Effects of dietary tryptophan levels and stocking density during the growing-finishing phase on broiler performance and immunity.

摘要

A total of 240 Cobb 500 broiler chicks, 18 day old were allocated to 10 treatments groups, each of which included 4 replicates. Experimental treatments consisted of a 5*2 factorial arrangement with 5 levels of L-tryptophan supplementation and 2 levels of stocking density (11.90 birds m -2 as the normal stocking density or 16.66 birds m -2 as the high stocking density). Crystalline L-Trp was added to the basal diet at 0.0 (100%, NRC), 0.25 (114%, NRC), 0.50 (128%, NRC), 0.75 (141%, NRC) and 1.00 (156%, NRC) g kg -1 diet to obtain the dietary Trp level at 1.8, 2.05, 2.3, 2.55 and 2.8 g kg -1. Increasing L-Trp level in the diet did not affect LBW, BWG and FCR (p>0.05). However, feed intake decreased significantly with L-Trp and it was minimized at 0.75 and 1.0 g kg -1 diet. Also, increasing L-Trp level had no effects on immunity, plasma total protein and glucose (GLU). However, adding L-Trp at 0.75 or 1.00 g kg -1 diet increased liver weight. Also, plasma cholesterol (CHO) levels decreased significantly (p<0.05) with L-Trp supplementation and the lowest levels occurred at 0.25 L-Trp (114%, NRC). In addition, plasma triiodothyronine (T3) and thyroxine (T4) levels were higher at 0.75 (141%, NRC) L-Trp supplementation (p<0.05). The normal stocking density resulted in better performance (p<0.05) compared with the high stocking density. However, stocking density did not affect plasma total protein, total Ig, IgG, IgM, GLU, CHO, T3 and T4 levels. Significant interactions between Trp level and stocking density were observed for plasma levels of CHO. Findings suggest that addition of L-Trp up to 0.25 g kg -1 achieve 114% of NRC recommendations of dietary Trp that has a positive effect on decreasing plasma total CHO levels.