Introduction
Tolpyralate is a pyrazole (Group 27) herbicide registered for the control of some annual grass and broadleaf weeds in corn (Anonymous 2021). Tolpyralate is applied postemergence and inhibits the 4-hydroxyphenyl-pyruvate dioxygenase (HPPD) enzyme in sensitive plants (Anonymous 2021). Tolpyralate is often co-applied with atrazine, a photosystem II–inhibiting herbicide from the triazines (Group 5) chemical family for improved control of velvetleaf (Abutilon theophrasti Medic.), common lambsquarters (Chenopdium album L.), redroot pigweed (Amaranthus retroflexus L.), waterhemp [Amaranthus tuberculatus (Moq.) Sauer], Palmer amaranth (Amaranthus palmeri S. Watson), common ragweed (Ambrosia artemisiifolia L.), kochia [Bassia scopfaria (L.) A.J. Scott], barnyardgrass (Echinochloa crus-galli), green foxtail (Setaria viridis L.), and giant foxtail (Setaria faberi L.) (Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018a, b; Tonks et al. Reference Tonks, Grove, Kikugawa, Parks, Nagayama and Tsukamoto2015). Earlier studies have shown inadequate control of annual grasses with tolpyralate + atrazine (Fluttert et al. Reference Fluttert, Soltani, Galla, Hooker, Robinson and Sikkema2022; Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018a; Osipitan et al. Reference Osipitan, Scott and Knezevic2018). The mixture of tolpyralate + atrazine with a residual grass herbicide may improve late-season weed control in corn, especially control of annual grasses. Additionally, utilizing other modes of action in a mixture with tolpyralate + atrazine may slow down the evolution of additional herbicide-resistant weeds in corn (Lamichhane et al. Reference Lamichhane, Devos, Beckie, Owen, Tillie, Messéan and Kudsk2017; Osipitan and Dille Reference Osipitan and Dille2017; Owen Reference Owen2016). Grass herbicides that can potentially be co-applied with tolpyralate + atrazine to improve residual weed control efficacy include acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor.
Group 15 herbicides, acetochlor, dimethenamid-p, flufenacet, pyroxasulfone, and S-metolachlor belong to the chloroacetanilide, chloroacetamide, oxyacetamide, isoxazoline, and chloroacetanilide chemical families, respectively. The Group 15 herbicides are taken up by both roots and shoots of sensitive weed seedlings and inhibit the biosynthesis of very-long-chain fatty acid elongases (Fogleman Reference Fogleman2018; Grichar et al. Reference Grichar, Besler and Palrang2005; Hopkins et al. Reference Hopkins, Donaldson, Komm, Palrang, Rudolph and Bloomberg1998; Shaner Reference Shaner2014). Acetochlor, dimethenamid-p, flufenacet, pyroxasulfone, and S-metolachlor control some small-seeded annual grass including barnyardgrass, Digitaria spp., fall panicum (Panicum dichotomiflorum Michx.), Setaria spp., and witchgrass (Panicum capillare L.) and broadleaved weeds including pigweeds, common ragweed, common lambsquarters, Polygonum spp., Solanum spp., and wild mustard (Sinapis arvensis L.) (Fogleman Reference Fogleman2018; Hopkins et al. Reference Hopkins, Donaldson, Komm, Palrang, Rudolph and Bloomberg1998; Shaner Reference Shaner2014; Steckel et al. Reference Steckel, Simmons and Sprague2003).
Pendimethalin is an herbicide from the dinitroaniline chemical family (Group 3) used preemergence or postemergence in corn to control annual grasses such as barnyardgrass, fall panicum, giant foxtail, green foxtail, yellow foxtail (Setaria pumila L.), smooth crabgrass [Digitaria ischaemum (Schreb.) Muhl.], and large crabgrass [Digitaria sanguinalis (L.) Scop.], and broadleaf weeds such as common lambsquarters and redroot pigweed (Hopkins et al. Reference Hopkins, Donaldson, Komm, Palrang, Rudolph and Bloomberg1998; Shaner Reference Shaner2014). Pendimethalin is absorbed by roots and shoots and is a microtubule polymerization inhibitor that inhibits mitosis and cell wall formation in sensitive plants (Shaner Reference Shaner2014).
