Lignins in grass biomass and forage crops are highly acylated with p-coumarate, but the function of p-coumarate and its impact on cell wall degradability are poorly understood. In this study, cell walls from maize cell suspensions were artificially lignified with coniferyl alcohol and increasing proportions of sinapyl alcohol to mimic the shift from guaiacyl to syringyl lignin deposition observed during maturation of most grass tissues. During lignification, 0, 15, or 30% of the sinapyl alcohol was substituted with sinapyl p-coumarate (the main precursor of p-coumaroylated grass lignins) to assess its impact on the formation and characteristics of syringyl-rich lignins and the fermentability of cell walls. Inclusion of sinapyl p-coumarate accelerated peroxidase inactivation, interfered with cell wall ferulate copolymerization into lignin, and modestly increased cell wall lignin content from 109 to 123 mg/g. Although sinapyl p-coumarate enhanced oxidative coupling of sinapyl alcohol, the accumulation of nonreactive quinone methide intermediate products likely hindered more extensive formation of syringyl lignin. Copolymerization of sinapyl p-coumarate into lignin had little impact on the in vitro degradation of non-pretreated cell walls by anaerobic rumen microflora, suggesting that p-coumarate esters do not markedly influence the inherent fermentability of grass cell walls. In ongoing work, we are characterizing the enzymatic hydrolysis of intact and chemically pretreated cell walls to further assess the effect of p-coumaroylated lignins on grass cell wall saccharification for ethanol production.