Introduction
Herbicides that inhibit the 4-hydroxyphenyl-pyruvate dioxygenase (HPPD) enzyme in susceptible plants represent the most recent herbicide mode of action successfully commercialized for weed management in Ontario, Canada. Although herbicides within the triketone and isoxazole chemical families of HPPD inhibitors have been used in North America for nearly two decades, the registration of topramezone in 2005 marked the first development of the pyrazolone family of HPPD inhibitors (Grossmann and Ehrhardt Reference Grossman and Ehrhardt2007).
Four HPPD inhibitors are available commercially for use in corn in the United States and Canada. These include isoxazoles (isoxaflutole), triketones (mesotrione and tembotrione), and pyrazolones (topramezone) (Health Canada 2018). Tolpyralate is a new pyrazolone herbicide molecule that was registered in 2017 in the United States and Canada, for use in field, pop, seed, and sweet corn (Anonymous 2017). Results from this study presented in a companion manuscript indicate that tolpyralate exhibits strong herbicidal efficacy in field environments (Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018); however, HPPD inhibitors vary widely in their application timing, use rates, and selectivity (Hawkes Reference Hawkes2012; Ontario Ministry of Agriculture, Food and Rural Affairs [OMAFRA] 2016). Mesotrione provides PRE and POST control of several broadleaf weeds, including velvetleaf, common lambsquarters Amaranthus, and Polygonum spp.; however, control of annual grasses, including green foxtail and barnyardgrass is variable (Creech et al. Reference Creech, Monaco and Evans2004; OMAFRA 2016). Tembotrione, applied POST, provides control of certain grass weeds, including giant foxtail (Setaria faberi Herrm.), witchgrass (Panicum capillare L.), and barnyardgrass, in addition to several annual broadleaf weeds (Bollman et al. Reference Bollman, Boerboom, Becker and Fritz2008; Williams et al. Reference Williams, Boydston, Peachey and Robinson2011). Isoxaflutole is applied PRE or early POST and provides control of grass and broadleaf weeds (Ahrens et al. Reference Ahrens, Lange, Mueller, Rosinger, Willms and Almsick2013; Pallet et al. Reference Pallet, Cramp, Little, Veerasekaran, Crudace and Slater2001). Conversely, topramezone controls both grass and broadleaf species, but is only applied POST (Anonymous 2016; Bollman et al. Reference Bollman, Boerboom, Becker and Fritz2008). Similarly, tolpyralate applied POST controls several grass and broadleaf species (Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018), but is reported to have limited residual activity in soil (Anonymous 2017; Kikugawa et al. Reference Kikugawa, Satake, Tonks, Grove, Nagayama and Tsukamoto2015). At this time, there is limited published information examining the interspecific selectivity of tolpyralate relative to other HPPD inhibitors.
Due to variation in selectivity and residual control, HPPD inhibitors are commonly applied with other herbicides, such as atrazine, in tank or preformulated mixtures (OMAFRA 2016). Atrazine is a photosystem II (PSII) inhibitor and is one of the most widely used herbicides in corn; it is applied to more than 55% of total corn hectares in the United States (USDA NASS 2015). Atrazine provides broad-spectrum annual broadleaf control, has flexible application timing, and a mode of action that is complementary to that of HPPD inhibitors due to their shared interaction with plastoquinone within PSII (Creech et al. Reference Creech, Monaco and Evans2004; Hess Reference Hess2000). Interactions between mesotrione and atrazine are widely reported. For example, a mixture of atrazine and mesotrione improves the control of common ragweed, Palmer amaranth (Amaranthus palmeri S. Watson), common cocklebur (Xanthium strumarium L.), ivyleaf morningglory (Ipomoea hederacea Jacq.), yellow nutsedge (Cyperus esculentus L.), redroot pigweed, and velvetleaf compared with mesotrione applied alone (Abendroth et al. 2006; Bollman et al. Reference Bollman, Boerboom, Becker and Fritz2008; Creech et al. Reference Creech, Monaco and Evans2004; Johnson et al. Reference Johnson, Young and Matthews2002). Kohrt and Sprague (Reference Kohrt and Sprague2017) found that the addition of atrazine to both mesotrione and tembotrione improved control of atrazine-resistant Palmer amaranth biotypes. Stephenson and Bond (Reference Stephenson and Bond2012) reported that the addition of atrazine to isoxaflutole applied POST improved the control of entireleaf morningglory (Ipomoea hederacea (L.) Jacq. var integriuscula Gray) and Palmer amaranth. Similarly, Bollman et al. (Reference Bollman, Boerboom, Becker and Fritz2008) found that the addition of atrazine to topramezone provided better control of common lambsquarters compared with topramezone applied alone.
