This study of low protein diets for pigs was reported 25 years ago by Johnston and coworkers (1999), in anticipation that the library of free amino acids for commercial use would expand during the next decade. Two amino acids were in use at that time (DL-methionine, L-lysine) with threonine expected to be available within a few years. The driving force for this expansion was to reduce dependency on SBM as the primary source of amino acids, while reducing diet cost. A secondary factor was to reduce nitrogen excretion.

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