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Sweetclover Hay Can be Toxic

August 11, 2020

Sweetclover can provide good nutrition to cattle because it is high in protein and energy when not mature.

However, sweetclover can become toxic to cattle if fed as hay, North Dakota State University Extension livestock systems specialist Karl Hoppe cautions.

Sweetclover is a biennial legume that lives for two years. It is a prolific seed producer because the plant will die after producing seed during the second year. New sweetclover plants must grow from seed.

The wet fall conditions of 2019 in many parts of the state created the perfect conditions for the first year’s growth of sweetclover. As a result, the easily recognizable yellow or white blossoms of sweetclover are a common sight this growing season. Without the blossoms, sweetclover leaves look similar to those of alfalfa, except sweetclover leaves are serrated around the entire leaf edge, whereas alfalfa leaves are only serrated at the tips.

Sweetclover grows rapidly, and the best time to hay it is early in the growing season when the plant is short, according to Hoppe, who is based at NDSU’s Carrington Research Extension Center. Sweetclover matures quickly, becoming tall and stemmy. The stem is hard and has low palatability, so cattle will not readily consume it at this stage.

Grazing sweetclover in pastures doesn’t usually cause digestive problems, although the possibility of bloat can occur.

Sweetclover contains a substance called coumarin when sweetclover is baled too wet. Mold can grow and convert coumarin into dicoumarol. Dicoumarol is a blood thinner (anticlotting agent) and will cause hemorrhaging. Simple bruises turn into large hematomas (large bulges underneath the skin that are filled with blood and fluid).

At higher concentrations of dicoumarol in the feed, cows can abort, blood can drip from the nostrils and/or sudden death may occur. The toxic effect may last for a month in a pregnant cow even after feeding toxic hay for just a few days.

Visual observation of mold in the hay bale is not a good indicator of toxicity. Small amounts of mold can result in toxicity. Testing for dicoumarol concentration in hay is available at the NDSU Veterinary Diagnostic Laboratory (

When sweetclover haying conditions allow for a quick dry-down with no rain or dew, and hay is stored away from moisture, coumarin does not get converted to dicoumarol, so toxicity should not be an issue.

“However, weather rarely cooperates and dicoumarol is usually present,” Hoppe says. “Pure stands of sweetclover are at most risk for toxicity simply because the hay is not diluted with other grasses. The risk also is increased when the plants are mature because the dense stems make drying difficult.”

Producers should pay close attention to grass hay with some sweetclover present because sweetclover poisoning may show up unexpectedly. A good rule of thumb is to test all hay that contains sweetclover for dicoumarol content.

Dilution is the way to feed cattle to avoid sweetclover poisoning. This can be accomplished by mixing the toxic hay with nontoxic hay. The amount of dilution depends on the concentration of dicoumarol and symptoms on the cattle.

Hay also can be fed on an alternating schedule, such as feeding hay containing sweetclover hay for two days, then going three to four days without feeding sweetclover. Don’t feed sweetclover hay for a month before or during events where bleeding occurs, such as during calving, surgical castration and dehorning.

If sweetclover is ensiled correctly and covered or put up as a baleage, then dicoumaral should not be present. However, incorrect moisture levels, inadequate packing and failure to cover the sweetclover will lead to molding and toxicity.

Sweetclover can provide good nutrition to cattle when managed properly to control potential toxicities. Testing and knowing the dicoumaral level is critical to managing this feed source safely to prevent poisoning. Be sure to document the storage location of bale lots containing sweetclover and the dicoumaral levels to prevent poisoning.

Source: North Dakota State University