Little research has been published on improved residual weed control with the co-application of residual grass herbicides with tolpyralate + atrazine for late-season weed control in corn. The aim of this study was to determine if the addition of a grass herbicide to tolpyralate + atrazine would improve late-season weed control in corn.
Materials and Methods
In 2021 and 2022, three field experiments were carried out at the University of Guelph Ridgetown Campus, Ridgetown, ON, Canada (42.45º N, 81.88º W), and three field experiments were completed at the Huron Research Station near Exeter, ON, Canada (43.32º N, 81.50º W). The experiments were designed as randomized complete block designs with four replicates. The experimental plots were 3 m wide and 8 m long at Ridgetown and 10 m long at Exeter. Glyphosate/glufosinate-resistant corn hybrids ‘DKC39-97 RIB’/’DKC 42-04RIB’ were seeded to a depth of 4–5 cm at 80,000 seeds ha–1 in rows that were 0.75 m apart (four rows in each plot).
Treatments consisted of a weedy control, weed-free control, and tolpyralate + atrazine (30 + 560 g ai ha–1) applied as a mixture and co-applied with acetochlor (1225 g ai ha–1), dimethenamid-p (544 g ai ha–1), flufenacet (360 g ai ha–1), pendimethalin (1680 g ai ha–1), pyroxasulfone (125 g ai ha–1), or S-metolachlor (1140 g ai ha–1). All tolpyralate + atrazine mixtures included methylated seed oil (MSO Concentrate®) at 1% v/v (Tables 1–7).
Table 1. Visible percent corn injury 1 wk after herbicide application (WAA) (n = 5), 4 WAA (n = 3), and corn yield (n = 6) for herbicide treatments consisting of tolpyralate + atrazine alone and in combination with various grass herbicides applied postemergence at Exeter and Ridgetown, ON, in 2021 and 2022. a
a Means within a column followed by the same letter do not differ significantly according to a Tukey-Kramer multiple-range test at P < 0.05.
b All tolpyralate + atrazine treatments included methylated seed oil (MSO Concentrate®) at 1% v/v.
Table 2. Visible percent control 4 and 8 wk after herbicide application (WAA), density and dry biomass 8 WAA for velvetleaf (ABUTH) treated with tolpyralate + atrazine alone and in combination with various grass herbicides applied postemergence at Exeter and Ridgetown, ON in 2021 and 2022 (n = 4). a
a Means within a column followed by the same letter do not differ significantly according to a Tukey-Kramer multiple-range test at P < 0.05.
b All tolpyralate + atrazine treatments included methylated seed oil (MSO Concentrate®) at 1% v/v.
Table 3. Visible percent control 4 and 8 wk after herbicide application (WAA), density and dry biomass 8 WAA for green and redroot pigweed species (AMASS) treated with tolpyralate + atrazine alone and in combination with various grass herbicides applied postemergence at Exeter and Ridgetown, ON in 2021 and 2022 (n = 5). a
a Means within a column followed by the same letter do not differ significantly according to a Tukey-Kramer multiple-range test at P < 0.05.
b All tolpyralate + atrazine treatments included methylated seed oil (MSO Concentrate®) at 1% v/v.
Table 4. Visible percent control 4 and 8 wk after herbicide application (WAA), density and dry biomass 8 WAA for common ragweed (AMBEL) treated with tolpyralate + atrazine alone and in combination with various grass herbicides applied postemergence at Exeter and Ridgetown, ON in 2021 and 2022 (n = 5). a
a Means within a column followed by the same letter do not differ significantly according to a Tukey-Kramer multiple-range test at P < 0.05.
b All tolpyralate + atrazine treatments included methylated seed oil (MSO Concentrate®) at 1% v/v.
