October 6, 2017
By Meaghan Anderson and Bob Hartzler
Marestail (Conyza canadensis) or horseweed as it is sometimes called is one of the most difficult weeds to manage in no-till soybean fields. While classified as a winter annual, the plant has significant emergence in both late summer and early fall, and in the spring. This extended emergence period greatly complicates management since the success of applying a postemergence herbicide is closely tied to plant size.
Why not apply a postemergence herbicide at time of planting? Attempting to control populations at the time of planting in the spring often results in control failures as fall-emerged plants are too large for acceptable control by planting time.
Fall-emerged marestail plants can be effectively controlled with either fall or early-spring herbicide applications; however, weather conditions frequently interfere with early-spring applications. Waiting until spring to try to control this weed allows the marestail that emerged in the fall to get too large for consistent control. For this reason, fall applications are a more consistent strategy.
Glyphosate-resistant marestail is widespread across Iowa. Applying 1 pound (acid equivalent) of 2,4-D per acre is recommended as the base treatment for marestail burndown treatments.
Glyphosate or other herbicide products may be tank-mixed with 2,4-D to provide broader spectrum control of winter annuals and certain perennial weeds. However, we generally do not recommend including residual herbicides in fall applications since they provide little benefit in managing weeds that emerge the following spring.
A bulletin developed by the Take Action on Weeds group, sponsored by the United Soybean Board, provides more detailed information on marestail management.
Most areas of Iowa received sufficient rain during September to promote germination of winter annuals. However, the only way to determine the benefit of fall herbicide applications for marestail is to scout fields before or after harvest.
A no-till field near Ames had two distinct sizes of marestail on Oct. 1. The larger seedlings probably emerged following mid-August rains, whereas the smaller seedlings likely emerged following rains Sept. 16-18. Applications can be made into November, but you should target applications for when the daily high temperatures are at least in the mid-50s.
Remember, fall applications are unlikely to eliminate the need for burndown treatments at planting. Marestail and other weeds adapted to cool temperatures (e.g., lambsquarters, giant ragweed) are likely to emerge before planting, making it necessary to control these weeds to provide the crop an even start with the weeds. Rye cover crops are effective at suppressing establishment of marestail and other winter annuals, and should be considered in future years.
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