Toxic Plants and Grasses for Horses in Colorado

Toxic Plants and Grasses for Horses in Colorado

Posted by Brian Burke on Monday, June 12th, 2017 at 12:06pm.

Riders Beware: A Guide to Toxic Plants and Grasses for Horses in Colorado

There are hundreds of different types of plants and grasses in Colorado and across the rest of America, which serve to provide a gastronomic feast for our equine companions. Whilst the vast majority of these plants are safe for horses to eat, some of them are not, and it is essential that all horse owners are aware of the toxic plants in Colorado that they and their horses should avoid. Whilst some of these plants may well leave your horse with an upset stomach, some are so toxic that they could lead to a risk of permanent damage or even death. This is why its so important to be safe and aware of what they are eating when you are riding outside with your horses and allowing them to eat from the land.

Not sure which plants and grasses are safe for your horses to eat and which are definitely not? Here is a quick and straightforward guide to the three most toxic plants in Colorado:

Bracken fern

Bracken fern is a moist green plant that grows in woodlands throughout the country. It is dangerous to horses because it contains high levels of thiaminase, a chemical which inhibits absorption of the essential thiamine found within vitamin B1. This is a concern because thiamine is essential in helping nerves to function properly-without it your horse could experience a significant neurological impairment. Whilst it would be incorrect to say that bracken fern could cause immediate brain damage, repeated and unchecked consumption could lead to serious mental health concerns. If you notice your horse seeking out this plant it is important to prevent their access to it immediately: horses seem to develop a taste for this unique plant and, once they have tried it, they will keep seeking it out. If you discover your horse has been eating bracken fern then large doses of thiamin over the course of a week or two can help to reverse any damage.


Hemlock is a multi-stemmed plant covered in small white flowers which is well known for its poisonous properties. Every part of the plant is dangerous: its leaves, flowers and even its seeds. When consumed by horses, hemlock can cause damage to the central and peripheral nervous systems. Most animals are put off by the smell of hemlock and won’t eat it, but if yours does then four to five pounds of the weed is the lethal dose. Symptoms of hemlock poisoning include nervousness, tremors and incoordination. This will then lead to a diminished heart rate and problematic breathing until death is ultimately caused by respiratory failure. There is no cure for the condition.

Yellow Star Thistle

Yellow Star Thistle has a distinctive appearance, thanks to the vibrant yellow flower that stands on the end of a stiff long stem. This is good news, as it means it should be easier to identify that your horse has begin to eat the weed and prevent them from consuming more of it immediately. Yellow star thistle contains a toxin which can have a negative effect on the brain by inhibiting the nerves that control the chewing reflex. Horses must consume a significant quantity of the toxin to be poisoned by it: 50 to 200 percent of their body weight over 30 to 90 days, however once they are displaying symptoms of yellow star thistle poisoning there is no cure. Horses who are affected will have tense and clenched facial muscles and will be unable to effectively chew their food, which can lead to weight loss. Unfortunately euthanasia is the only recommended course of action if the horse is too sick to eat meaning that, if yellow star thistles are abundant near your home, attentive horse care is the best course of action to prevent its consumption.


Oleander is a beautiful perennial shrub with thick leathery leaves and attractive flowers which grow in red, pink, or white. This is a plant that is grown in abundance in Colorado and, despite its pretty appearance, it is incredibly dangerous to horses. Oleander is dangerous because all parts of the plant contain the toxins oleandrin and neriin, which disrupt the beating of the heart. This is a plant which is deadly in small quantities and only 30 to 40 leaves are needed to kill a horse by causing an irregular heart beat until cardiac arrest occurs. Horses can survive if treated early with anti-arrhythmic drugs to stabilize the heart and charcoal to prevent the further absorption of the toxin, but in order for this treatment to be a success it is essential that it is sought immediately.

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