The amino acid requirements of pigs have been refined to standardized or apparent digestible values. Disappearance of amino acids from digesta indicates digestion and absorption but does not translate SID values directly to utilization or assimilation by the animal. Recent data indicate increased net (or productive) energy in SBM compared to previous estimates (NRC, 2012). The efficiency gain may be due to nitrogen retention. 

Objective: To compare the nitrogen retention efficiency of corn and soy protein and amino acids in the diets of young, growing pigs Materials and 

Methods: Twelve healthy growing pigs (18 kg BW) were used in a cross-over design with two protein sources (corn or soy) and synthetic amino acid additions (SAA). Treatment diets contained: 1) corn DDGS and corn fermented protein contained low (LowAA, 0.20% Lysine-HCl); 2) Corn protein and high synthetic amino acid levels (HiAA, 0.66% Lysine HCl);3) Corn-SBM with HiAAs (0.41% Lysine HCl and Met, Trp, and Thr.); 4) Corn SBM with no added SAA. Methionine (Met), threonine (Thr), tryptophan (Trp), valine and isoleucine were also provided as needed to HiAA diets. Pigs were adapted to either corn-soy or corn-DDGS-soy diets formulated to be equivalent in ME and SID lysine content and to meet or exceed estimated nutrient requirements (NRC, 2012). The pigs were placed in metabolism crates for total collection of urine and feces from d 1-4 and d 8-11 for determination of nitrogen excretion and retention. The pigs were fed 3X the maintenance requirement based on body weight in the a.m. and p.m. The data were analyzed using Proc Mixed procedures of SAS with significance at P< 0.05 and P< 0.01. 

Results: Fecal, urinary, and total nitrogen output increased in pigs fed corn protein compared to soy protein which together resulted in a higher relative nitrogen efficiency in pigs fed soy (P< 0.05). Despite equivalent dietary levels of SID lysine, the use of intact protein in both corn and soy protein diets increased total N retention and daily N retention more than 30%, when compared to the use of SAA (P< 0.05). Nitrogen efficiency, however, increased with SAA for either protein source (P < 0.01). 

Conclusion: The more efficient use of intact protein from soy or corn for protein retention indicates that valuing synthetic amino acids according to the SID coefficient does not result in equivalent protein and nitrogen retention in the pig. Also, the 30% reduction in nitrogen output of pigs fed soy protein when compared to corn protein has significant implications to the evaluation of soy relative to corn protein for sustainability initiatives. Feeding soybean meal increased nitrogen retention when compared to amino acid supplementation and soy protein is more efficiently used than corn protein.

Halbur, J., J. Hong, R. Samuel and E. Weaver. 2025. The effects of intact soy protein, corn protein or amino acid supplementation on nitrogen retention in pigs. 2025 American Society of Animal Science Midwest Section meeting, Abstract 183.