Controlling glyphosate/dicamba-resistant kochia in soybean
By Bruce Barker, P. Ag, CanadianAgronomist.ca
Kochia resistant to multiple herbicide groups is an issue on the Prairies. A kochia survey in southern Alberta in 2017 found that all kochia populations were resistant to Group 2 ALS inhibitors, 50 per cent were resistant to Group 9 glyphosate, and 18 per cent were resistant to Group 4 dicamba herbicides. Kochia populations with multiple resistance to Group 2 plus 4 plus 9 were found in 10 per cent of samples. This resistance issue is also emerging across Saskatchewan and Manitoba, as well as on the Great Plains of the United States.
Research results in Montana and Kansas can help guide western Canadian growers on how to manage glyphosate/dicamba resistant kochia in Roundup Ready 2 Xtend (glyphosate/dicamba-resistant) soybean. The key is layering pre-emergent with post-emergent herbicides. At a Montana State University research site near Huntley, a RR 2 Xtend soybean variety, DKB006-29, was drilled into six-inch (15-centimetre rows) using 170,000 seeds/acre (420,070 seeds/hectare) on May 10, 2017 and May 7, 2018, into a conventional till field. The glyphosate-resistant population used in this study had an eight-fold resistance to glyphosate, and the dicamba resistant population had a 6.8-fold resistance to dicamba.
At the site in Hays, Kansas, a different RR 2 Xtend soybean variety (AG34X7) 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 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 one litre per acre of Roundup WeatherMax (540 g ae/L). All PRE treatments were applied one day after soybean planting. POST treatments were applied when soybean plants had reached V3 to V4 growth stage. Pre-emergent (PRE) only herbicide treatments included sulfentrazone, pyroxasulfone, dicamba, sulfentrazone plus dicamba, pyroxasulfone plus dicamba, metribuzin plus flumioxazin plus imazethapyr, flumioxazin plus pyroxasulfone, and pendimethalin plus dimethenamid-P. The PRE fb POST application included the above treatments, followed by a glyphosate plus dicamba application.
Two other treatments were applied at the Kansas site: pyroxasulfone plus sulfentrazone fb glyphosate plus dicamba; and sulfentrazone plus metribuzin fb glyphosate plus dicamba.
In Montana, PRE-applied sulfentrazone provided complete, season-long control of glyphosate/dicamba-resistant (GDR) kochia among all tested PRE-alone programs. This compared to PRE pyroxasulfone or PRE dicamba with 70 per cent 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. The dicamba PRE alone rate was similar to the full western Canadian rate, and the dicamba PRE tank-mix rate was one-half the full rate. The dicamba POST rate was also one-half the full rate in the glyphosate tank-mix.
When dicamba was added to pyroxasulfone PRE, residual control of kochia was greatly improved to 95 per cent 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 Xtend 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 plus pyroxasulfone PRE produced a significant reduction to only 23 seeds/ft2 (230 seeds/m2), but that 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.
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 except for 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 plus dicamba POST program. This indicates the need for a two-pass dicamba-based program to protect soybean yield loss when only dicamba is used in the PRE application.
Average yield in the untreated plot was 57 bu/ac (3,800 kg/ha). Similar results were seen in the one-year of testing in Kansas for kochia control, seed set and yield.
While herbicide rates used in the study differ somewhat from some of the registered rates in Western Canada, the results of this research trial show the potential of layering herbicides to control glyphosate plus dicamba-resistant kochia in RR 2 Xtend soybeans. If you are interested in setting up a layering program to control resistant kochia, talk to your agronomist or company representative to determine the best approach.
Bruce Barker divides his time between CanadianAgronomist.ca and as Western Field Editor for Top Crop Manager. CanadianAgronomist.ca translates research into agronomic knowledge that agronomists and farmers can use to grow better crops. Read the full Research Insight at CanadianAgronomist.ca.
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