| Summary: | Secondary metabolites from weeds have potential as a natural herbicide and are safe for the main crop. This study aimed to screeningsecondary metabolites in Mikania micranthaextract and their effect on the agronomic, physiological and histological characteristics of barnyardgrass and lowland rice. A non-factorial randomized block design was used, with extract concentrations of Mikania micrantha (0; 20; 40; 60; 80; 100 %) and 2,4-D dimethyl amine herbicide as comparison. The M. micrantha extract presented flavonoids, tannins, alkaloids and saponins. The concentration of 20-100 % inhibited the barnyardgrass growth, but was considered safe for the lowland rice, and increased the root length and volume, while the concentration of 60 % increased the number of leaves and stomatal density in the lowland rice. The concentration of 20-100 % significantly inhibited the root volume, biomass, SPAD total chlorophyll and growth reduction of barnyardgrass, and it was classified as an inhibitor. The barnyardgrass tocixity also increased as the concentration grew from 60 to 100 %. The M. micrantha extract reduced the barnyardgrass growth by 63.5 %, being close to the percentage presented by the 2,4-D dimethyl amine (65.6 %).
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