The effect of heat on amino acids for growing pigs

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Abstract
Two growth experiments were conducted to determine the effect of heat on the utilization of ileal-digestible lysine from field peas (Pisum sativum cultivar Dundale) fed to growing pigs. Five lysine-deficient diets (0.36 g ileal-digestible lysine/MJ digestible energy (DE)) were formulated using raw field peas, and field peas heated to either 110°, 135°, 150°, or 165° for 15 min respectively in a forced-air dehydrator. Additional diets were formulated with supplements of free lysine to verify that lysine was limiting in the diets containing the raw peas, and peas heated to 150° or 165°. The growth performance and retention of ileal-digestible lysine by pigs given the diets was determined over the 20–45 kg growth phase. Heat had a significant quadratic effect (P < 0.01) on growth rate, with responses declining from 543 g/d with pigs given the raw peas, to 407 g/d for those given the peas heated to 165°. Similarly, crude protein deposition declined in a quadratic manner (P < 0.001) from 76 to 36 g/d for pigs fed on raw peas and peas heated to 165° respectively. Retention of ileal-digestible lysine was 0.85 in the pigs given the raw field peas and declined in a quadratic manner (P < 0.001) with the application of heat to 0.48 in those pigs given the peas heated to 165°. Pigs fed on field peas heated to 165° had increased (P < 0.05) liver weights. The results indicate that heat applied to protein concentrates, even at mild temperatures, renders lysine in a form that is apparently absorbed but inefficiently utilized by the growing pig. Consequently, ileal digestibility values for lysine in heat-processed meals are unsuitable for diet formulations.