Control of Glyphosate-Resistant Waterhemp with Two-Pass Weed Control Strategies in Glyphosate / Dicamba-Resistant Soybean

Waterhemp is a small-seeded, dioecious, broadleaf weed that emerges throughout the growing season. If left uncontrolled, waterhemp interference can reduce soybean yield up to 73%. Glyphosate-resistant (GR) waterhemp was first discovered in one county in Ontario in 2014; as of 2017, it has been found in two other counties. Glyphosate/dicamba-resistant soybean can be sprayed with glyphosate and/or dicamba preplant (PP), preemergence (PRE) and/or postemergence (POST). The objective of this study was to determine the control of GR waterhemp in glyphosate/dicamba-resistant soybean with PRE residual herbicides, glyphosate/dicamba applied POST or a two-pass program of a PRE residual herbicide followed by glyphosate/dicamba applied POST. At 8 weeks after application (WAA), pyroxasulfone (150 g ai ha), S-metolachlor/metribuzin (1943 g ai ha), pyroxasulfone/sulfentrazone (300 g ai ha) and flumioxazin/pyroxasulfone (240 g ai ha), applied PRE, resulted in 71%, 85%, 82% and 90% GR waterhemp control, respectively. The same PRE herbicides, followed by glyphosate/dicamba (1800 g ae ha) POST, improved control to greater than 96%. This study concludes that a two-pass program of an effective soil applied residual herbicide followed by glyphosate/dicamba POST controlled GR waterhemp in glyphosate/dicamba-resistant soybean.

widely used herbicide in the world, and controls susceptible waterhemp biotypes up to 30 cm in height [2].Currently, there are 41 weed species resistant to glyphosate globally, with four glyphosate-resistant (GR) weed species in Ontario [3].The weed species resistant to glyphosate in Ontario are waterhemp [(Amaranthus tuberculatus (Moq.)Sauer var.rudis (Sauer) Costea and Tardif], Canada fleabane (Conyza canadensis L. Conq.), common ragweed (Ambrosia artemisiifolia L.) and giant ragweed (Ambrosia trifida L.) [3] [4].Glyphosate-resistant weeds are prevalent in southern Ontario, and long-term, diversified, integrated weed management strategies need to be developed for the control of GR weeds [5].Glyphosate-resistant waterhemp was first discovered in Lambton County, Ontario in 2014 [6].By 2017, GR waterhemp had also been found in Essex County and Chatham-Kent County [5].All waterhemp populations surveyed in Ontario are resistant to Group 2 herbicides [acetolactate synthase (ALS) inhibitors], and frequently to Group 5 herbicides [photosystem II (PSII) inhibitors]; populations with multiple-sites of resistance decrease the number of herbicides available for controlling this weed species [5] [6] [7].Globally, waterhemp has been found resistant to six different sites-of-action: groups 2, 4, 5, 9, 14 and 27 [3].Herbicide resistance to five sites-of-action in one waterhemp biotype has been confirmed [3].This limits the efficacious herbicides for waterhemp control in some populations.
Waterhemp is a dioecious weed species in the Amaranthus genus that can produce up to 4.8 million seeds per plant when grown in the absence of competition [8].In the presence of competition, one study reported waterhemp produced an average of 309,000 seeds per plant when it emerged at the time of soybean planting.Waterhemp has been observed to emerge from mid-May until the end of October in Ontario [6] [9].There is a decrease in waterhemp germination and emergence with an increase in row shading; therefore, herbicides that provide residual control until canopy closure are important for waterhemp control [10].This highly competitive weed is a major issue in crop production, and when left uncontrolled, can decrease soybean yield as much as 73% [9].Schryver et al. [7] studied the efficacy of numerous PRE herbicides for the control of waterhemp in soybean.The most efficacious herbicides were pyroxasulfone (150 g ai ha −1 ), S-metolachlor/metribuzin (1943 g ai ha −1 ), pyroxasulfone/sulfentrazone (300 g ai ha −1 ) and pyroxasulfone/flumioxazin (240 g ai ha −1 ) which provided 87%, 93%, 95%, 97% control, respectively, 8 weeks after application (WAA) [7].

