TILLER DEVELOPMENT INFLUENCES SEASONAL CHANGE IN CELL-WALL DIGESTIBILITY OF BIG BLUESTEM (ANDROPOGON-GERARDII)

The productivity of cool-season forage grasses declines during midsumm er in the southern corn belt of the USA. Warm-season grasses are produ ctive during this time, but their digestibility decreases from spring to summer. The objective of this study was to investigate cell wall fa ctors contributing to the decline in in vitro dry matter digestibility (IVDMD) of big bluestem (Andropogon gerardii Vitman) leaf blades from late spring (May) to mid-summer (July) under three harvest regimens. Under frequent defoliation, decrease in cell wall digestibility accoun ted for much of the change in IVDMD. Among cell wall constituents, onl y xylose and alkali-labile phenolic acids increased significantly from spring to summer. Change in p-coumaric acid content and the ratio of p-coumaric acid to ferulic acid were both negatively correlated with t he decrease in cell wall digestibility. While glucose and uronic acid digestibility decreased from spring to summer, xylose was consistently the least digestible of the cell wall monomers. In warm-season grasse s grown under constant environments, tiller development causes an incr ease in leaf structural tissue, and the data suggest that the seasonal decline in leaf blade cell wall digestibility was partially due to ti ller development.