Expt 1. Six 200 kg Hereford heifers fitted with rumen and abomasal cannulas were fed ad lib. on alkali-treated oat straw sprayed with urea and minerals, with no supplement (O), or 700 g rolled barley (B) or cottonseed meal (C)/d, in a 3 x 3 Latin square with two replicates. 2. Intakes of the basal diet were not significantly affected by the supplements. Intakes of digestible organic matter (OM) were 3135, 3325 and 3515 g/d on diets O, B and C respectively. Abomasal OM flow on diet C was 13 and 12% higher than on diets O and B respectively (P less than 0.05) which was associated with a decrease in the proportion of OM intake apparently digested in the rumen (P less than 0.05) and an increase in the proportion of abomasal OM digested in the lower gut (P less than 0.05). 3. Total nitrogen and bacterial N flows at the abomasum were significantly higher (P less than 0.01 and P less than 0.05 respectively) on diet C than on diets O and B. Efficiency of bacterial N synthesis on diet C was 19 and 28% higher than on diets O and B respectively (P less than 0.05). 4. Dry matter (DM) pool size in the rumen was greater on diet C than on diets O and B (P less than 0.05). 5. Expt 2. Thirty-three 210 kg Friesian heifers were allocated to the same treatments as in Expt 1. Intakes of the basal straw diet were not significantly affected by the supplements.(ABSTRACT TRUNCATED AT 250 WORDS)
