We spend so much time researching what to feed our horses – but how about what not to feed them? Here’s a list of things that can harm your horses if ingested.

Did you know that dietary supplements, horse feeds, and forages may contain ingredients that are harmful to your horse? The risk is lowest for dietary supplements and commercially manufactured horse feeds, but increases for hay and pasture in which potentially harmful or toxic substances may be present. Here is a brief review of some common ingredients, plants, and other substances you should avoid feeding your horse.

Feeds and supplements – it can’t hurt to be cautious

A tremendous number of dietary supplements are available for horses. And since there is no national organization that reviews or approves equine dietary supplements, some caution is advised. My recommendation is to purchase supplements from companies that have a quality assurance program and research-proven products. These supplements can usually be safely given along with a commercial horse feed, if they’re provided at the recommended rates. The major concern is with dietary supplements that contain vitamins A and D. These are fat-soluble vitamins and are stored in the body tissues of the horse. Giving your horse a well-fortified feed along with a dietary supplement at higher than recommended rates of vitamins A and D could result in toxic levels over time, so carefully read and follow supplement directions.

Commercially manufactured horse feeds have a long history of safety. Harmful ingredients that could contaminate feed ingredients include mycotoxins and ionophores.

• Mycotoxins are compounds that can develop in plants from molds that grow in them; they can have harmful effects on horses. Many companies have testing procedures for specific mycotoxins in feed ingredients delivered to their mills, as well as supplier agreements with those companies with specific quality parameters. Also, the Food and Drug Administration has guidelines for maximum allowable levels of certain mycotoxins in horse feeds. For the concerned horse caretaker, contact a feed representative or the feed company and ask about their procedures for ensuring safety from mycotoxin contamination in their manufactured horse feeds.
• Ionophores are a class of antibiotics used as feed additives for cattle and poultry. They can cause harmful effects in horses eating ionophore-contaminated feed. Procedures to prevent ionophore contamination include computer-controlled sequencing of manufactured horse feeds, and medication-free feed mills. Check with your feed manufacturer and ask if they are HAACP (Hazard Analysis and Critical Control Points) certified, which provides international human food and animal feed safety programs that provide training, testing. and inspection to reduce the risk of food safety hazards.

Well-fed horses are less likely to consume toxic plants, which are typically not very palatable.

Keeping forage safe

The most common way a horse can ingest harmful ingredients or toxic substances is through hay or pasture. Hay can contain mycotoxins as well as other harmful substances from insects, animals, and other plants.

• Blister beetles: Poisoning occurs when horses eat alfalfa hay that contains these insects. Blister beetles contain a substance called cantharidin, which causes inflammation of the mouth, stomach, and intestines and may even result in death. Blister beetles with the greatest concentrations of cantharidin are found in the southern and western United States, so alfalfa hay purchased there has a higher risk of causing harmful symptoms.
• Botulism: Round bales in contact with decayed plant or animal matter can be a source of botulism poisoning from the bacteria Clostridium botulin. Botulism is a highly fatal neurologic disease for the horse. There is an equine botulism vaccine and antitoxin available for prevention and treatment.
• Foxtail: Mouth ulcers can occur when horses consume hay contaminated with the foxtail plant. Its prickly seed heads can become embedded in the horse’s lips, tongue, and gums and can produce blister-like sores and severe ulcerations.

Other common plants and weeds that are harmful to horses may grow in pastures or near fence lines. Additionally, leaves from certain trees or bushes can have toxic effects, and lightning strikes or trimming can result in branches and leaves falling into the pasture (see sidebar).

How to keep your horse safe

So what can you do to prevent feeding harmful substances to your horse? First of all, carefully follow feeding directions for feeds and supplements, and avoid giving him supplements at higher than the recommended rates. Find a feed manufacturer that is HAACP-certified to assure maximum feed safety, and purchase hay from a dealer or source that has a reputation for good quality and safety. Daily inspection of your horse’s feed and hay are important; if the look or smell is different, or the hay is moldy or especially dusty, don’t feed it!

Well-fed horses are less likely to consume toxic plants, which are typically not very palatable. Maintain your pasture with soil testing and fertilizer treatment at least every three years. Use herbicides to remove potentially harmful weeds from pasture. Local university extension agents and professional agronomists can provide information on fertilizer testing, weed identification, and pasture management. Observe your horses daily and walk the pasture on at least a weekly basis. Use the internet to find toxic plant guides, and observe your pasture, fence rows, areas around the barn, and exercise pens or arenas for toxic plants.

Preventing horses from ingesting harmful substances isn’t always possible. But by heeding these guidelines, you can greatly reduce the risks!