In the states of Rio Grande do Sul and Paraná, there are frequent reports of failure to control Conyza sumatrensis with chlorimuron-ethyl in soybean crops. Thus, the objectives of this study were to characterize Conyza sumatrensis leaves morphologically and evaluate herbicide control in biotypes of this weed at three stages. Two studies were conducted, with experiments in a greenhouse in a completely randomized design with four replications. In the first study, horseweed biotypes were collected and identified, and the second study evaluated the responses of herbicide rates and development stages. The herbicide rates were: 0.0, 6.25, 12.5, 25, 50, 100, 200 and 400, represented as a percentage of the dose registry of herbicides chlorimuronethyl (20 g ha-1) and glyphosate (720 g e.a. ha-1) applied in isolation or associated at three developmental stages of four Conyza sumatrensis (2, 5, 17 and 20) biotypes (height = 0.5-1 cm and 3-4 leaves, height = 1-2 cm and 6-7 leaves, height = 10-12 cm, 12-14 leaves). The variables analyzed were control, shoot dry weight and trichome and stomatal densities biotypes of the leaf surface at different stages of development. The results obtained demonstrate that the developmental stages affect the effectiveness of the herbicides, and applications at advanced stages of development decrease the effectiveness of control. The exception was biotype 5 of Conyza sumatrensis, which shows resistance to glyphosate, regardless of stage of development at the time of herbicide application. There was variation in the number of trichomes among biotypes at all stages of development, and the number of stomata decreased with the development of biotypes.
horseweed; weed; ALS; EPSPs; trichomes; stomata
