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Start & end early for good weed control

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.

5. The Pre/Post Combo Works Well

"A good pre/post combination program does best on weed control," says Gunsolus. "It can get over 90% kill, and controls a broad array of weed species quite economically. A post-only program gets in the 70% range. Getting your post-emergence weed control on 3- to 4-inch weeds is your goal. And if you're going to error, error on the smaller, earlier side." "What you don't want is to be out at the end of the season, trying to kill weeds in August. June and early July is when it needs to be done." Nothing's perfect Resistance isn't new or unexpected, says Gunsolus. No chemical or technique has ever been 100% effective, despite those picture-perfect soybean fields we admired a few years ago. In a 1999 Illinois finding, 10% of weed populations didn't have as much as a 50% kill response to a reduced rate of glyphosate.

"That was way back then, before we had such widespread use of one system,” says Gunsolus. Couple that with the fact that a single waterhemp plant can put out as much as 1,000,000 seeds. "New chemistry and herbicides aren't going to bail us out this time. Researchers have just about stopped looking for new chemistry.  "You still have control over which program you use, and which herbicides you chose to use. So, it's time to take control," he says. "We know what we need to do, let's do it.""

The full toolbox

We have a big toolbox to draw from to combat the issue of weed resistance to glyphosate, Gunsolus says.

  • Conventional herbicides    
  • LibertyLink or other mode of action technology   
  • Mechanical cultivation    
  • Crop rotation    
  • Harvest technology that traps weed seeds

He suggests that you target your herbicide-resistant crops in the crop you need it the most. Do you need it most in corn, or soybeans? Use another mode or practice in the other crops with greater herbicide options. "I say we need to be moving from the pro-active to the reactive stage now," Gunsolus urges. "We can get unified in our approach, and get past this satisfaction we've had with total post weed control. Start early, end early, that's my theme for effective weed control."

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