A total of 810 one-day-old, straight-run broilers were used to evaluate the effects of dietary nutrient density and feed additives included in the starter diet on their performance, intestinal microbiota, gut morphology, and immune response. A 3×3 factorial arrangement with three nutrient densities (100, 103.75 and 107.5%, as recommended) and three feed additives (no additives, 0.5 g/kg diet Maxi-Gen, and Maxi-Gen + Superzyme + Bio-Phytase at the rate of 0.5, 0.25 and 0.1 g/kg diet,

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... ), fed from 1 to 10 d of age. Similar commercial corn-soy grower and finisher diets fed to all birds from 10-24 and 24-42 d of age, respectively. There was higher (p<0.05) body weight gain and lower (p<0.05) feed conversion ratio in chicks fed starter diet with 107.5% nutrient density and Maxi-Gen with or without exogenous enzymes compared with those fed control diet at 10 and 42 d of age. Lactobacilli and Bifidobacteria counts in the cecal content were increased linearly as dietary nutrient density increased in 10-d-old birds (p<0.05). Higher duodenal and jejunal villus height and villus height to crypt depth ratio (p<0.05) were measured in the birds fed the starter diets with 103.75% and 107.5% nutrient density at 5 and 10 d of age. Total anti-SRBC and IgM titers were significantly higher in the broilers fed the 107.5% nutrient density diet containing feed additives at 35 day of age. It is concluded that higher nutrient density and the inclusion of feed additives in the starter diet may improve the growth performance, gut morphology, and immune response of broiler chickens.