Soybean production areas of the central region of Brazil are commonly infested by the weed beggarticks (a mixture of the species Bidens pilosa and Bidens subalternans), similarly to soybean in Argentina, commonly infested by the redroot pigweed (Amaranthus quitensis). These weeds are usually controlled by several herbicides among which the most used are the ALS inhibitors. The intensive and repetitive use of these herbicides in São Gabriel do Oeste, MS - Brazil and in the provinces of Córdoba and Tucumã (Argentina) has selected resistant populations of these weeds. Therefore, the objective of this research was carry out field and greenhouse experiments to study the management of these resistant populations, spraying as treatments the ALS-inhibitor herbicides chlorimuron-ethyl and imazethapyr and herbicides with alternative mechanism of action, protoporphyrinogen oxidase - PROTOX inhibitors (lactofen and fomesafen) and photosystem II inhibitor (bentazon). The field experiment was conducted at São Gabriel do Oeste, MS - Brazil, where the suspected resistant population of beggarticks has not been controlled by ALS-inhibitor herbicides, in the last years, which had been sprayed annually for, at least, eight years. It was concluded that the herbicides chlorimuron-ethyl and imazethapyr were not efficient in controlling the weed population at recommended rates. However, the herbicides with alternative mechanisms of action lactofen, fomesafen and bentazon, sprayed alone or in mixture with the ALS-inhibitor herbicides gave a good control of the weed. In the greenhouse experiment conducted with beggartick and redroot pigweed, whose seeds were collected from the suspected resistant sites, and seeds from a population that had never been sprayed with ALS-inhibitor herbicides, results similar to those obtained under field conditions were confirmed. Overall, it can be emphasized that the studied resistant populations of beggarticks and redroot pigweed are cross resistant to sufonylureas and imidazolinones, lacking, however, multiple resistance mechanisms to photosystem II inhibitors and PROTOX inhibitors, suggesting that these herbicides are good alternative for management of the resistant populations.
weed resistance; management; Bidens pilosa; Bidens subalternans; Amaranthus quitensis
