REGIONS OF NORMALIZED NEAR-INFRARED REFLECTANCE DIFFERENCE SPECTRA RELATED TO THE RUMEN DIGESTION OF STRAWS

The normalised and detrended (SNV-D) near infrared reflectance (NIR) spectra of partially rumen digested (0, 3, 8, 16, 24, 45 and 72 h incubations) residues of ten cereal straws (eight barley, two wheat) untreated and treated with ammonia were obtained. Cell wall fractions were also determined on the residues. For each straw the spectra of the 0 h incubation (washed in water only) was subtracted from the spectra relating to the other incubation times. This resulted in a clear pictorial image of the time course of digestion of the water-insoluble fraction of the straws. The SNV-D difference spectra was compared with similarly obtained difference spectra for simple log 1/R values and second derivative log 1/R values. The SNV-D spectral regions which became increasingly positive with digestion time were centred around 1672 and 2254 nm. These regions appear to indicate increased concentrations of indigestible cell wall material and changes in lignin content accounted for 90-94% of the variability in the 2254 nm peak amplitude. Spectral regions centred around 1498 and 2086 nm became increasingly negative with digestion time and the shape of these SNV-D difference spectra corresponded well with the SNV-D spectra of glucose and xylose. The important spectral regions were identical in untreated and ammonia-treated straws, although the changes tended to be of greater magnitude in treated material. It was concluded that SNV-D difference spectra can describe the time course of rumen digestion in spectral terms and the important spectral regions identified give a biological basis to the statistical wavelength selection used for prediction of digestibility.