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Precise nutrition: Correlations between the KOH protein solubility test and the coefficients of in vivo digestibility of 15 amino acids beyond lysine and arginine in commercial lots of soybean meal that have undergone the Maillard reaction

Ruiz, N., C. Parsons, and B. Parsons

At the 2022 International Poultry Scientific Forum (IPSF, Abstract M106), regression equations to estimate digestible lysine and digestible arginine in commercial soybean meal (SBM) using the KOH protein solubility (KOHPS) test were presented. The objective of this presentation is to further statistically analyze the correlations between KOHPS and the remaining 15 amino acids (AA). A total of 80 SBM samples were analyzed for KOHPS at Dairyland Labs, Arcadia, Wisconsin utilizing the newly standardized KOHPS method. Many of the samples had been overcooked and had undergone the Maillard reaction to varying degrees. These 80 samples came from a set of approximately 100 SBM samples previously analyzed for in vivo poultry amino acid digestibility and kept frozen during storage. The in vivo AA digestibility was determined utilizing the precision-fed rooster bioassay. A minimum of 4 cecectomized White Leghorn roosters per SBM sample were fasted for 24 h prior to being precision fed (crop intubation) 30 g. Excreta were collected for 48 h post feeding. The KOHPS values ranged from 86% to 13%. Linear and quadratic regressions of digestible AA coefficients on KOHPS were conducted using PROC GLM and broken line regressions were conducted using PROC NLIN in SAS. Regressions of digestible coefficients on KOHPS values were run for aspartic acid, threonine, serine, glutamic acid, proline, alanine, cysteine, valine, methionine, isoleucine, leucine, tyrosine, phenylalanine, lysine, histidine, arginine, and tryptophan. Effects were considered significant at P < 0.05. Only total concentrations of lysine and arginine were reduced as a consequence of Maillard reactions. Although for all AA their digestibility decreased as KOHPS descended from 86% to 13%, only 2 AA plateaued at KOHPS values of 80% and above: digestible lysine and digestible arginine. Aspartic acid was the next AA with a KOHPS value at plateau of 70%. In other words, the break point for the 1-slope broken line regression for digestible lysine occurred at 80% KOHPS (with a digestible lysine coefficient of 0.88), indicating that the minimum KOHPS value for SBM quality adequacy is 80% (IPSF, Abstract M106); therefore, none of the remaining AA are of significant concern from the quality of SBM stand point because their KOHPS values at plateau are well below 80% (ranging from 28 to 51%).

In conclusion, only digestible lysine and digestible arginine are of concern for prac-tical formulation purposes when KOHPS values are below 80%.