Four experiments are described which involved: (A) treating straw with urea and/or a cellulase enzyme (FS01 at 21t-1) when ensiling it with grass (60:40 grass:straw on a dry matter (DM) basis); (B) treating straw with urea (at 60 g kg-1 straw DM) at the time of ensiling, or adding urea to grass-straw silage at the time of feeding, and assessing the effects of these treatments on intake and digestibility of nutrients by sheep; (C) ensiling grass and straw either as a fairly homogeneous mixture or in alternate layers; (D) studying changes in straw (treated with urea and/or enzyme) contained in small nylon bags and buried in grass before ensiling. Intake of DM was increased slightly (P>0.05) by treating straw with urea, but by 20% when urea and enzyme were used together. These treatments had only slight impact on DM digestibility except where enzyme and urea were applied together. When the crude protein (CP) content of diets was low (less than 81 g kg-1 DM), urea increased the DM intake. Ensiling straw and grass in alternate layers increased the butyric acid content of silage. Animals ate 22% less DM from the silage made in layers than from that made from thoroughly mixed grass and straw (urea-impregnated straw). The straw composition was modified during fermentation and some loss of modified acid detergent fibre (MADF) from straw occurred during ensiling.