Most horses won’t experiment with potentially toxic plants unless the pasture contains slim pickings of safe plants. If it does, be aware that the toxic choices will more become appealing.
Most horses will avoid onion grass. However, this hardy grass often gets the jump on other grass species in early spring, and poisonings occur every year, usually in the Northeastern and Southern states. Onion grass can damage the red blood cells, resulting in the formation of “Heinz bodies,” which results in anemia. When the toxicity is severe, the horse’s urine may be red and the horse has a greatly decreased exercise tolerance.
A strong onion breath gives the diagnosis away. Treatment is removal of the horse from the pasture until the onion grass can be destroyed. Recovery is rapid once grazing is prevented.
Red Maple Trees
Most cases of red-maple-tree poisoning occur in the fall or when trimmed branches are left where horses can get them. Dried leaves have a higher concentration of toxin than live ones. Bark, live leaves and tender buds can also produce poisoning, and horses may turn to these if their pasture grasses are slow to come in.
Symptoms are similar to onion grass toxicity — Heinz bodies, anemia, red urine, decreased exercise tolerance — but more serious because the toxin prevents the hemoglobin from carrying oxygen. A characteristic of severe cases is that the blood and mucus membranes/gums are brown from a lack of oxygen. Treatment involves preventing access to the red maple but, beyond that, it’s largely supportive. Horses may need blood transfusion and oxygen therapy. High doses of vitamin C (15 to 22 grams for a 1,000-lb. horse, by IV, twice a day) have been reported to help.
Fortunately, nitrate poisoning is rare under natural conditions, and horses are more resistant to it than cattle or other ruminants. However, some horses may show signs of mild nitrate poisoning, including unexplained anemia and abnormal thyroid function. The heavy applications of nitrogen-containing fertilizers, including manure, to spring fields can lead to high nitrate levels in the plants. Brome, sudangrass, rye, johnsongrass and fescue are all grasses likely to accumulate nitrogen. Horses should be kept off nitrogen-fertilized pastures for seven to 14 days, longer if the fertilization was followed by a cold or dry period.
Defeating Toxic Plants
There are hundreds of potentially toxic plants throughout the United States. They can produce anything from mouth irritation to organ damage to death. Many toxic plants can be difficult to identify or look so much like common, safe plants you can overlook them.
Steps to reduce the risk of your horse ingesting them include:
• A visit to your local agricultural extension office to ask about toxic plants common in your area.
• Be sure you can identify all the plants in your field. If you see a heavy growth of a plant in your pasture that you can’t identify, play it safe by digging up a few samples and taking them to someone who can.
• Check plant growth along fence lines. Toxic plants that may not hold up well in high-traffic often thrive in this location.
• Continue to feed your horses well. A hungry horse is more likely to eat things he would otherwise normally avoid. Be alert for signs of bark or branch chewing that often indicate the horse is not getting enough to eat.
• Know the early warning signs of toxicity: a change in color of the gums, diarrhea, less spontaneous moving around, depressed attitudes, change in heart rate or respiratory rate and breathing pattern. If you suspect a problem, have the horse examined right away.
The safest way to eliminate toxic plants is to dig them up by the roots, kill them with boiling water or spray with a vinegar solution (available from organic gardening-supply houses). If you prefer a herbicide, glyphosate — the active ingredient in RoundUp — is a good choice. It degrades rapidly to a nontoxic substance when used as directed. However, plants treated with this herbicide may still need to be removed by hand after they have died, as the plant itself could still be toxic even though it’s dead.