Materials and Methods
This study was completed over a two-year period (2016, 2017) at three locations in southwestern Ontario for a total of six site-years.Two sites were on Walpole Island, Ontario (42.592650, −82.476869) and the third site was near Cottam, Ontario (42.128549, −82.744135).Each trial consisted of 13 treatments that were arranged in a randomized complete block design with four replications; treatments included a weedy and weed-free control.The six one-pass programs consisted of four different herbicides with residual activity applied PRE and two POST application timings of glyphosate/dicamba.The PRE herbicides were pyroxasulfone (150 g ai ha −1 ), S-metolachlor/metribuzin (1943 g ai ha −1 ), pyroxasulfone/sulfentrazone (300 g ai ha −1 ), and pyroxasulfone/flumioxazin (240 g ai ha −1 ).The POST treatments were glyphosate/dicamba (1800 g ae ha −1 ) applied early POST (EPOST), or glyphosate/dicamba (1800 g ae ha −1 ) applied late POST (LPOST).The five two-pass programs consisted of glyphosate/dicamba applied EPOST fb glyphosate/dicamba LPOST, and the above residual herbicides applied PRE fb glyphosate/dicamba LPOST.The PRE herbicides were applied after seeding soybean and before crop emergence; the EPOST herbicide applications were when waterhemp plants were up to 10 cm in height and the LPOST herbicide applications were when there were up to 10 cm waterhemp escapes in the pyroxasulfone treatment.All treatments were applied with a CO 2 pressurized backpack sprayer calibrated to deliver 200 L•ha −1 at 275 kPa through a 1.5 m boom fitted with four Turbo TeeJet Induction (TTI) nozzles spaced 50 cm apart (TeeJet Technologies, Wheaton, IL) resulting in 2.0 m spray width.
In crop cover sprays of glyphosate (900 g ae ha −1 ) were applied as needed to remove the confounding effect of other weed species in the experimental area.
Glyphosate was applied PP at the Walpole sites to control emerged weeds prior to seedbed preparation.Seedbed preparation at all sites was with a double disc followed by a cultivator. in a kiln at 60˚C for two weeks then weighed.In 2017, soybean yield was measured by harvesting two rows of each plot with an Almaco combine (Almaco, Nevada, IL).In 2016, soybean yield was measured from a two 1 m subsamples from two rows in the plot and threshed using a stationary Almaco thresher.
Moisture of soybean seed was adjusted to 14.5% before analysis.
Statistical analysis was performed using the GLIMMIX procedure in SAS (Ver.9.4, SAS Institute Inc., Cary, NC).The fixed effect was herbicide treatment, and the random effects were environment (combination of year and location) and block.Herbicide treatment means were separated using the Fisher's protected LSD test and adjusted using Tukey-Kramer.Alpha value was set at p = 0.05.The weedy and weed-free control was removed for analysis of visible weed control data; the weed-free control was removed from the waterhemp biomass and density data.PROC UNIVARIATE was used to test residuals for a normal distribution, errors independent of one another and homogeneity.As a result, an arcsine transformation was fit to all visible weed control data and a lognormal distribution with the identity link was fit for waterhemp density and biomass data.Yield data were not transformed.For presentation purposes, all transformed means were back-transformed.
At two locations in 2017 (Walpole 1 and 2), excellent control of waterhemp by the PRE herbicides was observed which delayed LPOST application.The LPOST

