Know What You Grow: Clover Toxicity and Horses

Taylor Fabus, Michigan State University Extension, and Lisa Skylis, MSU animal science student – Originally published August 25, 2017, Revised July 2021

Learn more about how clover affects your horse’s health, how to identify clover and manage its growth.


Horse grazing on clover.


Fresh summer pastures can provide your wallet some temporary relief with lower feed and hay bills. However, parts of the northwest are experiencing rain and humidity, providing the perfect breeding ground for clover in pastures to develop a fungus which in turn can cause slobbers in horses.


The fungus Rhizoctonia leguminicola, more commonly known as black patch for the bronze to black spots it creates on the stems and leaves, is usually found on white or red clover. While the clover plants themselves are non-toxic and nutritious, the fungus contains the toxin slaframine that causes undesirable symptoms in horses.


White Clover Source: Chris Evans, River to River CWMA, Bugwood.org


Red Clover Source: Richard Old, XID Services, Inc., Bugwood.org


Slaframine is known to stimulate the salivary glands of a horse and lead to an excessive amount of drooling, or “slobbers.” While it is a nuisance and rather unpleasant, the condition is not life threatening. The toxin slaframine can be found in pasture or dried hay. Once identified, the fungus can be effectively eliminated by removing the horses from the infected pastures and using a pasture-friendly broad leaf herbicide to eliminate any remaining broadleaf plants, such as clover.


Slobbers in horses is caused by the fungus Rhizoctonia leguminicola that can affect legume plants like clover and alfalfa. Source: Steven S. Nicholson, DVM , LSU - http://www.omafra.gov.on.ca/english/livestock/horses/facts/info_slobbers.htm


Another clover likely to grow in your overgrazed pastures in the fall is alsike clover. Although similar looking to other clovers, alsike clover can be identified by its flowers, which are dark pink at the base and light pink towards the tip. Unlike the relatively non-toxic red and white clovers, alsike clover is known to cause two more serious conditions in horses: photosensitization and big liver syndrome.


Alsike clover is more prevalent in overgrazed pastures. Source: Richard Old, XID Services, Inc., Bugwood.org


The toxin in alsike clover that causes these two severe conditions remains unidentified. Photosensitization occurs with short-term exposure and clinically appears to look like sunburn that becomes crusty, dies and may slough off entirely. Horses that remain inside for the day or are blanketed while outside may not show these more typical symptoms and, in these cases, photosensitization can present as oral lesions, diarrhea and even colic.


Long-term exposure to the alsike clover may lead to big liver syndrome, which is the progressive destruction of the liver. Symptoms of liver failure due to alsike clover consumption include loss of appetite, weight loss, depression, jaundice, colic and death. It goes without saying that if you observe any of these changes in your horse, contact a veterinarian immediately because liver failure is often fatal.


Overgrazed pastures and rainy and humid weather conditions can contribute to changes in pasture vegetation, and you should always be on the lookout for significant clover growth. Pasture management plans should be reviewed annually, especially if you continue to notice a problem with toxic plant growth. The easiest way to prevent your horses from consuming toxic plants is to know what you grow and ensure they have plenty of nutritious options so they will avoid toxic plants.


If you are unsure of a plant species, Michigan State University Extension suggests you bag it and use the following free guides, with pictures, to help you with identification:

This article was originally published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).