Comparison of ileal apparent, standardized and true digestibilities of amino acids in pigs fed wheat and lupine seeds

The aims of the investigation were to compare calculation methods for ileal apparent digestibility (AD) of amino acids (AA) and to demonstrate the calculation of ileal true digestibility (TD) using the regression approach and additionally, to compare the TD with the standardized digestibility (SD). Eight Goettingen minipigs, average 52 kg initial BW, were fitted with ileo-rectal anastomoses and fed consecutively with assay diets in four experiments according to a Latin rectangle design. To determine AD by using the ‘direct method’, diets contained wheat, lupine, or casein as the sole sources of protein. Furthermore, the ‘difference method’ was used by applying diets with wheat plus casein or wheat plus lupine. To determine AD using the regression (‘Reg2’) method, three graded levels of lupine mixed with wheat were fed. The TD and the basal ileal endogenous losses of AA (ELbAA) were determined using the regression approach (‘Reg1’) by feeding both 5 graded levels of lupine and 5 graded levels of wheat, each added with N-free mixture at 1000 g. In each experiment, one repetition comprised 10 days for adaptation and 4 days for quantitative collection of ileal effluents. Pigs were fed twice a day and the target dietary allowance was 35 g DM kg− 0.75 BW d− 1 in all experiments. There were no differences in the AD of any AA in lupine or in wheat when those were determined with the direct, difference or Reg2 method. The TD values of the most indispensable AA were higher than the AD (in lupine 8 to 20%-units for lysine and methionine and in wheat 6 to 11%-units for tryptophan and lysine, respectively). These differences reflected the ELbAA derived from the intercept (a) of the Reg1. The ELbAA were higher in lupine than in wheat for arginine, histidine, leucine, methionine, phenylalanine, tryptophan, cysteine, glutamine, and serine (P < 0.007 to 0.047). In conclusion, the TD of AA should be determined by Reg1 regression rather than calculation of SD, i.e. AD values corrected with means of ELbAA, because the latter cannot be assumed as constant.