Conclusions
Based on this study, visible control of GR waterhemp with the four PRE herbicides evaluated declined from >95 at the time of the LPOST application to 51 to 85% at 12 WAA.This decline in control supports the need for effective two-pass weed control programs for the control of GR waterhemp.The level of visible control of glyphosate-resistant waterhemp provided by some herbicide treatments depended on the field site.At two of the sites, the PRE herbicides used alone provided 100% control throughout the season, and therefore the LPOST application of glyphosate/dicamba was not necessary.
Waterhemp is a highly prolific seed producer and even some escapes can add seed to the seedbank; therefore, season-long control is required to limit waterhemp seed return to the seedbank.The EPOST application of glyphosate/dicamba was more efficacious at 2 and 4 WAA than LPOST.At 8 and 12 WAA, there was no difference in control; however, there was a difference in density.It is important to note that the waterhemp populations at each field site were not 100% resistant to glyphosate; therefore, glyphosate was an effective mode-of-action to some degree.Crop competition is important to reduce the germination and establishment of waterhemp, therefore, narrow soybean row widths, competitive cultivars and crop rotation should be considered in a long-term diversified weed management program.
Two-pass herbicides programs provided greater than 91% control of GR waterhemp in this study.Although all two-pass systems were efficacious and there was no difference in yield, it is important to choose the most efficacious herbicide program to reduce weed seed return to the soil while also ensuring that multiple herbicide modes-of-action are used over time.

Table 1 .
Soybean injury and weed control assessments were completed on the same day as the LPOST (WAA) application.Crop injury ratings were taken at 2 and 4 weeks after the LPOST application of glyphosate/dicamba.Injury was rated on a scale of 0 to 100, where 0% was no injury and 100% was soybean death.Visible weed control assessments were completed 2, 4, 8 and 12 WAA of glyphosate/dicamba applied LPOST.Control at 2 WAA was from five site-years instead of six due to human error, and control at 12 WAA was from four site-years since soybean harvest occurred before the 12 WAA rating.Data from glyphosate/dicamba applied LPOST was from 5 site-years, due to waterhemp plants being too tall at time of application for reliable weed control data to be determined.Visible weed control was rated on a scale of 0 to 100, where 0% was no control and 100% was complete control.Waterhemp density and biomass measurements were determined 8 WAA and soybean yield was obtained at maturity.Waterhemp density was measured by counting the number of plants and removing the aboveground biomass in two, 0.25 m −2 subsamples within each plot.Plants samples were dried

Table 1 .
Trial location, year, seeding date, herbicide application dates, and soybean growth stage at application in Ontario in 2016 and 2017.

Table 2 .
Means for visible waterhemp control before LPOST application in Ontario in 2016 and 2017.was applied prior to soybean canopy closure; however, the waterhemp escapes in the pyroxasulfone treatment had not reached 10 cm in height at the time of application.
Abbreviations: PRE, preemergence; EPOST, early POST; LPOST, late POST; app, application.a Means followed by the same letter with a column are not statistically different according to Fisher's Protected LSD (p = 0.05).b Visible control estimates based on comparisons made to weedy and weed-free control treatments.application

Table 3 .
Means for waterhemp visible control, density, biomass and soybean yield in Ontario averaged across six field sites in 2016 and 2017 a .Visible control estimates based on comparisons made to weedy and weed-free control.At 8 WAA, visible control of GR waterhemp with the PRE herbicides alone declined to 71% to 90% depending on the PRE herbicide.Glyphosate/dicamba applied EPOST, LPOST and EPOST fb LPOST provided 77%, 79% and 95% control of GR waterhemp, respectively (Table3).The two-pass programs of a Abbreviations: PRE, preemergence; EPOST, early POST; LPOST, late POST; app, application; WAA, weeks after application.a Means followed by the same letter within a column are not statistically different according to Fisher's Protected LSD (P = 0.05).b Waterhemp control with a single application of glyphosate/dicamba was unacceptable; glyphosate/dicamba applied EPOST or LPOST controlled GR waterhemp 69% and 71%, respectively at 12 WAA, reduced density 68% and 89% and biomass 79% and 92%, respectively.Although the sequential application of glyphosate/dicamba provided >90% control, this weed management program is not recommended due to increased selection intensity for dicamba-resistant waterhemp.
The two-pass programs of a residual herbicide applied PRE fb glyphosate/dicamba applied LPOST controlled GR waterhemp ≥ 93% at 12 WAA, reduced density and biomass ≥ 98%, and resulted in soybean yields that were equivalent to the weed-free con-B.K. Hedges et al.DOI: 10.4236/ajps.2018.971041431 American Journal of Plant Sciences trol.