A total of 981 pigs ([Fast LW × PIC L02] × PIC 800; initial body weight [BW] = 10.3 ± 0.19 kg) were used in a 21-d study to determine if nitrogen is a limiting factor to maintain feed efficiency when feeding low protein, AA fortified diets. Pens were randomly allotted to one of five dietary treatments in a randomized complete block design with BW as a blocking factor. There were approximately 20 pigs per pen and 10 pens per treatment. Experimental diets were corn-soybean meal-based. Treatments consisted of: 1) low level of feed-grade AA with a SID Lys:CP ratio of 6.0:1; 2) moderate level of feed-grade AA with a SID Lys:CP ratio of 6.5:1; 3) high level of feed-grade AA with a SID Lys:CP ratio of 7.0:1; 4) diet 3 with di-ammonium phosphate (DAP) added to achieve a SID Lys:calculated CP ratio of 6.5:1; 5) diet 3 with glycine added to achieve a SID Lys:calculated CP ratio of 6.5:1. All diets contained 0.4% titanium dioxide for determination of apparent total tract digestibility (ATTD) of dry matter (DM) and crude protein (CP). Feces were collected on d 21 from three pigs per pen. Overall (d 0 to 21), G:F worsened (linear, P = 0.002) as the SID Lys:CP ratio increased above 6.5 with additional feed-grade AA. Adding DAP or glycine to the high feed-grade AA diet improved (P ≤ 0.003) G:F compared to pigs fed the high feed-grade AA diet because of a reduction (P = 0.007) or numerical reduction (P = 0.109), respectively, in ADFI. There was a decrease (linear, P < 0.001) in blood urea nitrogen (BUN) as feed-grade AA increased. Adding nitrogen to the high feed-grade AA diet increased (DAP; P = 0.038) or tended to increase (glycine; P = 0.091) BUN. Pigs fed the moderate feed-grade AA diet tended (quadratic, P = 0.051) to have the lowest fecal DM. Pigs fed the diet containing DAP had increased (P = 0.005) fecal DM compared with pigs fed the high feed-grade AA diet. The ATTD of DM tended to increase (P = 0.060) for pigs fed the DAP diet compared with the pigs fed the high feed-grade AA diet. The ATTD of CP decreased (linear, P = 0.048) as the SID Lys:CP ratio increased. Pigs fed the diets containing either DAP or glycine had increased (P ≤ 0.026) CP digestibility compared with pigs fed the high feed-grade AA diet. These data suggest that diets for 11 to 23 kg pigs should be formulated to a SID Lys:CP ratio of 6.5 or lower and that adding a non-protein nitrogen source or non-essential AA to diets formulated above this ratio can improve G:F and CP digestibility.

Smallfield, J., M. Tokach, J. Woodworth, R. Goodband, J. DeRouchey, k. Gaffield, J. Gebhardt, K. Haydon, A. Warner and C. Hastad. 2025. Determining if nitrogen is a limiting factor to maintain feed efficiency when feeding low protein, amino acid fortified diets. 2025 American Society of Animal Science Midwest Section meeting, Abstract 140.