With all the challenges we have faced in 2019, I figure we’ll see a few more before December gets here. One that particularly concerns me is marestail that will germinate this fall because of the weed seed bank in some areas across the state. This past spring and summer most were unable to get residuals down in a timely manner. This has led to more escaped weeds than we would like, some of which is marestail that has, or will, go to seed.
So, how do we address this issue? What herbicides or combination of herbicides are most effective? Are there any other options?
One thing to remember, we normally get only one or two germination flushes in the fall. So, controlling the weeds with a good burndown is key to a fall management program. A combination of glyphosate, 2,4-D and dicamba is my favorite for a broad-spectrum burndown. Glyphosate for grasses and easy to kill broadleaves, then the 2,4-D and dicamba combination for the tougher broadleaves like marestail, chickweed, dandelion, etc. This combination also gives us a 3- to 4-week broadleaf residual to help with emerging weeds before freeze. On highly erodible land (HEL acres) we often do not use glyphosate so that we leave the winter annual grasses to help hold soil in place.
As far as residual herbicides in the fall, I prefer a minimal combination. Save the big expense for spring when we deal with many more flushes of weeds. For a marestail residual, I prefer simply metribuzin or metribuzin containing a premix. There are many premix options, so consult with your CCA on what combination works best for your farm.
Last, let’s look at a cover crop alternative to fall herbicide applications. While we have used cover crops here in Southeastern Illinois to hold soil in place and as weed control for over a decade, I realize it is still an untried practice in many areas. Cereal rye is a great option for both soil conservation and control of winter annuals like marestail. On corn stalks going into a soybean crop, the rye seeding rate should be 45-60 pounds per acre. For soybeans going into corn, rye seeding rate can be reduced to 30-45 pounds per acre. I know an organic soybean grower who is having great success with 150 pounds per acre seeding rate and controlling weeds very well into the summer. The biggest issue with cereal rye is timing of cover crop termination.
The point to a good fall burndown is to start with a clean field next spring—the easiest weed to kill is one that never emerges. A fall marestail herbicide program can make a huge difference in controlling them next spring. For more localized information contact your local Illinois Certified Crop Advisor.