1. Digesta were collected from eleven preruminant calves fitted with re-entrant (four calves in Expt 1 and three in Expt 2) or single cannulas (four calves in Expt 1) in the terminal ileum. Collection periods lasted 24 h (Expt 1) or 96 h (Expt 2).
2. Two milk-substitutes (fish and soya bean) and a control diet were given to the calves. In the control diet, protein was entirely provided by skim-milk powder. In the other two diets, protein was provided mainly by a partially hydrolysed white-fish protein concentrate or a soya-bean protein concentrate prepared by extracting soya-bean meal with hot aqueous ethanol.
3. In Expt 1, flow rates of fresh matter, dry matter, nitrogen and ash exhibited two maxima between 6 and 8 h after the morning meal and between 4 and 6 h (control and soya-bean diets) or 6 and 8 h (fish diet) after the evening meal. Minimum pH values were observed at times of maximum flow rate. Variations observed in the flow rates and pH values were larger with fish and especially soya-bean diets than with the control diet.
4. The apparent digestibility of the three diets in the terminal ileum was significantly higher in Expt 2 than in Expt 1: for N, the values were 0.92, 0.83 and 0.75 (Expt l), and 0.94, 0.87 and 0.88 (Expt 2) with the control, fish and soya-bean diets respectively.
5. The amount of N apparently absorbed in the terminal ileum represented 90–96% of the amount that disappeared from the whole digestive tract in Expt 1 and 95–99% in Expt 2.
6. In Expt 1 the amino acid (AA) composition of digesta changed little with the flow rate when the calves were given the control diet (from 158 to 179 g glutamic acid/kg AA). With the fish and soya-bean diets the AA composition was similar to that observed with the control diet when the flow rate was minimum, but differences became apparent as the flow rate increased (281 and 161 g glutamic acid/kg AA for the soya-bean and control diets respectively with maximum flow rate). In Expt 2, the mean compositions of the digesta were very similar to the means obtained in Expt 1.
7. Different comparisons with dietary, endogenous and bacterial proteins indicated that for the three diets a common mixture containing approximately 65% endogenous and 35 % bacterial proteins reached the terminal ileum. The quantity of dietary protein added to this mixture appeared to be very low with the control diet, but it increased with the flow rate in the case of the fish and soya-bean diets.
8. In Expt 2, the additional undigested protein in the small intestine was richer in glycine with the fish diet and in glutamic acid and aspartic acid with the soya-bean diet compared with the control diet. This undigested fraction probably originated mainly from the fish solubles and the glycinin of soya bean respectively.
9. With the control diet the apparent digestibility of threonine and cystine was always lower than the mean value for all AA while that of methionine was higher (0.92, 0.82 and 0.96 respectively). Digestibility of all AA was higher for the control diet than corresponding values for the fish and soya-bean diets; these differences were greatest for histidine with the fish diet (–0.11) and for glutamic acid with the soya-bean diet (–0.13).