Cell wall digestibility is the main factor limiting the nutritive value of forages. The identification of parameter to evaluate the quality of the fiber that actually has an impact on animal performance is of great importance to the breeding programs of sugarcane. The objective of this study is to evaluate the effect of changing fiber digestibility by time of harvest and by genotype on performance, intake, rumen kinetics and rumen ecosystem of steers fed sugarcane silage on the following parameters: 1) dry matter intake, 2) weight gain and change in body condition, 3) NDF rumen rate of digestion and passage, 4) ruminal fermentation parameters, and 5) quantification of cellulolytic, amylolytic and lactate-fermenting rumen bacteria. Fifty-six Nelore steers (350 kg body weight) will be used, being eight rumen cannulated steers. The experimental diets will be composed of 40% of forage on the dry matter basis. The experimental design will be a randomized block with 12 replicates (performance) and a duplicated 4x4 latin square design (kinetics and ruminal metabolism). The volume and total mass of ruminal contents will be determined. Samples will be taken from solid and liquid phases to determine the size of the ruminal compartment of digesta components. Diet, orts and rumen digesta will be analyzed for nutrient content. Calculations of NDF rumen passage rate (kp), digestion rate (kd) and apparent ruminal digestibility will be performed. Real-time PCR will be used to quantify the relative population of the major bacteria in the sample of rumen contents.
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