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Poisonous Plants & Feed-Related Poisonings
By Dr. Randel Raub, Purina Mills, LLC
Poisonous plants are very common in most areas of the western United States and can grow almost anywhere including in our own backyards. This means that our horses may have access to toxic plants without us ever realizing it. Though most horses find these plants to be distasteful, they can be found pretty much wherever they graze.
Poisonous plants can grow in our pastures. They can grow in the woods where we trail ride and graze our horses on overnight pack trips. Some leaves and bark from certain species of trees are toxic, as are some of our ornamental landscape plants that we may clip and dispose of over the fence where the horses live. Even a bale of hay harvested from an unsprayed field may contain toxic plants.
In considering this it may seem as if poisonous plants lurk everywhere, and indeed, there is more that you should know.
Most poisonous plants generally cause problems only under certain conditions and, fortunately, they are not very palatable. A plant that could be toxic typically is ignored if there is adequate grass or hay forage available. Overgrazed pastures allow noxious weeds to proliferate, so it is important to practice good pasture management.
Often, the horse may begin to consume toxic plants simply because, for whatever reason, he found himself in the same environment as the plant and it looked like a good thing to eat at the time. For example, in areas of severe inclement weather horses may seek shelter in a ravine where there is little grass to eat but various weeds that could be consumed.
Weather can be a causative factor, as well as various seasons. What may not be a "problem plant" during one time of year can become a threat during a different season.
Some plants are even relatively safe to graze part of the year, but then will become lethal as the season changes and their toxin levels rise. The amount of toxin in a plant will vary due to soil mineral imbalances, excessive moisture, fertilization, drought and herbicides.
Horses are at greater risk of plant poisoning if they have the opportunity to consume large quantities (5%-10% of their body weight) over a period of several weeks or months. It is rare that a horse will be poisoned by a single mouthful of the plant, with some exceptions such as yew and water hemlock.
Here is a partial list of some plants which are considered poisonous to the horse:
Bracken Fern (very common in the Northwest)
Horsetail
St. John’s Wort
Sagebrush
Tall Fescue
Black Walnut
Red Maple
Yellow Starthistle
Alsike Clover
Yews
Nightshades (potato, tomato, jimson weed, thorn apple)
Tansy Ragwort
Poison Hemlock
Larkspurs
Ergot (found in grasses including orchard grass, fescue, ryegrass, bromegrass)
Lupines
Death Camas
Oaks
Mountain Laurel
Azaleas
cherry, peach, plum tree leaves
Sudan Grass and Johnson Grass (some species-hay not affected)
Wild Blue Flax
Elderberries
Milkweeds
Purple Foxglove
Oleander, Yellow Oleander
Horse Chestnut
Buckeye Tree (some species)
Buttercups
Consult your local county conservation department for more specifics on poisonous plants that may be found in your area.
If your horse does consume toxic plants, the quantity consumed and the level of plant toxicity will directly affect the prognosis. Symptoms and effects of poisoning will vary with the specific organs affected and the degree to which they are affected. Recovery depends upon how quickly the condition has been discovered, how severe the degree of poisoning, and the method of treatment available. Currently, there are no known antidotes for most poisonous plants and often the result is death, reduced growth, birth defects, abortion, reproduction inefficiency, or emaciation.
Prevention, therefore, is the best cure.
- Make certain adequate forage (pasture and/or hay) is provided. Avoid overgrazing pastures. Provide salt and water to prevent your horses from searching for unusual things to eat.
- Prior to moving your horse to a new pasture or grazing area, check it out! Take note of what the horse is eating and don’t allow free access to unfamiliar areas.
- Remove toxic plants from your pastures. Check with your county extension office for recommendations for your area.
While pasture plants are primarily responsible for most horse poisonings, toxins can be present in some commercial horse feeds. These toxins are produced by molds, bacteria, plants, insects, contamination of plants, or errors in preparation.
The most common group of toxins that affects the horse is mycotoxins. Mycotoxins are poisons which are produced by molds. Mycotoxins can be present in both harvested feeds, including those responsible for moldy corn disease and aflatoxin poisoning, and unharvested feeds, such as in the case of fescue poisoning. Still, others will occur in both harvested and unharvested feeds as with ergotism, a fungus that affects cereal grain plants, especially oats, wheat, rye and some grasses.
