Management of Resistant Conyza spp. During Soybean Pre-sowing

Abstract

Horseweed (Conyza spp.) is a weed with a considerable presence and frequency throughout Brazilian crops, as well as presenting difficult chemical control. The objective of this study was to evaluate the efficiency of herbicide management, through single and sequential applications, in controlling Conyza spp., at two development stages with 4 to 6 leaves (3 to 5 cm) and 20 leaves (15-20 cm). The analyzed variables were: control percentage (7, 14, 21, and 28 days after application), dry matter content (28 days after application) and regrowth percentage (21 days after application). Data were submitted to analysis of variance at 5% probability and when there was a difference, the means were submitted to the Scott-Knott test. For Conyza spp. plants with a height between 3 and 5 cm and 4 to 6 leaves, the best treatments were T4, T8, T9, T10, T11, T12, T13, and T14, presenting a control percentage above 90%, 7 days after application. For Conyzaspp. plants with a plant development of 15 cm in height and 15 to 20 leaves, the highest control percentages and lowest regrowth percentages were with treatments T9, T13, and T14.

Multiple resistance of Conyza sumatrensis to three mechanisms of action of herbicides

“…After 2011, with the confirmation in Paraná State of biotypes of C. sumatrensis presenting multiple resistance to glyphosate and chlorimuron, other non-selective herbicides, such as paraquat, became more widely used in cropping systems, on burndown applications. Additionally, in the western region of Paraná State, the use of paraquat for pre-harvest soybean desiccation is extremely common, which further increases the selection pressure (Albrecht et al, 2017;Albrecht et al, 2018;Cesco et al, 2019).…”

“…Control of this weed species before soybean sowing usually requires sequential applications in burndown, combining glyphosate + 2,4-D followed by paraquat. However, the control provided by paraquat to fleabane has become less effective over time, most likely due to the selection of resistant biotypes caused by the intensive use of paraquat, which has been applied two to four times per growing season (Albrecht et al, 2017;Albrecht et al, 2018;Cesco et al, 2019).…”

“…Research conducted before the discovery of fleabane resistance to paraquat demonstrated satisfactory results for the use of ammonium glufosinate (Silva et al, 2014;Oliveira Neto et al, 2011;Moreira, Melo, Carvalho, Nicolai, & Christoffoleti, 2010). Also, to saflufenacil, which is a Protox inhibitor, and other new herbicides, positioned isolated and associated, in unique and sequential applications (Albrecht et al, 2018;Krenchinski et al, 2019;Cesco et al, 2019).…”

“…has gradually increased in cultivated agricultural areas in Brazil, primarily in soybean crops, becoming one of the primary weeds that interfere with this crop (Albrecht et al, 2018). After the harvest of off-season corn, interval occurs before sowing soybeans, and in this period, between the planting of the crops, fleabane develops significantly (Krenchinski et al, 2019;Cesco et al, 2019). This fact is due to the high adaptability of this plant to the production systems and the evolution of biotypes resistant to herbicides, which cause increasing losses to crops (Lamego et al, 2013;Albrecht et al, 2017;Cesco et al, 2019).…”

Management of Glyphosate-Resistant Hairy Fleabane and Contribution of the Physiological Potential of Seeds to Resistance

“…Diuron+paraquat and paraquat complement the use of glyphosate, but are not associated with it (VARGAS et al, 2007). In addition, ammoniumglufosinate and saflufenacil are effective in controlling hairy fleabane (CESCO et al, 2019). However, following soybean seedling emergence, only chlorimuron-ethyl and diclosulam act as selective herbicides.…”