A comparison of the predictions of digestible energy content of compound feeds for pigs by chemical or in vitro analysis

The comparison of the predictions of the digestible energy (DE) content of commercial compound pig feeds from the organic matter in vitro digestibility (OMD) and five recent and accurate multiple regression equations based on chemical components was examined. The OMD was determined by submitting each sample (17 compound feeds for lactating or pregnant sows and 23 for piglets and growing-fattening pigs) to three sequential incubations with enzymatic solutions in a closed system to simulate the gastric and pancreatic digestion and the degradation in the hind gut (2, 4 and 18 h of incubation, respectively). OMD values were converted into energy digestibility coefficients (dE), which together with gross energy (GE) content were used to calculate DE (=GE*dE). Chemical components (ash, ether extract (EE), crude protein (CP), crude fibre (CF), NDF and ADF) and GE content were used to predict the DE content using five multiple equations, selected in terms of their accuracy (r2 and/or RSD). The DE content was closely correlated with NDF (r, -0.91, p<0.01), while ADF and CF were correlated to a lesser degree (r,-0.75, and -0.79, respectively, p<0.01) and no significant relationship was found with ash. The average DE contents estimated with the multiple equations were slightly lower than those predicted in vitro (from 15.15-15.40 vs. 15.58 MJ/kg DM). The differences between the DE content estimated with the multiple equations and that predicted in vitro (ΔDE) were highly, and negatively, correlated with the ash content of the feeds (r from -0.68 to -0.84, p<0.01) and it was estimated that the ΔDE varied by 4-5% (ca. 0.71 MJ/kg DM) when the ash content (62 ± 14 g/kg DM) ranged from low to high levels (from -1 to +1 SD). All the ΔDE values had negative correlations with both CP (r, from -0.62 to -0.68, p<0.01) and EE (r, from -0.48 to -0.53, p<0.01). The fibre components, present in all equations either as NDF, ADF or NDF-ADF, were not significantly related with any of the ΔDE considered. The prediction of DE content based on the in vitro technique considered or chemical parameters showed a general coherency, the only exception concerning the ash fraction.