Molecular mechanism of lignification on tropical grasses and the effect of altering fiber lignification on animal performance

Abstract

The low fiber digestibility of tropical forages, more specifically, the rapid decline in nutritional quality with advanced maturity is an important aspect that limits weight gain and productivity of beef cattle. Improving the efficiency of roughages with greater fiber digestibility can contribute to the increase of the Brazilian livestock productivity. Therefore, it is our objective to: 1) identify genes involved with the lignification process of leaves and stems using Marandu-grass and Mombaça-grass, identify the homologous genes of the lignification process, and determine changes in lignin composition that occur during advance in maturity; 2) characterize the lignification process in the stem of two sugarcane genotypes, with high and low fiber digestibility, and harvested at three maturity stages; and quantify the expression of homologous genes involved in the lignification process; and 3) quantify the effect of altering sugarcane fiber digestibility by different genotype or time of harvest, on the intake, performance, digestibility, rumen kinetics, and on rumen microbial population. In the 1st experiment, Marandu-grass and Mombaça-grass will be collected at three maturity stages for functional characterization of genes related to fiber quality. The collected material will be separated into stem and leaf blade fractions for further processing and RNA extraction from each morphological component. Samples will also be characterized by chemical composition, fiber digestibility and monomer lignin composition. The overall gene expression of the two species, in both tissues, and at the three cutting ages, will be quantified by next-generation sequencing (RNA-Seq), and the differential expression of the identified candidate genes will be confirmed by real-time polymerase chain reactions. The expression of the homologous forms of genes involved with lignification, such as CCoAOMT, C4H, CAD, CCR and 4CL, will be quantified by qPCR.In the 2nd experiment, two genotypes divergent for fiber digestibility will be harvested in three maturity stages (9, 11 and 14 months after harvest), and two internodes will be collected for functional characterization of genes related to sugarcane fiber digestibility, similarly to that performed in Experiment 1. In the 3rd experiment, the effects of altering sugarcane fiber digestibility by either different genotypes or by time of harvest will be evaluated using Fifty-six Nellore steers (350 kg body weight), being eight rumen cannulated steers. 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. (AU)