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Reasons for and against preemergence herbicides

02/14/2013 @ 2:28pm

Jeff Gunsolus knows you'd rather avoid the move to more preplant and preemergence herbicides. He's the University of Minnesota Extension weed specialist, and he even understands your reasons for not particularly liking pre programs.

But he's also telling you to get over it. Preemergence herbicides for both corn and soybeans are the wave of the future for these reasons:

» They slow the development of resistant weeds by providing another site of action.

» They help reduce early-season weed competition.

» They improve the effectiveness of postemergence herbicides.
Here are four common complaints about preemergence programs that Gunsolus hears and his responses to each.

1. Preemergence herbicide programs cost more. True, a single pre pass application will probably cost $20 per acre or more compared to a glyphosate post-emergence pass that costs $10 per acre less. Just as competition and patent expiration drove down the cost of glyphosate, though, competition is hard at work in the pre market. Seed and chemical companies are now on board with the preemergence programs, and they routinely tout them in tandem with their in-season products. Those companies offer bundled programs (“buy our seed and chemical program, get a price break”) that give better value for the dollar.

“In a bad weed situation, we see a favorable return on investment for a pre program, similar to a two-pass glyphosate return,” Gunsolus says.

2. Application timing for preemergence herbicides couldn't be worse. That's true. You need to apply the pre herbicides during the prime planting window. Roundup Ready technology spoiled you with the “plant today, worry about weeds later” approach. The problem is, even though you may control those weeds later, they cost your crop water, nutrients, and yield.

Besides attacking weeds with a different site of action, Gunsolus says preplant herbicides increase yield potential by suppressing early weed competition. If you let weeds pass the common 4-inch threshold in soybeans, they've already cost you significant yield potential. In corn, those early weeds suck up nitrogen.

“The right preemergence products can help get your crop going in a weed-free environment, giving an earlier crop canopy in both corn and soybeans. Then, that canopy shade is like free weed control,” he says.

There's little you can do regarding applying herbicides at the height of planting season. “They can be applied right as you plant, so I like to say if it's a good day to plant, it's a good day to put down a pre herbicide, too,” says Gunsolus.

3. The preplant programs are more complicated. True. Total post weed control was easy: Apply just one product once or twice. Now, you will have to learn or relearn the pre system, along with your weeds and problem spots, and the best pre products for your situation.

“Some things in life just aren't fair,” he adds. “Pre programs are a proactive strategy when it comes to resistant weeds. If you want to stay in the business, this is something you have to learn. They use a different site of action on the weeds, and that increases their effectiveness and slows resistance to all products. They control weeds as they germinate, and that's easier than when they get bigger.”

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Farm Science Review, Day Two