Which plant's spoiled hay led to the discovery of the anticoagulant warfarin?

The plant whose spoiled hay led to the discovery of the anticoagulant warfarin was sweet clover.

In the 1920s, cattle in northern North America began bleeding uncontrollably after eating moldy or spoiled sweet-clover hay. Canadian veterinarian Frank Schofield showed that the damaged hay—not good hay from the same mow—caused the hemorrhagic disease. The clue eventually led researchers to a compound formed when fungi acted on coumarin in sweet clover.

Karl Paul Link’s University of Wisconsin team isolated and identified dicoumarol in 1940. Dicoumarol became the first important drug in a family of 4-hydroxycoumarin anticoagulants. Link’s later work produced warfarin, first introduced as a rodenticide in 1948 and approved for human medical use in 1954.

Strictly speaking, spoiled sweet clover directly revealed dicoumarol, the precursor and prototype that led to warfarin. That distinction explains why some historical accounts call the event the discovery of the “sweet-clover disease” or dicoumarol rather than warfarin itself. Warfarin works by interfering with vitamin K recycling.