Weedy rice resistance to imidazolinone herbicides and control with glyphosate

Abstract Background Weedy rice (Oryza sativa L.) is a serious competitor of rice, causing severe yield losses. It contaminates and reduces the quality of harvested grain. Objective (1) evaluate the response of Colombian weedy rice accessions to glyphosate and the mixture of imazamox + imazapyr (F-IMIs) at commercial rates; (2) determine the resistance level of one morphotype of weedy rice putatively resistant to the F-IMIs from the objective (1). Methods Ten weedy rice accessions were evaluated for response to F-IMIs (imazamox + imazapyr 49.5 and 22.5 g a.i. ha-1, respectively) and glyphosate (960 g a.e. ha-1). One putative ALS-resistant and five genotypically different accessions were subjected to dose-response assay to determine the level of resistance, or sensitivity, to F-IMIs. The doses included 0, 0.5, 1, 2, 4, 8, and 16; and 0, 0.15, 0.25, 0.5, 0.75, 1, and 2 times the commercial rate of F-IMIs for resistant and susceptible, respectively. Results The F-IMIs controlled weedy rice tested (> 71%), except one straw-hull-awned accession (< 5%). The GR50 of weedy rice genotypes ranged from 0.26- to 0.32-fold the commercial rate of F-IMIs, therefore the accessions tested, representing distinct genotypes were susceptible. The resistant strawhull-awned morphotype was 47.04-fold resistant to the F-IMIs based on GR25. Glyphosate controlled all morphotypes and genotypes >90%. Conclusions Most Colombian weedy rice can be controlled with IMI herbicides recommended for ALS-resistant rice; however, at least one population evolved resistance to IMI herbicides. Glyphosate is effective on weedy rice populations and can be used before planting rice or after rice harvest.