Lignification of switchgrass (Panicum virgatum) and big bluestem (Andropogon gerardii) plant parts during maturation and its effect on fibre degradability

Five cultivars of switchgrass (Panicum virgatum L) and four cultivars of big bluestem (Andropogon gerardii Vitman) were harvested at vegetative, boot and heading stages of maturity. Leaf and stem fractions were analysed for detergent fibre composition and 48-h ruminal in-vitro degradability, ester- and ether-linked non-core lignin phenolic acids, and core lignin composition. Big bluestem leaves contained more neutral detergent fibre than switchgrass, but general composition of the fibre did not differ. Stem fibre of switchgrass had relatively lower levels of cellulose and lignin at the vegetative stage than observed in big bluestem. Esterified and etherified p-coumaric and ferulic acid concentrations were generally higher in switchgrass plant parts. Yield of nitrobenzene oxidation products from core lignin was greater for switchgrass leaves, but very little difference in composition was noted. Leaf tissue contained lower concentrations of all lignin components than stems. Maturation resulted in increased total lignification, but all components did not respond in the same manner. Variation for all measures of lignification seemed to be as great within species as between the grass species. Degradability of fibre declined with maturation. The only species difference was that switchgrass fibre was more degradable at the vegetative stage. Relationships between lignification and fibre degradability were in agreement with some, but not all, previously reported relationships. Concentration of core lignin was only a significant predictor of fibre degradability when the relationship was examined across maturity stages. Within a stage of maturity, lignin composition was more closely related to fibre degradability than was concentration. However, the best predictors of fibre digestibility differed among species, plant part and maturity.

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