Sensitivity of Conyza sumatrensis biotypes to glyphosate and ALS-inhibiting herbicides at two growth stages.

Conyza sumatrensis is one of the most problematic weeds in Argentina, withfrequent control failures after glyphosate application. The objective of this research was to evaluate sensitivity of C. sumatrensis biotypes, at two growth stages (rosette and vegetative), to glyphosate and acetolactate synthase(ALS)-inhibiting herbicides. Dose-response experiments were performed using 0, 1/16, 1/8, 1/4, 1/2, 1, 2, 4, 8, and 16-fold of the recommended dose for each herbicide. At rosette stage, the resistance level for glyphosate was 9.8, while at the vegetative stage neither biotype was controlled with the recommended dose, with lower sensitivity of biotype R (RI = 2.3). Control of both biotypes using ALS-inhibiting herbicides was excellent at rosette stage, with lower doses than the recommended. At vegetative stage controls were good (80 %) with lowerdoses than recommended (0.16 X to 0.97 X), except for metsulfuron-methyl +chlorsulfuron in biotype S (1.75 X). These results may confirm the existence of abiotype resistant to glyphosate in Argentina and that ALS-inhibiting herbicides could control resistant biotypes and more developed individuals.