Levels of Ig in the serum of broiler chickens given experimental diets at day 28 and day 56. Abbreviations: ANT, basal diet supplemented with 30 mg/kg bacitracin; GML1, basal diet supplemented with 500 mg/kg α-GML; GML2, basal diet supplemented with 1,000 mg/kg α-GML; NCO, basal diet provided as the control. Bars represent mean ± SD (n = 8). Different lowercase letters (a, b) above bars represent significantly different means (P < 0.05).