Modeling the interaction of herbicide doses and nitrogen fertilizer on crop and weed biomass production in multiple weed species–wheat interference

Understanding the interaction between crop-weed competition and herbicide doses may, in many cases, result in a recommendation to reduce the consumption of the herbicide, which is both environmentally and economically important. Nitrogen fertilizer also influences the competitive balance through changes in weed density and biomass, and also contributes to environmental pollution. Hence, the evaluation of the interaction of herbicide doses and nitrogen fertilizer application is necessary for proper management of weeds and achieving acceptable economic yield along with minimizing environmental negative impacts. A field experiment was conducted as split plots in a randomized complete block design with four replications at Ramin Agriculture and Natural Resources University of Khuzestan. Nitrogen fertilizer included five levels of 0, 50, 100, 200 and 350 kg ha-1 of urea was considered as the main plot and the herbicide dose of “iodosulfuron-methyl sodium + mesosulfuron methyl + mefenpyr-diethyl” in six levels of 0, 0.2, 0.4, 0.6, 0.8 and 1 times the recommended dose (1.5 L ha-1) was considered as the sub plot. Half of the nitrogen fertilizer was distributed before sowing and the other half was distributed in mid-tillering stage of wheat. Four weeks after spraying (late of wheat tillering stage), destructive sampling was done using a quadrate with a surface area of 0.25 m2 from four points of each experimental plot. Total biomass of weeds and wheat were weighed in each plot after three days of placement in an oven at 80 °C. The response of weed and wheat biomass to herbicide doses was described, respectively, with dose-response and sigmoid models, and developed in response to an increase in nitrogen fertilizer consumption. Based on the combined model, the dosage required for reducing the weed biomass to less than 50 g m-2 with consuming of 140, 210 and 300 kg ha-1 of urea fertilizer was predicted to be 100, 85 and 60% of the recommended dose, respectively. Also, to achieve about 700 g m-2 of wheat biomass, using 230 kg ha-1 of nitrogen fertilizer plus 100% of the recommended herbicide dose, or applying 270 kg ha-1 of nitrogen fertilizer and 60% of herbicide recommended dose or applying 350 kg ha-1 of nitrogen fertilizer and 40% of the recommended dose of herbicide. Also, application of 230 kg ha-1 of nitrogen fertilizer plus 100% of the recommended herbicide dose or application of 270 kg ha-1 of nitrogen fertilizer and 60% of the recommended herbicide dose or application of 350 kg ha-1 of nitrogen fertilizer with 40% of the recommended herbicide dose led to the achievement of about 700 g m-2 of wheat biomass. Increasing the competitive ability of weeds in response to increasing nitrogen use led to a higher loss of wheat biomass at higher levels of fertilizer application. Application of herbicide at higher levels of fertilizer application, which the competitive ability of weeds in these conditions was much higher than that of low fertilizer levels, led to a greater increase in wheat biomass. Increasing environmental concerns has led to the development of low input systems (low fertilizer and herbicide use). However, our results showed that weeds grown in low nitrogen levels were much more tolerant to herbicide than plants grown at higher levels of fertilizer application.