Levantamento fitossociológico de plantas daninhas em arroz e soja na fronteira oeste do Rio Grande do Sul

Abstract

Weeds interfere in the growth of other plants by competing for nutrients, water, sunlight and soil space. They can also spread quickly and occupy large areas, further competing with the desired plants. They can also serve as hosts for diseases and insects. The objective of this work was to carry out a phytosociological survey of weeds in areas of rice and soybeans in the city of Itaqui, at different stages of crop development. The incidence of weeds during the development stages of the rice crop was evaluated in an experiment installed at the Federal University of Pampa and the collection of soybeans was carried out in a commercial field, located in the rural area. Samples were collected for rice with fallow soil; soil preparation; second fully expanded sheet; fully expanded sixth leaf and panicle initiation, and for soybean, soil preparation, fully expanded first unifoliate leaf; fully expanded second trefoil; beginning of flowering and pod formation. The indices evaluated were frequency, relative frequency, density, relative density, abundance, relative abundance, importance value index, relative importance and also the aggregation index. Ten random releases of a square of 0.25 m² were made, all the plants that remained inside were collected and identified. Thirteen families and 18 weed species were identified in the rice area, with the Asteraceae family having the highest number of species. When evaluating the importance value index, which is a complex index that involves three fundamental factors in determining the relative importance of a species in relation to the community, the weed Gamochaeta coarctata presented 35.84 and was identified in fallow, soil preparation, already in the collection at stage V2 (second fully expanded leaf) the highlight of the importance value index was with Echinochloa crus-galli (140.02), in V6 (sixth fully expanded leaf) the highest index of importance value was for Oryza sativa, (109.66), in stage R0 (initiation of the panicle) Urochloa mutica L. stood out (111.68). On the other hand, in the soybean cultivated area, 14 families and 22 species were observed, with the weed present in the samples of V1 (first unifoliate leaf), V3 (second fully expanded trefoil), R1 (beginning of flowering) and R3 (Pod formation), was Portulaca oleracea, although it is not the weed that obtained the highest importance value index values, it stood out for being present throughout the crop development cycle. The spatial distribution of these plants was mostly aggregated. It is concluded that the efficiency of the herbicide applied to control weeds in soybeans did not achieve the expected efficiency due to lack of irrigation and edaphoclimatic factors. In the rice area, several factors influenced weed control, including herbicide resistance, application failures, adaptations of some plants to flooded environments, among others. Therefore, it is extremely important to identify the plants present in the areas to be cultivated, for a better management decision, product application, in order to eliminate weeds to reach the maximum productivity of the crop of interest.