Adequacy of glyphosate doses in the Merremia cissoides (Lam.) Hallier f. control as a function of light intensity in the growth environments.

Shading interferes with the weed's biology, which can change their sensitivity to post-emergence herbicides. The objective was to evaluate the control of Merremia cissoides with glyphosate in full sunlight and shade conditions in two plant growth stages (30 and 73 days after sowing (DAS)). At 30 and 73 DAS, treatments were established in a 2 × 5 and 2 × 6 factorial scheme, respectively. In both experiments, the growth environments constituted the first factor, and the glyphosate doses the second factor. Shading promoted 50 and 40% reductions in glyphosate doses at 30 and 73 DAS, respectively. At 73 DAS, M. cissoides is 177.77 and 131.48% more tolerant to glyphosate than 30 DAS in shading and full sunlight, respectively. Due to the increase in glyphosate tolerance as the plant grows, the management of M. cissoides should be carried out until the stage of six fully expanded leaves. Increasing glyphosate doses reduced the quantum yield of photosystem II and electron transport rate (ETR) in both growth environments, with ETR data showing a high negative correlation with the control. The doses reductions promoted by shading and glyphosate application in the initial growth stage of M. cissoides reduces costs and the negative environmental impacts of this herbicide use.