AbstractFirst‐harvest direct‐cut, double‐chopped grass (190 and 164g DMkg−1in Experiments 1 and 2 resptectively) was ensiled without an additive or, in Experiment 1, with 30 kg t−1grass of an absorbent additive based on sugar beet pulp (Sweet ‘n’ Dry) or with 3·441 t−1grass of formic acid and, in Experiment 2, with 30, 50 and 70 kg t−1grass of Sweet ‘n’ Dry or with 50kg t−1grass of unmolassed sugar beet pulp. The preservation and nutritive value of the silage, in‐silo losses (including silage effluent production), silage intake and animal performance of adult and growing cattle were examined.In Experiment 1 all three silages were well preserved, although the formic acid‐treated silage displayed significantly lower pH, ammonia nitrogen (NH3N) [g kg−1total nitrogen (TN)] and volatile fatty acids (VFAs) than the other two silages. In Experiment 2 absorbent‐treated silages displayed significantly lower pH, buffer capacity (Bc), NH3N (gkg−1TN), CP, modified acid detergent fibre (MADF) and VFAs than untreated silage.Treatment of grass with the absorbent additives at ensiling resulted in reduced effluent production. In Experiment 1 each kilogram of Sweet ‘n’ Dry retained approximately 11 effluent, and in Experiment 2 silages made with Sweet ‘n’ Dry applied at 70kgt−1and sugar beet pulp applied at 50 kg t−1produced similar volumes of effluent and each kilogram of absorbent retained 1·0 and 1·31 of effluent respectively.In Experiment 1 sixty beef cattle [mean initial live weight (LW) 460 kg] were grouped according to LW and allocated to treatment at random. For untreated silage (unsupplemented or with 1 or 2 kg supplement head−1day−1), absorbent‐treated silage (unsupplemented or with 1 or 2 kg supplement head−1day−1) and formic acid‐treated silage (1 kg supplement head−1day−1) the daily silage DM intakes were 6·12, 6·21, 6·40, 7·65, 7·45, 7·11 and 7·85 (s.e. 0·280) kg respectively, the daily liveweight gains were 0·22, 0·56, 0·81, 0·59, 0·74, 0·81 and 0·75 (s.e. 0·071) kg respectively and daily carcass gains were 0·31, 0·47, 0·67, 0·47, 0·61, 0·70 and 0·57 (s.e. 0·043) kg respectively throughout a 75‐day feeding period.In Experiment 2, fifty‐six growing cattle (mean initial weight 312 kg) were grouped according to LW and allocated to treatment at random. For untreated silage (unsupplemented or with 1·5 kg Sweet ‘n’ Dry or 1·5 kg commercial concentrates head−1day−1), silage treated with Sweet ‘n’ Dry at 30, 50 and 70 kg t−1grass and silage treated with 50kg sugar beet pulp t−1grass the daily silage DM intakes were 5·46, 5·28, 5·33, 6·21, 6·27, 6·60 and 6·62 (s.e. 0·154) kg respectively and daily liveweight gains were 0·39, 0·75, 0·81, 0·63, 0·76, 0·94 and 1·75 (s.e. 0·052) kg respectively throughout a 122‐day feeding period. In this experiment 360g kg−1more absorbent was required when it