Lignin Impact on Fibre Degradation: 1—Quinone Methide Intermediates Formedfrom Lignin During In Vitro Fermentation ofCorn Stover*

Experiments were conducted to determine whether formation of quinone methide intermediates from lignin occurs during ruminal fermentation of corn stover, as indicated by nucleophilic addition reaction with sulphur-containing reducing agents. Corn stover leaf and stem fractions harvested at full maturity were incubated in buffered ruminal fluid without reducing agents or with (NH4)2SO4 (S-control), Na2S.9H2O, cysteine-HCl (cysHCl), or cysHCl plus Na2S.9H2O; and in only buffer with or without cysHCl plus Na2S.9H2O. Mixed reducing agents (cysHCl plus Na2S.9H2O) enhanced (P<0.001)in vitrofibre degradation after 48 h, tended to increase solubilisation of fibre (P=0.07) and dry matter (P=0.06) in buffer alone, and elevated (P<0.001) S-content of residual fibre.In vitroincorporation of S into the undegraded fibre was determined for corn stover fractions of varying lignin compositions that were harvested at two maturities (early dent and full maturity) in 2 years. Extent of fibre degradation was correlated with extent of S-incorporation (r=-0.54,P<0.001), and with lignin methoxyl content (r=-0.84,P<0.001). The negative association of lignin methoxyl content with digestibility is explained by the relative likelihood of quinone methide intermediate formation from guaiacyl and syringyl units in lignin.