The use of free amino acids allows formulating very low crude protein diets for piglets.

Reducing the dietary CP level with free AA supplementation allows reduction of N excretion and the risk of gut disorders while maintaining performance of pigs. We have recently reevaluated the Val, Ile, His, and Leu requirements in pigs, which enables formulating very low CP diets. However, requirements for undifferentiated N, and thus the extent to which the dietary CP content can be reduced, are unknown. Two experiments were conducted to test the effect of reducing the dietary CP content to meet an ideal AA profile on N retention and performance in 10- to 20-kg pigs. In the first experiment, we measured N retention in 6 blocks of 4 pigs each, all receiving diets with 1.15% standardized ileal digestible (SID) Lys. Each pig within a block was allotted to different levels of dietary CP (19.7%, 16.8%, 14.0%, and 12.7%). The reduction of CP content from 19.7% to 16.8% had no impact on N retention and decreased N excretion by 29%. A further decrease in CP content to 14.0% and 12.7% resulted in a reduced N retention (P < 0.01). In the second experiment, we measured performance in 14 groups of 6 pigs each, allotted to 6 levels of dietary CP with 1.00% SID Lys with soybean meal (17.6%, 15.6%, 13.5%, and 11.8%) or without soybean meal (14.0% and 13.0%). Decreasing the dietary CP content from 17.6% to 13.5% had no effect on performance. Daily gain was reduced in pigs receiving the 11.8% CP diet (P < 0.01). Feeding the diet formulated without soybean meal with 13.0% CP content resulted in reduced feed efficiency, but the addition of Glu to increase the CP content from 13.0% to 14.0% restored performance (P < 0.01). In conclusion, the use of L-Val, L-Leu, L-Ile, L-His, and L-Phe enables a 4% unit reduction in the CP content relative to SID Lys in diets for pigs. Soybean meal can be totally replaced using cereals and free AA. However, a further reduction in dietary CP:Lys level below 13.5% reduces feed efficiency, indicating that the supply of N for the synthesis of dispensable AA may be a limiting factor for the performance of pigs.