Additivity and associative effects of metabolizable energy and amino acid digestibility in barley and canola meal for White Pekin ducks

An experiment was conducted using the TMEn bioassay method to investigate t he additivity and associative effects of metabolizable energy and amino aci d digestibility in barley and canola meal for White Pekin ducks. Additivity was tested by comparing the difference between observed values determined in a complete diet and predicted values from measurements determined with i ndividual ingredients (barley and canola meal). Six ducks each were assigne d to diets of barley, canola meal, the complete diet, and dextrose. Dextros e-fed ducks were used for estimation of endogenous losses for calculation o f true amino acid digestibility. The observed AME, TME, AME(n), and TMEn va lues in the complete diet were 0.065, 0.083, 0.016, and 0.023 (kcal/g), res pectively, numerically higher than predicted values. Differences between ob served and predicted values were not significant (P>0.05), indicating that the AME, AME(n), TME, and TMEn in barley and canola meal were all additive. In general, observed values for apparent amino acid digestibility (AAAD) a nd true amino acid digestibility (TAAD) in the complete diet were higher th an those predicted from individual ingredients. Observed AAAD for lysine, h istidine, tryptophan, alanine, and aspartate were higher (P<0.05) than pred icted values, indicating that digestibilities of these amino acids were not additive. The mean of AAAD in canola meal (77.29%) was higher (P<0.05) tha n the observed values of barley (52.2%) and complete diet (64.55%). For TAA D values, differences between observed and predicted values were significan t for lysine, histidine, and tryptophan (P<0.05). The mean of TAAD in canol a meal, barley, and complete diet were 85.88, 80.87, and 81.33%, respective ly. The average difference between observed and predicted values for TAAD ( 1.18%) was smaller than that of AAAD (5.41%). These results indicated that ME values for barley and canola meal were additive in the complete diet but that digestibilities of some amino acids were not additive; they further s uggested that there were some associative effects of amino acids in barley and canola meal for ducks.