عنوان مقاله [English]
Introduction: Competition with weeds is the most important factor that reduces agricultural crop yield worldwide. The magnitude of yield loss is affected by numerous agronomic and environmental factors, such as weed density, time of emergence, morphological characteristics of weed and crops and etc. Several studies have shown that the crop and weed growth parameters including height, total dry matter, leaf area index and its distribution in the canopy are major determinants of the capability of species competition under interference conditions. Asian spider flower (Cleome viscosa L.) is a summer annual weed of the Capparidaceae family. Currently, very high densities of this weed grow in the fields of eastern Golestan province. There is no report on the extent of damaging of this weed in this province. So, current study was conducted to achieve these goals: 1) Investigate the effect of different densities of Asian spider flower on growth characteristics of soybean. 2) Determine the soybean yield loss due to competition with different densities of this weed species.
Materials and Methods: In order to investigate the soybean competitive ability (cultivar D P X) under interference condition with different densities of Asian spider flower (0, 3, 5, 10, 15, 20, 30, 45 plant m-2); an experiment was established based on randomized complete blocks design with three replications in Gorgan University of Agricultural Sciences and Natural Resources in 2015–2016 growing season. Sampling was done 25 days after planting in eight stages. In each sampling, leaf area, dry weight of soybean and Asian spider flower plants were measured. Vertical distribution of leaf area in canopy profiles of soybean and Asian spider flower was studied, too. For this purpose, plants were divided into segments in 20- cm interval and leaf area of each layer was measured separately. A two parameter rectangular hyperbolic function (Equation 1) used for estimating grain and biological crop yield loss. Finally the accuracy of the model was confirmed based on RMSE, r and the dispersion of the observed and predicted grain and biological yield loss of soybean around the one to one line.
Where YL is percent yield loss, D is weed density (plants m-2), and I and A, are model parameters. Parameter I is the percent yield loss per unit weed as D → 0, and A is the asymptotic yield loss as D → ∞.
Results and Discussion: The results showed that soybean height was not affected by interference with different densities of Asian spider flower. But maximum accumulation of dry matter of soybean and leaf area index were reduced under competition conditions. According to the fitted model, the maximum accumulation of the dry matter of Asian spider flower was obtained at 83 days after planting, while the maximum soybean dry matter accumulation, depending on the interference treatment, varied between 53 and 60 days after planting. The maximum leaf area index of soybean (in weed free treatment) and Asian spider flower were 6.41 and 0.96, and were obtained at 63 and 53 days after planting, respectively. Although Leaf area index of Asian spider flower was lower than soybean, but maximum percentage of leaf area of Asian spider flower was located at higher canopy layers than soybean (20-40 compare to 0-20 cm). The results showed that the maximum soybean grain and biological yield loss due to high densities of Asian spider flower were 60.42% and 54.52%, respectively. Also, soybean grain and biological yield reduction were 8.39 and 7.94, respectively, in case the first weed. Based on the results of this study (RMSE, r and correlation between observed and predicted grain and biological yield loss), it can be concluded that the present model has a good accuracy and its outputs can be cited. In general it can be said the competition effect of Asian spider flower on soybean biological yield was lower than grain yield.
Discussion: Overall our results showed that Asian spider flower weed could be considered as a strong competitor to soybean. According to the results, it seems that some characteristics of Asian spider flower including 1) representing greater height at the end of the growing season, 2) requiring a shorter time to reach maximum leaf area index and 3) arranging a greater part of plant leaf area at higher canopy layers compared to soybean, were the most important factors to increasing the competitive ability of this weed.