Sweet clover poisoning occurs when cattle eat spoiled sweet clover.
It causes bleeding problems, which can result in fetal deaths, bleeding under the skin and in joints, and sometimes a fatal anemia.
Coumarin, a normal constituent in sweet clover, causes the problem.
It is harmless on its own but is converted to toxic dicoumarol if sweet clover forage is allowed to spoil with mould growth. Large, round bales left out in adverse weather are most likely to contain dicoumarol in their outer layers.
Dicoumarol interferes with the synthesis of vitamin K, which is needed to produce several essential clotting proteins. These proteins prevent bleeding from minor injuries.
The symptoms of toxicity depend on how much dicoumarol is consumed and the age of the animals.
Cattle can be exposed for months before symptoms appear if just a small amount is consumed. In these cases, symptoms are sometimes never seen.
However, signs can occur in less than a month if large amounts are consumed. As well, younger cattle are more susceptible than older ones to poisoning.
A sudden death could be the first indication of dicoumarol poisoning, but it’s more common for producers to notice that several animals are stiff and lame. This is due to bleeding in joints and muscles.
Swellings can be visible or palpable under the skin, mainly on the hips, brisket and neck. The gums may be pale due to significant blood loss causing anemia.
Blood may be seen in the feces or urine, or it may drip from the nostrils.
Bleeding can be extensive and possibly life threatening during calving.
Dicoumarol can also pass through the fetal membranes and cause bleeding problems in the fetus, which can result in fetal death followed by resorption or stillbirth. Neonatal death is possible if the fetus survives calving.
Sweet clover poisoning is possible when cattle have a steady diet of sweet clover hay or silage.
To diagnose the condition, cattle must also have symptoms compatible with exposure to dicoumarol. A clotting problem can be confirmed with a laboratory test measuring prothombin time.
Sweet clover toxicity is normally a herd problem. This means clotting problems are unlikely to be caused by sweet clover if they are limited to one animal.
The anemia associated with sweet clover poisoning can be corrected by providing transfusions of whole blood.
A 1,000 pound animal requires two to 10 litres . Blood donors must be cattle that aren’t fed sweet clover.
Injections of vitamin K can also
restore blood clotting ability but can take 24 hours or more to act and it is expensive.
Prevention is key with sweet clover poisoning, and variety selection is the place to start.
Yellow clover (M. officinalis) and white clover (M. albus) contain substantial levels of coumarin, while Melilotus dentate was developed with low levels of coumarin and is thus a safer feed.
Another way to avoid toxicity is to bale sweet clover only when it is well cured and dry. As well, avoid tightly bound, large bales.
Not being able to see spoilage is no guarantee it is safe. A chemical test for dicoumarol can be performed if there are any doubts.
If sweet clover forage must be fed and it is suspected of containing dicoumarol, it can be fed for seven to 10 days followed by the same period of another roughage such as alfalfa or grass-legume hay.
This is more effective than continuously feeding a sweet clover-alfalfa mix. It prevents poisoning and development of symptoms, but bleeding times can be prolonged if the feed contains enough dicoumarol.
Cattle that are calving should not eat sweet clover for at least four weeks before parturition. This allows their blood to clot properly after calving.
Similarly, surgery such as castration and dehorning should be avoided until cattle have been on clean feed for a month or more.
Jeff Grognet is a veterinarian practising in Qualicum Beach, B.C.