Evaluation of four NIR spectrometers in the analysis of cattle slurry

• The performance of four NIR spectrometers were investigated and compared.
• Ninety-nine cattle slurries were characterised and used for instrument calibration.
• Calibrations were at least moderately useful for dry matter, total C and organic N.
• Benchtop instruments outperformed portable instruments for total and ammoniacal N.
• The higher resolution of the benchtop instruments explained their better calibrations.

The composition of animal slurries is basic information required to arrange fertilisation plans that allow satisfactory crop yields and avoid environmental pollution. Slurry management based on measured composition data requires the development and adoption of analytical methods that are rapid, affordable and applicable directly in the field. NIR spectroscopy is a candidate technique for this application. In the present work, the performances of four NIR spectrometers, differing in optics, detectors and price, were investigated and compared: one FT-NIR and one dispersive benchtop instrument; a less expensive diode array dispersive instrument also suitable for online applications; and a low-cost interferometric portable instrument. Ninety-nine slurries, collected from livestock farms in Lombardy (Italy), were characterised using reference methods for dry matter (DM), ashes (ASH), total nitrogen (TKN), ammonium nitrogen (AN), organic nitrogen (ON), total carbon (TC) and total phosphorus (TP) content, and were used to investigate the performance of the four NIR spectrometers. The calibrations obtained from the two benchtop instruments for DM, ASH, TKN and TC were classified “useful” or “moderately useful”, while for AN and TP they were “useful for screening purposes”. The two portable instruments provided less accurate calibrations, with the exception of DM and TC which, in the case of the diode array instrument, performed similarly to the benchtop spectrometers. The higher resolution and, to a lesser degree, the wider spectral range of the two benchtop spectrometers contribute to explain their better calibration performances for TKN and AN.