Table 5. Visible percent control 4 and 8 wk after herbicide application (WAA), density and dry biomass 8 WAA for common lambsquarters (CHEAL) treated with tolpyralate + atrazine alone and in combination with various grass herbicides applied postemergence at Exeter and Ridgetown, ON in 2021 and 2022 (n = 6). a
a Means within a column followed by the same letter do not differ significantly according to a Tukey-Kramer multiple-range test at P < 0.05.
b All tolpyralate + atrazine treatments included methylated seed oil (MSO Concentrate®) at 1% v/v.
Table 6. Visible percent control 4 and 8 wk after herbicide application (WAA), density and dry biomass 8 WAA for barnyardgrass (ECHCG) treated with tolpyralate + atrazine alone and in combination with various grass herbicides applied postemergence at Ridgetown, ON in 2021 and 2022 (n = 4). a
a Means within a column followed by the same letter do not differ significantly according to a Tukey-Kramer multiple-range test at P < 0.05.
b All tolpyralate + atrazine treatments included methylated seed oil (MSO Concentrate®) at 1% v/v.
Table 7. Visible percent control 4 and 8 wk after herbicide application (WAA), density and dry biomass 8 WAA for green or giant foxtail (SETSS) treated with tolpyralate + atrazine alone and in combination with various grass herbicides applied postemergence at Exeter and Ridgetown, ON in 2021 and 2022 (n = 5). a
a Means within a column followed by the same letter do not differ significantly according to a Tukey-Kramer multiple-range test at P < 0.05.
b All tolpyralate + atrazine treatments included methylated seed oil (MSO Concentrate®) at 1% v/v.
Herbicide treatments were applied postemergence when the naturally occurring weed population reached an average canopy height of 10 cm with a CO2-pressurized backpack sprayer calibrated to deliver 200 L ha–1 aqueous solution at 240 kPa. The boom was 1.5 m long with four Hypro ULD120-02 nozzle tips (Pentair, New Brighton, MN) spaced 50 cm apart, producing a spray width of 2.0 m. The weed-free control plots were sprayed with S-metolachlor/atrazine/mesotrione/bicyclopyrone (Acuron, 2230 g ai ha–1, Syngenta Canada Inc, Guelph, ON, Canada) applied preemergence followed by glyphosate (Roundup WeatherMAX®, 540 g ae L–1; Bayer CropScience Canada Inc., Calgary, AB, Canada).
Visible injury in corn was evaluated 1 and 4 wk after herbicide application (WAA), and weed control was evaluated 4 and 8 WAA on a scale of 0% to 100% (0% = no corn injury/no weed control and 100% = complete plant necrosis) relative to the weed-free/nontreated control. Weed density was determined by counting each weed species present in two randomly selected quadrats (0.25 m2 each) within each plot at 8 WAA. Shoot dry weight (biomass) was measured by removing each weed at the soil level within each quadrat, separating them by species, placing them in a paper bag, drying them (60 C) to constant moisture, and then weighing them. The two middle rows of corn in each plot were combined at harvest maturity with a small-plot combine; weight and seed moisture were recorded. Yields were adjusted to 15.5% moisture prior to statistical analysis.
Statistical Analysis
Data were analyzed with PROC GLIMMIX. The generalized linear mixed model fixed effect was herbicide treatment and random effects were environment (year–location combinations), environment-by-treatment interaction, and replicate within environment. The normal probability plot from PROC UNIVARIATE and the Shapiro-Wilk statistic was used to assess normality. Deviations from the assumption of homogeneity of variance were checked using studentized residual plots generated in PROC GLIMMIX. Visible percent weed control evaluations were arcsine-square root transformed prior to using a Gaussian distribution. Weed density and shoot dry biomass were analyzed using a log-normal distribution. Corn yield was analyzed using a Gaussian distribution. Least square means for all variables except yield were back-transformed to the data scale for presentation, with treatment differences reflecting the results of the analysis performed on the model scale. Treatment differences were determined using the Tukey-Kramer multiple-range test at a significance level of P < 0.05. When a treatment had zero variance due to assigned values (nontreated and weed-free controls), it was excluded from the analysis; comparisons between each treatment and the value zero were possible using the P value generated for each mean in the LSMEANS output.
