ObjectivesWeed is one of the main factors affecting yield of rapeseed. Controlling its growth cost effectively plays key role for improving the production efficiency of direct seeding rapeseed.

MethodsA field trial was carried out in Wuhan during October 2014-April 2015. Three seeding amounts of rapeseed (1.5, 4.5 and 7.5 kg/hm²) and five nitrogen application rates (0, 60, 120, 180 and 240 kg/hm²) were designed in the experiment. The biomass, N content and N accumulation of weed and rapeseed were analyzed when the shoot biomass reached maximum. The Pielou evenness of weed community was analyzed with the Shannon index.

ResultsThe biomass and N accumulation of rapeseed were increased with the increases of seeding rate and nitrogen rate. The average rapeseed biomass of all N treatments with 4.5 and 7.5 kg/hm² seeding rates were increased by 23.3% and 45.2% than those with 1.5 kg/hm² seeding rate, and the N accumulation was increased by 21.2% and 39.2%, respectively. Compared with no nitrogen treatment, the rapeseed biomass were increased by 0.9, 1.7, 2.2 and 2.7 times from N application 60 kg/hm² to 240kg/hm², respectively, and the N accumulation was increased by 1.0, 2.0, 3.5 and 4.4 times, respectively. The biomass and N accumulation of weeds were decreased with the increase of the seeding amount. The average weed biomass of all N treatments with 4.5 and 7.5 kg/hm² seeding rates were decreased by 16.8% and 25.8% than those with 1.5 kg/hm² seeding rate, and similarly the N accumulation was decreased by 17.3% and 29.4%, respectively. However nitrogen supply improved weed biomass and N accumulation,but the increases were much lower than those of rapeseed. The N content in weeds was higher than rapeseed at the same treatment, and both of them were improved with the increasing of N supply. However the rapeseed was more sensitive to N fertilizer. Under N 240 kg/hm², the average N concentration of rapeseed was increased by 46.2%, while that of weeds was increased by 24.1%, compared with the control. The ratios of N accumulation of rapeseed to weeds were less than 1 under the seeding rate of 1.5 kg/hm², and were increased with the increase of seeding rate. The ratio in the 7.5 kg/hm²sowing quantity and 240 kg/hm²N rate was up to 2.2, which suggested that nitrogen competition of direct seeding winter rapeseed could be improved by high density and high N application. However, the increase of N rate could change the structure of weed population, and decrease the evenness of weed community.

ConclusionThe sensitivity of rapeseed in response to the N fertilizer and sowing amount is greater than that of weeds. The nitrogen competition ability of rapeseed can be improved by increasing the amount of N fertilizer and seeding amount.