There have been few studies investigating the benefit of the addition of atrazine to tolpyralate. Tonks et al. (Reference Tonks, Grove, Kikugawa, Parks, Nagayama and Tsukamoto2015) reported that on average, the addition of atrazine to tolpyralate improved control of broadleaf species, including velvetleaf, Amaranthus spp., common ragweed, common lambsquarters, and kochia [Bassia scoparia (L.) A. J. Scott] at 30 d after application (DAA). However, the difference in control with tolpyralate alone or with atrazine at 30 DAA varied widely by species (Tonks et al. Reference Tonks, Grove, Kikugawa, Parks, Nagayama and Tsukamoto2015). Kohrt and Sprague (Reference Kohrt and Sprague2017) reported similar control of atrazine-resistant Palmer amaranth with tolpyralate alone and tolpyralate plus atrazine.
The results presented in the companion manuscript (Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018) indicate that the addition of atrazine to tolpyralate at a 1:33.3 ratio broadens the spectrum of weed control. Given that tolpyralate is reported to have limited PRE activity (Anonymous 2017), the addition of atrazine may also contribute to improved late-season control of select species; however, there is no published information in this regard. Therefore, the objectives of this study were to examine the effects of atrazine addition to tolpyralate and to compare tolpyralate efficacy in field environments to the efficacy of two other HPPD-inhibiting herbicides currently in use.
Materials and Methods
Experimental Methods
The results outlined in this paper describe the results from field studies conducted near Ridgetown and Exeter, Ontario, Canada from 2015 to 2017, as described in the companion paper (Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018). A total of six experiments with four replications each were conducted, arranged at each site in a randomized complete block design. Treatments investigated in this paper include tolpyralate applied at 30 g ha−1, representing the lowest current label rate (Anonymous 2017), and tolpyralate+atrazine applied at 30+1,000 g ha−1, representing a 1:33.3 mix ratio. This ratio was determined to be appropriate with consideration of preliminary work conducted by Tonks et al. (Reference Tonks, Grove, Kikugawa, Parks, Nagayama and Tsukamoto2015). Each of these tolpyralate treatments were compared against two current HPPD inhibitors applied at the registered POST label rate for field corn in Canada: mesotrione+atrazine (100+280 g ha−1) and topramezone+atrazine (12.5+500 g ha−1). Tolpyralate applications included methylated seed oil (MSO Concentrate®; Loveland Products Inc., Loveland, CO, USA) at 0.50% vol/vol plus 28% N urea ammonium nitrate (UAN) at 2.50% vol/vol as adjuvants. Mesotrione applications included a nonionic surfactant (Agral® 90; Syngenta Canada Inc., Guelph, ON, Canada) at 0.20% vol/vol, while topramezone treatments included blended surfactant (Merge®; BASF Canada Inc., Mississauga, ON, Canada) at 0.50% v/v plus UAN at 1.50% v/v. Treatments were applied when weeds reached 10 cm in height on average, using a four-nozzle handheld sprayer equipped with ULD12002 nozzles (Pentair, New Brighton, MN, USA), calibrated to deliver a 187 L ha−1 spray volume at 240 kPa.
Visible control was assessed against the nontreated check plots at 1, 2, 4, and 8 WAA, with each species assigned a percent value between 0 and 100, where 0 signifies no control and 100 signifies complete plant death/absence from plots. At 8 WAA, the reduction in density and biomass of each species provided by all treatments was determined by counting and harvesting all plants contained in two 0.5-m2 quadrats, placed randomly within each plot. Samples were allowed to dry at 60 C, and dry biomass was recorded.
For further information regarding experimental design, location characteristics, and technical methods, readers are referred to the companion manuscript derived from the same field study (Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018).