Results and Discussion
Corn Injury and Corn Yield
Tolpyralate + atrazine applied as a mixture and in combination with acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor caused 11% to 16% corn injury at 1 WAA; there was no difference among the herbicide mixtures assessed (Table 1). Tolpyralate + atrazine applied as a mixture and in combination with grass herbicides assessed caused 4% to 8% injury in corn at 4 WAA; tolpyralate + atrazine + dimethenamid-p resulted in higher corn injury than tolpyralate + atrazine + pendimethalin.
Weed interference reduced corn yield 60%. Decreased weed interference with tolpyralate + atrazine and when mixed with acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor resulted in corn yield that was comparable to the weed-free control.
Similar corn yields were reported in other studies conducted with tolpyralate + atrazine at comparable rates in corn (Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018a, b; Osipitan et al., Reference Osipitan, Scott and Knezevic2018; Tonks et al., Reference Tonks, Grove, Kikugawa, Parks, Nagayama and Tsukamoto2015). Osipitan et al. (Reference Osipitan, Scott and Knezevic2018) found that decreased weed interference with tolpyralate and tolpyralate + atrazine caused higher corn yields by 52% and 61%, respectively in comparison to the weedy control. Metzger et al. (Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019) found a significant yield enhancement in corn (10,400 kg ha–1 vs 4,300 kg ha–1) due to reduced weed interference with tolpyralate + atrazine in comparison to the weedy control. Other research has shown that reduced weed interference provided by the residual activity of pyroxasulfone or dimethenamid-p can result in corn yields that are similar to the weed-free control (Soltani et al. Reference Soltani, Brown and Sikkema2019; Stephenson et al. Reference Stephenson, Bond, Griffin, Landry, Woolam, Edwards and Hardwick2017). However, weed interference with S-metolachlor applied alone caused a 33% corn yield reduction in comparison to the weed-free control (Soltani et al. Reference Soltani, Brown and Sikkema2019).
Weed Control
Velvetleaf
Tolpyralate + atrazine controlled velvetleaf 82% and 74% at 4 and 8 WAA, respectively; there was no enhancement of velvetleaf control when acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor were added to the mixture of tolpyralate + atrazine (Table 2). Tolpyralate + atrazine decreased velvetleaf density and shoot dry biomass 77% and 92%, respectively; there was no further reduction in velvetleaf density and shoot dry biomass with the addition of acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone or S-metolachlor to tolpyralate + atrazine. In other studies, tolpyralate + atrazine controlled velvetleaf 92% to 96% in corn (Fluttert et al. Reference Fluttert, Soltani, Galla, Hooker, Robinson and Sikkema2022). Metzger et al. (Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019) observed 72% to 81% control of velvetleaf with tolpyralate + atrazine in corn. Tonks et al. (Reference Tonks, Grove, Kikugawa, Parks, Nagayama and Tsukamoto2015) reported >90% velvetleaf control with tolpyralate + atrazine 30 d after application in corn.
Pigweed Species (Green and Redroot Pigweeds)
Tolpyralate + atrazine applied as a mixture or in combination with acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor controlled pigweed species 96% to 100% at 4 WAA (Table 3). The inclusion of acetochlor or dimethenamid-p with tolpyralate + atrazine enhanced pigweed species control 4% at 4 WAA; the inclusion of flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor with tolpyralate + atrazine provided no significant enhancement of pigweed species control. Tolpyralate + atrazine applied as a mixture or in combination with acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor controlled pigweed species 95% to 100% at 8 WAA; pigweed species control was similar with all mixtures tested. Tolpyralate + atrazine decreased pigweed species density 88% and shoot dry biomass 96%. The inclusion of acetochlor with tolpyralate + atrazine decreased pigweed species density and shoot dry biomass 100%; the inclusion of dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor with tolpyralate + atrazine did not cause a further reduction in pigweed species density and shoot dry biomass relative to tolpyralate + atrazine. In other studies, tolpyralate + atrazine controlled pigweed species >90% in corn (Fluttert et al. Reference Fluttert, Soltani, Galla, Hooker, Robinson and Sikkema2022; Langdon et al. Reference Langdon, Soltani, Raeder, Robinson, Hooker and Sikkema2020; Metzger et al. Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019). Metzger et al. (Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018b) observed excellent control (96% to 98%) of pigweed species with tolpyralate + atrazine in corn. In another study, Metzger et al. (Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019) found 88% to 92% control of pigweeds with tolpyralate + atrazine in corn. In other studies, dimethenamid-p, pyroxasulfone, and S-metolachlor applied alone provided 99%, 99%, and 98% redroot pigweed control (Soltani et al. Reference Soltani, Brown and Sikkema2019). Additionally, the density of redroot pigweed was reduced 98%, 93%, and 92%; and shoot biomass was reduced 99%, 93%, and 91% with dimethenamid-p, pyroxasulfone, and S-metolachlor, respectively (Soltani et al. Reference Soltani, Brown and Sikkema2019).
