Vet Advice: Avoiding sweet clover poisoning

A variety of bacteria and moulds can grow in sweet clover once baled or put up as silage

Sweet clover is high in coumarin, which converts to dicoumarol – a potent vitamin K antagonist and anticoagulant– if the plant is spoiled or damaged.

Preparing forages and getting them stored in perfect condition seldom happens. Spoilage is often linked to the production of moulds and a broad spectrum of mycotoxins in grains. Syndromes in domestic livestock following consumption of feed containing mycotoxins varies depending on the species of animal involved, the stage of the production cycle when it is introduced, and the level consumed. With sweet clover hay and silage, spoilage presents a serious risk to livestock. Few large animal practitioners in Western Canada do not have a tragic story about sweet clover poisoning. Horses are particularly sensitive to mycotoxins. Leukoencephalomalacia (polio), aflatoxicosis, ergotism, fescue toxicity, slobbering disease, ryegrass staggers and mouldy sweet clover poisoning are all syndromes seen in horses exposed to mouldy feed.

The common spoilage organisms affecting sweet clover are soilborne. There are a variety of bacteria and moulds that grow in sweet clover when it is baled or put up as silage. Damp, unfavourable conditions, or failure to pack silage properly to exclude air promote spoilage organism growth. Weathered, large round bales, particularly the outer portions, usually contain the highest concentrations of dicoumarol.

The sweet clover plant is high in the compound coumarin, which is converted to dicoumarol if the plant is spoiled or damaged. Dicoumarol is a potent vitamin K antagonist and anticoagulant preventing normal blood clotting resulting in hemorrhages and associated symptoms. Vitamin K is needed as part of the blood clotting process. Dicoumarol concentrations of 20 to 30 mg/kg of hay ingested throughout several weeks are usually required to cause poisoning in cattle. The toxic agent crosses the placenta in pregnant animals, and newborn animals may be affected at birth. All species of animals studied are susceptible, but instances of poisoning involve cattle and, to a limited extent, sheep, pigs, and horses.

When vitamin K is inhibited, cattle can essentially bleed to death internally. Warfarin, which is found in rat, gopher and ground squirrel poisons, works in much the same way.

Symptoms include stiffness, lameness, dull attitude and hematomas or blood clots beneath the skin, particularly around the hips, brisket and neck. Mucous membranes can turn pale, indicating anemia. The toxin also can cause reproductive problems at sub-clinical levels. Poisoning occurs less often in silage than in hay and infrequently in pastured animals. The time between consumption of toxic sweet clover and appearance of clinical disease varies greatly and depends on the dicoumarol content of the particular sweet clover variety being fed, age of the animals and amount of feed consumed.

If the dicoumarol content of the ration is low or variable, animals may consume it for months before signs of disease appear. In affected animals, the first signs may be stiffness and lameness due to bleeding into the muscles and joints. It is not uncommon for producers to first become aware that sweet clover toxicity is an issue after cattle are dehorned, castrated, or at calving time when, unfortunately, animals succumb to hemorrhage.

This biennial plant has really expressed itself this year, and is abundant, due to the excellent moisture conditions in some areas. As a grazing resource, sweet clover can be excellent feed. Research from North Dakota State University has documented yearlings gaining over two pounds per head per day grazing sweet clover pasture.

Producers need to be very cautious when feeding sweet clover that is spoiled or mouldy. Here are some preventative steps:

  • Test suspect hay for dicoumarol. Your veterinarian should be consulted.
  • Plant only low-coumarin-content sweet clover (Melilotus dentata). Avoid contamination of pastures or hayfield with yellow (M. officinalis) or white (M. albus) sweet clovers, which have high levels of coumarin.
  • Sweet clover is a legume so it can also cause bloat. Most of the time, sweet clover is a part of a diversity of grasses, sedges, legumes and forbs on rangeland or pasture and doesn’t result in bloat (University of Nebraska, Lincoln). Providing a supplement with an ionophore such as Rumensin as well as the use of poloxalene (Bloatguard) several days before turning cattle into pasture with legumes can help reduce the risk of bloat.
  • Stack or bale sweet clover only when it is well cured and dry. Sweet clover stems can be quite large and hard to dry. Tight, dense bales that are damp can make the problem worse as mould formation will be greater at the core of the bale.
  • If you suspect hay could be toxic, feed it for seven to 10 days, then replace it with alfalfa or other forage for an equal period of time.
  • Do not feed sweet clover for at least three weeks before or during the calving period, or before any surgical procedures like castration or dehorning.

Unfortunately, it doesn’t take much dicou­marol to cause a problem. Hay containing greater than 10 parts per million of dicoumarol should be fed with caution. All cattle are susceptible, but younger cattle seem to be more susceptible than mature cows. Pregnant cows may abort or give birth to stillborn calves if they are consuming mouldy sweet clover hay.

Intravenous administration of whole blood is the best and most effective treatment. This may be difficult in large animals, because effective dosages range from two to 10 litres of fresh blood per 1,000 lbs. (450 kg) body weight. Care should be taken to ensure that donor animals are not receiving sweet clover feed. All clinically affected animals should receive a transfusion, which can be repeated if necessary. In addition, all severely affected animals should receive parenteral administration of synthetic vitamin K1 (phytonadione). Although more costly, vitamin K1 is more effective than K3 (menadione). Coagulation can be restored in about 24 hours.

Sweet clover is an opportunistic plant that takes advantage of good growing conditions when available. In grazing situations, clover is a good feed resource. In the feed yard or silage pit, be careful.

About the author


Dr. Ron Clarke

Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]).

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