endophyte produces alkaloid substances, which can be toxic for broodmares and foals. Common signs of fescue endophyte toxicity include prolonged gestation, lack of udder development, absence of milk production, abortion, thickened placentas, retained placentas, stillbirths and foal mortality. Horses other than broodmares aren’t affected by the endophyte.

Porr recommended an engineered variety of fescue, Max Q, for broodmares. This plant contains a non-toxic endophyte.

“Your other option is pulling them off the pasture for the last 30 to 60 days [of their gestation] and feeding them fescue-free hay. Or you can use the medications that help combat the endophyte,” said Porr.

Clover and other legumes also carry a risk, although a small one. A mold may form on the legume, and, when ingested, it causes slobbers, a condition that leaves horses drooling profusely.

“The only thing it does is affect the salivary glands. Some horses just stand there and drool,” said Porr. “As long as they have access to water, the worst thing I can think of is that the horse would become dehydrated if it didn’t have water. You can fix it by pulling them off the legume. In a couple of days they’ll be fine.”