Analysis of Alfalfa Root Carbohydrate Concentration by Near Infrared Reflectance Spectroscopy1

Abstract

Near infrared reflectance spectroscopy (NIRS) has been used to evaluate forage plants primarily for their feeding quality. Our objective was to determine whether NIRS could also accurately measure the root total nonstructural carbohydrate (TNC) concentration of alfalfa (Medicago saliva L.) established by various methods and grown to a wide range of maturities in several environments. We assayed root TNC concentration of two alfalfa cultivars harvested by eight cutting schedules in each of 2 years after establishment and of one cultivar during the year of establishment with and without companion crops harvested at various small grain maturation stages within each of 2 years. Calibration equations for TNC concentration were obtained by multiple linear regression of conventional laboratory assay values on NIR spectra from 54 randomly selected samples using a scanning monochromator NIR spectro‐computer system. Calibration equations were verified with 18 additional samples selected from the original population. Five wavelengths were required to produce the best equation for TNC concentration analysis for each of the data treatments (log 1/R, first derivative, and second derivative). The squared coefficients of multiple determination (R2) of the three equations ranged from 0.98 to 0.99. The lowest standard error of analysis by NIRS of TNC concentration in verification samples was 1.17 dag kg−1 (%) dry weight for the log 1/R data treatment. We conclude that NIRS can analyze the TNC concentration of alfalfa roots more conveniently than, and as accurately as, conventional laboratory techniques and that accurate and precise analysis is not limited to a single mathematical treatment of the NIR data.