Statistical Analysis
For each of the eight weed species described in the first part of this study (Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018), visible control data at 1, 2, 4, and 8 WAA, density reduction, and dry biomass reduction (8 WAA) were subjected to a mixed-model variance analysis using the GLIMMIX procedures in SAS v. 9.4 (SAS Institute, Cary, NC). A significance level of α=0.05 was declared for all tests. Variance was partitioned into random effects of environment (comprising location and year), block nested within environment, and the treatment*environment interaction, with treatment designated as the fixed effect. The appropriate model was assigned to each parameter based on the distribution and link function that best met assumptions that residuals were normally distributed, homogeneous, and had a mean of zero, as determined with a Shapiro-Wilk test and scatter plots of studentized residuals. Where appropriate, a normal distribution was used. Non-Gaussian data were analyzed using the Laplace method of integral approximation. Visible control data were assigned either a normal distribution with identity link or a beta distribution with a logit or cumulative log link, except for wild mustard control data at 4 WAA, which were arcsine square-root transformed to meet assumptions. Weed density and biomass were analyzed using a normal or lognormal distribution with identity link or a Poisson or negative binomial distribution with a log link. Least-square means of each parameter were computed on the analysis scale and converted to the data scale using the ilink option for all distributions, except lognormal, in which case, data were back transformed using the omega method within PROC GLIMMIX (M. Edwards, personal communication). Least-square means were compared across each of the four treatments using Tukey-Kramer’s multiple range test, and letter codes were assigned by specifying the lines option in the GLIMMIX procedure.
Results and Discussion
Means comparisons included control at each assessment timing and reduction in density and biomass provided by tolpyralate alone or with atrazine, mesotrione+atrazine, and topramezone+atrazine for the eight weed species discussed in the first part of this study (Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018). Means comparisons are presented in Tables 1–8.
Table 1 CHEAL-visible control at 1, 2, 4, and 8wk after application (WAA), density, and biomass of common lambsquarters following treatment with tolpyralate, tolpyralate+atrazine, topramezone+atrazine, and mesotrione+atrazine in field studies conducted in Ontario, Canada, in 2015, 2016, and 2017.Footnote a
a Means followed by the same letter within columns are not significantly different according to Tukey-Kramer multiple range test α=0.05.
b DM, dry matter.
Table 2 ABUTH-visible control at 1, 2, 4, and 8 wk after application (WAA), density, and biomass of velvetleaf following treatment with tolpyralate, tolpyralate+atrazine, topramezone+atrazine, and mesotrione+atrazine in field studies conducted in Ontario, Canada, in 2015, 2016, and 2017.Footnote a
a Means followed by the same letter within columns are not significantly different according to Tukey-Kramer multiple range test α=0.05.
b DM, dry matter.
Table 3 AMASS-visible control at 1, 2, 4, and 8 wk after application (WAA), density, and biomass of redroot pigweed/Powell amaranth following treatment with tolpyralate, tolpyralate+atrazine, topramezone+atrazine, and mesotrione+atrazine in field studies conducted in Ontario, Canada, in 2015, 2016, and 2017.Footnote a
a Means followed by the same letter within columns are not significantly different according to Tukey-Kramer multiple range test α=0.05.
b DM, dry matter.
Table 4 AMBEL-visible control at 1, 2, 4, and 8 wk after application (WAA), density and biomass of common ragweed following treatment with tolpyralate, tolpyralate+atrazine, topramezone+atrazine, and mesotrione+atrazine in field studies conducted in Ontario, Canada, in 2015, 2016, and 2017.Footnote a
a Means followed by the same letter within columns are not significantly different according to Tukey-Kramer multiple range test α=0.05.
b DM, dry matter.
Table 5 POLPE-visible control at 1, 2, 4, and 8 wk after application (WAA), density, and biomass of ladysthumb following treatment with tolpyralate, tolpyralate+atrazine, topramezone+atrazine, and mesotrione+atrazine in field studies conducted in Ontario, Canada, in 2015, 2016, and 2017.Footnote a
a Means followed by the same letter within columns are not significantly different according to Tukey-Kramer multiple range test α=0.05.
b DM, dry matter.
