Controlling Roundup Ready volunteer canola in Roundup Ready soybean

All 15 PRE followed by POST herbicide combinations provided better control of volunteer canola compared with the glyphosate-only control, but tribenuron (i.e. Express SG) followed by bentazon, and tribenuron followed by imazamox plus bentazon (i.e. Viper ADV) provided solutions that were low cost, registered in western Canada, and had the potential to minimize development of herbicide resistance in other weeds.

Volunteer Roundup Ready (GR) canola poses a challenge for soybean production because many growers who choose to grow Roundup Ready (GR) soybean also grow GR canola, and glyphosate alone will not control GR canola in GR soybean. The objective of this research study was to evaluate the efficacy of various combinations of PRE plus POST herbicides applied to volunteer GR canola in GR soybean.

Field experiments were conducted at Saskatoon, Scott and Indian Head, Saskatchewan and Carman, Manitoba in 2014 and 2015. Treatments were 15 different combinations of PRE and POST herbicides, and 0.33 L/ac glyphosate (450 g ae/ha) glyphosate applied alone as a control treatment.

PRE and POST herbicides and application rate for volunteer GR canola control in GR soybean

Mierau, et al. 2020

To simulate volunteer canola, GR canola was broadcast seeded across the trial at a target density of 4 plants/ft2 (40 plants/m2) to supplement the established seedbank and to allow for emergence of canola prior to application of the PRE herbicide treatments. Additional supplementation of the canola seed bank was carried out at 2 seeds/ft2 (20 seeds/m2) after application of the PRE herbicide treatments.

PRE herbicide treatments were applied when volunteer canola reached the one- to two-leaf stage. Soybean was sown 5 to 7 days after PRE herbicide applications at a target density of 180,000 plants/ac (444,600 plants/ha) at 1.2” deep. POST herbicide treatments were applied at the one- to two-trifoliate soybean crop stage. All PRE and POST treatments were tank mixed with 0.33 L/ac glyphosate.

Adequate volunteer canola control

All the POST herbicides in combination with the three PRE herbicides were effective in controlling volunteer GR canola at 7 to 10 days after treatment. They provided greater than 74% control, and most provided greater than 80% control.

All herbicide combinations provided greater than 77% efficacy at 21 to 28 days after treatment. Control efficacy was greater with herbicide treatments containing Group 2 products, especially cloransulam-methyl (FirstRate; not registered in western Canada) and imazamox plus bentazon. These combinations provided more than 80% weed control following all the PRE herbicides.

All herbicide treatments showed excellent crop tolerance throughout the growing season with less than 10% visual damage.

Soybean biomass and yield effects

All the herbicide combinations had greater biomass and seed yield compared with the glyphosate-only control treatment. The highest soybean yield was with the tribenuron PRE followed by bentazon POST, with a yield of 20 bushels per acre (1,354 kg/ha).

Of the herbicide combinations, the lowest seed yield was found in 2,4-D followed by fomesafen (Reflex) (16.3 bu/ac), and 2,4-D followed by bentazon plus imazamox (16.6 bu/ac), which were significantly lower than tribenuron followed by bentazon

The remaining herbicide treatments yielded statistically similar. The glyphosate-only treated control yielded around 9 bu/ac.

Volunteer canola biomass and seed contamination

Compared with the glyphosate-only control, all the other herbicide mixtures provided greater than 92% canola biomass reduction, and none were significantly different from each other.

All the herbicide combinations had significantly lower canola seed contamination than the glyphosate-treated control. The fewest volunteer canola seeds present in the harvested sample were in the 2,4-D followed by bentazon plus imazamox treatment with 2% seed contamination. This treatment also had very high soybean-shoot biomass and low volunteer canola-shoot biomass. The remaining treatments had seed contamination ranging from 3 to 5%. The glyphosate-only treatment had 36% canola seed in the harvested sample.

Based on herbicide costs alone, and because cloransulam-methyl is not registered in western Canada, the most cost-effective options for producers are tribenuron followed by imazamox plus bentazon at $20.65/ac ($51/ha) or saflufenacil (Heat) followed by imazamox plus bentazon at $21/ac ($52/ha). Considering the most efficacious and economical options would be tribenuron followed by bentazon and tribenuron followed by imazamox plus bentazon.

Managing herbicide resistance

Using herbicides with multiple modes of action is a strategy to slow the development of herbicide resistance. Fields receiving herbicide mixes containing three modes of action are 51 times less likely to have GR weeds than fields on which herbicides with only two modes of action were used. The herbicide combination in this study that contained at least three modes of action were treatments in which glyphosate was tank-mixed with a POST application of imazamox plus bentazon (Groups 2 and 6). Including saflufenacil as a PRE herbicide with this combination would mean four different modes of action were applied in a single year, which is beneficial for resistance management.

Since this study was completed, several more herbicide options were registered to control GR canola in GR soybeans, including Fierce (flumioxazin + pyroxasulfone; Groups 14 + 15), Valtera (flumioxazin; Group 14), Enlist Duo in Enlist E3 soybeans (2,4-D + glyphosate; Groups 4 + 9), and Roundup Extend in RR2 Xtend soybeans (glyphosate plus dicamba; Groups 4 + 9).

The Western Grains Research Foundation, the Agriculture Development Fund of the Saskatchewan Ministry of Agriculture, Pioneer Hi-Bred, and Monsanto funded this research. The University of Saskatchewan is thanked for in-kind support, including facilities and technical support staff.

Mierau A, Kurtenbach ME, Johnson EN, Gulden RH, Weber JD, May WE, Willenborg CJ (2020) Herbicide programs for control of glyphosate-resistant canola (Brassica napus) in glyphosate-resistant soybean. Weed Technol. 34: 540–546.

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