A comparison of four methods of estimating the rate and extent of cell wall degradation in grass silages

The feeding value of grass silages, in common with other forages, is constrained by the rate as well as the extent of cell-wall degradation in the rumen. Four methods of estimating the rate and extent of cell-wall degradation were compared using six replicates of each of six grass silages. The purpose was to select one biologically valid method which would be used on a larger number of silages, in order to provide data which would be used in the search for a lower cost, rapid method, suitable for extension work. The four methods compared were: (1) neutral detergent was used to remove the cell contents from freeze-dried, milled silage; the cell wall residue was then incubated in the rumen of sheep in polyester bags (pore size 43 μm) for different lengths of time (0, 3, 8, 16, 24, 45 or 72 h); (2) freeze-dried, milled silage was incubated in the rumen in bags for different lengths of time (as in (1)) and the residue treated with neutral detergent; (3) chopped fresh silage (1 cm) was incubated in the rumen in bags for different lengths of time (as in (1)) and the residue treated with neutral detergent after freeze-drying and milling; (4) freeze-dried, milled silage was incubated in buffered rumen fluid in sealed tubes for different lengths of time (as in (1)) and then treated with neutral detergent. Methods 2 and 3 had higher standard errors than Methods 1 and 4 and were rejected mainly for that reason. Method 4 was selected in perference to Method 1 because the cell wall degradation points fell on a smoother curve, a much larger number of samples could be started at the same time with the same batch of rumen fluid, the cost was lower and fewer fistulated animals were required.