Control of Glyphosate-Resistant Marestail in Identity-Preserved or Glyphosate-Resistant and Glyphosate/Dicamba-Resistant Soybean with Preplant Herbicides

Two studies, each consisting of six field experiments were conducted in growers’ fields in 2018 and 2019 to determine the optimal herbicide tankmixes, applied preplant (PP) for the control of glyphosate-resistant (GR) marestail in 1) identity-preserved and glyphosate-resistant soybean (Study 1) and, 2) glyphosate/dicamba-resistant soybean (Study 2). There was no significant injury in soybean with the PP herbicides evaluated in both studies. In Study 1, at 8 weeks after treatment (WAA), glyphosate + saflufenacil, glyphosate + 2,4-D ester, glyphosate + pyraflufen/2,4-D, glyphosate +, 4-D choline or glyphosate + halauxifen-methyl, applied PP, controlled GR marestail 93%, 58%, 60%, 67% and 71%, respectively. The addition of metribuzin to the tankmixes of glyphosate + saflufenacil, 2,4-D ester and pyraflufen/2,4-D increased the control to 98%, 91% and 95%, respectively. The addition of metribuzin + chlorimuron-ethyl to 2,4-D choline/glyphosate and glyphosate


Introduction
Marestail (Conyza canadensis L. Cronq.) is a highly competitive winter or summer annual weed from the Asteraceae family that is native to North America [1]. Globally, marestail is most abundant in north temperate zones which suggest that it has few specialized climatic requirements [1]. In Canada, marestail is found in all provinces except in Newfoundland [1]. In the past, marestail was found mainly in roadsides, orchards and recently abandoned fields. More recently it has become abundant in reduced-, strip-, and no-till crop production systems [1] [2].
Marestail is well adapted to coarse-textured, well-drained soils and is frequently found on sandy knolls [1]. Marestail begins flowering in late July, primarily through self pollination, although up to 4% out crossing is possible [3] [4]. Seeds reach maturity by late August to mid-September [5]. Each marestail plant can produce up to a quarter million seeds; the number of seeds is proportional to its height [1] [6]. Marestail seed disperses primarily by wind although it can be dispersed by moving water as well [7] [8]. Marestail has two primary emergence periods, in the fall (August to October) and spring (April to May) [3] [5]. Marestail is a very competitive weed and depending on the density and time of emergence can cause up to a 93% soybean yield reduction [9] [10] [11].
Glyphosate-resistant (GR) marestail was first confirmed in Ontario from seed collected in 2010 [11]. Glyphosate-resistant marestail is now present in 30 counties in Ontario from Essex county adjacent to the Michigan border to Glengarry county adjacent to the Quebec border [9] [10]. The challenge of managing GR marestail has been exacerbated in recent years with the evolution of multiple-resistant biotypes [11]. In Ontario, the average yield losses due to marestail competitiveness in soybean have been estimated to be 65% [12].
Earlier studies have reported variable GR marestail control in soybean in Ontario with most two-way herbicide tankmixes [9] [10]. Growers need consistent, full-season control of glyphosate-resistant marestail in soybean to maintain yield and be competitive in the global marketplace. In identity-preserved and glyphosate-resistant soybean, postemergence (POST) herbicide options generally do not adequately control GR marestail, in part because this troublesome weed can emerge throughout the growing season [9] [10]. Preplant (PP) herbicide mixtures that have burndown and extended residual activity are the preferred options for the control of GR marestail in GR soybean [9] [10]. New herbicides or herbicide combinations need to be identified that provide more consistent control of this problematic weed in Ontario.
In identity-preserved and glyphosate-resistant soybean, saflufenacil, 2,4-D ester, pyraflufen/2,4-D, 2,4-D choline/glyphosate, and halauxifen-methyl applied PP alone or in combination with metribuzin have the potential to provide consistent control of GR marestail. In glyphosate/dicamba-resistant soybean, glyphosate/dicamba or s-metolachlor/dicamba form the foundation for the control of GR marestail, however control is variable. Three-way tank mixtures with either saflufenacil or metribuzin need to be evaluated. To our knowledge, no study has cumulatively compared the efficacy of these herbicide tankmixes for the control of GR marestail in soybean under Ontario environmental conditions. The aim of this research was to identify herbicide tankmixes that provide consistent control of GR marestail in identity preserved or glyphosate-resistant soybean and glyphosate/dicamba-resistant soybean under Ontario growing conditions

