As with everything else, the extremely wet conditions this fall will have an impact on winter annual weed populations and plans for managing them.
Kevin Bradley, University of Missouri weed specialist says winter annual weeds like henbit, purple deadnettle and chickweed have already emerged in many corn and soybean fields throughout the southern Corn Belt, but the wet weather will likely limit the opportunities for fall herbicide applications.
"Our research indicates that applications of residual herbicides made in the early spring can provide similar levels of winter annual weed control as applications of these same herbicides in the fall," Bradley says. "In addition, our data indicate that early spring applications of residual herbicides provide better control of emerging summer annual weed seedlings than fall herbicide applications.
"This is especially the case with our current herbicidal options available in soybeans."
The wet weather may also have an influence on fall or winter herbicide applications that were planned to be made in wheat, Bradley says.
"Although I have been a proponent of this timing especially for the control of winter annual grass weeds in wheat, most of the herbicide applications that are made in wheat production in Missouri are typically made in the spring anyway," he says. "So, I don’t think it will hurt to just plan on making these applications in the early spring if necessary.
"Most of our winter annual broadleaf weeds are not very competitive and it is unlikely that they will have much of an impact until 'greenup' of wheat in the spring."
If you have winter annual grasses, Bradley says these weeds can be very competitive with wheat.
"I would try, if at all possible, to make a herbicide application for the control of these weeds sometime yet this year," Bradley says. "Although it has been really, really wet, and it certainly will be difficult to get across fields without tearing them up, I don’t believe all hope is lost just yet. I believe there will still be opportunities for fall or winter herbicide applications yet this season, but these opportunities are fading fast."
Bradley says the problem is the season is drawing near to where air temperatures will not be conducive to the application of systemic herbicides like glyphosate and 2,4-D, which are usually included with a residual herbicide in a fall herbicide program.
The weed specialist says these herbicides need air temperatures to be in the 50s for at least a couple of hours a day over a 3- or 4-day time period to penetrate and translocate effectively.
"One final thing to keep in mind is that many of these winter annual weeds germinate at two peak periods during the year in Missouri, usually September to October and February to March," he says. "With all the rainfall we have experienced this fall, it wouldn’t surprise me to see another big flush of these winter annual weeds in the early spring."