Effects of ageing on the in vitro fermentation of cell walls and cell contents of entire, fractionated and composite leaves of Italian ryegrass

Changes in fermentability of cell walls and cell contents of Italian ryegrass leaves at two stages of maturity were measured to explain the generally observed decline in herbage quality with ageing. A herbage fractionation method was developed to separate cell contents and cell walls. Cell walls were either untreated or protease‐treated. Fermentation characteristics of all cell wall and cell contents fractions, as well as whole and recombined leaves, were measured using cumulative gas and volatile fatty acid (VFA) production. The gas production profiles of all substrates contained two significant phases. After fractionation, addition of the profiles from separated cell contents and untreated cell walls resulted in the same profile as for the recombined leaf. The strongest reduction in gas and VFA production due to leaf ageing was observed for cell contents. The increased ratio between branched and straight chain VFAs suggested that there had been an increase in the protein fermentation. Treatment of cell walls may have removed some easily fermentable cell wall components, as seen in the small differences in gas and VFA production between whole and recombined leaves. It has been concluded that the phases of gas production, separable in kinetic fermentability studies of complex animal feeds, need to be interpreted with caution. The large reduction in fermentability of cell contents with plant maturity, compared with the cell walls, indicated the importance of determining the role of cell contents in herbage quality studies, as the cell contents clearly do not remain uniform. © 2000 Society of Chemical Industry