Five essential oil compounds in four doses (0, 50, 500, and 2000 mg kg-1 of fresh forage) were sprayed on ryegrass forage before ensiling. Thymol, eugenol, and cinnamaldehyde at 2000 mg and carvacrol at 500 and 2000 mg inhibited deamination. Cinnamaldehyde at 2000 mg resulted in silages with 9.8% higher true protein.