Introduction
Corn is an important crop for agriculture and the economy of Canada and more specifically in the province of Ontario, where more than 60% of Canadian corn is produced (OMAFRA 2017). On average, growers in Ontario seed nearly 900,000 ha of corn and produce approximately 9 billion kg of grain corn valued to be approximately Can$1.54 billion (OMAFRA 2017). Weed management is an important component of profitable corn production. A study conducted by the yield loss committee of the Weed Science Society of America has estimated 50% yield loss in corn if weeds are not controlled (Soltani et al. Reference Soltani, Dille, Burke, Everman, VanGessel, Davis and Sikkema2016). Early control of weeds is of particular importance, as early-season competition can substantially reduce corn yield (Hall et al. Reference Hall, Swanton and Anderson1992).
Glyphosate-resistant (GR) horseweed is a highly competitive winter or summer annual weed that has rapidly spread throughout Ontario after its discovery in 2010 (Byker et al. Reference Byker, Soltani, Robinson, Tardif, Lawton and Sikkema2013; Mahoney et al. Reference Mahoney, Shropshire and Sikkema2017; Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019; Weaver Reference Weaver2001). GR horseweed has now been identified in more than 30 Ontario counties (Budd et al. Reference Budd2016). GR horseweed can interfere with corn growth and has been shown to cause as much as 69% corn yield reduction if not adequately controlled (Ford et al. Reference Ford, Soltani, Robinson, Nurse, McFadden and Sikkema2014; Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019). Early-season GR horseweed control is important to decrease early-season weed competition, prevent corn yield loss, reduce weed escapes, and increase net returns. GR horseweed needs to be controlled before seeding with preplant (PP) herbicides that have burndown/residual activity to minimize weed interference and maximize corn yield and farm profitability (Brown et al. Reference Brown, Shropshire and Sikkema2016; Budd Reference Budd2016; Davis and Johnson Reference Davis and Johnson2008; Ford et al. Reference Ford, Soltani, Robinson, Nurse, McFadden and Sikkema2014; Mahoney et al. Reference Mahoney, Shropshire and Sikkema2017).
Tiafenacil is a new nonselective contact herbicide from the pyrimidinedione chemical family, developed by FarmHannong Co., Ltd., Korea for registration in corn, soybean [Glycine max (L.) Merr.], wheat (Triticum aestivum L.), cotton (Gossypium hirsutum L.), and other crops to control emerged grass and broadleaf weeds before crop emergence (Anonymous 2020; Park et al. Reference Park, Young, Jeong-Won, Myoung, Namsook, Taejoon, Gyung-Hee and Soon-Kee2018; Westerveld Reference Westerveld2021). Tiafenacil inhibits the activity of protoporphyrinogen IX oxidase enzyme in susceptible weeds, including velvetleaf (Abutilon theophrasti Medik.), Amaranthus spp., common purslane (Portulaca oleracea L.), and barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] (Haring and Hanson Reference Haring and Hanson2020; Park et al. Reference Park, Young, Jeong-Won, Myoung, Namsook, Taejoon, Gyung-Hee and Soon-Kee2018). Tiafenacil is registered as a useful herbicide for herbicide-resistant (HR) weed management, as it provides an alternative for the control/suppression of GR weeds, including GR Palmer amaranth (Amaranthus palmeri S. Watson), GR horseweed, and GR waterhemp [Amaranthus tuberculatus (Moq.) Sauer var. rudis] in corn and soybean (EPA 2020; Haring and Hanson Reference Haring and Hanson2020). HR weeds such as GR horseweed have created a challenge for farmers in terms of pest management, crop yield, and profitability of their farm enterprises (Haring and Hanson Reference Haring and Hanson2020). Tiafenacil is proposed for PP and PRE burndown use before crop emergence (EPA 2020). Tiafenacil degrades rapidly in the soil and has been shown to have a low risk to non-target organisms (EPA 2020).
Currently, tiafenacil is being considered for registration at rates of 25 to 75 g ai ha−1 for PP burndown in corn. Herbicides that are currently registered in corn can be applied as a partner with tiafenacil to extend and broaden the spectrum of weeds controlled (Anonymous 2020). Current registered herbicides with biological activity against GR horseweed in Ontario include bromoxynil, dicamba, and tolpyralate (Budd Reference Budd2016; Ford et al. Reference Ford, Soltani, Robinson, Nurse, McFadden and Sikkema2014; Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019). Mixing tiafenacil with these herbicides can increase the spectrum of weeds controlled and improve the consistency of control of GR horseweed and other troublesome weeds.
