Comparative dose-response curves between sourgrass (Digitaria insularis) resistant and susceptible biotypes to glyphosate

Abstract

The frequent use of glyphosate in citrus cropping systems of citrus in Brazil has selected resistant populations of sourgrass (Digitaria insularis) to this herbicide, and it is important to characterize this resistance levels for reasons of rational recommendations for management measures. Thus, an assay was conducted to elaborate comparative dose-response curves between two biotypes sourgrass weed, one resistant (R) and one susceptible (S) to glyphosate. The assay was conducted in a greenhouse from February to May 2012, with five doses of the herbicide glyphosate, based on the recommended dose of the herbicide 900 g ae ha^-1 ( 0.0, 450, 900, 1,800, 3,600 and 7,200 g ae ha^-1) , and the weed with 3-4 tillers at time of application. It was evaluated the percentage of the dry matter in relation to dose 0.0 of glyphosate, percentage of visual control, 14 days after herbicide application. The resistance factor R/S was 16.66, when based on dry matter and 20.26 when based on the percentage of control. Thus, the R biotype has a high degree of resistance, higher than those previously reported in the literature that are smaller than 10. High degree of resistance may indicate that the mechanisms of glyphosate resistance in this biotype are the result of changes at the site of action or super gene expression, and not to metabolism, as usually occurs in weeds with low values of R/S factor. Further studies are necessary to determine these mechanisms. It was also concluded that the management of this biotype should be based on alternative herbicides to glyphosate, because increments in the dose of glyphosate are probably not economically viable.