Reducing the dietary crude protein level with free amino acid supplementation allows reduction of nitrogen excretion and the risk of gut disorders while maintaining performance of pigs. French researchers recently reevaluated the valine, isoleucine, histidine, and leucine requirements in pigs, which enables the formulating of very low crude protein diets. However, requirements for undifferentiated nitrogen and thus the extent to which the dietary crude protein content can be reduced, are unknown. Two experiments were conducted to test the effect of reducing the dietary crude protein content to meet an ideal amino acid profile on nitrogen retention and performance in 10- to 20-kg pigs.

In the first experiment, pigs were allotted to diets contain different levels of crude protein (19.7%, 16.8%, 14.0%, and 12.7%) with 1.15% standardized ileal digestible lysine. The reduction of crude protein content from 19.7% to 16.8% had no impact on nitrogen retention and decreased nitrogen excretion by 29%. A further decrease in crude protein content to 14.0% and 12.7% resulted in reduced nitrogen retention (P < 0.01). In the second experiment, pigs were allotted to six levels of dietary crude protein with 1.00% standardized ileal digestible lysine by adjusting the soybean meal in the experimental diet. 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% crude protein diet (P < 0.01). Feeding the diet formulated with 13.0% CP content resulted in reduced feed efficiency, but the addition of glycine to increase the crude protein content from 13.0% to 14.0% restored performance (P < 0.01). In conclusion, the use of L-valine, L-leucine, L-isoleucine, L-histidine, and L-phenylalanine enables a 4% unit reduction in the crude protein content relative to standardized ileal lysine in diets for pigs. Cereal grains and free amino acids can replace protein ingredients in pig diets, provided, adequate levels of essential amino acids and the supply of nitrogen for the synthesis of dispensable amino acids are maintained for pig performance.