AbstractThe effects of two commercial cellulase/hemicellulase enzymes derived fromTrichoderma reeseion silage fermentation and aerobic stability were investigated in three laboratory‐scale experiments. In Experiment 1, perennial ryegrass (Lolium perenne) was treated with enzyme A at the rates of 0. 0·125, 0·250, 0·500 and 0·750 cm3kg‐1. In Experiment 2, Italian ryegrass (Lolium multiflorum) was treated with the same enzyme at the rates of 0, 0·250 and 0·500 cm3kg‐1and with 85% formic acid (3·5 cm3kg‐1). In Experiment 3, perennial ryegrass was ensiled untreated, with enzyme A (0·250 and 0·500 cm3kg‐1) and with 0·200 and 0·400 cm3kg‐1enzyme B which also contained glucose oxidase. All silages were well preserved. Aerobic deterioration was related to numbers of lactate assimilating yeasts. In Experiment 1, all silages were stable over a 4‐day period. In Experiment 2, enzyme A treatment delayed the temperature rise compared with the untreated control (P<0·05), whereas the formic acid‐treated silage was stable. In Experiment 3, the temperature rise in the silage treated with the higher level of enzyme B occurred one day later than in