عنوان مقاله [English]
Introduction: Competition for resources among plants has long been considered to generate stress for plants and to be important for determining the distribution of species, as well as their evolution. Competition can occur among the organs of a plant (intra plant competition), or negative interactions between the plants of a species (intra specific competition) or interference among different plant species (inter specific competition). Weeds have long been considered as the main competitor of crop plants. These plants can be problematic due to competition with crop plants over light, water and nutrients, decreases the quantity and quality of the product and the creation of a suitable refuge for insects and pathogens. The presence of weeds in soybean reduces seed yield and the rate of this decrease depends on weed density and stage growth stage. In agriculture areas, crop density is kept constant whereas weed density varies in accordance to local infestation degree. Therefore, variation in plant proportion of crops and weeds is established. Thus, in competition studies, it is important to measure the influence of plant density on the competitive process as well as the variation in plant proportion. There are several methodologies used to study plant competition. However, most researchers measured just the interference of weeds on crop growth and production without concerning about the competition process. Thus, it is important to use appropriate experimental designs and methods of analysis to understand the competition process not just by quantifying crop losses but in a mechanistic way. Replacement series experiments allow the control of plant density and proportion, where plant density is kept constant while plant proportion is changed for both studied species. This study aimed to investigate the response of soybeans to the interaction of three species of foxtail (Setaria glauca, S. verticillata and S. viridis) and competitions between the species of these three weeds.
Materials and Methods: These pot experiments were carried out at Sari Agricultural Sciences and Natural Resources University based on a completely randomized design in three replications. The first experimental treatments included planting ratios: 25:75, 50:50, 75:25 (soybean-weed) and pure stand of soybean and weed and in the other three experiments, each of the weeds compared in pairs with the ratios listed above. Density of soybean and weed in the sole stand was four plants per pot and in the planting ratio of 50:05 soybean-weed, two plants were considered from each plant. Also, in the planting ratio of 75:25 soybean and weed, 4 seeds were cultivated in each pot, wherein planting ratio of 75% three seeds of soybean or weed and planting ratio of 25% one seed of soybean or weed was planted. The traits studied in this experiment included seed yield, number of sub-branches, number of pods per plant, number of seeds per pod and 100- seed weight of soybeans. Weed biomass, height, number of leaves, number of spikes and number of tillers of weeds were also calculated in competition with each other. In addition, the ability of weed competition and the competition index were determined.
Results and Discussion: The results showed that the competitive ability of three species with soybean showed that the maximum soybean seed yield (15.56 g plant-1) were obtained in weed-free conditions (soybean monoculture) and increasing soybean density in interference with three species of foxtail reduce weeds biomass compare to the monoculture of weed. Although, survey competitive withstand ability of soybean to different densities of three species of foxtail showed that soybean is more tolerant to low densities (25%) of S. glauca, S. verticillata and S. viridis, but the competitive ability of soybeans compared to these three species was S. glauca > S. verticillata > S. viridis; because the increasing density of S. glauca, caused decreasing soybean yield with a gentle slope while, increasing density of S. viridis, made a decrement of soybean yield with a steep slope. The competitive ability of three species of foxtail together showed that increasing density of any species will diminished the growth index of other species when maximum height, tiller number, leaf number, spike number and biomass of all three species was observed in their monoculture.
Conclusion: The results of the competition of all three species of terns with each other show that inter-species competition has been more effective in reducing weed population life than the intra-species competition; Thus, S. viridis species could have a greater decreasing effect on S. verticillata and S. glauca species by creating more height, more tillers and leaves and as a result higher biomass production. So, the competitiveness between the three species was S. glauca <S. verticillata <S. viridis. According to the findings of the present study, by increasing the density of soybeans, weeds can be managed, especially in their low densities.