Clover Causing Sunburn and Liver Problems in Horses
Article adapted from M. Murphy, DVM, PhD and K.Martinson, PhD
University of Minnesota, July 7th, 2012
Clovers are usually considered a beneficial pasture forage. However, alsike clover (Trifolium hybridum) and red clover (Trifolium praetense) are considered toxic when infected with the mold Cymadothea trilolii that causes Black Blotch disease in clovers. When mold-infected clovers are ingested by horses, photosensitivity (sunburn) and liver damage can occur. Sunburn of non-pigmented skin, and also discharge of the mucous membranes of the mouth, eyes, and vulva, are often the first noticeable signs. In more severe cases, signs of liver failure include progressive weight loss, loss of appetite, depression, jaundice, circling, yawning, colic, recumbency, and eventually coma and death. As little as 20% infected alsike clover in the diet may result in poisoning, with clinical signs appearing two to four weeks later.
To determine if your clover has Black Blotch disease, go to the lowest, wettest area of the pasture, and look at the underside of the leaves on the lower 6 inches of the clover. Look for black or brown “blotches”; like a felt tipped marker was blotted on to the leaves. Look for this on all clover species in the pasture.
If you find the “blotches” on clover, rotate the horses off the pasture. If horses are showing signs of sunburn move to a shaded area to reduce the exposure to sun and potential sunburn. Horses will recover if they are kept out of the sun until the liver enzyme profile returns to normal.
Mow the infected pasture or cut it for hay; the mold cannot live on dried hay so hay from an infected pasture will not cause Black Blotch disease. The clover will generally grow back without the mold if environmental conditions are drier and warmer and you can begin grazing again.
Research has shown that alsike clover not infected with mold did not cause toxicity, whereas alsike clover infected with the mold did. However, this research has not been done with horses. In our experience, mold infected clover has been present in pastures of horses with photosensitivity, and we have not seen photosensitive horses in pastures of clover or alfalfa uninfected with the mold.
If the problems persist, because clovers tend to grow in bunches that promote moisture retention (most molds thrive in humid conditions), you may want to consider reducing or eliminating the clover from your pasture. Broadleaf herbicides (i.e. 2, 4-D) will moderately control clovers and could be used to thin stands. All herbicides have grazing restrictions so it’s important to read the herbicide label and follow all directions. Fertilizing a grass/clover pasture with nitrogen will also help the grasses better compete with the clovers and become more dominant in the pasture. Grasses need “applied” nitrogen while legumes (i.e. clovers and alfalfa) can fix it from the environment. Mowing and other practices to increase air flow to the pasture should also be done to reduce the amount of moisture retained in the thick stands of clover.