Winter and summer annual marestail (horseweed) produces about 200,000 seeds per plant that can travel up to 400' by wind. Marestail germinates more in the fall than any other time and goes dormant when buried deeper than 1" in the soil. Seed longevity has not been extensively studied, though there is a report of finding viable seeds after 20 years, according to Michigan State University Extension weed science.
As crops come out of the field, bare soil acts as a breeding ground for weeds. Managing them now can help you have a more successful planting season next year.
“Marestail is at the top of the list for the most troublesome fall weeds right now because of herbicide resistance to ALS and glyphosate,” says Bryan Young, a weed scientist with Purdue University.
Marestail has shown resistance to the following herbicides in at least one state: Photosystem I Electron Diverter (Group 22); EPSP synthase inhibitors (Group 9, includes glyphosate); ALS inhibitors (Group 2); Photosystem II inhibitors (Group 5); and PSII inhibitors (Group 7).
Gain control of marestail before rosettes reach 2" tall this fall—they’ll be much more aggressive and difficult to manage once they leave dormancy in the spring especially if they’re tall. Other weeds to evaluate are henbit, deadnettle, cheatgrass, annual ryegrass and chickweed.
“You might think about a fall tillage pass, cover crops or I’m considering a glyphosate-2,4-D application,” Young says. “I don’t want to go into planting without some management.”
If you’re considering fall tillage, know it won’t necessarily control all weeds—it’s great for marestail; but if you have troublesome grasses, it’s not the most effective management tool. Consider fuel costs and soil impact before moving forward with tillage.
“Cover crops could give growers a good bang for their buck—it just depends on what the grower wants to give cover crops credit for,” Young says. “Cover crops can be polarizing on weed management.”
While cover crops might provide soil health benefits, it could take a couple years to see those come to fruition. In addition, it costs anywhere from $10 to $25 per acre to seed cover crops, not including fuel. If you just want weed control, they’re likely your priciest choice.
“We do a fair amount of no-till before winter wheat and apply herbicides,” says William Curran, Penn State University Extension weed scientist. “The number of new products growers can use preplant in wheat has grown.”
If you’re not planting winter cereal crops, Curran says 2,4-D and glyphosate, or dicamba and glyphosate, are inexpensive options. But there’s a planting delay, so if you’re in a time crunch they aren’t the best choice.
“Sharpen is good on marestail with no planting delay,” Curran says. “You can use Prowl when wheat is spiking, and there are five or six other products like Zidua and Anthem Flex that target winter annual weeds.”
Bottom line: Don’t let tall weeds compete with seedling corn and soybeans next spring—gain control of problem fall annuals now.