Both internal and external bleeding may occur in livestock as a result of the presence in their blood of the anticoagulant dicoumarol. Dicoumarol is produced from coumarin, a harmless substance found in sweetclover, which breaks down if overheating or spoilage takes place during hay or silage making. Coumarin, in turn, is not normally present in the living sweetclover plant, but is produced as a result of freezing, drying, or, most usually, maceration (as is hydrogen cyanide in other species). Coumarin has a sweet smell, which will be familiar to anyone acquainted with new-mown sweetclover hay. However, it has bitter flavor. This may well confer on sweetclover an advantage which has been developed through many generations of natural selection. Insects dislike the bitter taste of coumarin and reject sweetclover in favor of other plants. Unfortunately, livestock palatability is reduced by this substance, as well.
Spoilage is very likely to occur in making sweetclover hay, since the plant's leaves are small and fine and dry quickly, while the stems are thick and retain moisture. In an attempt to retain a high proportion of leaves, sweetclover hay is often stacked before the stems are fully dry. Chopping, which is a feature of many modern hay-making systems and a common practice in silage making helps to overcome this problem.
As in the case of plants containing cyanogenetic compounds, the genetic control of the presence of plant substances from which dicoumarol might be produced is simple. It has been possible to breed cultivars in which dicoumarol production is negligible. Such cultivars are, however, low yielding.