The objective of this study is to evaluate the effects of different doses of sodium monensin on morphological and histological parameters of rumen epithelium of feedlot Nellore cattle. Sixty 20-mo-old yearling Nellore bulls with 400 kg of average initial body weight, back-grounded on pasture, will be used in this experiment. Animals will be blocked by weight, and randomly they will be assigned to one of five treatments according to the dose of sodium monensin per kg of ration dry matter: 1) 0 mg, 2) 9 mg, 3) 18 mg, 4) 27 mg, e 5) 36 mg, characterizing a complete randomized block design, with 12 replications per treatment (1 animal per pen). Animals will be fed for 84 days, included 14 days of adaptation using multiple step up diets: 68% concentrate for 7 days and 76% concentrate for 7 days. After the adaptation period a 84% concentrate diet will be fed for 72 days, which will contain: 16.0% sugarcane bagasse, 71.5% corn grain processed as cracked, 7.7% soybean meal, 1.2% urea, 3.0% mineral supplement, and 0.6% 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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