Published data on the effectiveness of tiafenacil applied alone or in mixture with other herbicides for GR horseweed control in corn are scarce. The objective of this research was to assess GR horseweed control in corn with tiafenacil alone or in combination with bromoxynil, dicamba, and tolpyralate and compare their efficacy with the industry standards in Ontario, glyphosate + dicamba/atrazine and glyphosate + saflufenacil/dimethenamid-p.
Materials and Methods
Four field experiments (randomized complete block design with four replications) were conducted in Ontario over 2 yr (2019 and 2020) in commercial fields near Ridgetown, ON, with confirmed GR horseweed populations. Treatments evaluated are listed in Table 1. Glyphosate (900 g ae ha−1) was included in all treatments, including weed-free and non-treated controls, to eliminate all other weed species and non-GR horseweed plants. Tiafenacil herbicide treatments included methylated seed oil (0.5% v/v). Plots were 2.25 wide (3 rows spaced 75 cm apart) by 8 m in length with a 2-m walkway between replicates. Corn was seeded with a no-till planter at approximately 80,000 seeds ha−1 to a depth of 4 cm.
Visible glyphosate-resistant (GR) horseweed percent control at 4, 8 and 12 wk after herbicide application (WAT), GR horseweed density and dry biomass at 8 WAT relative to the non-treated control, and corn grain yield at four sites in 2019–2020 near Ridgetown, Ontario. a
a Means within a column followed by the same lowercase letter do not differ significantly according to Tukey’s HSD at P < 0.05.
b All treatments included glyphosate at 900 g ae ha−1. Tiafenacil herbicide treatments included methylated seed oil (0.5% v/v).
c With oil-surfactant blend (Merge®, BASF Canada Inc., Mississauga, ON, Canada) at 1 L ha−1
Herbicides were sprayed 1 to 10 d before planting, when horseweed was up to 10 cm in height, with a CO2-pressurized backpack sprayer (1.5-m spray boom with 4 ULD-120-02 nozzles [Pentair, New Brighton, MN, USA] spaced 50 cm apart) calibrated to deliver 200 L ha−1 water volume at 240 kPa, producing a spray width of 2.0 m. Glyphosate (450 g ha−1) was applied POST to all trials to eliminate all other weed species, including glyphosate-susceptible horseweed.
Visible corn injury (0 to 100 = no injury to complete death) was estimated at 2 and 4 wk after emergence (WAE). GR horseweed control (0 to 100 = no to complete horseweed control) was estimated at 4, 8, and 12 wk after treatment (WAT). GR horseweed density and aboveground dry biomass were determined at 8 WAT by counting GR horseweed plants within two 0.25-m2 quadrats per plot, dried to a constant moisture and weighed. Corn grain yield was collected upon harvest maturity by combining the center two rows of each plot. Corn grain moisture was transformed to 15.5%.
Data were analyzed in SAS (Ver. 9.4, SAS Institute Inc., Cary, NC, USA). The generalized linear mixed model included treatment as the fixed effect and location-year (environment), replicate within environment, and treatment by environment interaction as random effects. Percent visible GR horseweed control and relative corn yield were analyzed utilizing a gaussian distribution, and relative GR horseweed density and dry biomass were analyzed utilizing a lognormal distribution in the GLIMMIX procedure. The distributions chosen were the ones that best met the assumptions of the analysis; studentized residual plots were used to assess homogeneity of variance, normality was checked using a normal probability plot, and the Shapiro-Wilk statistic and the Pearson chi-square/df ratio were checked to ensure there was no overdispersion. If a treatment was assigned a fixed value, as was the case for the non-treated and weed-free control for visible control and relative density and dry weight, those treatments were not included in the analysis. However, the remaining treatments could still be compared with the value zero using the P-value from the LSMEANS table produced by the GLIMMIX procedure. Treatment pairwise comparisons were subjected to Tukey’s adjustment, and a significance level of 0.05 was used for all comparisons. Data analyzed using the lognormal distribution were back-transformed for presentation of results.
