Vitamin A needs based on Ross manuals is 9500 IU/kg more than NRC (1994) recommendation, thus the effect of vitamin A on mucin2 gene expression, histological characteristics and performance were evaluated in Ross male broilers fed diets containing no vitamin A supplement, 1500 IU/kg vitamin A supplement (NRC requirement), 6250 IU/kg vitamin A supplement (average of NRC and Ross requirement) and 11000 IU/kg vitamin A supplement (Ross requirement). The dietary treatments consisted of an isonitrogenous corn-soybean meal-based diet. At the first day of chicken age, 24 pens were equalized to 12 birds per pen, in a completely randomized design and dietary treatments were randomly distributed for the 14-d period. Live performance measurements improved (P< 0.05) as dietary vitamin A supplement increased from 0 to11000 IU/kg. The least performance was related to no vitamin A supplement containing diet. Histological assays showed that villi height, crypt depth and villi surface increased as dietary vitamin A supplement increased .There was no effect of threonine on mucin2 gene expression and goblet cells density. According to these results, vitamin A needs in starter period in Ross (308) broilers is more than NRC (1994) recommendation.

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