Digestion of wheat protein in the preruminant calf: ileal digestibility and blood concentrations of nutrients

Abstract

Three milk substitute diets (control, SWP76 and SWP36) were given to ileo-caecal cannulated preruminant calves. In these diets, skim milk powder was either the only protein source or totally or partially (48%) replaced by a mixture of solubilized wheat protein concentrate (SWP) and whey powder (76% and 24%; crude protein basis), respectively. The apparent ileal digestibility of nitrogen was lower ( P<0.02) with the SWP76 and SWP36 diets than with the control (0.85, 0.89 and 0.91, respectively) and also with the SWP76 than with the SWP36. Similar trends were observed for most amino acids (AA), except for cystine which showed higher values with the SWP-containing diets than with the control ( P<0.05). Assuming the true digestibility of milk protein was total, the real digestibility would be ≥0.95 for most essential AA of SWP+whey proteins. Repetitive fractions rich in glutamine, proline and phenylalanine, which are abundant in wheat protein, appeared to partially escape digestion. The amount of undigested endogenous protein could also be slightly higher (7–17%) with the SWP-containing diets than with the control. The pre-feeding plasma concentrations were lower ( P<0.001) with the SWP76 than with the control for triglycerides and similar with both diets for free methionine and threonine. In contrast, most post-feeding values were higher ( P≤0.05) with the SWP76 than with the control. The SWP36 diet gave values or patterns intermediate to those recorded with the other diets. Most of these differences were consistent with a faster abomasal emptying for fat and protein with the SWP-containing diets, especially the SWP76. As far as threonine and methionine were concerned, they could also have been increased by the fact that larger parts of these AA were free in the SWP-containing diets.