The Agricultural Research Council (ARC, 1981) first estimated ideal amino acid (AA) ratios for growing pigs based on pig carcass AA composition despite turnover of each and individual amino acids differ in both whole body and individual tissues. Wang and Fuller (Br. J. Nutr. 62:77-89, 1989) later improved the ARC ideal protein but without providing estimates of histidine and arginine.

Subsequently in the late 1980s, a purified amino acid diet (devoid of intact protein) with the Illinois Final AA Pattern (IFP) being established was successfully developed to producing weight gains, feed intakes and nitrogen (N) and energy retention values that were equal to 20% CP corn-soybean meal-dried whey for young pigs in Dr. David Baker’ s Laboratory at the University of Illinois (J. Nutr. 121:979-984, 1991).

The purified diet was critical in this step as it provided an opportunity to develop an ideal AA pattern in that true digestibility of AAs are 100% (J. Anim. Sci. 70:3781-3790, 1992) and AA levels can be set precisely. Using this approach, two precision growth trials and one nitrogen balance experiment were conducted. Four indispensable AA patterns (IFP; the Illinois Ideal AA Pattern (IIP), a modification of IFP; the Wang and Fuller Ideal AA Pattern (WFIP); and the 1988 NRC AA requirement pattern for 10-kg pigs (NRCP)) were tested and fed together with a mixture of dispensable AAs consisting of glutamate, glycine and proline, with experimental diets being made isonitrogenous and isoenergetic within experiments and their AA levels being set above and below NRC requirement under conditions of both ad libitum and equalized feeding. When fed indispensable AAs above NRC levels, pigs gained weight and converted feed to gain at the same rate when fed the four indispensable AA patterns. When pigs fed below the NRC levels, daily gains and daily feed intakes were similar for pigs fed IIP and WFIP. Gain:feed ratio and gain per unit of N intake were highest for pigs fed IIP. N retained per gram of N intake from indispensable AAs was greater for IIP than IFP, WFIP or NRCP.