Seven field trials were completed over a three-year period (2016 to 2018) in southwestern Ontario, Canada, to assess weed control in conventional-till dicamba-resistant (DR) soybean with glyphosate/dicamba (2 : 1 ratio) applied postemergence (POST) at 3 doses (900, 1350, and 1800 g·ae·ha−1) and 3 application timings (up to 5, 15, and 25 cm weeds). There was minimal soybean injury (≤2%) from treatments evaluated. Glyphosate/dicamba applied at application timing of up to 5, 15, and 25 cm weeds, controlled
Multiple-herbicide-resistant weeds, especially glyphosate-resistant (GR) weed biotypes, have increased rapidly in respect to number of weed species and geographic areas affected over the past 12 years in Ontario. Presently, there are four weed species with confirmed resistance to glyphosate in Ontario, including
DR soybean will allow soybean growers to incorporate dicamba, a Group 4 benzoic acid herbicide with excellent activity on nearly 100 annual, biennial, and perennial broadleaved weeds, into their cropping system. Despite being in use for more than 50 years, there are only six weed species that are currently confirmed resistant to dicamba worldwide [
DR soybean was first introduced in Ontario in 2017, and currently, over 50% of soybean fields in eastern Canada are seeded to DR soybean cultivars. The current registered dose of glyphosate/dicamba (2 : 1 ratio premix) in DR soybean in Ontario ranges from 900 (600 + 300) to 1800 (1200 + 600) g·ae·ha−1 [
Earlier studies have mainly focused on GR weed control with glyphosate/dicamba in DR soybean in Ontario [
This research was established to assess control of common annual grass and broadleaved weeds in conventional-till DR soybean with glyphosate/dicamba (2 : 1 ratio premix) applied POST at three doses (900, 1350, and 1800 g·ae·ha−1) and three application timings (up to 5, 15, and 25 cm weeds) in Ontario.
There was a total of seven field experiments which included two experiments (one in 2017 and 2018) at Exeter, ON (43° 19′ 1.2108″ N, 81° 30′ 3.8736″ E), and five experiments (two in 2016, one in 2017, and two in 2018) at Ridgetown, ON (42° 26′ 41.46″ N, 81° 52′ 44.472″ W). The experiment was arranged as a factorial with four replications and established as a randomized complete block design. Factor one consisted of three glyphosate/dicamba doses (DOSE: 0, 900, 1350, and 1800 g·ae·ha−1) and the second factor was application timing based on weed height (TIMING: up to 5, 15, and 25 cm weeds).
Plots were 10 and 8 m long at Exeter and Ridgetown, respectively, and included four rows (spaced 75 cm apart) of DR soybean (“DKB 14-41”/“DKB 06-61”/“DKB 12-57”) seeded 4 cm deep at a seeding rate of 400,000 ha−1. Density of weed species present at each evaluation for Ridgetown and Exeter, ON, sites is shown in Table
Density of weed species present at each evaluation for Ridgetown and Exeter, ON, sites.
Weed species | Timing | Weed density (#m−2) | ||||||
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R-2016a | R-2016b | R-2017 | E-2017 | R-2018a | R-2018b | E-2018 | ||
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4 WAA | 57 | 19 | 19 | 11 | 11 | — | 6 |
8 WAA | 35 | 17 | 21 | 9 | 10 | — | 4 | |
10 WAC | 50 | 5 | 10 | 9 | 13 | — | 2 | |
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4 WAA | 9 | 14 | — | 12 | 54 | 23 | — |
8 WAA | 9 | 28 | — | 12 | 21 | 8 | — | |
10 WAC | 12 | 17 | — | 12 | 13 | 7 | — | |
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4 WAA | 33 | 10 | 12 | 12 | 21 | 23 | 5 |
8 WAA | 30 | 25 | 14 | 12 | 15 | 13 | 3 | |
10 WAC | 18 | 10 | 13 | 13 | 10 | 13 | 1 | |
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4 WAA | — | — | — | 9 | — | 3 | 19 |
8 WAA | — | — | — | 8 | — | 7 | 12 | |
10 WAC | — | — | — | 8 | — | 9 | 12 | |
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4 WAA | 99 | 114 | 70 | — | 3 | — | — |
8 WAA | 143 | 138 | 86 | — | 4 | — | — | |
10 WAC | 136 | 82 | 88 | — | 4 | — | — | |
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4 WAA | — | — | — | 26 | 19 | 28 | 25 |
8 WAA | — | — | — | 26 | 12 | 22 | 18 | |
10 WAC | — | — | — | 24 | 6 | 18 | 12 |
Visible injury to DR soybean was assessed on a scale of 0–100 (0 = no injury and 100 = complete death) at 1 and 4 weeks after herbicide application (WAA). Weed control was evaluated on a 0–100 scale (0 = no control and 100 = complete control) at 4 and 8 WAA and 10 weeks after the final herbicide treatment (WAC). DR soybean was harvested at maturity using a small-plot combine. DR soybean seed moisture and yield (13% adjustment) were then recorded.
