Effect of dietary β-glucan supplementation on growth performance, carcass characteristics and gut morphology in broiler chicks fed diets containing different theronine levels

The present study was conducted to investigate the effect of dietary β-glucan supplementation on performance, carcass characteristics and jejunal morphology in broiler chicks fed diets containing different threonine (Thr) levels. A total of 360 one-day-old Ross 308 broiler chicks were randomly distributed among 4 pen replicates of 6 dietary treatments according to a 2×3 factorial arrangement of treatments, including 2 different basal diets (control or diet supplemented with 30g/kg of β-glucan) and 3 different Thr levels (90, 100 or 110% of Ross recommendations during different growth periods). Trial lasted for 42days. In addition to performance parameters, 4 randomly-selected birds from each pen were slaughtered at the last trail day to evaluate carcass traits and morphological indices of jejunal epithelium. Results showed that dietary inclusion of β-glucan decreased (P<0.01) average daily feed intake (ADFI) during the starter and grower periods; consequently, it resulted in the significant decreases in average daily gains (ADG) during the starter (P<0.05), grower and entire (P<0.001) trial periods. In addition, feed conversion ratio (FCR) was increased (P<0.05) as a result of dietary β-glucan supplementation. Although ADFI was not affected by dietary Thr level, increasing Thr level increased (P<0.001) ADG, and in turn, caused the significant (P<0.01) improvements in FCR values during the starter, grower and entire trial periods. Dietary supplementation of 30g/kg of β-glucan increased (P<0.001) the relative weights of gizzard and small intestine. On the other hand, abdominal fat pad was decreased by supplemental β-glucan. Increase in dietary Thr up to 100% of recommended values resulted in an increase (P<0.01) in thigh percentage. Although dietary β-glucan inclusion decreased (P<0.001) villi height (VH), VH to crypt depth ratio (VH:CD) and villi absorptive surface area, it resulted in the significant (P<0.01) increases in submucosal and muscular layers thickness, and increased goblet cells count. On the other hand, dietary supplementation of Thr at the level of 110% of recommendations increased (P<0.01) VH:CD. The present findings indicated that dietary supplementation of β-glucan at high level diminished growth performance and impaired morphological indices of jejunal epithelial cells. On the other hand, Thr fortification of diet could compensate for a large part of these detrimental impacts on weight gain of broiler chicks.