Controlling glyphosate/dicamba resistant kochia in soybean

Effective pre-emergent or pre-emergent followed by post-emergent (two-pass) programs tested in this research on the U.S Great Plains should be proactively utilized by growers to manage glyphosate/dicamba resistant kochia in Roundup Ready 2 Xtend (glyphosate/dicamba-resistant) soybean. A similar approach may prove useful for western Canadian farmers.

The main objectives of this research were to evaluate the effectiveness of various pre-emergent herbicides applied alone or followed by a post-emergent treatment of glyphosate + dicamba (two-pass programs) for controlling glyphosate + dicamba resistant kochia (GDR kochia) in glyphosate/dicamba-resistant soybean (Roundup Ready 2 Xtend), and the effect of those herbicide programs on grain yields of RR 2 Extend soybean. The research was conducted near Huntley, Montana, and Hays, Kansas.

At the Montana State University research site near Huntley, the experimental field was under barley– sugar beet rotation for more than 5 years prior to initiation of this study. A RR 2 Xtend soybean variety ‘DKB006-29’ (available in western Canada) was drilled in 6-inch (15-cm) rows at 170,000 seeds/acre (420,070 seeds/ha) on May 10, 2017 and May 7, 2018 into a conventional till field. The glyphosate-resistant population used in this study had an 8-fold resistance to glyphosate, and the dicamba resistant population had a 6.8-fold resistance to dicamba.

At Hays, Kansas a different RR 2 Xtend soybean variety (not registered in western Canada) was no-till planted on 30-inch (76 cm) row spacing at 130,000 seeds/acre (321,100 seeds/ha) in 2018. The kochia population had a 5.5-fold level of resistance to dicamba, and also glyphosate resistance.

Thirteen herbicide programs were evaluated, including pre-emergent (PRE) alone and PRE followed by (fb) a post-emergent (POST) application. A non-treated control (weedy check) was included for comparison.

Glyphosate at 1,261 g ae/ha was included with all PRE treatments for burndown weed control. This is roughly equivalent to 1 L/ac of Roundup WeatherMax (540 g ae/L). All PRE treatments were applied 1 day after soybean planting. POST treatments were applied when soybean plants had reached V3 to V4 growth stage.

Herbicide programs tested to manage glyphosate/dicamba-resistant (GDR) kochia in GDR soybean


Layering pre-emergent treatments critical to control

In Montana, PRE-applied sulfentrazone (Group 14) provided complete, season-long control of GDR kochia among all tested PRE-alone programs. This compared to PRE pyroxasulfone (Group 15) or PRE dicamba (Group 4) with 70% control rated 10 days after PRE application. It should be noted, though, that the sulfentrazone rate was about 50 per cent higher than the western Canadian rate, and the pyroxasulfone rate was about one-half of the registered western Canadian rate.

When dicamba was added to pyroxasulfone PRE, residual control of kochia was greatly improved at 95% assessed 10 days after application. This shows that the kochia population that was resistant to POST applications of dicamba was moderately sensitive to PRE dicamba.

All PRE fb POST programs provided season-long control of GDR kochia in RR 2 Extend soybean.

Kochia plants in non-treated plots produced an average of 5,691 seeds/ft2 (56,910 seeds/m2) at soybean harvest. Kochia survivors from dicamba PRE-alone produced 301 seeds/ft2 (3,010 seeds/m2) and pyroxasulfone PRE-alone produced 253 seeds/ft2 (2,530 seeds/m2). Kochia survivors in plots treated with dicamba + pyroxasulfone PRE produced a significant reduction of only 23 seeds/ft2 (230 seeds/m2), which may still be enough to replenish the GDR kochia seed bank.

All other herbicide programs tested were effective in eliminating GDR kochia seed production in soybean.

Average yield in the untreated plot was 57 bu/ac (3,800 kg/ha).

All herbicide programs improved soybean grain yield with yields ranging from 69 to 76 bu/ac (4,610 to 5,090 kg/ha), with no significant differences for the majority of herbicide programs tested. An exception was the dicamba PRE-only program with a yield of 67 bu/ac (4,490 kg/ha) compared to 76 bu/ac (5,090 kg/ha) in the dicamba PRE fb glyphosate + dicamba POST program, indicating the need for a two-pass dicamba-based program to protect soybean yield loss when only dicamba is used in the PRE application.

Similar results were seen in the one-year of testing in Kansas.

In conclusion, PRE soil-residual herbicides investigated in this study can serve as a foundation for GDR kochia management, and can aid in preserving the utility of dicamba in RR2 Xtend soybean.

Yadav R, Kumar V, Jha P (2020) Herbicide programs to manage glyphosate/ dicamba-resistant kochia (Bassia scoparia) in glyphosate/dicamba-resistant soybean. Weed Technol. 34: 568–574.

NOTE: Herbicide application rates used in this study differ from those registered in western Canada. Consult with your company representative to establish a layering program similar to those in this research study.

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