Digestible Reactive Lysine in Processed Feedstuffs: Application of a New Bioassay

A new bioassay for determining digestible reactive lysine was applied to a range of processed feedstuffs. Semisynthetic diets containing various processed feedstuffs as the sole sources of protein and including chromic oxide as a marker were fed to growing rats. Digesta from the terminal ileum were collected and, with samples of the diets, analyzed for reactive lysine following reaction with O-methylisourea. True reactive lysine digestibility was determined by correcting for endogenous lysine loss at the terminal ileum of rats fed enzyme-hydrolyzed casein. True ileal digestibility of reactive lysine was similar to that of total lysine for blood meal, wheat meal, meat and bone meal, and soybean meal but significantly higher for dried maize (84.3% and 80.5%, respectively), an alfalfa-based mix (86.3% and 74.2%, respectively), heated skim milk powder (94.0% and 69.1%, respectively), and cottonseed meal (71.9% and 62.1%, respectively). When compared to digestible total lysine, digestible reactive lysine contents were lower for wheat (2.9 and 3.2 g kg-1), maize (1.9 and 2.6 g kg-1), heated skim milk powder (16.6 and 19.8 g kg-1), cottonseed meal (10.3 and 12.9 g kg-1), and the alfalfa-based mix (10.8 and 14.4 g kg-1). The new assay leads to different estimates of available lysine in processed feedstuffs compared to assays based on conventional analysis. Keywords: Lysine; digestibility; availability; protein; ileal