Digestible energy of sorghum grain for pigs could be predicted using a computer-controlled simulated digestion system

Prediction equations for digestible energy (DE) of sorghum grains fed to pigs have been generated based on chemical composition in our study. However, an in vitro method mimicking gastrointestinal digestion of pigs is more applicable to predict DE of feed ingredients in practice. This study evaluated in vitro DE (IVDE) and digestibility of gross energy (GE) in 28 samples of sorghum grain with a wide range in nutrient profiles using a 3-step computer-controlled simulated digestion system (CCSDS) and generated prediction equations based on IVDE and in vitro GE disappearance (IVGED) for previously determined DE and apparent total tract digestibility (ATTD) of GE. The IVDE (3305-4068 kcal/kg), IVDE/GE (0.72-0.91), IVGED (0.72-0.90), and in vitro dry matter (DM) disappearance (0.73-0.89; IVDMD) were quite variable and had a high negative correlation with tannin content in sorghum grain (r ≤ −0.90, P < 0.01). The IVDE or IVDE/GE was highly positively correlated with determined DE or DE/GE (r = 0.97 or 0.96, both P < 0.01), and linear relationship between DE and IVDE (kcal/kg of DM) or between DE/GE and IVDE/GE was observed in 28 sorghum cultivars: DE = 0.62 × IVDE + 1462 (R2 = 0.93, RMSE  = 40.9 kcal/kg, P < 0.001), or DE/GE = 0.626 × IVDE/GE + 0.331 (R2 = 0.92, RMSE = 0.01, P < 0.001). Differences of less than 50 kcal/kg or 0.02 were observed in 23 or 24 of the 28 samples between determined and predicted DE or DE/GE. Similarly, the positive relationship between ATTD of GE or DM and IVGED or IVDMD was high (r = 0.95 and 0.93, respectively, both P < 0.01), and the prediction equations based on in vitro values were 0.595 × IVGED + 0.351 (R2 = 0.90, RMSE = 0.01, P < 0.001) for ATTD of GE and 0.644 × IVDMD + 0.327 (R2 = 0.87, RMSE = 0.01, P < 0.001) for ATTD of DM. Differences between determined and predicted values were smaller than 0.02 in 25 or 24 out of the 28 samples for ATTD of GE or DM. In conclusion, tannin negatively affects the in vitro enzyme digestion of energy content in sorghum grain. The 3-step enzymatic method using CCSDS could be a promising in vitro digestibility technique to predict DE and digestibility of GE in sorghum gain for pigs.