Three experiments were conducted to determine if nitrogen is a limiting factor for growth performance when feeding low protein, amino acid (AA) fortified diets and determine the effects of standardized ileal digestible lysine to crude protein (SID Lys:CP) ratio on growth performance of 11- to 25-kg pigs. In Exp. 1, 981 pigs ([Fast Large White × PIC L02] × PIC 800; initially 10.3 ± 0.19 kg) were used in a 21-d study. Diets were corn-soybean meal-based consisting of: 1) a low level of feed-grade AA with a SID Lys:CP ratio of 6.0%; 2) a moderate level of feed-grade AA with a SID Lys:CP ratio of 6.5%; 3) a high level of feed-grade AA with a SID Lys:CP ratio of 7.0%; 4) diet 3 with added diammonium phosphate (DAP) added to achieve a SID Lys:CP ratio of 6.5%; and 5) diet 3 with L-Gly added to achieve a SID Lys:CP ratio of 6.5%. Average daily gain (ADG) was unaffected by dietary treatment but gain:feed ratio (G:F) decreased (linear, P=0.002; quadratic, P=0.054) as SID Lys:CP ratio exceeded 6.5%. Adding DAP or L-Gly to the high feed-grade AA diet increased (P ≤ 0.003) G:F compared to pigs fed the high feed-grade AA diet. In Exp. 2, 4,167 pigs (337×1050, PIC; initially 13.0 ± 0.27 kg) were used in a 14-d study. Diets were corn-soybean meal-based, and treatments arranged in a 2×5 factorial with main effects of SID Lys (1.15 or 1.30%) and SID Lys:CP ratio (6.00, 6.22, 6.46, 6.72, and 7.00%). Overall ADG was unaffected by dietary treatment; however, a SID Lys:CP × SID Lys interaction was observed for G:F (linear, P=0.012) where increasing SID Lys:CP ratio decreased(linear, P<0.001) G:F at both SID Lys levels with a more pronounced effect in diets formulated to 1.15% SID Lys. Lastly, Exp. 3 used 5,059 pigs (PIC 800 × Camborough and DNA 600 × 241; initially 11.0 ± 0.90 kg) in an 18-d trial. Treatment diets were arranged in a 2×6 factorial with main effects of dried distillers grains with solubles (DDGS; 0 or 15%) and SID Lys:CP ratio (6.01, 6.22, 6.45, 6.70, 6.97, and 7.26%). Overall, ADG was unaffected by dietary treatment, but a SID Lys:CP × DDGS interaction was observed (linear, P<0.001) where G:F increased then decreased (quadratic, P<0.001) in diets without DDGS, whereas in the diets with DDGS, G:F decreased (quadratic, P≤0.002) as SID Lys:CP ratio increased above 6.45%. In summary, a SID Lys:CP ratio greater than approximately 6.5% decreased G:F, but adding a protein or non-protein nitrogen source to low protein diets formulated above this ratio improves G:F.

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Smallfield, J.,M. Tokach, K. Gaffield, R. Goodband, J. Woodworth, J. Derouchey, J. Gebhardt, K. Haydon, A. Warner, C. Hastad, D. Shawk, N. Gainey, H. Cemin, and J. Soto. 2026. Effects of standardized ileal digestible lysine:crude protein ratio and the use of non-protein nitrogen on growth performance of 11- to 25-kg pigs. Journal of Animal Science, 2026, 104. https://doi.org/10.1093/jas/skag013

Read the abstracts/reports here for Experiments 1, 2, and 3.