Effects of oven- or freeze-drying on chemical composition and NIR spectra of pasture silage

In order to assess effects of drying method on chemical composition and NIR spectra of pasture silage, 20 samples of silages from permanent pastures of different qualities were subsampled and dried either by forced draught oven at 65°C or freeze-dried. Samples were analysed for crude protein (CP), crude fibre (CF), neutral detergent fibre (NDF), acid detergent fibre (ADF), gross energy (GE) and in vitro digestible organic matter in the dry matter (DOMD) and composition expressed on a dry-matter (DM) basis, either obtained as oven 105°C DM (DM105°) or toluene DM (DMtol). Effects of the drying method on chemical composition were estimated by paired comparisons. Near-infrared reflectance (NIR) spectra were taken and the difference spectra between treatments for each sample were plotted in order to visually inspect effects of treatments, either as log 1/R or transformed by their first derivative. Principal components explaining spectral variability were computed and samples graphically displayed according to the eigenvalues of the first (1 and 2), or second (2 and 3) pairs of principal components. Oven-drying resulted in a reduction in CP (p<0.02) content and an increase in CF (p<0.1), NDF (p<0.001) and ADF (p<0.06) content of silages, when expressed on a DM105° basis. When expressed on a DMtol basis, a reduction in CP (p<0.02) and DOMD (p<0.06) content was observed with oven drying, but fibre fractions were not affected (p>0.1). Effects of the drying treatment were visually apparent on NIR spectra when plotted as log 1/R against wavelengths, mainly as a baseline shift. Difference spectra (OD − FD), both as log 1/R and its first derivative, showed consistent absorption bands, indicative of different molecular responses of OD and FD silages to incident light. Plotting samples according to eigenvalues of the first two or second two principal components suggests that drying methods affect the distribution of samples. This is also indicative of an effect of drying treatment on spectral features and reinforces the general advice that sample processing should be consistent in order to avoid adding errors to NIR analysis.