Regions of normalised near infrared reflectance difference spectra related to the rumen degradation of fresh grass, grass silage and maize silage

The chemical composition and in situ rumen degradability of dry matter (DM) was determined on five samples each of fresh grass, grass silage and maize silage. After rumen incubation (for 0, 3, 8, 16, 24, 45 and 72 h) the normalised and detrended (SNV-D) near infrared reflectance spectra of the dried, milled forage residues were obtained. For each forage the SNV-D spectra of the 0-h incubation (i.e., washed in water only) was subtracted from the spectra relating to the other incubation periods. The SNV-D difference spectra obtained were compared with the DM degradability data and with the SNV-D spectra measured on cellulose, extracted lignin, starch, glucose, sucrose and fructose. In all forages, spectral regions associated with low degradability were in the 1620 to 1690 nm and 2170 to 2290 nm bands and these corresponded well with the SNV-D spectra of extracted lignin. Spectral regions associated with high degradability (1430 to 1630 nm; 2020 to 2190 nm (2230 in grasses)) were in good agreement with the SNV-D spectra of cellulose and various sugars. Correlations between spectral peak amplitude and rate of peak height change with the extent and rate of DM degradation respectively were weak although a negative correlation (−0.48) was observed between peak amplitude around 2250 nm and the extent of degradation of the insoluble DM. It was concluded that SNV-D difference spectra can describe the time course of rumen degradation of these forages and that spectral regions associated with low degradability are most likely related to lignin. However, further nutritional interpretation of these spectra may be limited because of, for example, the similar and overlapping absorption wavelengths of various carbohydrates whose nutritional value may be different.