First-harvest, precision-chopped Lolium perenne grass with a dry matter (DM) content of 132·0 g/kg and water-soluble carbohydrate (WSC) and crude protein (CP) contents of 24 g/kg and 106 g/kg DM, respectively, was ensiled in farm scale silos following the addition of formic acid (FA) (2·5 l/t) or molassed sugarbeet pulp (MSBP) (50 kg/t). Mature twin-bearing ewes (n=56) were allocated to one of seven treatments and offered either FA-treated silage or MSBP silage ad libitum. The trial commenced on day 91 of pregnancy, when the ewes were offered FA-treated silage (T1), FA-treated silage+soyabean meal (T2), MSBP silage (T3), MSBP silage+soyabean meal (T4), FA-treated silage+MSBP (T5), FA-treated silage+MSBP+soyabean meal (T6) or FA-treated silage+a barley-based supplement (150 g CP per kg DM) (T7, control). Soyabean meal supplementation was only offered in the last 3 weeks of pregnancy, with the objective of achieving a total CP intake of 220 g/ewe per day. Compared with the FA-treated silage, the MSBP silage had a higher nitrogen content (P<0·01) and a lower neutral detergent fibre (NDF) content, but the two silages did not differ in DM, acid detergent fibre (ADF) or gross energy contents. Silage DM intakes (SDMI) were higher on the MSBP silage-based diets (T3, T4) than for any other treatment (P<0·05). The addition of soyabean meal significantly increased SDMI in T2 in the last week of pregnancy (P<0·05), but there was no response in SDMI to soyabean meal supplementation in any of the other treatments. Ewe liveweight changes (kg) of −0·4, 4·1, 7·8, 10·2, 7·1, 9·6 and 10·8 (S.E.±1·2), body condition score changes of −0·96, −0·59, −0·42, −0·33, −0·37, +0·01 and −0·27 (S.E.±0·1) and total lamb birth weights (kg) of 8·0, 8·7, 9·2, 9·9, 8·6, 8·0 and 10·4 (S.E.±0·5) were recorded for ewes in T1 to T7 respectively.