Noxious Weeds That Can Be Harmful To Horses

At our general meeting on November 15th, 2003, Maureen Connor, a weed specialist, was kind enough to come and give a talk to the members regarding noxious weeds and how they could be dangerous to our horses. She gave a wonderful talk, complete with slide presentation and dried weed specimens. She also gave out calendars and weed identification books to the members to assist them in identifying and controlling noxious weeds. Here is a small portion of that information. We hope it will encourage you to research the topic more and become active in the war on weeds!

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 was estimated at $107 million dollars, according to the 1978 Journal of Range Management. So, you can see this is a big problem, one we all need to be educated about if we are to help control the spread of noxious weeds and to keep our horses safe and our pastures healthy.

Did you know that horses can develop picas (addictions) to toxic plants once they start eating them? In general, most 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! This doesn’t mean you only find toxic plants in corrals; they grow wherever conditions are right including pastures, on mountainsides, near streams, you name it! That means there could be toxic weeds found in hay, so you should always look in 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? Be aware that herbicides can make toxic plants MORE palatable (for instance, Houndstongue). It is important to 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 very helpful sources of information. There is a Noxious Weed Trust Fund that helps people with the financial burden of weed control. Access to these funds is available through local weed management such as the County Weed Coordinator. It would also be wise to contact your veterinarian to discuss possible health problems related to toxic plants, and proper treatment for those problems.

Which weeds could be harmful to my horse? Realize that 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. That being said, some weeds that have possible toxic effects are as follows:

HOUNDSTONGUE:All stages of plant growth may be toxic and could cause permanent liver damage. If a horse eats 6% Houndstongue of it’s daily intake for 2 weeks, it may accumulate a lethal dose. Prognosis is death in less than 6 months due to liver failure in that situation. Houndstongue is easily identified by it’s seeds, which look like teardrop shaped, small burrs. You’ve probably found them in your horse’s mane or tail, or in your socks or shoelaces at some point! Do yourself a favor-if you find Houndstongue seeds stuck somewhere they shouldn’t be, don’t pick them off and drop them on the ground. That is how the weed spreads. Pick them off and throw them in the garbage! At least landfill sites have the ability to keep such pests from growing as they have methods to keep anything from living in a landfill site!

TANSY RAGWORT: 4-8% 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. It can contaminate the milk of grazing animals, and honey.

YELLOW STAR THISTLE:Unfortunately, horses LIKE it. Fortunately, it is not real common in Montana yet. If ingested it can cause Chewing Disease, where a horse cannot chew and swallow it’s food or water properly. This condition is incurable and is fatal. The same toxin is found in Russian Knapweed.

RUSSIAN KNAPWEED: (See Yellow Star Thistle). This too can cause Chewing Disease. Not to be confused with Spotted Knapweed, which is very common in Montana. Spotted Knapweed is a noxious weed, but it 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. Seeds can spring up to 15 FEET from the mother plant! The tap root is enormous and can be as deep as the soil is-we’re talking a tap root that can be underground 20 feet with little sprouts all the way down, just waiting to spring forth a new plant! Herbicides, flea beatles, sheep and goats are all ways to try and control it’s spread, however it is extremely difficult to control.

ST. JOHNSWORT: Yes, this is the same thing you find in the alternative medicine or health food section of stores. AKA Goat Weed or Klamath Weed, 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 it’s body, which of course is very painful and fatal. It has been said that there is enough of this plant in Montana alone to fill all the needs of those who use it for alternative medicine in the entire world.

KOCHIA: Can cause Nitrate Poisoning in cattle. Kochia is very common, especially around corrals. It can shift it’s 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.

CANADIAN THISTLE: While not particularly dangerous to horses, this plant can cause Nitrate Poisoning in cattle, so we thought we’d mention it! Many horse people also own cows!

YELLOW TOADFLAX: Can cause gastrointestinal upset in horses. It looks like a Snapdragon with it’s pretty yellow flowers.

WHITE BRYONY: This is a climbing plant. 15 berries are enough to possibly kill a child.

HOARY ALYSSUM: Can cause laminitis and edema (swelling) in limbs.

COCKLEBURR AND BURDOCK:These are not toxic but bear mentioning as the spiny burrs from these plants have been known to cause corneal ulcers in equines.
These and other weeds could have harmful effects on your livestock. Symptoms may go unnoticed until it is too late to save the animal. Horses who have had previous problems with weed toxicity are more vulnerable to future encounters with toxic plants, as previous damage could exist that you were unaware of.

It is important to the well being of our livestock and our environment to control these noxious weeds and prevent their spread. The best way to do this is to get involved, get educated, and be persistant! This is not a short term project, but something you have to be diligent about year after year. Remember, the cheapest and safest method of weed control is PREVENTION!
Here are some links to aid you in researching plants that may be harmful to equines:

Or, you can always utilize a search engine on the internet, as there is a multitude of information out there!

*Cara Moore and MHJA are not responsible for typographical errors or misstated information. We encourage you to utilize your County Extension Service, Veterinarian, County Weed Coordinator, University, or Commercial Weed Sprayer to obtain complete and factual information pertaining to this material before making any decisions regarding weed control or any diagnosis of health problems you believe may pertain to weed poisoning.