INTERFERENCE OF ALEXANDERGRASS (Brachiaria plantaginea (Linck) Hitchc.) AND OF SOME BROADLEAF WEEDS IN SOYBEAN

Abstract

A field experiment was carried out in Eldorado do Sul, RS, during the 1994/95 growing season, in order to evaluate the influence of weed classes (grasses and broadleaves) in reducing soybean yield, as a function of time of their control. Soil preparation and soybean seeding were performed in the conventional way. The experimental design was randomized blocks; where check treatments consisted of one weedy check and another in which soybean plants were pulled out 14 days after emergente (DAE) and Alexandergrass (Brachiaria plantaginea) was chemically controlled. Additional treatments consisted of chemical control of Alexandergrass alone or of general control of weeds, performed chemical or mechanically at 14, 21, or 28 DAE. Soil covering by weeds and dry matter produced were evaluated two times. The highest values of soil covering and of weed dry matter production were obtained where soybean and Alexandergrass were eliminated. As time progressed, weeds showed reduction in soil covering; nevertheless, dry matter production increased. All treatments that included weed control overcame the weedy check in grain yield; however, they did not differ among them. Results show that weed control applied between second and fourth week after soybean emergency preserve crop yield, that Alexandergrass is the main responsible by yield decrease, and, also that the presence' of broadleaf weeds is reduced by Alexandergrass occurrence, causing disappearance of some species.