Introduction

Concerns about plants found in pastures include whether they may be poisonous to horses. In general, a horse usually would need to consume a large quantity of a specific toxic species for it to be an issue. However, a few ornamental plants are toxic in small quantities, even only a couple of mouthfuls. Prevention is the best approach to dealing with plant toxicity to horses. There are some things you can do to prevent your horse from having issues.   

Plant Toxicity Levels

Most toxic plants must be consumed in large quantities before clinical signs appear. An example is bracken fern which becomes toxic after the horse consumes a diet of three to five percent bracken fern for at least 30 days. The horse taking a quick bite on the trail ride is unlikely to be affected.  

On the other hand, some ornamental plants are extremely toxic.  

• Oleander is the most toxic, with consumption of 0.005 percent of a horse’s body weight in oleander (that is 0.05 lbs or 0.8 oz of oleander for a 1,000 lb horse to help you keep this in perspective), being a lethal dose (meaning it will kill the horse).  
• Yew, another ornamental plant, has a lethal dose of 0.1 to 0.5 percent of body weight (one to five pounds of yew for a 1,000 lb horse).  
• Rhododendron has a lethal dose of 0.2 percent of body weight (two pounds of rhodo for a 1,000 lb horse).  
• Milkweed, commonly found in CT, has a lethal dose of 0.05 to 2 percent of body weight depending on the variety (lethal dose of half a pound to 20 lbs).   
• A horse must consume a diet of 25% dry matter in onions, which is very unlikely.  
• Red maple also concerns horse owners, but the horse must consume the wilted or dried leaves, such as those found after a storm, and the fatal dose is 6.6 lbs, with a poisonous dose of 3.3 lbs.   

Prevention Strategies

Rather than talking more about what signs we would see in the animal, this fact sheet will focus on preventing horses from getting sick or even dying from consuming toxic plants.  

Here are some ideas for plant toxicity prevention:   

• When in doubt, avoid the horse having contact. Try not to let the horse eat things other than its normal diet; 

• Have your neighbors come for a visit and tell them in a friendly way never to feed your horse any clippings from their yard;   

• Obtain hay from a reputable farmer;   

• Keep your pasture strong and healthy by doing soil testing, adding fertilizer if needed and removing any weeds early, before they get established. Remove them mechanically, or by applying an herbicide, carefully following the instructions on the label;   

• Make sure horses have sufficient forage, either through a free choice supply or using slow feeders made with smaller openings to allow easy keepers to eat more continuously; 

• Before any new plantings, do some research and see if that plant is toxic to horses; 

• Make sure you know the source of your shavings. Avoid black walnut since bedding with as little as 5-20% of black walnut shavings can result in laminitis in horses;  

• After a storm or whenever you see them, remove dried and wilted maple leaves from the horse pasture.   

Although individual horses may have some strange tastes for certain plants, in general taking these steps can keep your horse safe. Availability and quality of forage (pasture and/or hay) definitely plays a role, by giving them something safe to consume.  

There should be no need to go out and chop down all the red maple trees.   

Resources

Burger, S. M., & Breakthrough Publication Editors (Eds.). (1996). Horse owner’s field guide to toxic plants (in consultation with A. P. Knight). Breakthrough Publication. 

Burrows, G. E., & Tyrl, R. J. (2001). Toxic plants of North America. Iowa State University Press. 

Knight, A. P., & Walter, R. G. (2001). A guide to plant poisoning of animals in North America. Teton New Media. 

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