Prediction of the concentration of standardized ileal digestible amino acids in distillers dried grains with solubles

Values for the concentration of standardized ileal digestible (SID) CP, Lys, Met, Thr, and Trp from 34 sources of distillers dried grains with solubles (DDGS) were obtained from a series of 5 experiments with the objective of predicting the concentration of SID AA from physical and chemical assays. The concentration of NDF, ADF, hemicellulose, acid detergent insoluble CP (ADICP), and KOH soluble protein (SolCP) were measured and calculated in all DDGS sources. Likewise, particle size was measured and color of each source of DDGS was determined with a Minolta colorimeter and HunterLab spectrometer and was expressed as lightness (L*), redness (a*), and yellowness (b*). The HunterLab spectrometer also provided optical density that was recorded between 400 and 700 nm. Front face fluorescence was measured at 360 nm excitation and recorded from 380 to 600 nm. Multiple linear regression and principal components analyses were performed to predict the concentration of SID AA among DDGS sources, and predicted means as well as predicted residual sums of squares (PRESS) were calculated to estimate accuracy and precision of the model. Some correlations (P < 0.05) were observed between ADF, hemicellulose, ADICP, and SolCP with SID CP and AA but were generally low (r < 0.51). There was a greater association (R-2 = 0.40; P < 0.05) between L* and SID Lys among DDGS sources when L* was less than 50 than when samples had L* values greater than 50. In addition, a* was negatively correlated (P < 0.05) with SID CP (r = -0.41), Lys (r = -0.59), and Met (r = -0.50) whereas b* tended to be positively correlated (P < 0.10) with SID Lys (r = 0.31) and Trp (r = 0.30) and was correlated (P = 0.05) with SID Met (r = 0.43) and Thr (r = 0.36). There were no correlations between NDF or particle size with SID CP and AA. Optical density, along with CP, was highly predictive of SID Lys (R-2 = 0.97; PRESS = 0.05), Thr (R-2 = 0.94; PRESS = 0.06), and Trp (R-2 = 0.93; PRESS = 0.004) but not SID Met (R-2 = 0.39; PRESS = 0.12). Front face fluorescence was also highly predictive of SID Lys (R-2 = 0.99; PRESS = 0.07), Met (R-2 = 0.95; PRESS = 0.05), Thr (R2 = 0.99; PRESS = 0.008), and Trp (R-2 = 0.99; PRESS = 0.006). In conclusion, correlations between ADICP, SolCP, NDF, particle size, and color measurements with SID AA concentrations were poor, but optical density and front face fluorescence methods appear to provide good predictions of SID AA concentrations in DDGS. However, these prediction equations need to be validated using samples of DDGS from a separate data set.