Call it horseweed or call it marestail. In the end the result is the same. It's a pesky weed that's made noise in soybeans, particularly no-till soybeans, over the past several seasons. And while resistance to glyphosate is documented, it's tough to know without further follow-up where marestail escaped because of resistance vs. where it escaped because herbicides weren't applied or timed properly.
That's how Glenn Nice, Purdue University weed control specialist, sums up the situation with marestail in Indiana in 2011. This winter the most calls to Nice and co-workers Bill Johnson, Tom Jordan and Tom Bauman about possible glyphosate-resistant marestail have come from farmers working between Interstate 70 to the south and Interstate 80 to the north. However, that doesn't mean it's not a problem in other places.
What's at stake if you let marestail control slip and allow the weed to get a foothold is considerable yield loss, Nice observes. In one experiment, Ohio State University tested three treatments: burndown fails to work; burndown works but no residual herbicide was applied; and burndown works and a residual herbicide was applied. Soybean yields were 51, 57 and 65 bushels per acre, respectively. At $12 per bushel, a 14 bushel shift amounts to $168 per acre. There's obviously incentive to pay attention to marestail, and invest in control when necessary.
Those results suggest spring burndown is important. Options for corn, Nice says, include glyphosate at 1.5 pounds per acre active ingredient plus 2,4-D, or 2,4-D plus Gramoxone at three to four pints per acre plus metribuzin.
For soybeans, options are 2,4-D plus Canopy, 2,4-D plus Gramoxone at the same rate plus metribuzin, Expert or Sharpen plus glyphosate.
One secret is timing, Nice says. If you're applying a burndown, marestial plants shouldn't be more than four to six inches tall. That's a pretty small plant.
If you're going pre-emergence this spring for marestial, premixes with atrazine or atrazine in tank mixes will suppress or even control marestail in corn. In soybeans, options include Canopy DF/EX, Firstrate, AuthroityFirst, Gangster, Envive and Valor XLT. Be aware that Ohio State University weed scientists have identified marestail populations with multiple resistance to both glyphosate and ALS herbicides.
If you're going post for marestial in corn, any of the growth regulators herbicides labeled in corn should work, Nice says. That includes 2,4-D and dicamba. You can use Ignite with atrazine plus oil on Liberty Link corn.
For soybeans in postemergence applications, consider Firstrate or Classic with glyphosate. Be aware of the possibility of resistance to ALS herbicides as well as glyphosate that's been reported in Ohio. You could also use Ignite if marestails are small and you're applying over Liberty-Link soybeans.
For more information on marestial control, visit: www.btny.purdue.edu/weedscience/
The entire 2011 Weed Control Guide issued jointly by Purdue and OSU is available for $13.50. Or you can download portions of the Guide free from the Purdue Website.
To purchase the Guide, visit: http://estore.osu-extension.org/ or call 614-292-1607.