Levantamento de plantas daninhas em soja cultivada sob diferentes espaçamentos em savana amazônica

The management of weeds in the soybean crop, by reducing the planting spacing, is an important grown factor on which the success of production depends. The objective of this study was to evaluate the floristic, ecological and phytosociology composition of weeds in soybean grown under different spacing in Amazonian savanna area, Boa Vista, RR, Brazil. Thus the BRS 7980 soybean cultivar was sown in a conventional system, in four blocks, using three rows spacing (45, 55 and 65 cm). The weeds sampling was performed using an iron square (0.25 m2), which was thrown randomly four times in each block, totaling 4.0 m2 of sampled area. A total of 16 weed species were recorded, distributed in ten genera and five families, of which Fabaceae and Malvaceae predominated. Four species were present in all spacings: Mimosa pudica L., Urochloa decumbens Stapf., Calopogonium muconoides Desv. and Waltheria communis A.St.-Hil. The different spacings were not indicators of the floristic composition of weeds. The low index of similarity between species indicates that the plant community was affected by the different spacing. Urochloa decumbens Stapf. obtained greater prominence in the evaluated phytosociology parameters, requiring special attention in the control. The largest dry weed mass was obtained in the 55 and 65 cm spacing. The predominant type of propagation, life form and life cycle were by seeds, grass and perennial, respectively. The predominant photosynthetic route was type C3, especially at 45 cm. Therefore, the grown of soybean in 45 cm spacing between rows causes a reduction in the ecology and phytosociology composition of weeds in Amazonian savanna area. The different spacing between rows is not an indicator of the floristic composition of weeds in soybean cultivation in the Amazonian savanna.