THE RATE AND EXTENT OF CELL-WALL DEGRADATION IN-VITRO FOR 40 SILAGES VARYING IN COMPOSITION AND DIGESTIBILITY

The rate and extent of cell-wall degradation was recorded in 40 silage s in order to explore the variation between silages and provide materi al and results to use in the search for a lower cost method. The metho d used in the present study was selected from four compared previously (Wilman et al., 1996. Anim. Feed Sci. Technol., 63, 99-109). Freeze-d ried, milled silage was incubated in buffered rumen fluid in sealed tu bes for 0, 3, 8, 16, 24, 45 or 72 h and then boiled with neutral deter gent. The constants a, b and c in the equation p = a+b(1-e(-ct)) (Orsk ov and McDonald, 1979. J. Agric. Sci. Cambridge, 92: 499-503.) (where p is the percentage of cell wall degraded after t hours of incubation) were calculated for each silage. The rate of cell-wall degradation in the early stages of fermentation was much higher in a lucerne silage than in the grass and forage maize silages. Mixtures of lucerne and fo rage maize were intermediate in rate of cell-wall degradation. Among 2 8 grass silages, 'c' varied from 0.029 to 0.066, indicating wide varia tion in rate of cell-wall degradation; 'c' was negatively correlated w ith the cell-wall content of the silage (r = -0.81), suggesting that t he cell wall from the younger, leafier crops was degraded more quickly than that from more mature crops. The cell-wall degradation curve fro m 3 to 72 h of incubation was well described by the equation p = (a b)(1 - e(-c(t-to))), where t(o) is the lag period before rapid degrada tion began. In the case of the 28 grass silages there were strong corr elations between '(a + b)' and cell-wall degradation after 72 h (r = 0.92) and between 'c' and cell-wall degradation after 24 h as a percen tage of cell-wall degradation after 72 h (r = +0.95), suggesting that, for some purposes, it may be sufficient to have only 0, 24 and 72 h o f incubation.