Effects of different sources of protein on the growth performance, blood chemistry and polypeptide profiles in the gastrointestinal tract digesta of newly weaned piglets.

To determine the effects of different sources of protein on the growth performance of newly weaned piglets, 72 newly weaned piglets were randomly assigned to three groups fed different diets (soya bean, casein and dried distillers' grain with solubles (DDGS) feeds). Casein and DDGS feeds consisted of soya bean feed in which 5% of the CP was replaced with casein- or DDGS-derived CP respectively. Blood and chyme samples were collected from each piglet 2 h post-feeding on days 0 and 28 of the feeding period. The DDGS feed decreased DMI (p = 0.024) and increased FCR (p = 0.025) due to lower nitrogen utilization (p = 0.078) than those of other feeds. Total amino acid content in chyme demonstrated that casein feed digested rapidly in the duodenum (p = 0.005), whereas DDGS feed was digested primarily in the distal jejunum (p = 0.003) and ileocecum (p = 0.002). However, polypeptide profiles in chyme exhibited a pattern different from those of amino acids. There were no differences in the polypeptide profiles in the stomachs of piglets fed soya bean or casein feeds (p > 0.05), but soya bean group had greater amounts of small polypeptides (mass under m/z 3000 Da) in the duodenum (p = 0.052) than other groups. In contrast, the DDGS feed group had more large polypeptides (m/z 3000-4000 Da) in the stomach than the other groups (p < 0.001). In addition, 10 pairs of polypeptides with matching masses were identified in the plasma and digesta, indicating that polypeptides may have been transported across the intestinal epithelial cells and into the blood. Taken together, substitution of 5% of the CP in soya bean meal-based feed with DDGS-derived CP decreased the growth performance of newly weaned piglets due to poor digestibility and N utilization of DDGS feed, as well as untimely digestion of casein feed.