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Beyond herbicides

Herbicides are the main way to manage weeds. Still, there are such huge weed seed banks forming in the Corn Belt that herbicides aren't often sufficient to manage weeds.

“We are planting two summer crops with nearly identical planting and harvesting dates,” says Bob Hartzler, Iowa State University Extension weed specialist. “We are using similar weed-control tactics in both crops. The system is set up for large acres and as little labor as possible. Cultural practices are something that need to be looked at more closely.”

That's particularly true now that herbicide-resistant weeds are on the upswing. Here are five cultural and chemical ways to manage weeds.

1. Plant narrow rows. Narrow rows have almost the same weed-management benefit as a midseason cultivation, says Hartzler. That's because 15-inch rows block more sunlight via early canopying than wider rows. With no sunlight, weed seeds can't germinate. Thus, narrow rows can help nix late-emerging weeds like waterhemp from surging through the canopy.

“Waterhemp that escapes later in the season might not impact yields, but it produces lots of seed, which causes problems in future years.”
White mold is one drawback to narrow rows. Bear in mind, though, that white mold doesn't always cause a problem every year. A field infested with waterhemp does.

“I don't want to belittle diseases, but waterhemp poses a greater threat to profitability than white mold,” says Hartzler.

2. Prevent weed seed movement. “We will stress this more as herbicide-resistant weeds become more of a problem,” says Hartzler.

Harvesting clean fields before weedy ones is a practical way of preventing weed seed movement.

One way Australian farmers halt weed seed movement is with the Harrington Seed Destructor. This device attaches to the back of a combine and sifts weeds from the rest of the chaff. Weed seed is then burned. “That's what we may have to do in the future to manage herbicide-resistant weeds,” says Hartzler.

3. Use multiple herbicide modes of action effectively. “Some of the tank mixes are being used in a way that are not effective in managing resistance,” says Hartzler. “They are sometimes used at too low of a rate that would not control weeds when used alone. If a herbicide doesn't control a weed by itself, it won't manage resistance in a tank mix.”

4. Apply herbicides when weeds are the recommended size. “That would seem to be a gimme,” says Hartzler. There still are cases, though, where farmers wait to apply herbicide mixes to save on application trips. Although glyphosate kills big weeds, it won't bring back the yield potential lost by early weed completion.

5. Scout fields routinely. This can tip you off to early weed infestations that a postemergence herbicide application can nip. Late-season scouting can also detect escapes of waterhemp that can be pulled to prevent seed formation. 

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