Table 6 SINAR-visible control at 1, 2, 4, and 8wk after application (WAA), density, and biomass of wild mustard following treatment with tolpyralate, tolpyralate+atrazine, topramezone+atrazine, and mesotrione+atrazine in field studies conducted in Ontario, Canada, in 2015, 2016, and 2017.Footnote a
a Means followed by the same letter within columns are not significantly different according to Tukey-Kramer multiple range test α=0.05.
b DM, dry matter.
Table 7 SETVI-visible control at 1, 2, 4, and 8wk after application (WAA), density, and biomass of green foxtail following treatment with tolpyralate, tolpyralate+atrazine, topramezone+atrazine, and mesotrione+atrazine in field studies conducted in Ontario, Canada, in 2015, 2016, and 2017.Footnote a
a Means followed by the same letter within columns are not significantly different according to Tukey-Kramer multiple range test α=0.05.
b DM, dry matter.
Table 8 ECHCG-visible control at 1, 2, 4, and 8wk after application (WAA), density, and biomass of barnyardgrass following treatment with tolpyralate, tolpyralate+atrazine, topramezone+atrazine, and mesotrione+atrazine in field studies conducted in Ontario, Canada, in 2015, 2016, and 2017.Footnote a
a Means followed by the same letter within columns are not significantly different according to Tukey-Kramer multiple range test α=0.05.
b DM, dry matter.
Common Lambsquarters
At 1 WAA, tolpyralate controlled common lambsquarters 60%, while the addition of atrazine to tolpyralate improved control to 93% (Table 1). Tolpyralate+atrazine and topramezone+atrazine provided similar control; however, tolpyralate+atrazine provided better control than mesotrione+atrazine. The numerical increase in common lambsquarters control across the four treatments at 1 WAA follows the respective rate of atrazine used with each HPPD inhibitor. However, Woodyard et al. (Reference Woodyard, Bollero and Riechers2009) reported similar control of common lambsquarters with atrazine applied POST at 280 and 560 g ha−1 10 DAA, suggesting the differences observed in this study at 1 WAA may be secondary to the rate of atrazine. At 2 WAA, tolpyralate alone, tolpyralate+atrazine, topramezone+atrazine, and mesotrione+atrazine each provided >90% control of common lambsquarters, and the four treatments were not different from one another. These results did not vary widely from the preliminary results described previously by Tonks et al. (Reference Tonks, Grove, Kikugawa, Parks, Nagayama and Tsukamoto2015), who reported 86% control of common lambsquarters with tolpyralate. Mesotrione+atrazine applied POST at the doses used in this study have previously been reported to provide effective control (93% to 99%) of common lambsquarters (Armel et al. Reference Armel, Wilson, Richardson and Hines2003; Whaley et al. Reference Whaley, Armel, Wilson and Hines2006; Woodyard et al. Reference Woodyard, Bollero and Riechers2009). Similarly, Bollman et al. (Reference Bollman, Boerboom, Becker and Fritz2008) found that topramezone+atrazine applied POST, following S-metolachlor PRE, provided 100% control of common lambsquarters. At both 4 and 8 WAA, tolpyralate alone provided similar control to tolpyralate+atrazine and both industry-standard HPPD inhibitors; however, the addition of atrazine to tolpyralate led to a greater reduction in common lambsquarters density and biomass than tolpyralate applied alone. Topramezone+atrazine and mesotrione+atrazine provided a similar reduction in density and biomass to tolpyralate applied alone or with atrazine.
Velvetleaf
At 1 WAA, tolpyralate+atrazine provided better control of velvetleaf than tolpyralate alone; however, at 2, 4, and 8 WAA, no differences were observed between treatments (Table 2). These results suggest that the addition of atrazine to tolpyralate may increase speed of velvetleaf control despite the low biologically effective dose (BED) (3.2 g ai ha−1) of tolpyralate for velvetleaf control determined in the first part of this study (Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018). Speed of weed control may have important physiological implications, with more rapid control ultimately shortening the duration of weed–crop competition. At 4 and 8 WAA, all treatments provided ≥95% control of velvetleaf, and there were no differences among treatments with respect to reduction in velvetleaf density or biomass. Similar results were outlined by Tonks et al. (Reference Tonks, Grove, Kikugawa, Parks, Nagayama and Tsukamoto2015), who reported that tolpyralate, tolpyralate+atrazine and topramezone+atrazine controlled velvetleaf 87%, 94%, and 93%, respectively. Likewise, mesotrione has been found to exhibit excellent foliar activity on velvetleaf (Creech et al. Reference Creech, Monaco and Evans2004; Johnson and Young Reference Johnson and Young2002).
