Abstract
ABSTRACT: Spermacoce latifolia, S. verticillata, and Richardia brasiliensis (family Rubiaceae, tribe Spermacoceae) are glyphosate-tolerant weeds in the soybean producing areas of Brazil. The weed shifts to glyphosate-tolerant weeds across soybean-producing areas has shown the need for adoption of practices that conserve the efficacy of glyphosate. This study evaluated the effect of single- and double-knockdown herbicide applications on the control of S. latifolia, S. verticillata, and R. brasiliensis prior to soybean sowing. Trials were designed as a randomized block and treatments were arranged as a factorial. Factor A was three systemic herbicide treatments 10 days before sowing (DBS), while Factor B was three contact herbicide treatments applied 0 DBS (“sow and apply”). The single- and double-knockdown applications were followed by post-emergence applications of glyphosate or 2,4-D + glyphosate when the crop reached three leaves. The efficacy of control of each weed species was visually evaluated 14 days after sowing (DAS), as well as 0 and 28 days after post-emergence application (DAA). The double-knockdown applications generally provided higher levels of control for the target weeds of the Rubiaceae family than the single-knockdown applications. Glyphosate + 2,4-D, glyphosate + 2,4-D + diclosulam or glyphosate + [halauxifen-methyl + diclosulam] followed by glufosinate or paraquat achieved at least 90% of control of S. latifolia and R. brasiliensis, but did not control S. verticillata (<80%). Glufosinate at 457 g a.i. ha-1 was equivalent to paraquat at 400 g a.i. ha-1 as a contact herbicide in the double-knockdown applications, especially on the first assessment dates.
Highlights
The Rubiaceae is the fourth-largest family of the angiosperms by number of species, containing about 611 genera and approximately 13,100 species distributed throughout most of the world (Govaerts et al, 2007)
The efficacy of Spermacoce latifolia control has varied as a function of the interaction between systemic and contact herbicide treatments in the evaluation at 14 days after sowing (DAS) (Table 3)
The herbicide treatments 3 through 8 achieved the highest levels of S. latifolia control in this evaluation date, with average values of efficacy of control ranging from 87% to 99% (Table 4)
Summary
The Rubiaceae is the fourth-largest family of the angiosperms by number of species, containing about 611 genera and approximately 13,100 species distributed throughout most of the world (Govaerts et al, 2007). Herbaceous species of Rubiaceae are widely distributed in the agricultural areas of Brazil, especially those from the Spermacoce tribe (Ikeda et al, 2008; Marques et al, 2011). These species are generally tolerant of a broad array of environmental conditions such as acid soil, drought, and even soils highly contaminated by arsenic and iron ore (Campos et al, 2014). Among the glyphosate-tolerant weeds that occur in the soybean areas, Spermacoce latifolia (Aubl.) and S. verticillata (L.) are some of the most common species of the Spermacoce tribe. As glyphosate does not effectively control these tolerant weeds, other herbicide modes of action must be mixed with glyphosate (Vidal et al, 2010)
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