The effects of oxidants and surfactant additives on silage and forage fermentation were studied. First, the effects of two nonionic surfactants, Tweens 60 and 80, on rumen protease and cellulase activity and in vivo digestibility was investigated. At 0.25% of either Tweens 60 or 80 in the in vitro reaction mixture, both protease and cellulase activity were enhanced (P < .05). The inclusion of 0.5% Tween 80 in the grass hay diet fed to sheep, increased dry matter intake (DMT) by about 40%, and apparent nutrient digestibility by up to 10 percentage units. Hence, the potential benefits of Tween 80 in ruminant forage rations were established. Secondly, the effect of Teepol HB7 on mixed rumen protease and cellulase activity, and digestibility. Up to 0.3% Teepol increased SH masking significantly, without adverse effects on cellulose degradation and microbial protein adsorption. The effective degradation of canola meal protein was decreased (P < .05), while the in vitro degradation of ruminal fluid soluble protein was reduced by more than 40% with the addition of 0.1% Teepol. In vivo rumen ammonia nitrogen concentration also dropped (P < .05). However, DMI and apparent nutrient digestibility were not affected (P < .05). Thirdly, the proteases of orchard grass were characterized according to their mechanistic reactions and pH dependence. The rate of proteolysis (umol glycine equivalent/g/h) declined with increasing acidity (pH 7-4) and ensiling period. Phenylmethanesulfonylfluoride and p-chloromercuribenzoate inhibited proteolysis by up to 90% in the first 5 d after ensiling. It was concluded that orchard grass proteases are primarily thiol in nature. Lastly, the effect of thiol reagents on orchard grass silage fermentation was evaluated on the same material as in the latter experiment. Tween 80 resulted in a rapid decline in pH, but the final TCA-precipitable protein was higher (P < .05) than in the control. The true protein content and pH of the other anionic-surfactant based and KIO₃ treatments was higher (P < .05) than that of the control. It was concluded that Tween 80 could be used to increase silage fermentation and Teepol HB7 or KIO₃ to reduce proteolysis. Keywords: rumen, silage, protein, thiol, surfactant