Relationships between Cell Wall Digestibility and Lignin Content as Influenced by Lignin Type and Analysis Method

Abstract

Difficulties in measuring the lignin content of forage and biomass crops confounds efforts by plant breeders and molecular biologists to grasp how shifts in lignin chemistry affect fiber digestibility. In this study, relationships between digestibility and lignin content were examined by artificially lignifying Zea mays L. cell walls with bulk polymers composed of p‐hydroxyphenyl‐guaiacyl (HG) and guaiacyl (G) units or with endwise polymers composed of G, guaiacyl‐syringyl (GS), and guaiacyl‐syringyl‐p‐coumaric acid (GSpCA) units. Regressions of in vitro rumen gas production or nonfermented polysaccharides (NFP) vs. lignin content accurately measured by an indirect spectrophotometric method revealed that G‐bulk and especially syringyl (S)‐rich endwise polymers were the least inhibitory to digestion. While providing biased estimates at low lignin contents, the acetyl bromide‐soluble lignin (ABSL) assay correctly ranked how endwise polymers affected the digestibility of highly lignified cell walls, but the method required unique absorption coefficients to assess relationships for bulk polymers. Conversely, the Klason lignin (KL) method provided more accurate estimates at low lignin contents, but varying recovery of KL among lignin types created bias for highly lignified cell walls. Overall, our results indicate that S‐rich lignins are the least inhibitory to digestion, but detecting desirable shifts in lignin chemistry will require the judicious use of several analytical methods.