Abstract
ABSTRACT Hairy fleabane (Conyza bonariensis L.) is a major weed of the conventional crop systems. Therefore, the objectives of the present study were to assess the responses of glyphosate-susceptible (S) and -resistant (R) C. bonariensis at various developmental stages and evaluate the physiological potential of seeds to propose alternative herbicides for the control of this weed. Two experiments were performed in replicates. The first experiment was performed in a greenhouse, arranged in a 2 x 3 x 10 factorial design. Specifically, two hairy fleabane biotypes (S and R) at different developmental stages (I, II, and III) were subjected to various treatments (glyphosate, chlorimuron-ethyl, metsulfuron-methyl, diclosulam, ammonium glufosinate, paraquat, paraquat+diuron, diquat, 2,4-D, and control). Percentage control was evaluated at 7, 14, 21, and 28 days after the application of the treatments (DAT), and shoot dry mass (SDM) was measured at 28 DAT. The second experiment was performed in a laboratory to evaluate the physiological potential of seeds based on the weight of 1000 seeds (TSW); shoot length (SL), radicle length (RL), total length (TL), fresh seedling mass (FSM), dry seedling mass (DSM), accelerated aging (AA) and cold test (CT), and germination (G) in response to cold and accelerated aging. The alternative herbicides tested effectively controlled biotype R up to the stage -I. Seeds of biotype R showed higher physiological potential in terms of all analyzed variables and exhibited greater tolerance to adverse conditions during seedling establishment. Therefore, strategies for the management of glyphosate-resistant hairy fleabane should aim at preventing new seed production.
Highlights
The evolution of resistance to glyphosate, a primary herbicide used for the management of this weed, has led to a gradual increase in its infestation in agricultural areas of Brazil, with Conyza bonariensis L. and C. canadensis L. being abundant in the south of the country (LAMEGO; VIDAL, 2008; HEAP, 2020)
The effectiveness of glyphosate did differ significantly across development stages, this herbicide had no effect on biotype R. These results indicate that the stage of application and degree of control are correlated, which is consistent with previous reports
A significant loss of C. sumatrensis control with chlorimuron-ethyl has been reported in plants taller than 10 cm due to the higher density of foliar trichomes, which blocked herbicide penetration (SANTOS et al, 2015)
Summary
The evolution of resistance to glyphosate, a primary herbicide used for the management of this weed, has led to a gradual increase in its infestation in agricultural areas of Brazil, with Conyza bonariensis L. and C. canadensis L. being abundant in the south of the country (LAMEGO; VIDAL, 2008; HEAP, 2020). Infestation of glyphosate-resistant hairy fleabane during the establishment and initial development of crop led to substantial losses in soybean production, compelling the search for alternatives to this herbicide. For the management of Conyza spp. at the soybean pre-sowing stages, several glyphosate-related herbicides, such as 2,4-D, diclosulam, metsulfuron-methyl, chlorimuron-ethyl, diuron+paraquat, and/or paraquat, are used. In addition to the use of alternative herbicides, the efficiency of management is associated with the stage of weed development and weed control may even be impossible at advanced developmental stages (VARGAS et al, 2007; MOREIRA et al, 2010)
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