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Start & end early for good weed control

01/06/2012 @ 2:53pm

Jeff Gunsolus speaks with fervor -- and worry -- in his voice. He has watched the advent of total post-emergence weed control programs in corn and soybeans over the last decade, and some of his worst fears are materializing.

The weeds are fighting back. Gunsolus has more than a passing interest in the topic. He's a University of Minnesota Extension weed scientist. One of his jobs is to help farmers develop weed control systems. It's getting more difficult, not less so. "We need changes," he says, "and every state has the same problem. Waterhemp that is resistant to total post programs, particularly glyphosate-based products, is moving north." There are others

Waterhemp isn't alone, he says, with ragweed and lambsquarters not far behind.

"They are the same weeds that gave us the most trouble in the 1990s, before the total post-emergence glyphosate-based programs.  "A lot of times, we get injury, but not a kill on some weeds," says Gunsolus. "They're just stunted, but they survive and produce seed. And the bigger the weeds when we go after them, the harder they are to kill. "Let's stop and think about how we are doing things. In some cases we are using the same product, on the same fields, on the same weeds, year after year. Evidence mounts that we have more weeds that are resistant to glyphosate. It's really not a debate anymore, it's here. The only question now is, are we going to be pro-active on the front end, or not?"

So, what's his plan?

1. Add a preemergence herbicide to your post program.

This targets smaller weeds, when they are easier to kill.

"One of my favorite sayings is: If it's a good day to plant, it's also a good day to apply a pre-emergence herbicide," says Gunsolus.

2. A pre adds another action mode.

A preemergence herbicide will belong to a different chemical class than glyphosate, which is only effective on emerged weeds. Most weeds that show resistance to glyphosate will be killed by the pre-emergence chemical, reducing the size of the resistant seed bank. "Ultimately, the whole issue we are dealing with is how to reduce the weed seed bank," he says.

3. Pres enhance fertility.

Going after weeds earlier does more than just kill small weeds, says Gunsolus. It also increases the efficiency of your fertilizer program, because those weeds are tapping the same nutrients as your corn and soybean roots.

"Twelve-inch-tall weeds can take up to 60 pounds of nitrogen per acre away from the crop," he says.

4. Pres aid early competition.

Pre-emergence weed kills provide an early competitive advantage to your crop. In some cases, if you get that early flush of weeds with a pre-emergence program, you get season-long advantages. For instance, there's evidence that waterhemp emerging after the V8 stage in soybeans does not produce viable seed.

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