Cereal rye (cereale secale L.) is a winter crop well suited for cultivation especially besides high yield areas because of its relatively low demands on the soil and on the climate as well. In 2016 about 4.9% of arable land in Germany was cultivated with cereal rye (Statistisches Bundesamt, 2017).

Unlike other crops such as wheat, there is little research on cereal rye for site specific farming. Furthermore, also in a cereal rye cultivation it is necessary to minimize nitrogen loss. This is especially important in low yield areas to avoid any lodging of the plants because of an overdose of mineral nitrogen (N). Therefore, an efficient fertilization strategy becomes important. To achieve this goal, some essential crop parameters such as above-ground plant biomass, N-content and N-uptake are necessary. The objective of this work was to evaluate the suitability of various vegetation indices to describe the N-uptake of cereal rye.

Over a period of nine years, from 2007 to 2016 two different kinds of plot experiments were conducted. The first one was a fertilization experiment, which consisted of only one variety of cereal rye treated with nine different N-rates from 0 up to 250 kg N per hectare (ha) which were applied at different growth stages. The second experiment consisted of four different varieties (population varieties as well as hybrids) and was treated with two different N-rates (100 and 160 kg N per ha). Each of the two experiments was designed as a double plot with four replications, in which one was for non-conducting reflectance measurements of the canopy during the vegetation period, and at the end harvesting with a plot combine; whereas the neighboring plot was for biomass sampling. Biomass samples for nitrogen analysis and sensor measurements have been performed four times during the growing season at certain growth stages.

The results indicated that certain vegetation indices (VI) calculated from reflectance measurements represented well the N-uptake of the crops at different growth stages. Especially simple ratios, calculated from two different wavelengths, as well as the Red Edge Inflection Point (REIP) represent best the N-uptake and the influence of the different cultivars was negligible. In comparison, the coefficients of determination of the widely known NDVI were only good at an early growth stage and, in addition, the higher the N-uptake was, the more NDVI showed a saturation effect at later growth stages. By using the NDVI-formula with other spectral areas to describe crop parameters the results were different. The saturation effect almost disappeared and the coefficient of determination increased significantly.