Common Ragweed
Tolpyralate + atrazine applied as a mixture or in combination with acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor controlled common ragweed 94% to 99% and 91% to 99% at 4 and 8 WAA, respectively (Table 4). The addition of acetochlor enhanced common ragweed control 5% at 4 WAA, and the addition of acetochlor and dimethenamid-p enhanced common ragweed control 7% to 8% at 8 WAA; the inclusion of other grass herbicides did not enhance common ragweed control (Table 4). Tolpyralate + atrazine decreased common ragweed density 88%; the inclusion of acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor did not enhance the reduction in common ragweed density. Tolpyralate + atrazine decreased common ragweed shoot dry biomass 98%. The inclusion of acetochlor with tolpyralate + atrazine reduced common ragweed shoot dry biomass 100%; the inclusion of dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor with tolpyralate + atrazine did not cause a further decrease in common ragweed shoot dry biomass relative to tolpyralate + atrazine. In other research, tolpyralate + atrazine controlled common ragweed >90% in corn (Fluttert et al. Reference Fluttert, Soltani, Galla, Hooker, Robinson and Sikkema2022; Langdon et al., Reference Langdon, Soltani, Raeder, Robinson, Hooker and Sikkema2020; Metzger et al., Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018a). Metzger et al. (Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018b) observed 93% to 100% control of common ragweed with tolpyralate + atrazine in corn. Tonks et al. (Reference Tonks, Grove, Kikugawa, Parks, Nagayama and Tsukamoto2015) observed common ragweed control of 89% with tolpyralate and 95% with tolpyralate + atrazine. Metzger et al. (Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019) observed excellent (up to 99%) common ragweed control with tolpyralate + atrazine in corn. In other studies, solo application of dimethenamid-p, pyroxasulfone, and S-metolachlor provided only 56%, 77%, and 42% control of common ragweed in corn, respectively (Soltani et al. Reference Soltani, Brown and Sikkema2019). Additionally, the density of redroot pigweed was reduced by only 52% with dimethenamid-p and 63% with pyroxasulfone (Soltani et al. Reference Soltani, Brown and Sikkema2019). Inadequate common ragweed control (7% to 28%) was also observed in other studies with preemergence application of dimethenamid-p, pyroxasulfone, and S-metolachlor in corn (Steele et al. Reference Steele, Porpiglia and Chandler2005).
