Lysine oxidation by growing pigs receiving diets containing free and protein-bound lysine

Knowledge of amino acid availability in feedstuffs is central to accurate diet formulation. Dietary lysine oxidation was evaluated as a means of predicting dietary lysine availability. Growing pigs (30 kg) were offered control (1.43% lysine), free lysine-supplemented soybean meal-, or cottonseed meal-supplemented diets. Supplemented diets provided equivalent total lysine (approximately 24.2 g/d at 30 kg BW) but availabilities of lysine, determined by slope-ratio assay, in the free lysine, soybean meal, and cottonseed meal were 100, 90, and 30%, respectively. Feed was offered in eight equally spaced meals per day to achieve three times maintenance energy intake. Following a meal containing L-[U-14C]lysine (1 mu Ci/kg BW), lysine oxidation, as 14CO2 expired, was lower (P < .05) for the control diet but not different between the other three diets, contrary to the hypothesis. Lysine oxidation following an intravenous bolus dose was lowest (P < .05) for pigs fed the control, highest (P < .05) for pigs fed the cottonseed diet, and intermediate (P < .05) for pigs fed the free lysine-supplemented diet (27.1, 80.2, and 47.5, dpm/kg x 10(-2), respectively). Plasma lysine concentration was lower (P < .05) and lysine specific radioactivity tended to be higher (P < .10) following a meal containing cottonseed than following a meal containing free lysine, indicating that the lysine pool was smaller in pigs receiving the cottonseed-meal diet.(ABSTRACT TRUNCATED AT 250 WORDS)