If you baled sweet clover this year, beware. South Dakota State University and North Dakota State University Extension beef specialists have issued new warnings about sweetclover hay.

If sweetclover wasn’t baled at the right time and managed properly, it can be toxic to cows, say Karl Hoppe, North Dakota State University Extension livestock systems specialist, and Krista Ehlert, South Dakota State University Extension range management specialist.

Cows may bleed internally, drip blood from their noses, abort calves and die suddenly.

You may have no early warning that cows are being poisoned by sweetclover hay because the bleeding is internal. One early tipoff is black, tarry manure. It is caused by internal bleeding into the intestinal tract, Ehlert says.

What happens

Sweetclover contains a substance called coumarin, a compound that gives the plant its sweet smell. When sweetclover is put up too wet, mold grows in the bale and converts coumarin to dicoumarol. Dicoumarol is a blood thinner and will cause hemorrhaging.

Bailing sweetclover in large, dense bales can make the problem worse. The core may be damp, and mold will form.

“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,” Hoppe says. “However, weather rarely cooperates and dicoumarol is usually present.”

Pure stands of sweetclover are riskiest for toxicity simply because the hay is not diluted with other grasses, Hoppe says. The risk also is increased when the plants are mature because the dense stems make drying difficult.

Grass hay that contains some sweetclover presents a danger, too. Sweetclover poisoning may show up unexpectedly.

Visually checking hay for mold won’t tell you if it is safe to feed, Hoppe says. The conversion of coumarin to dicoumarol can occur even if there is only a little mold in the hay.

It doesn’t take much dicoumarol to affect cows. A pregnant cow may bleed internally for months after eating sweetclover for just a couple days, Hoppe says.

What to do

The best thing to do is to test sweetclover hay for dicoumarol. The NDSU Veterinary Diagnostic Laboratory offers the service.

If the hay contains dicoumarol, you might still be able to feed it if you can dilute the hay. 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 for two days, then going three to four days without feeding sweetclover. However, don’t feed sweetclover hay for a month before or during events where bleeding occurs, such as during calving, surgical castration and dehorning.

Grazing sweetclover doesn’t usually cause the cows digestive problems, although bloating is possible, Ehlert says.

Management to prevent bloat should focus on feeding livestock before turning them out to graze so that they are not hungry and thus are less likely to rapidly eat a large amount of sweet clover (or any other legume with bloat potential).

Ruminants can adapt to forage with high bloat potential over a period of several days. One strategy is to pull them off the pasture for part of each day for several days or feed hay on the pasture to limit sweet clover intake until a tolerance is developed.

Poloxalene (Bloatguard) and ionophores (Rumensin) also help prevent bloat. Be sure to start feeding these substances several days before turning the cattle out so they can adapt before consuming sweet clover or other legumes, Ehlert says.

SDSU and NDSU Extension contributed to this article