1. Two nitrogen-balance experiments were performed with growing rats to test the effect of dietary fibre level, protein quality and antibiotic inclusion on microbial activity, N excretion patterns and energy digestibility. Each experiment involved eight dietary treatments in a 2 x 2 x 2 factorial design, with five rats per treatment. The eight treatments resulted from a combination of two protein treatments, two fibre treatments and two antibiotic treatments. In Expt 1 the protein was provided as barley, or barley plus 2 g L-lysine hydrochloride/kg dry matter (DM) (at 15 g N/kg DM) and in Expt 2 as soya-bean meal or soya-bean meal plus 2 g DL-methionine/kg DM (at 15 g N/kg DM). In both experiments the basal diet was provided with or without additional fibre as 100 g barley husk/kg DM and with or without antibiotic as 7 g Nebacitin/kg DM.
2. With both barley and soya-bean meal, true protein digestibility (TD) was improved with the addition of amino acids. Only with the soya-bean meal diets was TD increased with Nebacitin treatment, with the effect of Nebacitin and methionine being additive. Barley husk slightly reduced the TD of soya-bean meal.
3. The effect of treatments on biological value (BV) was considerable. Lysine increased BV of the barley diet from 0.741 to 0.815 whereas Nebacitin reduced BV from 0.799 to 0.757. Methionine increased the BV of soya-bean meal from 0.754 to 0.911 while BV was reduced by Nebacitin from 0.843 to 0.821 and by barley husk from 0.845 to 0.820.
4. Net protein utilization (NPU) was markedly improved by the addition of amino acids and reduced by the addition of Nebacitin. Barley husk reduced NPU with soya-bean meal diets only.
5. Amino acid addition had no effect on energy digestibility of either diet, but Nebacitin reduced this value by approximately 5% on the barley diets, with a smaller reduction on the soya-bean meal diets. Fibre reduced energy digestibility of both diets.
6. On both diets, blood urea (BU) decreased with addition of amino acids. Nebacitin increased BU with the barley diets, but decreased BU with the soya-bean meal diets. With the soya-bean meal diets, the decrease with a combination of barley husk and methionine was particularly marked.
7. Microbial activity, as measured by caecal ATP activity, was strongly reduced by antibiotic addition with both protein sources. On the soya-bean meal diets ATP activity was increased by the addition of methionine.
8. The results demonstrated a difference in response to the addition of fibre and the first limiting amino acid to barley and soya-bean meal, as measured by several indicators of protein and energy utilization.