Epiphytic microbial populations, reducing sugar (RS) concentrations and in vitro fermentation were measured in unsterilised and sterilised barley silage and corn silage treated with an exogenous enzyme preparation. Enzyme treatments comprised spraying silages (10 ml kg−1 DM) with a xylanase/β‐glucanase enzyme preparation or with water or autoclaved enzyme (controls). In Experiment 1, autoclaving the silages increased (P < 0.05) concentrations of RS present 24 h after treatment. Enzymes increased (P < 0.05) RS in both fresh and autoclaved silages, more so (P < 0.05) with autoclaving than without. Autoclaved enzymes did not affect RS concentrations relative to the (water) controls. Aerobic exposure of corn silage numerically increased microbial numbers; no microbial viability was detected in autoclaved silages. Enzymes increased (P < 0.05) total bacterial counts in all silage types, and this effect was more pronounced on aerobically exposed silage. In Experiment 2, RS concentrations were determined 0 and 24 h after enzyme treatment of silages unsterilised or sterilised with ethylene oxide (EO). Irrespective of EO or silage type, RS concentrations were higher (P < 0.05) 24 h after enzyme treatment than immediately after (0 h). In enzyme‐treated silages, 24 h RS concentrations were higher (P < 0.05) with EO than without. Reducing epiphytic microbiota by sterilisation conserved enzyme‐liberated RS in the silages. Aerobic exposure before applying enzymes enhanced RS‐associated increases in microbial numbers. Enzymes effectively released RS from silages, but utilisation of the RS by inherent epiphytic micro‐organisms may limit their availability to the ruminant. By releasing RS, exogenous enzymes may encourage silage deterioration if the interval between treatment and feeding is excessive. During in vitro incubation of freeze‐dried silages (Experiment 3), incorporation of 15N into microbial N was decreased by enzyme applied to fresh silage (before drying), but increased by enzyme applied after the silage had been dried. Moisture level of feeds and epiphytic microbial populations may influence the effectiveness of enzyme treatment for enhancing utilisation of feeds by ruminants.