Field Invaders

ROCKVILLE, Md. (DTN) -- The healthiest crop in some fields this fall isn't corn or soybeans -- it's weeds.

Towering above soybeans and poking out of cornfields, waterhemp, marestail, Palmer amaranth and giant ragweed escapes represent a lot more than lost yield. They promise to produce a seedbank that will clog fields for years if left uncontrolled, said Purdue University weed scientist Bill Johnson.

"Harvesting fields like that is one of the worst things you can do, because combines are great weed seed spreaders," he explained.

Let's review the limited options for weed control at this point in the season:

CHEMICAL CONTROLS

Most solutions to late-season weed escapes can't be found in a jug, Johnson noted.

For grassy weeds like barnyardgrass, foxtail, johnsongrass and shattercane, herbicides may do some damage control. "You can go in and spray one of the post-emergence grass herbicides and maybe reduce seed production," Johnson said.

Some growers opt to spray vine weeds like morningglory and burcucumber in brown-silk corn fields with 2,4-D, but the benefits will be limited at this stage, he added.

"There are so few instances where the value of that herbicide treatment will be greater than the yield you're losing by driving over beans or corn again," Johnson said.

At this stage, broadleaf weeds like Palmer amaranth, waterhemp, marestail and giant ragweed are so large that they cannot be controlled by any herbicides, he added.

Growers are also likely to run into pre-harvest interval periods and other label restrictions such as growth stage cut-offs or maximum volumes used.

Finally, if the broadleaf weeds have produced viable seeds, no herbicide can limit the future population of weeds each one will soon drop into the fields, Johnson noted.

HAND WEEDING

If you're serious about getting a weedy field under control, hand removal is your best option at this point, Johnson said.

First, check the seedheads of the weed escapes. If the seeds are still green, white or yellow, they're probably not viable yet. If they drop to the ground during removal, they won't germinate.

Nonetheless, taking these younger weed escapes out of the field for burning or composting is still the best practice.

"If you just drop the weeds on the ground, they can re-root," Johnson warned. "All they have to do is find water. It will slow them down for a while, and they might not produce as much seed, but they can still come back."

If the seedheads produce brown or black seeds, the seeds are viable and these weed escapes are quite dangerous. You may need to bag them before moving them, so the seeds don't drop into the field during the removal process, Johnson said.

It's best to remove weeds from the roots, or at least cut them off well below the soil line, Johnson added.

"If you chop the weed and leave green material above the ground to capture sunlight, it can go ahead and re-grow and put on a seedhead," he warned.

WHEN HAND WEEDING ISN'T AN OPTION

If part of the field is too infested for hand weeding, it might be time to consider simply destroying that section of the field, Johnson said.

The wet spring left many growers with drowned spots in the field where weeds quickly overcame the struggling crop, so this is a likely scenario this summer, he noted.

For those lost spots that won't be taken to harvest, use whatever you can -- tillage, mowing or herbicide passes.

"Be careful with herbicide carryover," Johnson cautioned. "And I'd do it sooner rather than later, because you're trying to hit weeds before they have viable seed."

If you find yourself having to harvest infested fields that are beyond spot control, the last -- and weakest -- recourse is to clean the combine out as carefully as possible, Johnson said.

Air compressors can be used to blow out leftover seed, but keep in mind that the tiny seeds from weeds like Palmer amaranth and waterhemp are nearly impossible to remove completely.

"If possible, harvest these infested fields last rather than first," Johnson said.

For help identifying weed escapes, see this guide from the University of Illinois: http://bit.ly/….

For details on controlling late-season Palmer amaranth escapes in particular, see this recent article from Ohio State University: http://bit.ly/….

Emily Unglesbee can be reached at Emily.unglesbee@dtn.com

Follow Emily Unglesbee on Twitter @Emily_Unglesbee

(PS/AG)

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