June 28, 2021
The prolonged dry period we experienced this spring, along with herbicide shortages, has led to several weed control challenges. While there is little that can be done at this time to control weeds in this year’s crop, we can start setting the stage for successful weed control for the 2022 growing season.
Identifying weed problems, collecting weed seed for resistance screening and using fall herbicide applications or tillage to manage problematic perennial, biennial or winter annual weeds are all steps that can be taken this fall to help improve weed control for the 2022 growing season.
Additionally, planting a cover crop, such as cereal rye, this fall can also help provide suppression of winter annual and some early-emerging summer annual weeds.
Scouting for weed escapes
Fall is the perfect time to scout for weed escapes and problem weed areas. When harvesting, note where and what weed species are present in the field. The majority of these weeds will have already produced mature seeds that will end up in the soil seed bank and be a priority for control next season.
Correctly identify these weeds to start planning effective 2022 weed management strategies, noting that these weed escapes may be resistant to one or more herbicides.
Currently in Michigan, different populations of horseweed (marestail), waterhemp, Palmer amaranth, common ragweed and giant ragweed are resistant to glyphosate (Roundup) and, in some cases, also to additional herbicides.
A detailed list and map of confirmed herbicide-resistant weeds in Michigan can be found on the MSU Plant and Pest Diagnostic website. These weeds can be screened for resistance, and for more information on how to collect seed and submit samples for testing, consult the fact sheet “Tips for Collecting Weed Seeds.” If herbicide-resistant weeds are suspected, harvest those fields or areas last to reduce the spread.
Fall herbicide applications
After harvest, fall herbicide applications offer an excellent opportunity to clean up fields with perennial and biennial broadleaf weeds. In the fall, sugars and carbohydrates present in these weeds move from the leaves to underground storage structures.
As a result, greater amounts of systemic herbicides, such as glyphosate, 2,4-D and dicamba, will move with the sugars and carbohydrates to underground structures where they kill the plant. Fall applications can also be used to help manage several winter weeds, including common chickweed and glyphosate-resistant horseweed (marestail).
Seeds of these winter annuals lack dormancy and can germinate immediately after dropping. Above-average horseweed populations across the state have led to an immediate increase in fall-emerging weeds.
During optimal years, fall burndown herbicide applications should be made by mid-October before the first hard freeze and when daytime air temperatures are at least 50 degrees F. Fall herbicide applications can be made when daytime temperatures range from 40 to 60 degrees, but weeds may be killed slower because absorption and translocation of herbicides such as glyphosate and 2,4-D are lower.
When temperatures are below 40 degrees for a prolonged period after herbicide application, weed control will be reduced. If a hard freeze occurs, evaluate the condition of the weeds in your field before herbicide application. Frost may cause leaf damage (water-soaked leaves that turn black and die) and reduced herbicide absorption.
Some perennials, such as Canada thistle and dandelion, and some winter annual weeds can survive light frosts, allowing herbicide applications after active weed growth has resumed (appearance of new green leaves). This is usually after multiple days with nighttime temperatures above 35 degrees, followed by 50-degrees-or-above daytime temperatures and may extend applications through late-October or early-November.
However, some perennial weeds such as hemp dogbane and common milkweed complete their life cycles by late summer and do not tolerate frost well, so herbicide applications should not be delayed until late fall.
Cover crop establishment
Timely fall-planted cover crops will ensure establishment and can help suppress weeds. When establishing a cover crop, such as cereal rye, plant into a field as weed-free as possible. This gives the cover crops an establishment advantage on yet-to-emerge winter annual weeds.
In some cases, a herbicide application or tillage to manage emerged weeds may be needed before planting the cover crop. Research has shown that these cover crops help reduce winter annual weed growth the following spring.
For more information on fall herbicide applications and planning weed management strategies for the 2022 growing season, consult the 2021 MSU Weed Control Guide for Field Crops.
Burns is an MSU assistant professor in the department of plant, soil and microbial sciences and can be reached at [email protected] or 517-353-0223. Sprague is an MSU Saginaw Valley Research and Extension Center professor and Extension specialist and can be reached at [email protected] or 517-353-0224.
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