Molds that produce mycotoxins grow in many feeds including all cereal grains and forages. Typically they will occur in those feeds that contain over 12%-20% moisture, in humid conditions, and in alkaline conditions. A plant can become more susceptible to mold due to insect damage, incorrect application of fertilizer or pesticides, and drought.
Under certain conditions it doesn’t take mold very long to grow, sometimes within only a few hours! Because weather plays such a key factor in the production of mycotoxins, it may be a regional concern due to growing conditions in that specific geographical area.
Lets take a look at a few of the more prevalent poisonings of horses.
Moldy corn disease is probably the most common cause of poisoning, with older horses being more at risk than younger horses. It is a horrible disease that affects the brain and liver. It occurs worldwide and affects only the equine species. The mycotoxin responsible for this disease is found in the grain prior to harvesting.
Aflatoxin poisoning is the most common mycotoxin that directly affects most domestic animals, but is rarely reported in horses. This is due to the fact that the type of mold that causes aflatoxin poisoning is present in those grains that are not typically used in the production of horse feed, and rarely is found in hay. As opposed to moldy corn poisoning, the mycotoxin responsible is usually not present prior to harvest, but is produced during storage. Liver damage is the primary result of aflatoxin poisoning, with young horses being especially susceptible.
Ergot is toxic to all animals and people alike. It is a mycotoxin produced only during the time that the plant is flowering and the seed is maturing. Ergot production is more likely to occur during times of damp weather and is inhibited during those periods of drought. Ergots are black to brown-colored hard masses that project from the plant seedheads. They may be present in any cereal grain and seedheads of other grasses including rye and wheat. Rarely are horses affected by ergotism though ergot poisoning has been shown to cause reproduction problems, reduced growth, and behavioral changes.
Blister beetle poisoning occurs from horses consuming alfalfa hay harvested after midsummer and is a common problem throughout a large portion of the U.S. and Canada. Washington State-grown and harvested alfalfa hay appears to be free of the blister beetle. There have been very few reports of blister beetle poisoning in our state and those reported were unclear as to the actual source of the alfalfa hay.
There are over 300 different mycotoxins, and while mycotoxins do serve a useful purpose in the production of antibiotics such as penicillin, they have no place in the preparation of horse feed! So how do you know if the grain you have purchased to feed to your horse is safe?
Just as good pasture management is the best control to minimize plant poisoning, prevention is also the best option when it comes to selecting ingredients for the manufacturing of commercial horse feeds.
When raw grains arrive at the feed mill, they should be tested prior to purchase using a blacklight test and/or a chemical test. Those grains that result in positive test results for mycotoxins should not be accepted by the feed manufacturer. A blacklight, or NIR (Near Infrared Reflectance Spectroscopy) test, will also show levels of fat, fiber, moisture and protein content.
Another important consideration in commercial feed production is the use of mold inhibitors. Molds in feeds can decrease the nutrient value, affect palatability, create respiratory problems, and cause toxic reactions. Mold inhibitors stop mold growth, but do not kill mycotoxins already present.
One popular mold inhibitor is propionic acid, a colorless, oily, water-soluble liquid commonly used by most commercial feed manufacturers. It is added only at very low levels. Propionic acid is also used in making bread. Though other mold inhibitors are also available they tend not to be used in the manufacturing of horse feeds for various reasons including lower effectiveness, cost, and fungal resistance.
Here are a few suggestions to help you in the prevention of feed-related poisonings.
- Don’t feed moldy or questionable feeds. Throw it away or return it (along with the feed bag it came in) to the dealer for exchange.
- Make certain that at least 50% of your horse’s diet is forage (with the exception of manufactured complete feeds that contain forage.)
- Purchase only the amount of grain that you expect to feed within the next 3 to 4 weeks.
- Buy grain concentrates that contain mold inhibitors.
- Feed at least twice a day. Always have fresh water and salt available.
- Clean out feed troughs daily and remove any unconsumed feed.
- Store feed where moisture levels do not exceed 13-15%.
- Purchase grain concentrates only from those feed companies that test raw grains prior to manufacturing.
- Watch your horses when they eat. Any reluctance to consume either grain or hay is an alert for your inspection. If you observe a change in your horse’s condition or behavior that causes you to become suspicious, call your veterinarian. Remove the feed that you feel may be the problem.
- Follow manufacturer’s recommendations. If you have any questions regarding the feeding rate, ingredients, or manufacturing standards consult your dealer or equine nutritionist.
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