The present study aims to evaluate the influence of different sources of non-degradable protein in the rumen on the population of methanogenic Archaea and methane production in Nelores cattle finished in feedlot. It will be used 8 Nellore steers, average live weight of 380 kg, castrated and cannulated in the rumen. The animals will be allocated in individual stalls and distributed in a cross over design with three experimental periods, three protein sources and eight replications. The animals will be distributed in the following treatments: 1) soybean meal; 2) Corn Gluten 60; 3) Soybean meal protected. Total diets will be offered twice a day. The leftovers of the food will be withdrawn and weighed before the morning offer, in order to adjust the daily dry matter intake (DM). The consumption will be measured from the 15th to the 19th day of each experimental period, as well as the total collection of feces for five consecutive days, from the 15th to the 19th day and kept under refrigeration (4 º C). Dry matter (DM), organic matter (OM), Crude protein (PB), neutral detergent fiber (NDF), acid detergent fiber (FDA), lignin, Ether Extract (EE) and metabolizable Energy (MS) will be evaluated in the food, leftovers and feces samples. Samples of ruminal content will be collected on the 20th day of each experimental period. These will be refrigerated and taken to the laboratory to obtain a microbial pellet in order to concentrate the microorganisms and obtain higher DNA yields during the extraction. The qPCR method will be used for the quantification of Archaeas. The methane production will be estimated by the equation proposed by Ellis et al. (2007).
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