Digestion, absorption and utilization of single-cell protein by the preruminant calf

Two experiments of Latin square design, with four Friesian bull calves fitted with re-entrant duodenal cannulas at 4–10 d of age, were made to study the effect of giving varying levels of single-cell protein on the abomasal outflow and composition of digesta. 2. In Expt 1, diets in which 0,220,440, and 660 g/kg milk protein were replaced by the bacterial protein Pruteen were compared from 14 d of age. In Expt 2, which began at about 61 d of age, a comparison was made of diets in which 0, 220, 440 and 660 g/kg milk protein were replaced by the yeast protein Toprina. 3. Collection of abomasal outflow was made for 8 h after feeding for 2 d within each 6 d period of the Latin square design experiment. The amount of diet offered daily was 50 g dry matter/kg body-weight 0.75. Polyethylene glycol (PEG), which was used as an indigestible marker, total nitrogen (TN), protein-N (PN), fat, and potassium, sodium and chloride ion outflows were measured together with pepsin (EC 3.4.23.1) and chymosin (EC 3.4.23.4) activities, pH and titratable acidity. 4. In Expt 1 there was little difference in the outflow of liquid digesta between diets and about 0.9 of the dietary PEG was recovered within the 8 h collection period. With increasing amounts of Pruteen in the diet, outflows of TN, PN, fat and Na + increased, and the pH of digesta increased. However, the volume of 'apparent secretion' into the abomasum (outflow - intake), pepsin activity, chymosin activity, titratable acidity, (outflow of C1 - -outflow of Na + ) as a measure of outflow of HC1, and outflows of K + and of C1 - were reduced. All outflows decreased with the time interval after feeding, except (Cl - -Na + ) outflow. 5. In Expt 2, the same trends as in Expt 1 were apparent, but since one calf had to be slaughtered and the experiment had to be analysed as a randomized block, only PN and K + outflows and pH were significantly affected by dietary treatment, with K + outflow increasing, rather than decreasing, with increasing concentration of single- cell protein in the diet. 6. Reduced proteolysis in the abomasum, associated with a faster and greater outflow of protein as a result of poor or no coagulation of protein in the abomasum, and a reduction in secretion of enzymes and in acidity may partly explain the poor protein digestibility and growth rate obtained in other experiments when diets containing more than 100 g single-cell protein/kg diet (about 200 g protein/kg total protein) were given to young calves.