Determination of nitrogen and ash contents in total herbage and botanical components of grassland systems with near infra-red spectroscopy

Near infra-red reflectance spectroscopy (NIRS) was used to develop calibration equations to measure nitrogen and ash contents in grassland samples. Four populations of samples were collected or prepared: total herbage, with a heterogeneous and complex botanical composition, and its botanical components (grasses, legumes and forbs). A set of samples from each population was selected to develop the specific calibration equations using three mathematical data treatments (log 1/R, first and second derivative). Six and seven wavelengths were selected by multiple regression to predict nitrogen and ash contents, respectively. Calibration equations were evaluated by comparing the values obtained by reference methods (Kjeldahl test and dry ashing measurements) with those predicted by NIRS. The three data treatments generally provided similar results as regards estimations of nitrogen contents. The correlation coefficients varied from 0.94 to 0.98, and the standard errors of prediction ranged from 1.10 to 1.49 g kg−1 (total herbage), from 0.73 to 0.98 g kg−1 (grasses), from 0.99 to 1.30 g kg−1 (legumes) and from 0.76 to 0.80 g kg−1 (forbs). The most suitable treatments to predict ash contents were log 1/R and the first derivative. The best performance was obtained for legumes, using log 1/R, with a correlation coefficient of 0.95 and a standard error of prediction of 3.54 g kg−1. The calibration equations became more accurate as the components of the sets of samples became botanically simpler. Prediction accuracy was greater when the specific calibration equations for each population of samples were used.