Status of weed control in imidazolinone-herbicide resistant rice in Rio Grande do Sul

Abstract Background: Rice fields in the state of Rio Grande do Sul (RS) are one of the largest areas where the herbicide resistant system is utilized worldwide. Weed management is important to achieve high rice yields, and herbicide resistance is the main limiting factor. Knowledge of practices used by farmers is important to understand the dynamics of weeds in rice fields. Objective: To evaluate the frequency of utilization of weed management practices and the main consequences related to herbicide resistance associated with the utilization of the imidazolinone-herbicide resistant rice in RS. Methods: The study was carried out during the 2017/2018 season based on a survey of 103 consultants about weed management in the Clearfield® flooded rice system. Data were analyzed by non-metric multidimensional scaling (NMDS) and word cloud analysis. Results: The main weeds in Clearfield® rice are weedy rice and barnyardgrass, with Cyperus sp. also mentioned. Currently the main reasons for low weed control efficacy are resistance to herbicides, delay in beginning the flooding and application to plants at advanced developmental stages. The consultants’ perceptions indicate that the number of cases of herbicide resistance and area are increasing to several weed species and herbicide modes of action. The predominant rice establishment system is minimum tillage. Conclusions: Barnyardgrass and weedy rice are still a problem after 15 years of utilization of the imidazolinone-herbicide resistance system, and the adoption of best herbicide practices and integrated weed management is necessary in the rice crop in RS.