Caracterización de la resistencia de Amaranthus spp. para el herbicida glifosato en Uruguay

The species Amaranthus hybridus, Amaranthus tuberculatus, and Amaranthus palmeri constitute the most significant weed complex in Uruguayan agricultural systems. In recent years, effective control of these species with glyphosate has proven not effectively. This study aimed to confirm the presence and level of glyphosate resistance in these three Amaranthus species, and to assess various chemical alternatives for managing resistant biotypes. To confirm glyphosate resistance, dose-response assays were conducted for each species, employing a 5x8 factorial design. Factor A encompassed 5 biotypes with varying resistance levels, while Factor B involved 8 glyphosate doses, each with four replications. Plant mortality (% control) was assessed at 7, 14, and 21 days after treatment (DAT), and aerial dry matter (Reduction in DM) was measured at 21 DAT. Resistance factors (RF) were calculated from the C₅₀ and GR₅₀ data obtained. To evaluate herbicides alternatives to glyphosate, an independent trial was conducted for each Amaranthus species with a completely randomized experimental design, and the treatments were arranged in a 5x6 factorial design where factor A corresponded to 5 biotypes of Amaranthus spp. with different susceptibility/resistance, and factor B corresponded to five herbicide treatments: diclosulam, imazethapyr, 2,4-D, dicamba, and fomesafen, along with a control (no application), with four replications utilized.. % Control and reduction in DM were determined at 21 DAT. Both sets of trials were fully replicated twice. The studied biotypes exhibited a high level of resistance to glyphosate, with RF ranging from 12 to 113 for A. hybridus, 1 to 47 for A. tuberculatus, and 4 to 30 for A. palmeri, thereby formally confirming glyphosate resistance in Uruguay. Additionally, results obtained for alternative herbicides indicated evolving resistance of different Amaranthus species to ALS inhibitors and fomesafen. However, 2,4-D and dicamba remain viable alternatives for managing glyphosate-resistant biotypes. Thus, the adoption of other chemical tools such as pre-emergent, along with herbicides featuring distinct mechanisms of action, combined with non-chemical weed management practices like cover crops, should be promoted to mitigate selection pressure and the emergence of new herbicide resistance cases in Uruguay.