Be careful if you are putting up hay with sweet clover in it or are allowing livestock to grass pastures that contain sweet clover.
Roger Gates, South Dakota State University (SDSU) extension range specialist, explains why:
Sweet clover contains the compound coumarin. If sweetclover becomes moldy, coumarin converts to dicoumarol, which interferes with blood clotting. Excessive bleeding may result in livestock, including horses.
Molds can colonize the stem of sweetclover. Even if you do not see it, there still may be mold present. Mold may multiply if the stem is damaged from trampling by cattle when grazing. While the risk of poisoning to grazing cattle is small, cattle may not readily graze sweetclover due to the bitter taste of coumarin.
High moisture content makes managing sweetclover as a hay or silage more of a challenge. If hayed too wet, mold can form. If sweetclover is to be utilized as hay, sufficient dry down time is necessary. Sweetclover hay needs to be baled at less than 18% moisture to minimize the chance for mold growth.
Moldy sweetclover silage will also produce dicoumarol. It is critical that sweetclover is chopped fine to aid packing and exclude air. It is also important that the silage be covered to exclude outside air from entering the silage mass.
--Source: SDSU AgBio Communications