The results allowed concluding that the management methods of weed ryegrass must be adopted in the period between 11 and 21 days after crop emergence, which is described as a critical period of control of this weed. The wheat grain yield loss competing with ryegrass reached 59% when grown with ryegrass. For ETL, the linear regression model of the rectangular hyperbola adequately estimates grain yield losses in the presence of ryegrass. The cultivar presenting the lowest values of ETL, that is, less capacity to live with the weed, was TBIO Alvorada. The other cultivars presented similar ETL values.

Maize is one of the world's major cereals, and alexandergrass is the most damaging weed in the crop. The objective of this work was to evaluate the relative competitive ability of maize hybrids in the presence of alexandergrass through replacement series experiments. The experiments were set up in a complete randomized block design, with four replications. Firstly, for the maize hybrids as well as for the alexandergrass, the plant population was determined in which the final production becomes constant. were composed of plastic pots with a capacity of 8 dm3, and in each experimental unit was placed in the proportion corresponding to each treatment (100: 00, 75:25, 50:50, 25:75 and 0: 100) in each experimental unit, the species competing or not between them. For each hybrid, a separate experiment was considered with the objective of evaluating intra and interspecific competition. The analysis of the competitiveness of the species was carried out through diagrams applied to the replacement experiments and by the relative competitiveness indexes. At 50 days after the emergence of the maize the leaf area (LA) and the dry mass of the aerial part (DM) of the plants were checked. The relative competition was occured among the weed and each maize hybrid, being negatively affected for both of species, independently of the proportion of weed causing reductions in LA and DM of the maize. Interspecific competition causes less damage to LA and DM of species than intraspecific competition. There is basically competition for the same environment resources (water, CO2, sun light and nutrients) between maize and alexandergrass, being the crop more competitive than the weed.

This work is related to the project of Scientific Initiation of the second author. Universidade Federal da Fronteira Sul 1,

This study aimed to assess the interference and threshold level (TL) of Sida rhombifolia, the arrowleaf sida, competing with different soybean cultivars. The treatments comprised different soybean cultivars (NS 6909, NA 5909, DM 5958, Brasmax ELITE, Brasmax LANÇA, and SYN 13561) and densities of arrowleaf sida plants per square meter (m-2) (0, 2, 3, 4, 9, 15, 16, 23, 22, and 58; 0, 2, 3, 3, 6, 6, 10, 11, 18, and 47; 0, 3, 4, 7, 8, 10, 11, 13, 15, and 24; 0, 1, 4, 6, 12, 18, 19, 31, 44, and 50; 0, 4, 5, 6, 9, 13, 17, 20, 20, and 47; 0, 2, 3, 5, 9, 11, 15, 18, 29, and 30, respectively) for each cultivar. Cultivars NS 6909, NA 5909, and Brasmax Lança were more competitive than DM 5958, Brasmax Elite, and SYN 13561. The TL values varied from 0.55 to 0.95 plants m-2 for cultivars NS 6909, NA 5909, and Brasmax Lança, which exhibited greater competitiveness with arrowleaf sida. The lowest values of TL varied from 0.26 to 0.61 plants m-2 for cultivars DM 5958, Brasmax Elite, and SYN 13561, which had less competitiveness with weed. Highlights The rectangular hyperbola model adequately estimates the grain yield losses of soybean cultivars. The density of arrowleaf sida plants was the variable that best fitted the rectangular hyperbola model.  Soybean cultivars NS 6909 IPRO, NA 5909 RG, and Brasmax Lança IPRO are the most competitive in the presence of arrowleaf sida. The levels of economic damage decrease with the increase in grain yield, in the soybean price, with the reduction in the cost of controlling arrowleaf sida and the herbicide efficiency.

Among the factors which affect crop productivity, there are weeds which compete for environment resources. The objective of this work was to determine the periods of interference of alexandergrass on the morphophysiology and productivity components of black beans. The experiment was conducted in a complete randomized block design with four replications. The bean cultivar of the black type used was IPR Uirapuru, being the treatments separated in two models of interference: in the coexistence group, the crop of the bean lived with the alexandergrass by growing periods of 0, 7, 14, 21, 28, 35, 42 and throughout the cycledays after emergence and throughout the cycle; in the control group the crop was kept free of the infestation for the same periods described previously. At 42 DAE were evaluated the variables related to plant morphology and physiology (height of bean plants, number of trifoliate leaves, dry mass plant, number of grains pods, number od pods plants, stomatal conductance, internal CO2 concentration, photosynthetic activity, efficiency of carboxylation, efficient use of water, transpiration rate). Considering the results, it is possible to conclude that the critical period of interference prevention (CPIP) comprises 24 to 50 DAE, the period before the interference (PBI) was 24 DAE and the total period of interference prevention (TPIP) was 50 days and that the morphological and physiological variables of the plants were negatively affected in the coexistence with the alexandergrass when compared to the treatments kept free of weeds throughout the crop cycle.