This work was carried out with the objective of comparing, under controlled and non-competitive condition, the growth of glyphosate-resistant and susceptible biotypes of Conyza canadensis and C. bonariensis; to quantify the effects of resistance selection pressure on the biotypes. Two trials were developed with treatments organized according to a factorial scheme 9 x 2, where nine were periodical growth evaluations and two were biotypes of each species. The variables evaluated per plant were: leaf area and dry mass (shoot, root and total); to determine absolute growth rate from the total dry mass. The glyphosate-resistant biotype of C. canadensis exhibits slower growth and smaller accumulation of leaf area and dry mass than the susceptible biotype. Lower leaf area and dry mass were also registered for the glyphosate-resistant biotype of C. bonariensis when compared to the susceptible one, however with more subtle differences than those identified for C. canadensis. Absolute growth rate of susceptible biotype was superior to that of resistant ones for both species. The pressure of selection to glyphosate resistance had negative impacts on the biotype growth.
horseweed; resistance; development; ecology; populations
