1. Pigs weaned at 2 d of age were given diets in which a functional fish-protein concentrate (FFPC) supplied 0 (diet U), 350 (diet H), 525 (diet I) or 700 (diet G) g crude protein (nitrogen × 6·25, CP)/kg total CP. The remainder of the CP was supplied by dried skim-milk and whey. The diets were reconstituted (200 g dry matter (DM)/l), from two spray-dried powders (diets U and G) and these were mixed in the proportions 1:1 and 1:3 (v/v) to give diets H and I respectively. All four diets were given at hourly intervals on a scale based on live weight, and supplemented with vitamins and minerals. All diets contained the same amounts of calcium, phosphorus, sodium and potassium.
2. In an experiment to 28 d of age with ten pigs/diet, performance was inversely related to the amount of FFPC in the diet. One pig (diet I) and four pigs (diet G) died after scouring. In healthy pigs, N retention (g/d per kg live weight) was not affected by partial replacement of dried skim-milk (diet H).
3. Digestion of protein was studied in a further experiment with eight pigs/diet, killed at 7 d of age and at 1 h after a feed. The amount, pH, DM and total N contents of the digesta in the stomach were also inversely related to the amount of FFPC in the diet.
4. FFPC had no consistent effect on proteolytic enzymes. Greater amounts of trypsin and chymotrypsin were present in the distal compared with the proximal region of the small intestine, but source of dietary protein had no effect.
5. The proportion of non-protein-N in the total N in digesta from the small intestine was lower in pigs receiving FFPC suggesting that this protein may be more resistant to proteolysis than milk proteins. The apparent absence of an increase in enzyme secretion when FFPC was given could, therefore, explain the decrease in performance.
6. Some general effects of non-milk proteins from this series of experiments in the baby pig are discussed. It is suggested that performance may be inversely related to the rate of gastric emptying, as indicated by the amount of DM in the stomach. Unlike milk, other sources of protein did not coagulate in the stomach, and reduced the pH, DM and total N contents of the digesta. Giving diets containing a single-cell protein did appear to stimulate proteolytic enzyme secretion, unlike fish or soya-bean proteins.