Results and Discussion
Corn Injury and Yield
Corn was not injured at 2 and 4 WAE by any of the herbicides tested (data not shown). GR horseweed interference caused a 32% reduction in the grain yield of corn (Table 1). Metzger et al. (Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019) reported 47% yield loss in corn when multiple herbicide–resistant horseweed was not controlled. Ford et al. (Reference Ford, Soltani, Robinson, Nurse, McFadden and Sikkema2014) reported a 69% corn yield loss due to GR horseweed interference. Westerveld (Reference Westerveld2021) reported 69% soybean yield loss due to GR horseweed interference. Glyphosate (900 g ha−1) + tiafenacil at 12.5, 25, and 37.5 g ha−1 applied PP alone or in combination with bromoxynil (280 g ai ha−1), dicamba (300 g ai ha−1), or tolpyralate (30 g ai ha−1) resulted in corn yield that was statistically similar to the weed-free control and the industry standards in Ontario, glyphosate + dicamba/atrazine and glyphosate + saflufenacil/dimethenamid-p (Table 1). Similarly, Metzger et al. (Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019) reported corn grain yield comparable to the weed-free control with glyphosate + dicamba, bromoxynil, and tolpyralate applied PP in corn. Westerveld (Reference Westerveld2021) reported yield comparable to the non-weedy plots with tiafenacil applied PP at 25 and 50 g ai ha−1 alone and in mixes with other herbicides in soybean.
Horseweed Control
All herbicide treatments evaluated provided at least 63% GR horseweed control relative to the non-treated control (Table 1). Adding an additional mix herbicide improved control in most cases. At 4 WAT, glyphosate (900 g ae ha−1) + tiafenacil at 12.5, 25, and 37.5 g ai ha−1 controlled GR horseweed 63%, 68%, and 72%, respectively (Table 1). Glyphosate + tiafenacil at 12.5, 25, and 37.5 g ai ha−1 plus bromoxynil (280 g ai ha−1) improved GR horseweed control to 81%, 88%, and 87%, respectively. Similarly, glyphosate + tiafenacil at 12.5, 25, and 37.5 g ai ha−1 plus dicamba (300 g ai ha−1) improved GR horseweed control to 86%, 88%, and 88%, respectively. Glyphosate + tiafenacil at 12.5, 25, and 37.5 g ai ha−1 plus tolpyralate (30 g ai ha−1) improved GR horseweed control to 90%, 90%, and 91%, respectively. There was no difference in GR horseweed control among the tiafenacil mixes evaluated, nor was there a significant tiafenacil rate response. However, although not significant, treatments with 12.5 g ai ha−1 of tiafenacil tended to give slightly lower control compared with the higher tiafenacil rates. The industry standards in Ontario, glyphosate + dicamba/atrazine and glyphosate + saflufenacil/dimethenamid-p, controlled GR horseweed 95% and 100%, respectively.
At 8 WAT, the control of GR horseweed with glyphosate + tiafenacil at 12.5, 25, and 37.5 g ai ha−1 decreased to 55%, 60%, and 67%, respectively (Table 1). As at 4 WAT, additional mix herbicides improved control in most cases. Glyphosate + tiafenacil at 12.5, 25, and 37.5 g ai ha−1 plus bromoxynil (280 g ai ha−1) controlled GR horseweed 76%, 85%, and 84%, respectively. Glyphosate + tiafenacil at 12.5, 25 and 37.5 g ha−1 plus dicamba (300 g ai ha−1) controlled GR horseweed 81%, 85%, and 85%, respectively. Glyphosate + tiafenacil at 12.5, 25, and 37.5 g ai ha−1 plus tolpyralate (30 g ai ha−1) controlled GR horseweed 88%, 86%, and 89%, respectively. Glyphosate + dicamba/atrazine and glyphosate + saflufenacil/dimethenamid-p controlled GR horseweed 95% and 100%, respectively.
At 12 WAT, there was a further trend to reduced GR horseweed control with tiafenacil (Table 1). Glyphosate + tiafenacil at 12.5, 25, and 37.5 g ai ha−1 controlled GR horseweed 48%, 51%, and 63%, respectively (Table 1). Glyphosate + tiafenacil plus bromoxynil (280 g ai ha−1), dicamba (300 g ai ha−1), or tolpyralate (30 g ai ha−1) controlled GR horseweed 72% to 83%, 79% to 84%, and 83% to 86%, respectively. The industry standards in Ontario, glyphosate + dicamba/atrazine and glyphosate + saflufenacil/dimethenamid-p, controlled GR horseweed 95% and 100%, respectively.
