Glyphosate-resistant Russian thistle found in Montana
This article highlights the evolution of glyphosate (Roundup, RT3, and other generics)-resistant Russian thistle in Montana. Russian thistle (Salsola tragus L.), a native of southeastern Russia, is one of the most problematic broadleaf weed species in the dryland no-till cropping systems of Montana and the U.S. Great Plains. It is a summer annual weed belonging to the goosefoot family that reproduces by seed. Russian thistle commonly invades dryland crop production fields, irrigation canals, roadsides, railroads, ditch banks and other disturbed non-cropland areas in this region. It is known to reduce crop yields, hinder harvest operations and pose problems in summer fallow.
Russian thistle exhibits low seed dormancy and persistence in the soil seedbank. Seeds can germinate early in the spring, and seedlings can exhibit an extended period of emergence during the summer. Russian thistle plants are round in shape with a bushy appearance and can grow up to 4 feet tall. It has a deep tap root system that can extend up to 5 feet depth in the soil. Leaves are alternate and linear in shape. In general, Russian thistle exhibits indeterminate flowering that normally starts during mid-summer. At maturity, Russian thistle plants break off at the ground level and tumble with the prevailing wind, dispersing seeds to a long distance. A single mature plant can produce almost 250,000 seeds.
In the fall 2015, seeds of a Russian thistle population surviving glyphosate applications were collected from a chem-fallow field in Chouteau County, Montana. The field was under wheat-fallow rotation and had a history of repeated glyphosate applications. Almost 100 plants screened from the population survived the field-use rate of glyphosate (@32 fl oz/a of 4.5 lb ae/gal Roundup Powermax). Dose-response experiments conducted by Prashant Jha and Vipan Kumar, weed researchers at the MSU Southern Agricultural Research Center in Huntley, showed up to 4.5-fold levels of resistance to glyphosate. Furthermore, “the confirmed glyphosate-resistant Russian thistle biotype survived two times the field-use-rate (1 oz/acre) of Ally Extra (Group 2, Sulfonylurea herbicide), confirming multiple resistance to glyphosate and sulfonylurea herbicides,” said Kumar. The demographic spread of glyphosate-resistant Russian thistle in Montana is unknown, and the underlying mechanism(s) of glyphosate resistance is under investigation at the MSU Southern Agricultural Research Center in Huntley.
“This report confirms the first case of evolution of glyphosate-resistant Russian thistle. In Montana, this is third weed species (after kochia and marestail) that has developed resistance to glyphosate. The discovery of glyphosate resistance in Russian thistle is a concern for Montana producers,” said Jha.
Growers need to be proactive in managing Russian thistle populations in their fields. Weed control efforts should aim at preventing the soil seedbank replenishment. Utilize multiple, effective modes of action herbicides to manage the problem. Glyphosate applications in fallow should include 16-26 fl oz/acre of 2,4-D LV6 (equivalent rates for other formulations). Incorporating spring or fall soil-applied residual herbicides can aid in reducing the weed seedbank. Gramoxone, Sharpen + 2,4-D, Distinct + 2,4-D can be used to control glyphosate-resistant Russian thistle (less than 5 inches) in chemical-fallow. Always follow the label for herbicide use rates and adjuvants.
“Diversification in crop rotation with the inclusion of pulse crops such as peas during the fallow period will aid in diversifying herbicide modes of action and using PRE soil-residual herbicides,” said Jha. Russian thistle seed bank should be proactively managed (zero tolerance to seed production) with effective herbicides labeled in wheat.
For information on preventing and managing glyphosate-resistant weeds in Montana, please refer to the MSU Extension research bulletin No 4602: Glyphosate-resistant kochia in Montana: Herbicide Recommendations and Best Management Practices for Growers.
For further questions or testing suspected resistant weed samples, contact: Prashant Jha, weed scientist, at firstname.lastname@example.org or 406-348-3400; Vipan Kumar, research scientist, at email@example.com, MSU Southern Agricultural Research Center, Huntley, Montana; Peggy Lamb, research scientist, at firstname.lastname@example.org, Northern Agricultural Research Center, Havre, Montana, 406-265-6115.
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