Sweet clover can provide abundant and nutritious feed value to cattle diets but the potential for toxicity is far less sweet. Bloat is a common side effect of grazing too much sweet clover, but wet and moldy conditions can provide the perfect environment for poisoning.

Sweet clover hay has varying amounts of a substance called coumarin. When confronted with moldy or damaged clover, coumarin is converted to a toxic substance called dicoumarol, according to a publication produced by the University of Wisconsin Extension (UW).

Dicoumarol presents a risk because it is an anti-clotting agent that will result in livestock hemorrhaging. The substance interferes with vitamin K metabolism and synthesis, which assists in blood clotting. This same vitamin K interference is what causes death of rodents after ingesting rodenticide, commonly referred to as rat poison.

“I have had inquiries about potential toxicity of moldy sweet clover and recommendations on how to feed that forage,” Michelle Mostrom, DVM, toxicologist at North Dakota State University told WLJ, “but to date no positive dicoumarol concentrations in forage this spring [or] summer.”

However, Mostrom said she expects labs to have more testing requests this fall and winter with cases of potentially moldy sweet clover hay.

Not all moldy sweet clover will be toxic, but the absence of mold doesn’t necessarily guarantee the absence of dicoumarol either. Poisoning tends to occur less often in silage than hay, and infrequently in pasture. Toxic levels could remain for several years in storage, but if an animal consumes sweet clover for three weeks with no signs of toxicity, the chances are unlikely.

Ruminant animals are usually more prone to sweet clover toxicity than others, and mature animals have a greater resistance. Species more selective in forage choice such as horses and swine are less likely to experience toxic symptoms.

Bleeding caused by sweet clover toxicity happens internally, so it may be difficult to catch at first. South Dakota State University Extension (SDSU) recommends keeping a lookout for black, tarry looking manure. Preliminary symptoms may include stiffness, lameness, dull attitude, and blood clots under the skin over the body, according to UW.

Hemorrhaging will decrease the amount of oxygen brought to the lungs, which will result in respiratory stress. There may also be blood present in feces or urine. Dicoumarol can be transferred through the placenta, causing hemorrhaging in a fetus, which can result in an abortion or stillbirth.

In some cases, a cow may die before giving any visible symptoms.

Treatment, prevention, and management

Animals found exhibiting symptoms of sweet clover toxicity may be saved if caught early enough. A direct blood transfusion from an animal with no trace of toxicity and an intramuscular administration of vitamin K will help counteract the effects of the dicoumarol. Animals should be removed from the sweet clover and be given alfalfa hay that is high in vitamin K and calcium.

UW recommends caution when feeding hay that may be spoiled or moldy. Toxicity may be avoided by planting varieties of sweet clover that has low levels of coumarin.

An issue this year for producers has been the frequent rains that have made it difficult to cure hay adequately for baling, said SDSU. Waiting for a break in the wet weather has led to more mature sweet clover and challenges in drying the plant for hay.

With consistent wet conditions, bale silage may be the preferred route to go to preserve the forage from mold growth. Cold temperatures in the winter should limit mold growth when feeding the bales.

If sweet clover hay is suspected of being toxic, the hay may be fed for seven to 10 days and then replaced with high quality alfalfa hay. The intermittent feeding of alfalfa appears to neutralize the toxicity of the sweet clover hay. Sweet clover should not be fed for two weeks before or after calving. — Anna Miller, WLJ editor

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