1. Expt 1. Six 200 kg Hereford heifers fitted with rumen and abomasal cannulas were fed adlib. 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, Bland C respectively. Abomasal OM flow on diet C was 13 and 12% higher than on diets O and B respectively (P < 0.05) which was associated with a decrease in the proportion of OM intake apparently digested in the rumen ( P < 0.05) and an increase in the proportion of abomasal OM digested in the lower gut ( P < 0.05).
3. Total nitrogen and bacterial N flows at the abomasum were significantly higher (P < 0.01 and P < 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 0 and B respectively ( P < 0.05).
4. Dry matter (DM) pool size in the rumen was greater on diet C than on diets O and B (P < 0.05).
5. Expt 2. Thirty-three 210 kg Friesian heifers were allocated to the same treatments as in Expt I. Intakes of the basal straw diet were not significantly affected by the supplements. Intakes of digestible OM were 3.80, 4.16 and 4.34 kg/d on diets O, B and C respectively.
6. Rumen ammonia and plasma urea levels were significantly higher on diet C than on diets O and B ( P < 0.05).
7. Live-weight gains were 679, 838 and 1051 g/d on diets O, B and C respectively ( P < 0.01) and live-weight gain/MJ metabolizable energy intake was highest on diet C (P < 0.01).
8. It was concluded that the growth response to barley was attributable to the increase in energy supply, and that the additional response to cottonseed meal was attributable to greater protein absorption arising from greater abomasal flows of bacterial and dietary proteins.