Alexandergrass population in Mangueirinha County, Paraná State, Brazil, was controled by ACCase inhibitor herbicides for more than ten years. However, no control was observed in soybean season 95/96. Dose-response experiments were caried out in greenhouses of Horticulture Department-ESALQ/USP and Embrapa Soja in order to study the level of resistance and cross resistance to aryloxiphenoxypropionic and cyclohexanodione herbicides. The supposedly resistant biotype (B- 196) and susceptible biotype were treated with ACCase inhibitors at rates corresponding to 0, 1/2, 1, 2, 4 and 8 fold the recommended rate. The herbicides and their recommended doses were haloxyfop-methyl (120 g/ha), fluazifop-p-butil (188 g/ha), sethoxydim (230 g/ha), clethodim (108 g/ha), propaquizafop (125 g/ha) and fenoxaprop-p-ethyl (82 g/ha). The adjuvants were added following the recommendation of each herbicide. It was estimated the GR50 values, R/S ratios and S-R (the difference between control percentage of both biotypes at recomendated rates). The results of resistance level among the biotypes and herbicides showed differences, with variable GR50 values for herbicides and biotypes. The S-R values were 97 and 11; 96 and 62; 99 and 86; 0.6 and 4; 20 and 17; 88 and 35 for the herbicides haloxyfop-methyl, fluazifop-p-butil, sethoxydim, clethodim, propaquizafop and fenoxaprop-ethyl in Piracicaba and Londrina, respectively. The Brachiaria plantaginea biotype B-196 is resistant to ACCase inhibitor herbicides and exhibited different levels of cross resistance to the herbicides, except clethodim. Sethoxydim, fluazifop-p-butil, haloxyfop-methyl and fenoxaprop-ethyl were the herbicides which the biotype showed high levels of resistance compared to clethodim and propaquizafop.
Alexanderweed; graminicides; chemical control; doses response curve