AbstractFour grass silages were made from perennial ryegrass ensiled after a 1h wilt in 2‐t silos without additive application, with application of formic acid or with one of two enzyme mixtures of hemicellulases and cellulases (enzyme 1 and enzyme 2). Effluent losses were monitored over the ensiling period (130 d).Analyses of the silage showed that formic acid‐treated silage had lower concentrations of lactic acid than the other silages. Both enzyme‐treated silages had lower levels of cellulose, acid detergent fibre (ADF) and neutral‐detergent fibre (NDF) than the untreated and formic acid treated silages. Effluent production was highest with enzyme‐treated silages.The silages were subsequently fed to growing steers equipped with rumen cannulae and T‐piece duodenal cannulae. Apparent whole‐tract digestibilities of organic matter constituents were significantly lower (P<0·05) with both enzyme‐treated silages (untreated; 0·736, formic acid; 0·722, enzyme 1; 0·694, enzyme 2; 0·703). Both untreated and enzyme 2‐treated silages sustained higher nitrogen digestibilities (g g−1intake) (untreated; 0·675, formic acid; 0·636, enzyme 1; 0·630, enzyme 2; 0·662) and N retentions (g d−1) untreated; 16·0, formic acid; 14·0, enzyme 1; 11·6, enzyme 2; 16·6), but none of these differences was significant. When formic acid‐treated silage was offered, there was a greater amount of organic matter apparently digested in the rumen (ADOMR). Non‐ammonia nitrogen and microbial nitrogen flows at the duodenum were similar on all diets. The efficiency of microbial protein synthesis was highest with enzyme 2‐treated silage and lowest with formic acid‐treated silage (untreated, 35·4; formic acid, 25·2; enzyme 1, 30·4; enzyme 2, 39·4),