LIGNIN IMPACT ON FIBER DEGRADATION .1. QUINONE METHIDE INTERMEDIATES FORMED FROM LIGNIN DURING IN-VITRO FERMENTATION OF CORN STOVER

Experiments were conducted to determine whether formation of quinone m ethide intermediates from lignin occurs during ruminal fermentation of corn stover, as indicated by nucleophilic addition reaction with sulp hur-containing reducing agents. Corn stover leaf and stem fractions ha rvested at full maturity were incubated in buffered ruminal fluid with out reducing agents or with (NH4)(2)SO4 (S-control), Na2S.9H(2)O, cyst eine-HCl (cysHCl), or cysHCl plus Na2S.9H(2)O; and in only buffer with or without cysHCl plus Na2S.9H(2)O. Mixed reducing agents (cysHCl plu s Na2S.9H(2)O) enhanced (P < 0.001) in vitro fibre degradation after 4 8 h, tended to increase solubilisation of fibre (P = 0.07) and dry mat ter (P = 0.06) in buffer alone, and elevated (P < 0.001) S-content of residual fibre. In vitro incorporation of S into the undegraded fibre was determined for corn stover fractions of varying lignin composition s 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 cont ent (r = -0.84, P < 0.001). The negative association of lignin methoxy l content with digestibility is explained by the relative likelihood o f quinone methide intermediate formation from guaiacyl and syringyl un its in lignin.