Q: I have two clients whose horses are having liver problems. Some of the light colored horses also have signs of sunburn. Their pastures are filled with alsike clover. Is the alsike clover causing this?
A: Clovers (i.e. red, alsike, and white clover) are usually considered a beneficial pasture forage. However, alsike (and other clovers) is considered toxic when infected with the mold Cymodothea trifolii that causes Black Blotch disease in clovers and legumes (i.e. alfalfa). When mold infected clovers are ingested by horses, photosensitivity (sunburn) and liver damage can occur.
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", rotate the horses off the pasture to a shaded area to reduce the exposure to sun and potential sunburn, mow the pasture or cut it for hay; the mold can not live on dried hay. After the pasture has had a chance to regrow and dry out, you can begin grazing again.
There is research being done on rabbits in Canada demonstrating 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.
Because clovers tend to grow in thick bunches that promote moisture retention (most molds thrive in humid conditions), if the problems persist, 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 do 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. Grasses need "applied" nitrogen while legumes (i.e. clovers and alfalfa) can fix it from the environment. Not fertilizing a grass-legume mixed pasture will heavily favor the legume species and over time, the legumes will become the dominant species in the pasture. 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.
By M. Murphy, DVM, PhD; and K. Martinson, PhD; University of Minnesota