Materials and Methods
Two studies, each consisting of six field experiments were conducted in growers' fields with heavy infestations of GR marestail in 2018 and 2019 (3 in each year for each study) to evaluate the control of GR marestail in identity-preserved or glyphosate-resistant (Study 1) and glyphosate/dicamba-resistant (Study 2) soybean.
Experiments were arranged in a randomized complete block design with 4 replications. Treatments for Study 1 and 2 are presented in Table 1 or Table 2, respectively. Plots were 2.25 wide (3 soybean rows spaced 75 cm apart) by 8 m in length with a 2 m walkway between blocks. Soybean was seeded with a three-row no-till planter at approximately 400,000 seeds per ha −1 to a depth of 4 cm.
All herbicide treatments were applied PP in spring, when the marestail plants were approximately 10 cm in diameter/height using a CO 2 pressurized backpack sprayer equipped with a handheld 1.5 m spray boom, with four ULD120-02 (Pentair, New Brighton, MN, USA) nozzles spaced 50 cm apart that produced a spray width of 2.0 m. The sprayer was calibrated to deliver 200 L·ha −1 of spray solution at 240 kPa.
Evaluation of visible crop injury and weed control occurred at 4 and 8 weeks after the herbicide application (WAA). These parameters were evaluated on a percent scale and crop injury/weed control in each plot received a rating between 0 and 100, where 0 represents no injury or weed control and 100 represents complete death of the soybean or weed species. GR marestail density and biomass (aboveground dry weight) were determined 8 WAA by counting the number of marestail plants (density) and cutting the plants within two randomly placed 0.25 m −2 quadrats per plot. Biomass was determined by harvesting the aboveground section of GR marestail plants within each quadrat and drying them in a paper bag at 60 C in a kiln for a minimum of 48 hours. Soybean yield was measured at crop maturity by harvesting two rows of soybean in each plot with a small plot combine. Grain moisture content and weight were recorded; grain yield was presented in tonnes ha −1 at 13% grain moisture.  for both studies, were analyzed using the Gaussian distribution, without any transformation. Density and dry biomass of GR marestail, as well as soybean seed moisture at harvest, were analyzed using the lognormal distribution for both studies. In instances where the untreated or weed-free controls were assigned a value (either 0 or 100), those treatments were excluded from the analysis. However, treatment means could still be independently compared to the value zero by using the P value included in the LSMEAN output. Tukey's adjustment was applied to pairwise comparisons prior to determining treatment differences (P < 0.05). Treatment means transformed for analysis were back-transformed for presentation of results.
GR marestail interference reduced soybean yield 60% in this study. Reduced GR marestail interference with all the herbicide treatments evaluated resulted in soybean yield that was similar to the weed-free control. Other research has reported that GR marestail interference reduced soybean yield 73% [10].

Hedges et al. (2019)
GR marestail interference reduced soybean yield 53% in glyphosate/ dicamba-resistant soybean ( Table 2). Reduced GR marestail interference with all the herbicide treatments evaluated resulted in soybean yield that was similar to the weed-free control ( Table 2). Results are comparable to Hedges et al. (2019) [24] who reported 67% soybean yield reduction due to GR marestail interference in glyphosate/dicamba-resistant soybean. Eubank et al. (2008) [22] reported up to a 97% reduction in soybean yield due to GR marestail interference. However, Byker et al. (2013) [11] reported a 35% to 42% reduction in soybean yield due to GR marestail interference in soybean.
In glyphosate/dicamba-resistant soybean, generally there was a significant improvement of GR marestail control with the addition of saflufenacil to glyphosate/dicamba or S-metolachlor/dicamba. In contrast, there was no significant improvement of GR marestail control with the addition of metribuzin to glyphosate/dicamba or glyphosate + S-metolachlor/dicamba. Glyphosate/dicamba + saflufenacil and glyphosate + S-metolachlor/dicamba + saflufenacil, applied PP, provided consistent season-long control (≥97%) of GR marestail. Glyphosate/dicamba, glyphosate/dicamba + metribuzin, glyphosate + S-metolachlor/dicamba or glyphosate + S-metolachlor/dicamba + metribuzin did not provide consistent control of GR marestail in glyphosate/dicamba-resistant soybean.
Reduced GR marestail interference with all the herbicide treatments evaluated resulted in soybean yield that was similar to the weed-free control in conventional/glyphosate-and glyphosate/dicamba-resistant soybean.