About the influence of weeds on spring barley NDVI determined from MODIS satellite data
A comparison of field data on aboveground barley biomass on a certain plots in Tula Region with NDVI MODIS values presented by the internet service VEGA was conducted. The crop fractions, as well as total weight of crop and weeds aboveground biomass, were used as the indicators of crop status. As a result of investigations it was found that NDVI in the middle of the growing season is predefined by the combined effect of cultivated plants and weeds, and at the end of the season the role of weeds becomes predominant. Theoretically, NDVI for this period can be used as a proxy of general weedness of crops. In the case of NDVI usage as a predictor of crop yields it seems better to use index values, received during the first half of the vegetative season. Based on specifics of crop phenology and physiology, one can conclude that the same results can be received not only for barley, but also for wheat, triticale, and rye. The results obtained are valid only for the areas with the same agro-technologies and crop rotations. They should be considered when using the VEGA data for monitoring crop status at the level of individual fields and when predicting crop yield.