Common Lambsquarters
Tolpyralate + atrazine as a mixture or in combination with acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor controlled common lambsquarters 97% to 100% at 4 WAA and 96% to 100% at 8 WAA (Table 5). The inclusion of acetochlor with tolpyralate + atrazine enhanced common lambsquarters control 3% at 4 WAA and 4% at 8 WAA; in contrast, the inclusion of the other grass herbicides with tolpyralate + atrazine did not enhance control of common lambsquarters (Table 5). Tolpyralate + atrazine alone or in a mixture with acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor decreased common lambsquarters density 96% to 99% and shoot dry biomass 99% to 100%. Tolpyralate + atrazine + acetochlor and tolpyralate + atrazine + dimethenamid-p decreased the density of common lambsquarters to the extent that it was comparable to the weed-free control. All of the mixtures tested except tolpyralate + atrazine + flufenacet reduced common lambsquarters shoot dry biomass so that it was comparable to the weed-free control (Table 5). In other research, tolpyralate + atrazine controlled common lambsquarters >90% in corn (Fluttert et al. Reference Fluttert, Soltani, Galla, Hooker, Robinson and Sikkema2022; Langdon et al. Reference Langdon, Soltani, Raeder, Robinson, Hooker and Sikkema2020). Metzger et al. (Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018b) observed excellent (93% to 100%) control of common lambsquarters with tolpyralate + atrazine at 2, 4, and 8 WAA in corn. In another study, Metzger et al. (Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019) found up to 99% control of common lambsquarters with tolpyralate + atrazine in corn. In other studies, dimethenamid-p, pyroxasulfone, and S-metolachlor applied preemergence controlled common lambsquarters 68%, 84%, and 60%, respectively (Soltani et al. Reference Soltani, Brown and Sikkema2019). Additionally, common lambsquarters density was reduced 98%, 93%, and 92%; and common lambsquarters shoot dry biomass was reduced 74%, 92%, and 66% with dimethenamid-p, pyroxasulfone, and S-metolachlor, respectively (Soltani et al. Reference Soltani, Brown and Sikkema2019). Jha et al. (Reference Jha, Kumar, Garcia and Reichard2015) observed 12% to 65%, 15% to 66%, and 33% to 77% control of common lambsquarters with acetochlor, pyroxasulfone, and dimethenamid-p in corn, respectively.
Barnyardgrass
Tolpyralate + atrazine in a mixture or in combination with acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor controlled barnyardgrass 90% to 100% at 4 WAA and 78% to 99% at 8 WAA, respectively (Table 6). The co-application of acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor with tolpyralate + atrazine increased the control of barnyardgrass 9% to 10% at 4 WAA and 21% at 8 WAA (Table 6). Tolpyralate + atrazine did not decrease barnyardgrass density and shoot dry biomass in comparison to the weedy control (Table 6). The mixture of tolpyralate + atrazine with acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor reduced barnyardgrass density 75% to 92% and shoot dry biomass 97% to 99%; there was no variation between the grass herbicides tested (Table 6). In other studies, Fluttert et al. (Reference Fluttert, Soltani, Galla, Hooker, Robinson and Sikkema2022) reported only 64% to 72% barnyardgrass control with tolpyralate + atrazine. However, Tonks et al. (Reference Tonks, Grove, Kikugawa, Parks, Nagayama and Tsukamoto2015) observed 90% control of barnyardgrass with tolpyralate + atrazine 30 d after application. Metzger et al. (Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2018b) observed 86% to 97% barnyardgrass control with tolpyralate + atrazine. In another study, Metzger et al. (Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019) reported 71% to 97% barnyardgrass control with tolpyralate + atrazine sprayed at various application timings in corn. In other studies, preemergence applications of dimethenamid-p, pyroxasulfone, and S-metolachlor caused 80%, 56%, and 61% barnyardgrass control in corn (Soltani et al. Reference Soltani, Brown and Sikkema2019). Stephenson et al. (Reference Stephenson, Bond, Griffin, Landry, Woolam, Edwards and Hardwick2017) found 93% to 96% barnyardgrass control with pyroxasulfone applied preemergence in corn. Similarly, Yamaji et al. (Reference Yamaji, Honda, Kobayashi, Hanai and Inoue2014) showed 100% barnyardgrass control with pyroxasulfone applied preemergence in corn.
