Between weeks 3 and 18 of lactation, 141 British Friesians all received concentrate (metabolizable energy (ME) 12·9 MJ/kg dry matter (DM), 202 g crude protein (CP)/kg DM) at 8·0 kg fresh weight/day. In addition, cows received ad libitum either grass silage (ME 11·7 MJ/kg toluene DM; 219 gCP/kg DM) or a mixture containing grass silage and 15, 25 or 40% (DM basis) of winter barley straw (cv. Igri) either untreated (UTS) or treated (TS) with 4% NaOH. Forage treatments were imposed at either week 3 or week 9 of lactation.
The mean forage DM intake of cows receiving grass silage only was 8·60 kg/day. When 15, 25 and 40% UTS was introduced into the rations at week 3 or 9 of lactation, forage DM intakes were 884, 8·43, 7·62 and 8·63, 8·83, 819 kg/day, respectively; the corresponding values for TS were 8·88, 855, 8·99 and 8·97, 8·98, 9·35 kg7sol;day. The milk yield of cows receiving grass silage only was 22·1 kg/day. When 15, 25 and 40% UTS was introduced into the rations at week 3 or 9 of lactation, milk yields were 21·1 kg/3, 20·1 kg/1, 20·1 kg/5 and 20·1 kg/1, 21·4, 20·5 kg/day, respectively; the corresponding values for TS were 21·8, 22·1, 21·3 and 22·5, 21·4, 22·6 kg/day. When averaged across the three inclusion rates, TS significantly increased forage intake (P < 0·05) and milk yield (P < 0·01) compared with UTS. Intermediate values were recorded for the diet with grass silage alone. The inclusion of TS in the ration did not affect milk yield compared with grass silage alone. No major changes in milk composition were recorded by the inclusion of either TS or UTS. However, the yields of milk fat, protein and lactose for the TS treatments were higher than for UTS and were significant for protein (P < 0·01) and lactose (P < 0·001). Cows with UTS in their ration tended to have lower live weight gains than either the control group or treatments with TS.
When compared with grass silage only, the inclusion of TS or UTS decreased digestibility coefficients and ME contents, determined in vivo using wether sheep fed at maintenance. However, the treatment of straw with NaOH (4%) increased its digestibility coefficients of dry matter from 0·-393 to 0·668, of organic matter from 0·445 to 0·686, of acid detergent fibre from 0·442 to 0·713, gross energy from 0·438 to 0·640 and ME content from 7·0 to 9·4 MJ/kg DM.