Quantifying the Effects of Herbicide Dose and Wild Mustard (Sinapis arvensis L.) Density on Wheat and Weed Biomass Production

The effect of weeds on crop yield has been widely studied and models predicting the relationship between weed abundance and crop yield are useful for simulating yield loss and assisting with developing management guidelines. The herbicide doses recommended by manufacturers are selected to give reliable weed control without crop damage. There is a good potential to apply lower herbicide doses within competitive cropping systems. Wild mustard (Sinapis arvensis L.) is one of the most problematic annual weed in wheat fields of Iran. Herbicides especially the sulfonylurea family are widely used for weed control in wheat. “Iodosulfuron-methyl sodium mesosulfuron methyl mefenpyr-diethyl” is a new sulfonylurea herbicide being developed for postemergence application on weeds in wheat fields of Iran. Environmental and economic costs now require the optimisation of herbicide effects. This study was therefore conducted to evaluate S. arvensis competitive ability with wheat under sprayed conditions.
A field experiment was carried out at Research Station of Ramin Agriculture and Natural Resources University of Khuzestan in 2015-16. The experiment consisted of four replicates of a split-plot factorial design, with two levels of wheat cultivar (Chamran and Verinac) as the main plot treatments. Other two factors including doses of herbicide (“Iodosulfuron-methyl sodium mesosulfuron methyl mefenpyr-diethyl”) in five levels of 0.2, 0.4, 0.6, 0.8 and 1 of recommended doses and densities of S. arvensis in four levels of 0, 12, 24 and 36 Plants m-2 were implemented in subplots. The competitive effect of the different densities of S. arvensis decreased by increasing doses of “Iodosulfuron-methyl sodium mesosulfuron methyl mefenpyr-diethyl”. Further, same interaction was observed in the standard dose–response curve. The combination of the rectangular hyperbolic model and the standard dose–response curve adequately described the complex effects of herbicide dose and weed competition on wheat biomass. Parametric estimates were used with the model to predict wheat biomass and estimate the doses of “Iodosulfuron-methyl sodium mesosulfuron methyl mefenpyr-diethyl” required to restrict wheat yield loss caused by S. arvensis to an acceptable level.
The results showed that weed competitivity (β) at no-herbicide treatment was smaller in cv. Chamran than in cv. Verinac, indicating that Chamran was more competitive than Verinac. Herbicide performance, as a result of crop competitivity, was also greater in Chamran with smaller LD50 than Verinac. Using the estimated parameters and the combined model, weed biomass was predicted separately in cvs Chamran and Verinac. The predictions showed that S. arvensis grows better in cv. Verinac than in cv. Chamran. For instance, the model predicted that S. arvensis biomass at 36 S. arvensis plants m-2 with no herbicide treatment was equal to 189.19 g m-2 in cv. Verinac and 171.76 g m-2 in cv. Chamran at selected assessment date, whereas at the same weed density but using half of the recommended dose of “Iodosulfuron-methyl sodium mesosulfuron methyl mefenpyr-diethyl” herbicide, weed biomass was predicted to be approximately 82.74 g m-2 in cv. Verinac and 39.91 g m-2 in cv. Chamran. Using the final model and estimated parameters, crop biomass was predicted. Biomass production of wheat with the utilization only half of the recommended dose of herbicide at the highest assessed density of S. arvensis were predicted to be 569.31 g m-2 in cv. Verinac and 720.49 g m-2 in cv. Chamran. . It is speculated that improved crop competitivity may help to minimize herbicide use. Many studies have found that improvement in crop competitivity was achieved by selecting competitive cultivars. A main aim of the modelling approach to crop:weed competition is to predict crop yield production. Incorporating other factors, i.e. herbicide dose, considerably complicates the prediction process. However, the model presented here provides a valuable tool for predicting the effect of these factors.
Results indicates that the standard dose-response model can be modified to a combined model by replacing parameter W0 (the weed biomass at no-herbicide treatment) with the rectangular hyperbolic model. The theoretical outputs of the combined model appear robust and indicate that there are opportunities for reduced herbicide use in the field. Increased crop competitivity by selecting competitive cultivars or increasing crop density may achieve better herbicide performance for crop yield.