Ten Hereford ✕ Friesian steers prepared with rumen, duodenal and ileal cannulae were offered ad libitum access to either grass (G), white clover (W) or red clover (R) silage or mixtures of the grass silage with the clover silages (GW and GR; 60: 40 dry matter (DM) basis). The experiment was conducted as a two-period change-over design within each clover treatment, with grass silage as an experimental control. The clover silages had higher concentrations of DM, nitrogen (N), and total fatty acids and lower concentrations of fibre (P < 0·05) than the grass silage. Rumen fermentation characteristics were similar between treatments apart from a slight elevation of n-butyric acid levels with white clover silage diets (P < 0·05). Duodenal flows of DM, organic matter (OM), total and microbial nitrogen (MN) were significantly higher (P < 0·05) with the white clover silage treatments compared with the grass silage diet. However there were no significant differences in the efficiency of microbial protein synthesis with a mean value of 27·1 g MN per kg OM apparently digested in the rumen. Duodenal flows of total fatty acids per unit intake were not significantly different, although there was a trend (P < 0·1) for increased flows with the white clover silage diets. Duodenal flows of cis-vaccenic acid (P < 0·01), linoleic acid (P < 0·05), α-linolenic acid (P < 0·05) and cis-9 trans-11 conjugated linoleic acid (CLA) (P < 0·05) were higher for the clover silages, particularly white clover silage. The increased flows of α-linolenic and linoleic acids remained significant after correcting for differences in DM intake. Biohydrogenation of linoleic (mean = 0·83) and α-linolenic (mean = 0·86) acids was extensive for all diets but the latter was significantly lower (P < 0·01) for the diets based on red clover silage. These results suggest potential for modifying the fatty acid composition of ruminant products by feeding clover silages.