Foxtail Species (Giant and Green Foxtails)
Tolpyralate + atrazine in a mixture or in combination with acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor controlled foxtail species 80% to 99% at 4 WAA and 69% to 98% at 8 WAA (Table 7). The inclusion of acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor with tolpyralate + atrazine increased foxtail species control 15% to 19% at 4 WAA and 24% to 29% at 8 WAA; there was no variation in foxtail species control between the grass herbicides when co-applied with tolpyralate + atrazine. Tolpyralate + atrazine did not decrease foxtail species density in comparison to the weedy control. The co-application of acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor with tolpyralate + atrazine decreased foxtail species density 83% to 92%; there was no variation between the grass herbicides tested. Tolpyralate + atrazine reduced foxtail species shoot dry biomass 91%. The co-application of acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor with tolpyralate + atrazine decreased foxtail species shoot dry biomass 98% to 100%; there was no variation between grass herbicides tested. In other studies, Fluttert et al. (Reference Fluttert, Soltani, Galla, Hooker, Robinson and Sikkema2022) reported 72% to 75% control of foxtails with tolpyralate + atrazine. Metzger et al. (Reference Metzger, Soltani, Raeder, Hooker, Robinson and Sikkema2019) observed 68% to 72% control of foxtails with tolpyralate + atrazine. In other studies, preemergence applications of dimethenamid-p, pyroxasulfone, and S-metolachlor provided 96% green foxtail control in corn (Soltani et al. Reference Soltani, Brown and Sikkema2019). Additionally, the density of green foxtail was reduced 99%, 95%, and 95%; and shoot biomass was reduced 99%, 93%, and 97% with dimethenamid-p, pyroxasulfone, and S-metolachlor, respectively (Soltani et al. Reference Soltani, Brown and Sikkema2019). Geier et al. (Reference Geier, Stahlman and Frihauf2006) observed 95% green foxtail control with the preemergence application of pyroxasulfone, but the control was 88% with the preemergence application of S-metolachlor.
Tolpyralate + atrazine in a mixture or in combination with acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor caused up to 16% and 8% injury at 1 and 4 WAA in corn, respectively. Weed interference reduced corn yield by 60%; reduced weed interference with tolpyralate + atrazine mixture with the grass herbicides tested provided corn yield that was comparable to the weed-free control. The inclusion of the grass herbicides tested with tolpyralate + atrazine did not enhance velvetleaf control. However, the co-application of the grass herbicides tested with tolpyralate + atrazine generally enhanced control of the other weed species evaluated, especially barnyardgrass and foxtail species in corn. The mixture of acetochlor with tolpyralate + atrazine generally provided the most consistent weed control. The co-application of the grass herbicides tested with tolpyralate + atrazine improves the range of weed species controlled and may help reduce the evolution of new herbicide-resistant weed biotypes in corn. Future research is needed to determine if the addition of a grass herbicide to tolpyralate + atrazine would improve the control of other late-emerging weed species in corn.
Practical Implications
There is limited information available on the benefit of adding acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor to tolpyralate + atrazine for weed management in corn. Results show that the co-application of the aforementioned grass herbicides with tolpyralate + atrazine did not increase injury in corn. Weed interference decreased corn yield 60% in this study; however, tolpyralate + atrazine alone or in a mixture with acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor provided corn yield that was comparable to the weed-free plots. The inclusion of acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor with tolpyralate + atrazine did not enhance velvetleaf control. The co-application of acetochlor or dimethenamid-p with tolpyralate + atrazine enhanced pigweed species control 5% and common ragweed control 7% to 8% at 8 WAA, but the inclusion of flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor with tolpyralate + atrazine does not enhance pigweed species or common ragweed control. The inclusion of acetochlor with tolpyralate + atrazine improved common lambsquarters control 4% at 8 WAA, but the co-application with the other grass herbicides with tolpyralate + atrazine gave no additional benefits. The inclusion of acetochlor, dimethenamid-p, flufenacet, pendimethalin, pyroxasulfone, or S-metolachlor with tolpyralate + atrazine increased barnyardgrass control 21% and green foxtail control 24% to 29% 8 at WAA. The application of tolpyralate + atrazine plus a grass herbicide provided species-specific improvement in weed control in corn except for velvetleaf.
Acknowledgments
We thank Grain Farmers of Ontario (GFO), and the Ontario Agri-Food Innovation Alliance for the funding of this study. No other competing interests have been declared.
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