Pigweed Species
Similar to common lambsquarters and velvetleaf, differences were observed among treatments in control of pigweed species (Amaranthus spp.) at 1 WAA but not at later assessment timings. The addition of atrazine to tolpyralate provided 96% control of pigweed species at 1 WAA, which was superior to tolpyralate applied alone, but not different from either mesotrione+atrazine or topramezone+atrazine (Table 3). Tolpyralate alone provided equivalent control of pigweed species compared with tolpyralate+atrazine, topramezone+atrazine, and mesotrione+atrazine at 2, 4, and 8 WAA. Tolpyralate+atrazine provided a greater reduction in pigweed density and biomass than tolpyralate alone, but results were similar to both topramezone+atrazine and mesotrione+atrazine. Similar activity with these HPPD inhibitors has been reported in other Amaranthus spp. (Hugie et al. Reference Hugie, Bollero, Tranel and Riechers2008; Kohrt and Sprague Reference Kohrt and Sprague2017; Tonks et al. Reference Tonks, Grove, Kikugawa, Parks, Nagayama and Tsukamoto2015; Woodyard et al. Reference Woodyard, Bollero and Riechers2009). Kohrt and Sprague (Reference Kohrt and Sprague2017) observed no difference in control of atrazine-resistant Palmer amaranth with tolpyralate (40 g ai ha−1) compared with tolpyralate+atrazine (40+560 g ai ha−1). Similarly, Armel et al. (Reference Armel, Wilson, Richardson and Hines2003) found that mesotrione (105 g ai ha−1) with or without atrazine (560 g ai ha−1) controlled smooth pigweed (Amaranthus hybridus L.) 99%. Tonks et al. (Reference Tonks, Grove, Kikugawa, Parks, Nagayama and Tsukamoto2015) reported an average of 89% control of Amaranthus spp. including Palmer amaranth, redroot pigweed, and waterhemp [Amaranthus tuberculatus (Moq.) J.D. Sauer] with tolpyralate (30 g ai ha−1). Thus, consistent with results obtained in the BED study (Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018), tolpyralate exhibits high activity on Amaranthus spp.
Common Ragweed
At 1 WAA, there was a greater range in common ragweed control than in other broadleaf species. Tolpyralate+atrazine provided 34 percentage points better control than tolpyralate alone and 26 percentage points better control than mesotrione+atrazine, but results were not significantly different from topramezone+atrazine (Table 4). Control with mesotrione+atrazine was similar to tolpyralate alone and topramezone+atrazine treatments. At 2 WAA, tolpyralate and tolpyralate+atrazine provided similar control of common ragweed. Tolpyralate+atrazine and topramezone+atrazine provided similar control of common ragweed, which was greater than mesotrione+atrazine. At 4 WAA, tolpyralate, tolpyralate+atrazine, and topramezone+atrazine each provided better common ragweed control than mesotrione+atrazine. Previous research has shown variable control of common ragweed with mesotrione, but improved control when co-applied with atrazine (Armel et al. Reference Armel, Wilson, Richardson and Hines2003; Bollman et al. Reference Bollman, Boerboom, Becker and Fritz2008; Whaley et al. Reference Whaley, Armel, Wilson and Hines2006). At 8 WAA, tolpyralate+atrazine controlled common ragweed 100%, which was equivalent to tolpyralate alone and topramezone+atrazine control and greater than mesotrione+atrazine control. It is possible that poorer control of common ragweed with mesotrione+atrazine at 8 WAA may be reflective of the comparatively lower rate of atrazine applied. However, tolpyralate alone provided 95% control of common ragweed in this study when no atrazine was applied, suggesting that the observed differences can be attributed to the HPPD inhibitors. Additionally, tolpyralate alone provided an equivalent reduction in common ragweed density and biomass compared with topramezone+atrazine and mesotrione+atrazine treatments. In contrast, there was a greater decrease in common ragweed density and biomass with tolpyralate+atrazine compared with other treatments. Therefore, the addition of atrazine to tolpyralate provided no significant benefit in visible control at 2, 4, and 8 WAA, but it provided more complete weed necrosis at 8 WAA, contributing to a greater reduction in common ragweed density and biomass compared with all other treatments. Similarly, the results from this study suggest that treatments with atrazine at 500 or 1,000 g ha−1 provided a greater numerical decrease in common ragweed density than tolpyralate alone or mesotrione+atrazine. Therefore, it is likely that these higher rates of atrazine contributed to better residual control of late-emerging seedlings, which were counted during harvests, but contributed little to biomass measurements. Overall, the results from this study are similar to those of Tonks et al. (Reference Tonks, Grove, Kikugawa, Parks, Nagayama and Tsukamoto2015), who reported 89% and 95% control of common ragweed with tolpyralate and tolpyralate+atrazine, respectively.
