Effect of Integrated Application of Canola (Brassica napus L.) Green Manure and Planting Density on Weeds Control and Sunflower (Helianthus annuus L.) Yield

Weeds are the most important limiting factor in agricultural systems and if they are not controlled, crop yields can be reduced up to 90% depending on the competitiveness of weeds and between 10 and 100% according to some literatures. One of the benefits of green manures is their ability to suppress weeds. Also, a higher crop density can reduce the biomass of weeds by reducing the amount of available light. Green fertilizers are plants cultivated in order to improve physical, chemical and biological soil properties and provide the necessary nutrients for the optimal crop growth. The use of different types of green manure is a suitable management for sustainable production in all agricultural ecosystems. In this way, the highest amount of light reaches to the crop and the least amount reaches to the competing weed. Increasing crop density can be an effective way to increase crop share of total source inventory and reduce weed dry weight. Rapeseed plant with allopathic substances in addition to food and medicinal uses, is also known as a weed controller providing a suitable environment for growing other plants by controlling weeds. This experiment was, therefore, performed to investigate the effect of canola green manure and planting density on weed control and sunflower yield.

the present study was conducted as split-plot design based on randomized complete block with three replications at the Research Farm of the Bu-Ali Sina University in 2015-2016 growing season, to evaluate the effect of canola green manure and crop density on weeds dynamic and sunflower yield. Weed free, using and non-using canola green manure were set as main-plots and sunflower densities (6, 8 and 10 plants.m-2) were considered as sub-plots. Rapeseed was mixed with soil at the beginning of flowering stage and after two weeks, sunflower was planted in the main plots with the desired planting densities. The amount of rapeseed biomass at the time of return to the soil was about 6250 kg of shoot dry matter per hectare. In each sub-plot, six rows with five meters long were planted. Weeds were sampled to determine their density and dry weight in three stages (three weeks after emergence, flowering and physiological maturation of sunflower) with three replications by 1 × 1 m2 quadrat. Weeds were counted by species and their biomass was measured after drying for 72 hours in an oven at 80 °C. Also, two square meters were taken from each experimental plot after applying the margin effect (one row on both sides and half a meter from the top and bottom of the planting row) so as to measure the yield of sunflower seeds. Weed control efficiency index was used to evaluate the ability of green manure and plant density in controlling weed of sunflower field. Data analysis was done by SAS ver. 9.1 and comparison of means was performed using LSD test at the significance level of 95%.

Maximum weeds density and biomass were found at the treatment of 6 plants.m-2 under non-using green manure. In comparison with this treatment, cultivation of sunflower with 10 plants.m-2 under using green manure increased weed control efficiency (WCE) up to 79% and decreased biomass of Amaranthus retroflexus, Chenopodium album, Setaria viridis, Convolvulus arvensis and total weeds up to 70, 76, 99, 92 and 72 %, respectively. Sunflower yield at the 10 plants.m-2 density was the highest and the same under weed free and weed infested by using green manure. The interaction effect of treatments on density and biomass of sunflower field weeds was significant. The highest and lowest density and biomass of weeds were obtained from uncontrolled at a density of six plants and green manure at a density of 10 plants, respectively. Also, the highest yield of sunflower seeds (411 g.m-2) was attributed to the density of 10 plants in weed control treatment, which was insignificantly different with the density of 10 sunflower plants in the application of green manure, which had a yield of 379 g.m-2. This can be attributed to the role of green manure in controlling weeds, increasing microbial activity, increasing food mobility and better use of water and nutrients. It seems that at low densities, the absorption of solar radiation and dry matter production are lower due to the low leaf area. Other researchers have attributed increased corn yield at high densities to greater uptake of solar radiation by the canopy.

Since the goal of sustainable agriculture is applying minimum herbicides and alternative management methods to control weeds, application of green manure and normal crop density can be a good strategy in this regard. Therefore, sunflower cultivation with density of 10 plant.m-2 and using green manure suppressed weeds and produced the highest yield. Therefore, this treatment can help to reduce herbicide application, and it will be an effective step for promotion of sustainable agriculture.