By Erin Burns and Christy Sprague
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, using fall herbicide applications or tillage to manage problematic perennial, biennial, and/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 in different fields. 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 seedbank and be a priority for control next season. Correctly identifying these weeds provides the opportunity to start planning effective weed management strategies for the 2022 growing season. Many of these weed escapes may also be resistant to one or more herbicides.
In order to effectively control these weeds in the future it will be important to know if the weed populations you have in your fields are herbicide resistant. Currently, in Michigan different populations of horseweed (marestail), waterhemp, Palmer amaranth, common ragweed and giant ragweed have been confirmed resistant to glyphosate (Roundup) and in some cases also to additional herbicides. A detailed list and map of confirmed herbicide-resistant weeds identified in Michigan can be found at Michigan State University Plant & Pest Diagnostics.
If you suspect herbicide-resistant weeds in your fields, you can get these weeds screened for resistance. For more information on how to collect seed and submit samples for herbicide-resistance testing, consult the fact sheet “Tips for Collecting Weed Seeds.” Also, if you feel that you have herbicide-resistant weeds in any of your fields, you may want to harvest those fields or areas in those fields where herbicide-resistant weeds are present last to reduce the spread of these weeds to non-infested fields.
Fall herbicide applications
After harvest, fall herbicide applications offer an excellent opportunity to clean up fields where perennial and biennial broadleaf weeds are present. In the fall, sugars and carbohydrates present in perennial and biennial 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 annual weeds, including common chickweed and glyphosate-resistant horseweed (marestail). Seeds of these winter annuals lack dormancy and can germinate immediately after dropping from the plant in the fall. Given the above average horseweed populations across the state, this will lead to an immediate increase in fall emerging weeds.
During optimal years, fall burndown herbicide applications should ideally be made by mid-October before the first hard freeze and when daytime air temperatures are at least 50 degrees Fahrenheit. Actively growing weeds are the key to consistent control. However, fall herbicide applications can be made when daytime temperatures range from 40-60 degrees Fahrenheit, but weeds may be killed slower at these cooler temperatures. For example, at cooler temperatures absorption and translocation of herbicides such as glyphosate and 2,4-D are lower compared with applications at warmer temperatures. Therefore, these applications take longer to kill the plant.
When temperatures are below 40 F 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 prior to 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 and herbicide applications can be made after active weed growth has resumed (appearance of new green leaves). This is usually after multiple days with nighttime temperatures above 35 F followed by 50 F or above daytime temperatures and may extend the period of some of these fall applications through late-October/early-November. However, some perennial weeds such as hemp dogbane and common milkweed complete their lifecycles by late summer and do not tolerate frost well, so herbicide applications should not be delayed until late fall.
Cover crop establishment
Using fall planted cover crops to help suppress weeds is a growing practice. Getting into fields timely in the fall after harvest to plant cover crops is essential for successful establishment and subsequent weed control. When establishing a cover crop, such as cereal rye, it is important to plant the cover crop into as close as a weed-free field 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 prior to planting the cover crop. Research has shown that these cover crops help reduce winter annual weed growth the following spring.
Source : msu.edu