Distribution of Nitrogen between Crop and Weeds in Spring Cereals

The nitrogen content of the above-ground biomass of crop and weeds was assessed one week after crop heading in the beginning of July in spring barley and spring wheat. Crops were sown with a combined sowing and fertilizing machine at five seed rates. The trial plots received nitrogen, 90 kg ha−1. No herbicides were applied. In spring barley sown at a normal seed rate, less than 5% of the nitrogen in the above-ground biomass was accumulated in weeds. In spring wheat stands the corresponding amount averaged 10–15%. The nitrogen concentration in the crop biomass averaged 1.8% of the dry weight and that in mixed weed species 3.2%. Under normal growth conditions the N concentration in crop and weeds was relatively constant at the time of crop heading. Variations in N accumulation between crop plants and weeds are explained by the different biomass production of the plants.