In Experiment A, perennial ryegrass (Lolium perenne) and a mixture of perennial ryegrass and white clover (Trifolium repens) (GL), were ensiled in laboratory silos after mixing with shredded barley straw in various proportions (straw, 0, 20 and 40% of dry matter) with or without a cellulase enzyme preparation. In Experiment B, chopped perennial ryegrass was mixed with straw [60:40 on a dry-matter (DM) basis], with or without fertilizer grade urea (2% of straw DM) and enzyme. The GL silages were lower in lactic acid content and higher in pH and ammonia nitrogen than the grass silages. Straw decreased the contents of crude protein, soluble carbohydrate and in vitro digestible organic matter and increased modified acid detergent fibre (MADF) content of silages. Straw also changed the composition of fermentation products by decreasing the ratio between lactic acid and acetic acid in the silage. Urea treatment increased lactic acid production and decreased MADF and hemicellulose. Enzyme treatment had no significant effect on the quality characteristics of either type of silage.