A total of 240 male broilers were used in a 21-d experiment to study the effect of reference diet type on nitrogen corrected-apparent metabolizable energy (AMEn) of soybean meal (SBM) and canola meal (CM). The two reference diets were corn-SBM or corn-CM. To prepare experimental diets, graded levels of SBM or CM at 300 and 450 g/kg were added to each of the two basal diets to make a total of 10 diets. The experimental diets were fed to birds from d 14-21. Excreta were collected on d 20 and 21. The AMEn values were calculated using the difference or the regression methods for SBM using SBM- and CM reference diets, and for CM using SBM- and CM-reference diets. The data for AMEn determined using the difference method were analyzed as 2×2×2 factorial to determine the effect of reference diet (corn-SBM and corn-CM), protein feedstuff (SBM, and CM), and substitution level (300 and 450g/kg) of assay feedstuff in the reference diets. There was significant (P < 0.05) three-way interaction between reference diet, feedstuff, and substitution level. For AMEn of SBM determined using CM reference diet, AMEn increased (P < 0.05) as the inclusion level of SBM in the CM reference diet increased. The AMEn of CM decreased (P < 0.05) with increasing inclusion level of CM in the SBM-reference diet. Data on AMEn determined by the regression method were analyzed as a 2×2 factorial to determine the effect of the reference diet (SBM- or CM-based) or feedstuff (SBM or CM). There was no significant reference diet × feedstuff interaction. AMEn was greater (P < 0.01) when determined in a corn-CM reference diet compared with corn-SBM reference diet but there was no significant difference between AMEn of SBM and CM. For comparison of AMEn calculated by the two methods, the data were analyzed as a 2×2×2 factorial to incorporate the effect of reference diet type (SBM- or CM-based), protein feedstuff (SBM or CM) and method (difference or regression). There were no significant 3- or 2-way interactions. AMEn calculated using the regression method gave greater (P < 0.01) values compared to the difference method. In addition, AMEn was greater (P < 0.01) when determined using a corn-CM reference diet compared to corn-SBM.
In conclusion, the reference diet used AMEn assays for protein feedstuffs should be taken into consideration, and the difference and the regression methods do not give similar AMEn values for SBM or CM.