Digestion Kinetics and Cell Wall Composition of Brown Midrib Sorghum ✕ Sudangrass Morphological Components

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Abstract
Microbial degradation of plant fiber is an important factor limiting the utilization of forages by ruminant animals. The objectives of this study were to determine the composition and rate and extent of digestion of cell walls of normal and brown midrib (bmr) genotypes of two sorghum ✕ sudangrass [Sorghum bicolor( L.) Moench] hybrids and to assess the relationship between cell wall composition and digestibility. Plants, harvested at three stages of maturity, were separated into leaf blade, leaf sheaths, and stems and analyzed for fiber constituents, structural carbohydrates, and, alkali‐labile phenolic composition. Fiber digestion kinetics were determined utilizing in vitro batch cultures. Brown midrib genotype were lower (P < 0.05) in neutral detergent fiber(NDF) and acid detergent lignin concentrations than normal genotypes. Leaf components had a greater (P< 0.05) NDF concentration than stems; however, leaf cell walls were much less lignified than stem cell walls. There were no differences between genotypes (P > 0.05) in neutral sugar composition or in the xylose to arabinose(X:A) ratio. Morphological components varied in the concentration of all neutral sugars, except mannose, and in the X:A ratio. Brown midrib genotypes had a significantly lower trans‐p‐coumaric acid concentration and a p‐coumaric acid to ferulic acid ratio than normal genotypes. There was no difference (P>0.05) between genotypes in the rate of NDF digestion, which averaged 0.0413 and 0.0.95 h‐1 for bmr and normal genotypes, respectively; however, bmr genotypes. had a greater (P<.05)72 h extent of NDF digestion when compared to normal genotypes. The results of this study indicate that the p‐couramic acid concentration and the linear xylan content are important factors limiting the rate and extent of cell wall digestion.