Effect of Planting Pattern and Density on Control of Weeds and Yield of Red Bean (Phaseolus vulgaris L.) Varieties in the Presence of Bentazon Herbicide

Bean (Phaseolus vulgaris L.) is so sensitive to intercompetition especially with weed species. Weed management is an essential factor for the success of an agricultural production system. The use of high competitiveness figures is one of these approaches. Another way to manage properly is just to reduce herbicide dose using through the mixed herbicides in crops. By intercropping, filling the empty space prevents weed development in the area. Mixing or simultaneous cultivation of two or more species in one plot of land is one of the oldest agricultural systems in the world. Planting density is another important factor in determining plant yield. Planting density not only determines competition for light and nutrients, but also controls the distribution and allocation of dry matter between plant organs. The studies showed that by increasing bean planting density from 20 to 30 and 40 plants m2, yield increased by 15.4% and 24.7%, respectively, and the weed biomass also increased by a density of 20 to 40 by 30 percent. The use of high competitive cultivars and increasing planting density are the main strategies to increase the competitive ability of beans against weeds. This study was carried out to investigate the methods of pure and mixed cultivation of bean cultivars with different densities for their effect on reduction of bentazone herbicide dose, weed control and crop yield.

In order to investigate the effect of planting density and reducing herbicide doses on weeds biomass and crop yield in pure and mixed cultivation of red bean cultivars, a field experiment was carried out at Agronomy Research Field of Ferdowsi University of Mashhad during 2013 - 2014. A factorial experiment (with three factors) was conducted in a completely randomized block design with 4 replications. Experimental treatments including bean planting densities in three levels consisting of optimum density (Goli 40 and Akhtar 50 /m2), 20 and 40 % higher than the optimum density of both Akhtar and Goli cultivars, and Bentazone herbicide concentrations (0, 50 and 100 %) based on the recommended dose (2.5 liters per hectare), and separate and mixed cultivation of Akhtar (standing) and Goli (ascending) cultivars were performed in rows (1:1). Weed biomass was evaluated every two weeks after spraying by 100*25 cm2 cadaver from two middle rows. At the end of the growing season, a 1*1 m2 staff was used to determine the performance. Data were analyzed by SAS 9.1 software and averages were compared with the LSD test at 5% probability level and the graphs were analyzed using SigmaPlot 12.0 software.

The results showed that for the weed biomass, the mixed cultivation of Akhtar and Goli cultivars was more than pure cultivation due to its success in suppressing high competitive weeds through rapid space cover, which can be attributed to this feature. It was used to reduce the herbicide dose. Also, the highest and lowest grain yield (respectively 463 and 132 g/m2) were found for the pure cultivation of Akhtar cultivar at concentration of 100 and 0 % herbicide, respectively. However, in flowering and mixed cultivars, the highest grain yield was observed at 372.2 and 341.3 g/m2, at 50% herbicide concentration, which was also observed in biological yield. Results of interaction effect showed that the highest biological yield in intercropping (1166 g/m2) was observed for 50% herbicide concentration and 40% planting density. It can be, therefore, concluded that reduced doses of herbicides can only be used if the crop has high competitive ability and planting density is increased, which can reduce the competitive ability of weeds. The use of high competitive crops and increased density of crops have the greatest potential in suppressing weeds and reducing herbicide dosage. The previous studies also showed that limiting bean growth leads to a decrease in leaf area index and bean growth rate, which in turn reduces its competitiveness against weeds.

The use of high-strength cultivars in mixed cultivation due to their overlap, space conquest, and ability to compete with weeds along with increased plant density has a high potential for weed suppression and can be used in other ways to reduce herbicide dose. The results of this experiment showed that there was no significant difference between biological yield at 50% and 100% herbicide concentrations in the mixed cultivation and the highest biological yield was observed in the mixed cultivation.