The objective of this study will be to evaluate the length of adaptation period to high concentrate diets on rumen health of Nellore and ½ Angus/Nellore cattle finished in feedlot. Forty-eight yearling bulls (370 kg), 36 Nellore and 36 ½ AngusxNellore, backgrounded on pasture will be randomly allocated in 24 pens with concrete floor (24 m² and 2,0 m bunk per animal) according to the treatments: T1: Nellore adapted for 9 days, T2: Nellore adapted for 14 days, T3: ½ AngusxNellore adapted for 9 days, and T4: ½ AngusxNellore adapted for 14 days), characterizing a completely randomized block design with a 2 x 2 factorial arrangement of treatments (2 genotypes and 2 lengths of adaptation period). Each treatment will be composed by 6 replications (pens) in this study. After 9 or 14 days of adaptation, yearling bulls will receive the finishing diet containing 86% concentrate. At the beginning of experimental period and every 28 days, yearling bulls will be weighed. After 9 or 14 days of adaptation period the animal will be fed will be fed with 86% concetrate diet which will contain: 14.0% sugarcane bagasse, 66.5% corn grain processed as cracked, 16.0% cotton seed, 1.2% urea, 0.8% mineral supplement, and 1.5% limestone. At harvest, rumens of all animals will be emptied and washed, and a rumenites score will be performed. Consequently, samples from cranial and ventral sac of rumen epithelium of each animal will be collected to determine the number and area of papillae per cm2, absorptive surface area, height and thickness of papillae, thickness of keratinized layer, and mitotic index.
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