Abstract
Digestibility of AA in feed ingredients for pigs has been generally determined by feeding semipurified diets containing test ingredients as a sole source of nitrogen. However, malnutrition caused by feeding semi-purified diets may affect the digestibility of AA. Therefore, the hypothesis of this study was that the standardized ileal digestibility (SID) of AA in distillers’ dried grains with solubles (DDGS) are not affected by the addition of casein in experimental diets. In Exp. 1, 20 growing pigs were allotted to dietary treatments including 2 diets containing either 480 g/kg DDGS or 308 g/kg DDGS and 60 g/kg casein. The SID of Lys and Phe for DDGS in the diet containing casein were greater (P < 0.05) than those without casein. Exp. 2 was conducted with 20 growing pigs assigned to dietary treatments containing 0, 55, 110, or 165 g/kg casein while the concentration of DDGS decreased at 466.8, 311.2, 155.6, or 0 g/kg. The SID of most indispensable AA in DDGS linearly (P < 0.05) decreased with increasing concentration of casein. Experiment 3 was conducted to verify that the results of Exp. 1 were affected by the addition of casein or by the concentration of DDGS or both. Twenty growing pigs were assigned to dietary treatments prepared as a 2 × 2 factorial arrangement with the concentration of DDGS at 466.8 or 155.6 g/kg and casein at 0 or 110 g/kg. The SID of most indispensable AA in DDGS at 466.8 g/kg were greater (P < 0.01) than in DDGS at 155.6 g/kg regardless of dietary casein. Overall, the addition of 60 g/kg casein to experimental diets may increase the SID of AA in low-protein quality ingredients but the addition of higher concentration of casein and low concentration of dietary protein from test ingredients may decrease the SID of AA.