Results are consistent with other studies that have shown 75% to 84% horseweed control at 3 d after treatment (DAT) and 71% to 73% horseweed control at 7 DAT with tiafenacil applied PP at 25, 50, and 75 g ai ha−1 (Haring and Hanson Reference Haring and Hanson2020). Mahoney et al. (Reference Mahoney, Shropshire and Sikkema2017) reported GR horseweed control of 93% to 96% with bromoxynil + atrazine and dicamba/atrazine applied POST at 8 WAT in corn. However, in their study, glyphosate mixtures with 2,4-D, mesotrione, and topramezone provided only 65% to 76% control of GR horseweed, while atrazine, tembotrione/thiencarbazone-methyl, glufosinate, and halosulfuron mixtures provided minimal control (36% to 57%) of GR horseweed in corn (Mahoney et al. Reference Mahoney, Shropshire and Sikkema2017). In soybean, Westerveld (Reference Westerveld2021) found up to 66% control of GR horseweed at 2 and 4 WAT with tiafenacil applied PP at 25 and 50 g ai ha−1. Their study also showed that when metribuzin was added to tiafenacil, GR horseweed control increased to 93% (Westerveld Reference Westerveld2021). Other pyrimidinedione herbicides such as saflufenacil in combination with metribuzin or glyphosate/dicamba were shown to provide excellent (98% to 100%) GR horseweed control in soybean (Westerveld Reference Westerveld2021).
All herbicide treatments evaluated caused a decrease in GR horseweed dry biomass and density. Glyphosate + tiafenacil at 12.5, 25, and 37.5 g ai ha−1 reduced density of GR horseweed 64%, 43%, and 83% and dry biomass of GR horseweed 69%, 55%, and 83%, respectively (Table 1). Glyphosate + tiafenacil at 12.5, 25, and 37.5 g ai ha−1 plus bromoxynil (280 g ai ha−1), dicamba (300 g ai ha−1), or tolpyralate (30 g ai ha−1) reduced GR horseweed density 82% to 94%, 76% to 89%, and 91% to 95%, respectively, and reduced GR horseweed biomass 93% to 98%, 94% to 98%, and 97% to 98%, respectively. The industry standards in Ontario, glyphosate + dicamba/atrazine and glyphosate + saflufenacil/dimethenamid-p, reduced density and dry biomass of GR horseweed 99% and 100%, respectively. Mixture responses are similar to other studies that have shown that tiafenacil (25 and 50 g ai ha−1) mixed with other herbicides such as metribuzin can reduce GR horseweed density as much as 94% and biomass 73% in soybean (Westerveld Reference Westerveld2021). Metzger et al. (Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019) found 98% to 100% reduction in density and dry biomass of GR horseweed with tolpyralate, dicamba/atrazine, tolpyralate + atrazine, and bromoxyil + atrazine in corn. Mahoney et al. (Reference Mahoney, Shropshire and Sikkema2017) reported 99% to 100% reduction in density and dry biomass of GR horseweed with bromoxynil + atrazine and dicamba/atrazine applied POST in corn. In the same study, glyphosate mixtures with 2,4-D ester, mesotrione + atrazine, and topramezone + atrazine reduced density 89% to 96% and dry biomass 97% to 99% in corn (Mahoney et al. Reference Mahoney, Shropshire and Sikkema2017).
In conclusion, glyphosate plus tiafenacil applied PP at 12.5, 25, and 37.5 g ai ha−1 alone or mixed with bromoxynil, dicamba, or tolpyralate caused no injury in corn. Reduced GR horseweed interference with all the herbicide treatments tested resulted in corn grain yield that was similar to the weed-free control. This research also suggests that tiafenacil applied alone is not as efficacious as the industry standards for the control of GR horseweed. However, the mixes of glyphosate + tiafenacil (25 or 37.5 g ai ha−1) + bromoxynil, dicamba, or tolpyralate have the potential to provide acceptable GR horseweed control in corn, although in many cases not quite up to the levels of the industry standards, dicamba/atrazine or saflufenacil/dimethenamid-p. Mixing herbicides with different modes of action may help reduce the evolution of herbicide-resistant weeds.
Acknowledgments
The authors would like to thank the Grain Farmers of Ontario (GFO) for funding this research. No other conflicts of interest have been declared.
