The effect of different protein sources in milk-substitute diets on abomasal acidity and proteolytic activity was studied in Friesian calves, aged 20-58 d (Expt 1). The diets contained 'mildly' preheated, spray-dried skim-milk powder (MHM), severely preheated, spray-dried skim-milk powder (SHM), fish-protein concentrate (FPC) or solvent-extracted soya-bean flour (SF) as the main protein source. 2. Gastric juice was collected from abomasal pouches before feeding and at 15 min intervals for 8 h after the morning feed. Samples of digesta were obtained from the abomasum at 1 h intervals during the same period. 3. Digesta pH was lower and titratable acidity higher 0-3 after giving the diet containing MHM than when any of the other three diets was given. 3. Acid secretion from the pouches for the different diets was in the order: FPC greater than MHM greater than SHM greater than or equal to SF. 5. Protease secretion from the pouches, assayed at pH 2-1, was in the order: MHM greater than SHM = FPC greater than SF. 6. The effect of dry matter (DM) intake and concentration on abomasal acidity was also studied in calves given diets which contained MHM (Expt 2). This diet was reconstituted at either 100 or 149 g DM/kg liquid diet and fed at either 32-5 or 49-0 g DM/kg live weight 0-75 per d. Samples of abomasal digesta were collected as in Expt 1. 7. A high intake of DM at a low DM concentration resulted in low acidity of the digesta in the first 3 h after feeding, which suggested a dilution effect. Comparison of two diets of different DM concentration, which were fed in the same volume of liquid, indicated that the greater the DM intake, the greater was the amount of acid secreted. 8. It is concluded that the protein sources varied in their ability to stimulate abomasal acid and protease secretion and it is suggested that this may relate to calf performance.
