Interference and Economic threshold level of Alexander Grass in Soybean as a Function of Cultivars and Weed Populations

In the soybean crop occurs high losses of grains production, in function of weeds. It is noteworthy that among the most damaging weeds in the soybean crop is the alexander grass (Urochloa plantaginea). The objective of the project was to test mathematical models and identify explanatory variables to determinate the level of control of alexander grass in the soybean crop estimated in function of cultivars and populations of the competitor. The experiment was installed in the UFFS experimental area in Erechim, RS in the agricultural year 2016/17. The treatments were composed of soybean cultivars (NS 5445 IPRO, NS 5959 IPRO, SYN 13561 IPRO, SYN 1059 RR e BMX Elite IPRO) and 12 alexander grass populations that competed with each soybean cultivar. We evaluated plant population, leaf area, ground cover and dry mass of the aerial part of alexander grass. The plant population presents a better fit to the model of the rectangular hyperbole, and this model can estimate the grain productivity losses due the alexander grass interference. The cultivars SYN 1059 RR, BMX Elite IPRO and NS 5445 IPRO were the most competitive in comparing with the others in the presence of alexander grass. The values of economic threshold level ranged from 0.96 to 2.16 plants m2. The increase in grain productivity, commercial price of soybeans, herbicide efficiency and reduction in control cost decrease the economic threshold level for weed control, justifying application of control measures for lower densities of alexander grass.