High Levels of Glyphosate Resistance in Conyza canadensis from Agricultural and Non-Agricultural Sites in Ohio and Iowa

Abstract

Glyphosate is an important herbicide worldwide, but its efficacy has been compromised where weed species have evolved glyphosate resistance. To better understand evolutionary outcomes of continued and strong selection from glyphosate exposure, we characterized variation in resistance in self-pollinating Conyza canadensis (horseweed) in Ohio and Iowa, where glyphosate resistance was first reported in 2002 and 2011, respectively. In 2015, we collected seeds from a total of 74 maternal plants (biotypes) from no-till soybean fields vs. non-agricultural sites in each state, using one representative plant per site. Young plants from each biotype were sprayed with glyphosate rates of 0x, 1x (840 g ae ha−1), 8x, 20x, or 40x. Resistant biotypes with at least 80% survival at each dosage were designated as R1 (1x), R2 (8x), R3 (20x), or R4 (40x). Nearly all Ohio agricultural biotypes were R4, as were 62% of biotypes from the non-agricultural sites. In Iowa, R4 biotypes were clustered in the southeastern soybean fields, where no-till agriculture is more common, and 45% of non-agricultural biotypes were R1–R4. Our results show that resistance levels to glyphosate can be very high (at least 40x) in both states, and that non-agricultural sites likely serve as a refuge for glyphosate-resistant biotypes.

No evidence for early fitness penalty in glyphosate‐resistant biotypes of Conyza canadensis: Common garden experiments in the absence of glyphosate

“…As noted above, we assumed that seeds from the same maternal plant were full sibs and we referred to these maternal seed families as individual biotypes. In a previous study, we confirmed that progeny from the same maternal plant had consistent levels of resistance to glyphosate (Beres, Ernst, et al, ). Here, we used common garden experiments to compare fitness‐related traits among a subset of the 74 biotypes described in Beres, Ernst, et al ().…”

“…In a previous study, we confirmed that progeny from the same maternal plant had consistent levels of resistance to glyphosate (Beres, Ernst, et al, ). Here, we used common garden experiments to compare fitness‐related traits among a subset of the 74 biotypes described in Beres, Ernst, et al (). Seeds were collected from one maternal plant per population from both agricultural and nonagricultural habitats in southern Iowa in 2015 (Figure ).…”

“…Seeds were collected from one maternal plant per population from both agricultural and nonagricultural habitats in southern Iowa in 2015 (Figure ). Glyphosate resistance for each biotype was characterized by spraying greenhouse‐grown rosettes with one of five dosages: 0×, 1× (840 g ae ha −1 ; manufacturer's recommended application rate, which equates to 0.6725% glyphosate (v/v); AquaMaster ® , 648 g/L, Monsanto Co.), 8×, 20×, and 40× (see Beres, Ernst, et al, for details).…”

“…Triangles (▲) correspond to biotypes from soybean field habitats, and circles (●) correspond to those from nonagricultural habitats; further site details are listed in Table . Adapted from Figure in Beres, Ernst, et al, ; maps were generated using ArcGIS ® software ver. 10.2.2 for Desktop (Environmental Systems Research Institute Inc., www.esri.com)…”

“…Horseweed can be managed by spring and fall tillage, and seeds fail to germinate when buried at depths greater than ~0.5 cm (Brown & Whitwell, 1988;Nandula, Eubank, Poston, Koger, & Reddy, 2006). However, after the commercialization of RoundUp Ready ® crops facilitated the increased adoption of no-tillage crop production systems, horseweed became more abundant and more difficult to manage, especially after many populations evolved resistance to glyphosate (Beres, Ernst, et al, 2018;Davis et al, 2009;Davis, Gibson, & Johnson, 2008;Webster & Nichols, 2012). Uncontrolled or poorly managed horseweed populations can form nearly monospecific stands with high-population densities and may reduce soybean yields by 90% (Bruce & Kells, 1990).…”