Nitrogen fertilizer application and competitive balance between spring wheat (Triticum aestivum L.) and volunteer oilseed rape (Brassica napus L.)

Abstract

We investigated the effect of nitrogen fertilizer on the competitive balance between spring wheat (Triticum aestivum L.) and volunteer oilseed rape (Brassica napus L.). The nature of the response of the species was also studied through evaluations of canopy structure and N absorption. Wheat and weed were sown in the field in monocultures and mixtures, receiving each of three nitrogen fertilizer levels (0, 40 and 100 kg/ha N). Aerial biomass production and N content were determined at four times during the experiment. Light interception and plant height were measured twice and relative leaf area was periodically evaluated using a point quadrat method. Nitrogen fertilizer increased biomass yield of both wheat and oilseed rape, but significantly altered the competitive balance between the two species. The crop was more competitive than the weed at low soil N availability. However, they were equally competitive at the highest N fertilizer level; significant crop losses due to weed competition were observed only under this situation. Total light interception was greater under the 100 kg/ha N fertilized (87%) than under the unfertilized treatment (40%). Observations of the structure and development of the canopies showed that the crop had access to relatively more light at low than at high soil N when in competition with volunteer oil seed rape. The oilseed rape N absorption was significantly lower in crop-weed mixtures than in monocultures for any N fertilizer treatment, but crop N absorption was only reduced at high N fertilizer doses. Soil fertility manipulation has to be considered part of wheats integrated weed management programs.