Montana State currently estimates that 8-10% of Montana is infested with noxious weeds. The losses from livestock eating noxious weeds in 17 western states were estimated at $107 million dollars, according to the 1978 Journal of Range Management.
Horses will not seek out weeds to eat if there are other nutritious items available. However, there is no guarantee that your horse won’t eat one, either. Horses in dry lots are particularly vulnerable as noxious weeds tend to grow in areas like corrals where the ground has been stressed. A general rule of thumb is, if it’s growing in your corral, it can’t be good. There could also be toxic weeds found in hay, so you always check your hay or the farmground it was cut from to make sure there are no weeds. And remember, where there are weeds, there are seeds, which means feeding seed infested hay in a place that didn’t have that weed could spread the weed there!
What do you do if you find noxious weeds on your property? Contact someone knowledgeable about weeds and weed control to accurately identify the weed and determine the best method of controlling that weed in your particular situation. Your County Extension Agent, County Weed Coordinator, University, or a commercial weed sprayer can be sources of information.
Which weeds could be harmful to my horse? Weed poisoning is a tricky subject with no absolutes. Age, size and general health of horse, type of plant ingested, amount of plant ingested, time of year/conditions when plant grew before it was ingested and other factors can all affect how toxic, if at all, a plant is to your horse. Weeds that have possible toxic effects are:
HOUNDSTONGUE: All stages of plant growth may be toxic and could cause permanent liver damage. If a horse eats 6 percent Houndstongue of its daily intake for two 2 weeks, it may accumulate a lethal dose. Prognosis is death in less than six months due to liver failure in that situation. Houndstongue is easily identified by its seeds, which look like teardrop shaped, small burrs. Pick them off and throw them in the garbage.
TANSY RAGWORT: 4-8 percent of a horse’s body weight could be a lethal dose. Like Houndstongue, it also affects the liver. It has been found to maintain it’s toxicity in hay.
YELLOW STAR THISTLE: Unfortunately, horses like it. Fortunately, it is not common in Montana. If ingested it can cause Chewing Disease, where a horse cannot chew and swallow food or water properly. The condition is incurable and is fatal. The same toxin is found in Russian Knapweed.
RUSSIAN KNAPWEED: (See Yellow Star Thistle). Can cause Chewing Disease. Not to be confused with Spotted Knapweed, which is very common in Montana. Spotted Knapweed is a noxious weed, but is not toxic to horses.
TALL BUTTERCUP: The toxin in this plant makes horses (and humans) lips swell on contact, so it is unlikely (but not impossible) that horses would eat it. If ingested it could contribute to gastrointestinal upset or colic.
LEAFY SPURGE: Plant contains latex, which can cause sensitivities in the skin or eyes-especially on white or light colored horses including Paints and Appaloosas. Leafy Spurge is common in Montana, and is extremely difficult to control. Herbicides, flea beatles, sheep and goats are all ways to try and control it’s spread.
ST. JOHNSWORT: St. Johnswort can cause serious photosensitivity in horses who eat it. This condition can become so serious, a horse’s entire skin might slough away from the body.
KOCHIA: Can cause nitrate poisoning in cattle. Kochia is very common, especially around corrals. It can shift its response to herbicides rapidly, so changing your spray every year can help.
COMMON TANSY: It isn’t likely that your horse would eat this, as it is very strong. In fact, some old fashioned remedies called for making a tea out of this plant. However, it can cause abortion, colic, cardiac and respiratory suppression, so make sure your horse doesn’t have access to it.
ANADIAN THISTLE: While not particularly dangerous to horses, this plant can cause nitrate poisoning in cattle.
YELLOW TOADFLAX: Can cause gastrointestinal upset in horses. It looks like a Snapdragon with its pretty yellow flowers.
HOARY ALYSSUM: Can cause laminitis and edema (swelling) in limbs.
COCKLEBURR AND BURDOCK: These are not toxic, but the spiny burrs from these plants have been known to cause corneal ulcers in equines.