Samples of 25 grass silages and of one maize, one lucerne and one whole crop wheat silage, of known digestibility, chemical composition and cell-wall degradability, were scanned on an near infrared spectrometer (NIRS). Also scanned were samples of the cell-wall (i.e. neutral detergent fibre (NDF)) fraction of the silages and the cell-wall residues produced by incubating the silage in buffered rumen fluid for 3, 8, 16, 24, 45 or 72 h and boiling with neutral detergent solution. The data points were transformed by the standard normal variate and detrending (SNV-D) procedure. Difference spectra were obtained by subtracting the spectrum of the 72 h residue from those of each of the 3, 8, 16, 24 and 45 h residues. The aim was to investigate the extent to which NIRS may be useful as a means of learning more about the digestibility and composition of silages and predicting the rate and extent of cell-wall degradation.
The difference spectra of all the silages examined were of similar pattern, with prominent positive regions, indicating components of high degradability at ∼1430–1620 and 1960–2230 nm, a prominent negative region, indicating components of low degradability, at 2230–2400 nm, and other negative regions at 1140–1430 and 1620–1820 nm. Among the 25 grass silages, the spectral regions with the strongest positive correlation with digestibility included those around 1676 and 2228 nm, which corresponded approximately with regions of strong negative correlation with lignin (1672 and 2246 nm) and NDF (1680 and 2214 nm). There were spectral regions of strong positive correlation with fractional rate of cell-wall degradation h−1, indicated by c, around 1684 and 2214 nm, which corresponded with regions of strong negative correlation with silage NDF (1680 and 2214 nm); there were regions of strong negative correlation with c at around 1414 and 1922 nm, which corresponded with regions of strong positive correlation with silage NDF (1406 and 1922 nm). The array of information provided by near infrared (NIR) spectra has the potential to enhance understanding of digestion and cell-wall degradation.