Standardized Ileal Digestibility of Reactive Lysine in Distillers Dried Grains with Solubles Fed to Growing Pigs

Abstract

The Maillard reaction can occur during the production of distillers dried grains with solubles (DDGS) as a result of the addition of condensed solubles to the wet distillers cake during drying. The Maillard reaction can lead to the formation of unavailable or unreactive lysine as a result of binding of reducing sugars to the ε-NH₂ group of Lys. The Lys that remains unbound is called reactive Lys. The conventional procedure to measure the apparent ileal digestibility (AID) and standardized ileal digestibility (SID) of total Lys in DDGS may overestimate the amount of digestible Lys in DDGS because this procedure does not discount the unreactive Lys in DDGS, although only the reactive Lys is available for use by animals. By measuring the ileal digestibility of only the reactive Lys, it is expected that estimation of the amount of bioavailable Lys in DDGS will be more accurate. The objective of this experiment, therefore, was to test the hypothesis that the concentrations of AID and SID of reactive Lys are lower than the concentrations of AID and SID of total Lys in DDGS. Twelve DDGS sources, diets containing each of these 12 DDGS sources, and ileal digesta from pigs fed these diets were obtained from two previous experiments. Samples were guanidinated with O-methylisourea and analyzed for the concentration of homoarginine. It was assumed that only the reactive Lys, but not the unreactive Lys, would be transformed to homoarginine. This procedure, therefore, allows the separate measurement of reactive and unreactive Lys in DDGS-containing diets and ileal digesta. Subsequently, the ileal digestibility of reactive Lys can be calculated. The AID and SID of reactive Lys (percent) and the concentration of apparent ileal and standardized ileal digestible reactive Lys (g/kg) were then calculated and compared with the previously calculated values for total Lys. Results showed that only 76% of the total Lys in DDGS is reactive. The AID and SID of reactive Lys in DDGS (average = 60.1 and 66.9%, respectively) were similar to the AID and SID of total Lys (60.9 and 66.5%, respectively). When calculated as grams per kilogram, the concentration of standardized ileal digestible reactive Lys (3.9 g/kg) was lower (P < 0.05) than the concentration of standardized ileal digestible total Lys (5.1 g/kg). Thus, 24% of the concentration of standardized ileal digestible total Lys that was calculated using the conventional ileal AA digestibility procedure was unreactive Lys. The implication of these results is that the conventional AA digestibility procedure overestimates the concentration of digestible Lys in DDGS, and measurement of the concentration of digestible reactive Lys may more accurately estimate the amount of Lys in DDGS that is bioavailable to the pig.