Abstract: The objective of this study was to optimize the loading sample size and filter medium of The American Oil Chemists' Society (AOCS) standard method for the determination of acid insoluble ash (AIA) to simplify the assay procedure and improve the accuracy. The repeatability, additivity and accuracy of AIA determined with AOCS modified method and the AOCS standard method were compared to obtain a reliable technique for accurately evaluating the dietary apparent metabolizable energy (AME) for broilers. Two experiments were conducted in the current study. In experiment 1, three batches determination of AIA content for each of 6 samples (3 diets, 3 excreta) using two methods (AOCS modified method, AOCS standard method) in a completely randomized design was to compare the reproducibility of the 2 methods. Then, the actual AIA content of 6 mixed diets and 6 mixed excreta samples were determined by the 2 methods. Meanwhile, the AIA value was calculated according to the composition of the mixed diet and mixed excreta and their AIA content. The difference between the determined and the calculated values was compared to test the additivity of the 2 methods by a paired experimental design. In experiment 2, a paired experimental design was used to compare the difference between the calculated AME of 13 diets using two AIA determination methods and the AME measured by the total collection method to test the accuracy of the 2 methods to evaluate the dietary AME. The results showed that the total coefficient of variation (TCV) of AIA determined by the AOCS modified method in each of 3 diets and 3 excreta samples ranged from 2.13% to 5.83% and 1.71% to 2.83%, respectively, while the TCV of AIA determined by the AOCS standard method ranged from 4.69% to 15.37% and 1.50% to 5.79%, respectively. The difference between the determined and calculated AIA values of 6 mixed diets and 6 mixed excreta by the AOCS modified method ranged from 0.004% to 0.017% and 0.002% to 0.029%, respectively. The difference between determined and calculated values of those samples by AOCS standard method ranged from 0.037% to 0.071% and 0.015% to 0.072%, respectively. The differences of 0.021 to 0.251 MJ·kg^-1 DM were observed between the AME evaluated by AIA determined with the AOCS modified method and AME determined by total collection. Whereas the differences of 0.025 to 1.799 MJ·kg^-1 DM were observed between AME evaluated by AIA determined with the AOCS standard method and AME determined by total collection. In conclusion, the AOCS modified method is superior to the AOCS standard method in the reproducibility and additivity of AIA determination and in the accuracy of evaluating dietary AME.

Key words: acid insoluble ash, determined method, metabolizable energy, broiler