By Erin Burns Michigan State University Extension
In general, 2019 was a difficult year for weed control with the large number of acres that were planted late or not at all leading to higher weed populations and extensive weed seed production in some fields. Below are reminders for fall weed control that will set the stage for successful spring 2020 weed control given the drastic increase in weed seedbank populations from 2019.
Fall herbicide applications
Seeds of many winter annual weeds for example horseweed (marestail) lack dormancy and can germinate immediately after dropping from the plant this 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 be made by mid-October before the first hard freeze and daytime air temperatures are at least 50 degrees Fahrenheit. Actively growing weeds are the key to consistent control.
Due to spring conditions, many corn and soybean harvests are late this year. Herbicides can be applied at daytime temperatures ranging from 40-60 F, 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 has occurred, 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 winter annual weeds may tolerate a frost and herbicide applications can be made after active weed growth has resumed (appearance of new green leaves), usually after multiple days with nighttime temperatures above 35°F followed by 50 F or above daytime temperatures.
Results from Michigan State University Extension weed scientist Christy Sprague’s research in no-till soybeans found that fall applications of 2,4-D, dicamba, or Sharpen will control fall emerged horseweed and are cost-effective. Tank-mixtures with glyphosate are needed to control other winter annual and perennial weeds outside of horseweed (many populations throughout the state are resistant to glyphosate). For detailed information on controlling horseweed season long see the fact sheet “Herbicide-resistant horseweed (marestail) in Michigan: keys to management in no-till soybean” by Sprague.
Prevent plant acres
Depending on the weed management plan implemented earlier in the season (cover crops, mowing, tillage, and/or herbicides) fall management is important in preparations for a successful 2020 spring.
If grass cover crops were planted and did not establish well and weed flushes have broken through consider applying a growth regulator herbicide (example: 2,4-D) to control broadleaf weeds and prevent seed production. Herbicide application is dictated by intended use of the cover crop (biomass, forage, seed), consult the herbicide label prior to making an application.
If mowing or tillage were part of your management plan, continue this into the fall to prevent any weed seed production and ensure resulting weed growth will be killed by frost. Alternatively, consider applying the herbicides outlined above to control the final flush of fall weeds.