The GLIMMIX procedure in SAS [
Injury symptoms in soybean included leaf drooping and chlorosis. At 1 WAA, there was minimal visible soybean injury (<2%) from the herbicide treatments evaluated (Table
Significance of main effects and interaction for percent visible soybean injury 1 WAA, soybean moisture content at harvest, and soybean yield treated with glyphosate/dicamba at various POST application rates and timings at five Ridgetown, ON, sites (2016–2018) and two Exeter, ON, sites (2017-2018).
Main effects | Injury† (%) | Moisture (%) | Yield (T·ha−1) |
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5 cm weed height | 0.2a | 15.2b | 2.8b |
15 cm weed height | 0.5a | 14.9ab | 3.1ab |
25 cm weed height | 1.6b | 14.6a | 3.2a |
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Untreated control | 0a | 16.1b | 1.7b |
900 g·ae·ha−1 | 0.1b | 14.6a | 3.4a |
1350 g·ae·ha−1 | 0.7c | 14.6a | 3.5a |
1800 g·ae·ha−1 | 1.3c | 14.5a | 3.5a |
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R × T | NS | NS | NS |
Soybean seed moisture content was less when glyphosate/dicamba was applied to up to 25 cm tall weeds compared to when it was applied to up to 5 cm tall weeds (Table
The control of
Significance of main effects and interaction for percent visible control of
Main effects |
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4 WAA | 8 WAA | 10 WAC | 4 WAA | 8 WAA | 10 WAC | 4 WAA | 8 WAA | 10 WAC | |
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NS |
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5 cm weed height | 96b | 92b | 87b | 99a | 95b | 93b | 99b | 94b | 89b |
15 cm weed height | 99a | 96b | 94b | 99a | 98ab | 97ab | 100a | 96b | 95ab |
25 cm weed height | 99a | 99a | 99a | 99a | 100a | 99a | 100a | 99a | 99a |
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NS | NS |
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NS | NS |
Untreated control | 0c | 0c | 0c | 0c | 0b | 0b | 0 c | 0b | 0b |
900 g·ae·ha−1 | 97b | 94b | 90b | 98b | 96a | 95a | 99b | 94a | 92a |
1350 g·ae·ha−1 | 98ab | 97ab | 95ab | 99ab | 98a | 97a | 100a | 97a | 95a |
1800 g·ae·ha−1 | 99a | 99a | 97a | 100a | 99a | 99a | 100a | 99a | 98a |
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R × T | NS | NS | NS | NS | NS | NS | NS | NS | NS |
The control of
The control of
The control of
Significance of main effects and interaction for percent visible control of
Main effects |
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4 WAA | 8 WAA | 10 WAC | 4 WAA | 8 WAA | 10 WAC | 4 WAA | 8 WAA | 10 WAC | |
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5 cm weed height | 84b | 81b | 82b | 90b | 51b | 37c | 94b | 96b | 95b |
15 cm weed height | 95a | 94a | 96a | 98a | 80b | 77b | 99a | 99a | 99a |
25 cm weed height | 96a | 97a | 96a | 100a | 99a | 99a | 100a | 100a | 99a |
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NS |
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NS | NS | NS |
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NS |
Untreated control | 0c | 0b | 0c | 0b | 0b | 0b | 0c | 0c | 0b |
900 g·ae·ha−1 | 84b | 87a | 88b | 95a | 77a | 74a | 98b | 98b | 98a |
1350 g·ae·ha−1 | 94a | 93a | 93ab | 97a | 80a | 75a | 99ab | 99ab | 98a |
1800 g·ae·ha−1 | 96a | 95a | 95a | 98a | 87a | 79a | 100a | 100a | 98a |
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R × T | NS | NS | NS | NS | NS | NS | NS | NS | NS |
The control of
The control of
Based on these results, glyphosate/dicamba applied postemergence at 900, 1350, and 1800 g·ae·ha−1 causes minimal injury in DR soybean. Presumed weed interference delayed DR soybean maturity and reduced DR soybean yield as much as 51%. There was an increase in DR soybean yield with the application of glyphosate/dicamba, but there was no difference in yield among application doses. DR soybean yield was lower with the early application of glyphosate/dicamba; the authors suggest that this was due to increased weed emergence and interference after the early application. There was a trend for increased weed control as the glyphosate/dicamba dose was increased and the application timing was delayed. Based on these results, glyphosate/dicamba at the manufacturer’s label doses adequately controls
The numerical data used to generate tables in this manuscript can be provided upon request to the authors.
The authors declare that they have no conflicts of interest.
Financial support for this study came from the Grain Farmers of Ontario.