Abstract
The aim of this study was to compare control of three stages biotypes (rosette, bolting, and repro- ductive stage) of Conyza bonariensis glyphosate susceptible biotype (S) and tolerant biotype (T) to glyphosate with glyphosate and mixtures of acetolactate synthase (ALS)-inhibiting. For glyphosate, the dose-response curves confirmed that injury of the biotype T relative to biotype S was signifi- cantly lower for both rosette and bolting stages. Resistance index (RI) for this herbicide was approximately 4 for both weed stages. At bolting, for both biotypes doses much higher than the recommended dose were required. For acetolactate synthase (ALS)-inhibiting herbicides, at the rosette stage, control of both biotypes was excellent with doses significantly lower than the recommended rate. All herbicides within this group showed a very low I50 relative to the recommended rate. These results indicate that biotypes difficult to control with glyphosate at the rosette and bolting stage may be controlled using acetolacte synthase (ALS)-inhibiting herbicides. Control for the reproductive stage was poor. A management program based on the combination of glyphosate with pre- and post-emergence acetolactate synthase (ALS) herbicides may be effective to control the weed.
Highlights
A correlation of r = 0.99; P ≤ 0.01 was obtaineb between C. bonariensis control at 30 DAT using visual evaluations and plant weight determinations
No significant interactions were detected in the analysis between years for any of the variables so data were pooled over the experiments for each biotype
The dose-response curves confirmed that injury of the biotype T relative to biotype S was significantly lower for both growth stages which concurs with other studies in C. canadensis [7]
Summary
No-till favours germination and emergence of small-seeded weed species including Conyza spp. Applications of glyphosate in no-till fallow have increased significantly achieving initially very good control of Conyza spp. in the region of the study [9]. Control of some populations is difficult in many sites of the Argentine soybean region, no glyphosate resistant Conyza spp. biotypes have been reported. The selection of these tolerant biotypes may be due to suboptimal growth stage at application [10], overreliance on glyphosate and long-term use of sublethal doses [11]. Resistance to five modes of action with Conyza canadensis (L.) Cronq. In Argentina, a better understanding of the differences in control between biotypes is essential for developing effective long-term management strategies
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