Rate and Extent of Digestion of Cell Wall Components of Brown-Midrib Sorghum Species

Cell wall composition and structure are important factors influencing utilization of forages in diets of ruminant animals. Our objective was to compare the cell wall composition and rate and extent of digestion of cell walls of normal and brown-midrib (bmr) genotypes of sorghum [Sorghum bicolor (L.) Moench.) species. Brown-midrib ‘Greenleaf’ sudangrass [Sorghum bicolor (L.) Moench.], and ‘Redlan’ ✕ ‘Greenleaf’ and Redlan ✕ ‘Piper’ sorghum ✕ sudangrass hybrids were planted in the field in a Zanesville silt loam soil (finesilty, mixed, mesic Typic Fragiudalf) along with corresponding normal genotypes. Stover was harvested and analyzed sequentially for fiber components, total nonstructural carbohydrates, and neutral sugar and phenolic acid composition of cell walls. Samples were incubated in buffered rumen fluid for 0, 6, 9, 14, 21, 48, and 72 h and rate constants were determined for neutral detergent fiber (NDF), cellulose, and hemicellulose digestion. The NDF, cellulose, and lignin concentrations of bmr genotypes were lower than the corresponding normal genotypes. This resulted in 7% higher in vitro dry matter digestibility of bmr tissues over the normal genotype. Rate constants for digestion of NDF, cellulose, and hemicellulose differed, averaging −0.083, −0.088, and −0.079 h⁻ for the bmr genotypes, compared to −0.064, −0.068, and −0.066 h⁻ for the normal genotypes, respectively. The bmr genotypes had a higher extent of NDF, cellulose, and hemicellulose digestion when compared to the normal genotypes. The lignin concentration in the cell wall of bmr genotypes was initially lower than normal genotypes, but as digestion progressed the lignin concentrations of both genotypes merged at 138 g kg⁻ NDF. This indicates that the proportion of lignin in the cell wall may be a limiting factor for extent of cell wall digestion. A portion of the lignin initially present was apparently digested, with extent of lignin digestion at 111 g kg⁻ for the bmr genotypes compared to 186 g kg⁻ for its normal counterpart. Ferulic acid (FA) concentration on a NDF basis was higher and p-coumaric acid (PCA) concentration lower in the bmr genotype, indicating considerable differences in linkages may exist between core lignin and structural carbohydrates. The FA/PCA ratio was 0.75 and 0.34 for the bmr and normal genotypes, respectively. This agrees with the suggested positive relationship between cell wall digestion and the FA/PCA ratio.