Methods to Determine Amino Acid Digestibility in Corn By-products

While corn contains only limited quantities of crude protein and amino acids, corn by-products often contain crude protein and amino acids in appreciable amounts. The reason for this difference is that corn by-products are often produced after processing that primarily removes the nitrogen-free extracts (i.e., the starch) from the corn kernel. Because starch is approximately 70% of the total corn kernel, the quantity of crude protein and other nutrients is concentrated in the product that is left over after processing (Table 1). Corn by-products, therefore, are valuable sources of crude protein and amino acids in diets for monogastric animals. As is the case with most other by-products, variations in the concentrations and the digestibility of the amino acids in corn by-products exist. The main reason for these variations is differences in processing technology and procedures. Currently, the most used corn by-product in the feeding of monogastric animals is distillers dried grain with solubles (DDGS). This product is produced by the ethanol industry and contains mainly the crude protein, fat, fiber and minerals from the corn kernel whereas the majority of the starch has been removed in the fermentation process. In general, the concentrations of crude protein, amino acids, fiber and minerals in DDGS are close to three times that than in corn. Thus, the crude protein concentration in DDGS is usually 26 – 28% and the concentration of amino acids is also approximately three times higher than in corn (Table 1). Because of the relatively high concentration of crude protein and amino acids, DDGS contributes significantly to the dietary concentration of amino acids if included in diets for pigs and poultry. It is, therefore, necessary to estimate the digestibility of these amino acids. It is the objective of the current contribution to summarize existing knowledge about the methods that are available to measure amino acid digestibility in corn by-products. Because of the importance of DDGS in the feeding of pigs and poultry in the US, the summary will be based on experiences with DDGS. However, the concepts discussed may also be applied to other corn byproducts.