Grain Overload in Horses

By Dr. Devon England

You come home after a long day at work, and your feed room door is wide open and grain bins are empty. What do you do? Before you call your vet, figure out what grains were eaten, try to estimate how much and determine which horse or horses got into the feed. Did your horse just get into some hay or alfalfa pellets, or did he get into a high sugar grain like Omolene 300? Did your horse get into chicken or cattle feed? If he did, is it a medicated feed, containing an ionophore like monensin or lasalocid?

Grain overload is an emergency, with better outcomes, the sooner the horse is treated. So, call your veterinarian as soon as you figure out a possible grain overload has occurred. Based on what feed and how much feed your horse consumed, your veterinarian can try to determine the risks to your horse and come up with a treatment plan. Signs of grain overload include colic, lethargy, inappetence, diarrhea and lameness. These signs will vary depending on how long ago grain ingestion occurred and the severity of grain overload.

If your horse got into hay or alfalfa pellets, these are low in sugar and therefore not a high risk for problems. Your horse may just have a mild belly ache (colic) episode. If your horse got into a feed like Omolene 300, they are at risk for grain overload. How severely they are affected will depend on a few factors including how much grain was eaten (5lb vs 30lb), if the horse normally receives that grain or doesn’t receive any grain, and the size of the horse vs the amount of grain eaten. Eating 5lb of grain for a full-size horse who normally receives grain may not be a problem, but that same amount of grain eaten by a small pony who normally only receives hay will be a problem.

Treatment for grain overload should be initiated as soon as possible. This includes lavaging the stomach to remove any remaining grain. Stomach lavage is only successful in removing feed within the first couple of hours. Horses will be given mineral oil or charcoal to decrease absorption of breakdown products (acids) from grain fermentation and protect the intestinal lining from damage. Intravenous fluids are given, because horses can quickly become dehydrated with grain overload, due to digestion products from the grain drawing in a lot of water into the colon. Anti-inflammatories, lidocaine, and ice boots are used in more severe cases to decrease the risk of laminitis. In more severe cases with damage of the intestinal lining, antibiotics may be used to prevent infections associated with bacterial escape from the damaged intestine.

If your horse got into medicated feed containing an ionophore, your horse is at high risk for heart failure and muscle damage, on top of grain overload. Ionophores are toxic at very low doses in horses (2mg/kg). These drugs are added to livestock feed to decrease parasites (coccidia), to help with feed efficiency, to decrease risk of bloat and to decrease methane emissions. If your horse has eaten ANY amount of medicated feed, they should be immediately seen by your veterinarian, even if they are not showing any clinical signs. Horses can have no clinical signs or be mildly lethargic in cases where small amounts of medicated feed were eaten but still have evidence on blood work or muscle and heart damage. Clinical signs include initial inappetence or colic signs, weakness or neurologic signs in the hind end, muscle fasciculations (twitching), laying down more, and not being able to stand. There is no anti-dote to ionophore toxicity. Treatment may include mineral oil, IV fluids, anti-inflammatories and supportive care, depending on how soon toxicity is identified. In severe cases, horses can die or develop heart failure, necessitating euthanasia. In less severe cases, especially if identified and treated early, horses can recover fully.

Grain overload can come with serious complications if severe or medicated feeds are involved, especially if treatment is delayed. Secure your feed room as best as possible and keep medicated livestock feeds in a separate location to prevent cross-contamination. And if you have any concerns about your horse getting into feed, call your veterinarian as soon as possible.

This article was originally published in the February 2025 issue of Oklahoma Farm & Ranch.