Ladysthumb
There was considerable variation in control of ladysthumb observed with all treatments, potentially due to interspecific competition within plots that could have prevented thorough spray coverage of ladysthumb foliage in the lower part of the weed canopy. At 1 WAA, all treatments provided equivalent control; however, treatment separation was present at later assessment timings. Consistent with findings described in the first part of this study (Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018), the addition of atrazine to tolpyralate improved ladysthumb control at 2, 4, and 8 WAA (Table 5). At 2, 4, and 8 WAA, topramezone+atrazine provided control which was similar to tolpyralate alone, tolpyralate+atrazine, and mesotrione+atrazine. At 4 WAA, mesotrione+atrazine provided better ladysthumb control than tolpyralate alone; however, by 8 WAA, control with both treatments was equivalent. At 8 WAA, tolpyralate+atrazine, topramezone+atrazine, and mesotrione+atrazine all provided similar control of ladysthumb. Mesotrione+atrazine reduced ladysthumb density more than tolpyralate alone; however, all treatments provided a similar reduction in biomass. Previous research has not investigated tolpyralate efficacy on this species; however, in agreement with relatively higher BED values outlined in the first part of this study (Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018), ladysthumb appears to exhibit greater tolerance to tolpyralate than other broadleaf species, including common lambsquarters, velvetleaf, pigweed species, and common ragweed. These results suggest that the addition of atrazine is necessary to achieve adequate control (>80%) of this species. Comparable findings have been reported by Rahman et al. (Reference Rahman, Trolove and James2013), who found that the addition of atrazine to topramezone was required for control of another Polygonum species, prostrate knotweed [Polygonum aviculare (L.)].
Wild Mustard
Consistent with findings presented previously (Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018), wild mustard showed high tolerance to tolpyralate. At 1 WAA, bleaching symptoms were visible on wild mustard plants across all treatments; however, tolpyralate+atrazine provided better control than tolpyralate alone (Table 6). At 1 WAA, tolpyralate+atrazine, topramezone+atrazine, and mesotrione+atrazine provided similar control of wild mustard. At 2, 4, and 8 WAA, tolpyralate+atrazine, topramezone+atrazine, and mesotrione+atrazine controlled wild mustard ≥99%, which was better than tolpyralate applied alone. Similarly, density and dry weight were reduced to zero with tolpyralate+atrazine, topramezone+atrazine, and mesotrione+atrazine. Consistent with these results, atrazine applied alone POST is reported to have excellent efficacy on wild mustard (OMAFRA 2016), suggesting there may be little benefit to inclusion of the HPPD inhibitor. However, mesotrione applied POST has been reported to control wild mustard (Cornes 2005). Because atrazine was not applied alone in this study, distinctions cannot be made between the relative wild mustard control provided by atrazine versus the respective HPPD inhibitor.
