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This study has aimed to diagnose and compare the predominant weed species in cowpea crop in the Amazon region and to evaluate the effectiveness of herbicides applied in pre-emergence. The weed sampling was obtained before planting, the cowpea cultivar was BRS Aracê, by collecting all the plants present within a 0.25 m² hollow frame, randomly thrown 40 times in experimental area to calculate the phytosociological indices. The herbicide efficiency experiment was conducted in a randomized block design with four replications, in time subdivided plot model. Treatments were applied one day after crop planting, their doses (g/ha ai) were: metribuzin (360), sulfentrazone (600), S-metolachlor (1200), pendimethalin (750), oxadiazon (1000), alachlor (2400), metribuzin + pendimethalin (360 + 750), metribuzin + alachlor (360 + 2400), manual weeding and weedy check. The evaluations of weed control were done at 21, 28 and 35 DAP. The most important weed in phytosociology was Tridax procumbens. Cultivation of the crop with weeds caused a reduction of more than 66% in crop productivity. The application of the metribuzin herbicide prevented the emergence of all cowpea seedlings while oxadiazon satisfactorily controlled the weeds without affecting the crop yield. [ABSTRACT FROM AUTHOR]

The objective of this study was to evaluate the weed community in green sugarcane field reform with three tillage managements and oilseed crops in succession. The experiment was conducted during the 2007/08 season on a sugarcane area harvested without prior burning in the last five cuts. Treatments were arranged in a split-plot design with four replications. Whole plots were conventional-till, minimum-till and no-till cropping systems and the subplots were fallow or crop rotation with peanut, soybean or sunflower. After 180 days of sugarcane harvest, weeds have been identified, the number of weeds.m-2 was counted and biomass of weeds above ground were taken. All data were subjected to analysis of variance using a split-plot model and the means were compared by Tukey's test at 5% probability. The phytosociological indices were calculated. No-till system in the reform of green sugarcane significantly reduces the number and biomass of weeds. The use of soybean and peanuts in rotation with sugarcane in field reform was beneficial to controlling the weed population and suppressing weed species difficult to control as Cyperus rotundus and Commelina bengalensis. [ABSTRACT FROM AUTHOR]