Three methods for preventing the occurrence of toxic dicoumarol concentrations [> 20 mg kg −1 dry matter (DM)] in sweetclover [ Melilotus officinalis (L.) Lam. and M. alba Desr.] hay were tested. In Method 1, the high coumarin (HC) cultivars ‘Madrid’, ‘Goldtop’, ‘Yukon’ and ‘Arctic’ and the low-coumarin (LC) cultivars ‘Norgold’ and ‘Denta’ were evaluated for dicoumarol production in artificially spoiled hay in 1982 and for DM yield in 1981 and 1982. The HC cultivars averaged 4567 kg DM ha −1 over the 2 years, while LC cultivars averaged 3996 kg DM ha −1. Spoiled hay from HC cultivars averaged 91 mg dicoumarol kg −1 DM, while spoiled hay from LC cultivars had only 5 mg kg −1. In Method 2, propionic acid (PA), sodium diacetate (SD), NaCl and Hay Treet (HT; a commercial preservative) were compared in two trials as preservatives of sweetclover hay at 266 g kg −1 moisture concentration. Hay Treet was also compared as a preservative of sweetclover hay re-wetted to 400 g kg −1 moisture concentration in a third trial. Propionic acid prevented dicoumarol formation and heating in re-wetted hay while SD, NaCl and HT did not. Hay Treet at 100 g kg −1 prevented heating and dicoumarol formation in hay re-wetted to 400 g kg −1 moisture, but HT at 3 and 10 g kg −1 did not. In the last experiment, sweetclover hay was treated with NH 4OH or anhydrous ammonia in an attempt to de-toxify the hay. Neither treatment significantly reduced dicoumarol concentrations.