Green Foxtail
At 1 WAA, tolpyralate+atrazine was the most efficacious treatment for control of green foxtail. Control of green foxtail with tolpyralate was similar when applied alone or in combination with atrazine at most assessment timings; however, at 1 WAA, tolpyralate+atrazine provided 18 percentage points better control than tolpyralate alone (Table 7). Tolpyralate+atrazine also provided better green foxtail control than topramezone+atrazine and mesotrione+atrazine. At 2, 4, and 8 WAA, tolpyralate applied alone or in combination with atrazine and topramezone+atrazine provided better control of green foxtail than mesotrione+atrazine. These results are consistent with those reported in the literature, which have documented poor control of Setaria spp. with mesotrione (Armel et al. Reference Armel, Wilson, Richardson and Hines2003; Creech et al. Reference Creech, Monaco and Evans2004; Kaastra et al. Reference Kaastra, Swanton, Tardif and Sikkema2008), but acceptable control with topramezone (Bollman et al. Reference Bollman, Boerboom, Becker and Fritz2008; Grossmann and Ehrhardt Reference Grossman and Ehrhardt2007; Kaastra et al. Reference Kaastra, Swanton, Tardif and Sikkema2008; Whaley et al. Reference Whaley, Armel, Wilson and Hines2006). Reduction in green foxtail density and biomass was equivalent with tolpyralate applied alone or with atrazine and was greater than with topramezone+atrazine and mesotrione+atrazine. Results from the first part of this study (Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018) determined the BED of tolpyralate in green foxtail to be 29.6 g ai ha−1 8 WAA when applied alone. Thus, the dose of 30 g ai ha−1 examined in this analysis is likely to diminish any contribution to green foxtail control provided by atrazine; however, control data collected at 1 WAA suggest that atrazine may improve speed of green foxtail control with tolpyralate.
Barnyardgrass
Control of barnyardgrass followed trends similar to green foxtail control but was more variable with all treatments. Tolpyralate alone or with the addition of atrazine provided similar control at all assessment timings (Table 8). At 1 WAA, tolpyralate+atrazine and topramezone+atrazine provided similar barnyardgrass control, while mesotrione+atrazine provided poorer control. In agreement with these results, mesotrione+atrazine has not been found to provide adequate control of barnyardgrass in Ontario in previous studies (OMAFRA 2016). In contrast, both Creech et al. (Reference Creech, Monaco and Evans2004) and De Cauwer et al. (Reference De Cauwer, Rombaut, Bulcke and Reheul2012) found that mesotrione provided complete barnyardgrass control; however, those studies were conducted in a greenhouse environment. At 2 WAA, tolpyralate+atrazine provided better control of barnyardgrass than topramezone+atrazine and mesotrione+atrazine. Topramezone has previously been reported to control barnyardgrass at doses similar to those used in this study (De Cauwer et al. Reference De Cauwer, Rombaut, Bulcke and Reheul2012). At 4 and 8 WAA, tolpyralate alone and tolpyralate+atrazine provided similar barnyardgrass control. No statistically significant difference was observed among treatments in barnyardgrass density or biomass at 8 WAA; however, the numerical differences across treatments may have biological significance.
Conclusions
The addition of atrazine to tolpyralate improved the control of ladysthumb and wild mustard, while there was no improvement in control of common lambsquarters, velvetleaf, pigweed species, common ragweed, green foxtail, or barnyardgrass compared with tolpyralate applied alone. Tolpyralate+atrazine and topramezone+atrazine provided equivalent control of common lambsquarters, velvetleaf, pigweed species, common ragweed, ladysthumb, wild mustard, green foxtail, and barnyardgrass 8 WAA. Tolpyralate+atrazine and mesotrione+atrazine provided equivalent control of common lambsquarters, velvetleaf, pigweed species, ladysthumb, and wild mustard; in contrast, tolpyralate+atrazine provided better control of common ragweed, green foxtail, and barnyardgrass than mesotrione+atrazine. The co-application of tolpyralate with atrazine at the 1:33.3 ratio used in this study resulted in more rapid control of all species compared with tolpyralate alone, with the exception of ladysthumb and barnyardgrass. Additionally, the rate of atrazine used with tolpyralate in this study may have contributed to extended residual control of late-emerging weed seedlings, particularly in the case of common ragweed. However, it is unclear what ratio of tolpyralate to atrazine is required for these effects to occur. Future research on the optimal ratio of atrazine to use in combination with tolpyralate would help to maximize weed control and reduce the selection pressure for resistance to a single herbicide mechanism of action, while minimizing environmental loading of herbicides.
Acknowledgments
The authors thank Christy Shropshire and Todd Cowan for their technical contributions to this study. Funding for this project was provided in part by the Grain Farmers of Ontario and through the Growing Forward (GF 2) program administered by the Agricultural Adaptation Council (AAC) and by ISK Biosciences Inc. No conflicts of interest have been declared.
