Energy determination of corn co-products in finishing pigs and the use of an in vitro organic matter digestibility assay to predict in vivo energy

Twenty co-products from various ethanol plants were fed to finishing pigs to determine DE and ME and to generate 8 equations to predict DE or ME based upon each ingredient's chemical analysis. Additionally, a 3-step enzymatic assay was used to determine if in vitro OM digestibility would predict in vivo DE or ME or improve the prediction estimate of DE or ME for corn co-products. Co-products included: DDGS (7), HP-DDG (3), bran (2), germ (2), gluten meal and feed, dehulled degermed corn, dried solubles, starch, and corn oil. The in vitro OM digestibility for each co-product was determined in triplicate using procedures as described by Boisen and Fernandez (1997). For the in vivo study, the control diet was based on corn (97.1%), limestone, dicalcium phosphate, salt, vitamins, and trace minerals. All but two test diets were formulated by mixing the control diet with 30% of a co-product. Dried solubles and oil were included at 20% and 10%, respectively. Eight groups of 24 gilts (n=192, 112.7 final BW +/- 7.9 kg) were randomly assigned to a test diet and each diet was fed to a total of 8 pigs. Gilts were placed in metabolism crates and fed an amount equivalent to 3% BW daily for 9 d followed by collecting feces and urine separately for 4 d. Ingredients were analyzed for GE, CP, moisture, crude fat, crude fiber, ash, total dietary fiber (TDF), NDF, and ADF. Gross energy was determined on the feed, feces, and urine to calculate DE and ME for each ingredient. The in vitro OM digestibility ranged from 33.3 to 100% for corn bran and corn oil, while DE ranged from 2,517 to 8,988 kcal/kg DM and ME ranged from 2,334 to 8,755 kcal/kg for corn gluten feed and corn oil, respectively. Although in vitro OM digestibility was correlated to in vivo DE and ME (r = 0.62 and 0.63, P < 0.01), it did not improve the prediction of DE or ME from ingredient analysis. Stepwise regression resulted in the equations: DE = (0.985 x GE) - (31.856 x TDF) - (34.010 x Ash) (r2 = 0.96, SE = 275, P < 0.01) ME, kcal/kg = (0.949 x GE) - (32.238 x TDF) - (40.175 x ash) (r2 = 0.95, SE = 306, P < 0.01). These results indicate that OM digestibility, DE, and ME vary substantially between corn co-products and the best predictors of DE and ME are GE, TDF, and ash.