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Weeds do the Backstroke

20:25PM Sep 25, 2008
The Midwest is producing a bumper crop of weeds this year. Floods and other unfavorable weather may have wiped out or limited crop yields, but the conditions seem to be just what weeds like.
Iowa State University extension weed scientist Mike Owen notes that flooded fields in river bottoms and "wet holes” within fields are teeming with undesirable weeds this growing season. Across the Midwest fields are dotted with spots that were replanted too late or were too wet for replant options. As a result, these areas had significant weed growth and stand to be big contributors to the "seedbank” in the soil.
"Farmers should be prepared to manage higher densities of weeds expected next year,” says Owen. "Fall is the time to be thinking about weed management for 2009, and as always, the watchword is stewardship.”
Weeds will never more obvious than from the combine during harvest. Owen suggests observing patterns of weeds in the fields. "I've noticed many fields have oblong-shaped patches of common waterhemp,” he says. "The direction almost always follows the tillage and harvest patterns and typically there are dead plants next to live plants. This should scream out impending concerns about herbicide resistant populations.
"Think back to what you did last spring and summer to control weeds. If you only sprayed glyphosate or any other single herbicide, rethink the tactic,” Owen suggests.
Wet spots with extreme weed pressure will need special consideration next spring. "Additional weed control tactics in those areas will likely be necessary and that area of the field should be managed differently,” Owen says. "Each field should be considered separately when weed management plans are formulated. Simplicity and convenience in weed management has an economic and environmental cost.”
The best way to protect against yield losses from early season weed competition is the use of an early pre-plant herbicide application, Owen says. "It's a good hedge against early weeds, but as we saw this year, it isn't foolproof,” he adds. Herbicides applied early preplant do not necessarily require incorporation and can be applied when it is too early to plant and before weeds begin to germinate.  Furthermore, the probabilities of timely rainfall necessary to move the herbicide into the active weed seedbank are high; thus when weed germination occurs, the herbicide is in position to control the weeds. 
Owen says it's important to recognize that early preplant herbicides will control many of weeds that germinate first during the season.  The first weed germination flushes occur in greatest numbers and are most detrimental to potential crop yields.  As a result of the early preplant herbicides, crops are protected from yield-robbing weeds and growers have more time to make post-emergence applications. 
Keep in mind that no herbicide, whether applied early preplant or otherwise, will provide season long control of weeds.  Soil-applied residual herbicides applied early preplant can also delay or minimize the risk of weeds becoming resistant to routine applications of post-emergence herbicides.
It's not enough to know where the trouble spots are located. You also need to know what weeds you're dealing with. Most weeds are easier to identify in the fall than they are as seedlings in the spring.
Need help figuring out horseweed from waterhemp? You'll find an online weed identification tool at

You can email Pam Smith at



This article appeared in a recent issue of Farm Journal's Crop Technology Update eNewsletter. To sign up for a free subscription, click here.