1. Half (diet M) or total replacement (diet N) of dried skim-milk by fish-protein concentrate and dried whey was compared with a diet containing dried skim-milk (710 g/kg) as the only source of protein (diet A). All three diets were prepared as spray-dried powders, and contained 20 g starch and 270 g soya-bean oil/kg. The diets were calculated to be isonitrogenous, but upon analysis had crude protein (nitrogen × 6.25) contents (g/kg) of 230, 270 and 291 in diets A, M and N respectively.
2. The diets were reconstituted as liquids containing 200 g dry matter (DM)/l and given to pigs weaned at 2 d of age during a 26 d experiment. The diets were fed at hourly intervals to a scale based on live weight. Protein digestion was studied in pigs, 6 d of age, and killed 1 h after a feed.
3. Live-weight gain was slightly improved when half the dried skim-milk was replaced, but the feed: gain ratio (g DM consumed/g live weight gain) was unaltered. Total replacement of dried skim-milk, markedly reduced performance and increased the incidence of scouring and mortality.
4. Increasing the proportion of fish protein reduced the apparant digestibility (AD) and the retention of N (g/d per kg live weight). N retention, but not AD, decreased with age.
5. Total replacement of dried skim-milk decreased the amount of digesta in the stomach, its pH value, and the proportion of DM and total N in the digesta, but non-protein N as a proportion of total N was increased.
6. The total pepsin activity in the stomach digesta was decreased by the substitution with fish protein, but there was no change in the activity of the stomach tissue. Trypsin and chymotrypsin activities in the pancreas were not affected by the source of dietary protein, but in the digesta of the small intestine the activity of these two enzymes was markedly reduced when the diet contained fish protein.
7. Total replacement of dried skim-milk with fish-protein concentrate and dried whey may increase the rate of flow of digesta through the small intestine, which together with the reductions in the apparent digestibility of N and amounts of trypsin and chymotrypsin in the digesta could reduce the efficiency of protein digestion and adversely affect performance.