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Reasons for and against preemergence herbicides

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.”

4. There's a greater risk of crop injury with pre products. This may be true, says Gunsolus. Young corn and soybean plants don't have resistance genes to preemergence herbicides, as they do for glyphosate. Still, injury potential can be reduced by reading and following product labels. They will tell you about any interactions the pre may have with insecticides, other pesticides, or weather issues.

“If crops have a little herbicide injury, they usually grow out of it,” he says.

If you want to get serious about your future herbicide-management plans, adopt a long-range plan and strategy, says Gunsolus. Write a multiyear plan that lists crops on each field for each year and your pesticide plan for each.

“Your goal is diversification of effective herbicide site of action on your weed species,” he says. “You can only do that by taking the long view of your cropping and herbicide plans. This is not as easy as a Roundup Ready program, but it is better than fighting a heavy weed infestation.”

Don't give up

Using a preemergence residual herbicide is one way to snap early-season weeds.

There's a big if, though. Without tillage, rainfall is required to move preemergence herbicide into the soil. That didn't always happen during a dry 2012.

“The probability of getting timely rainfall for incorporation was not reliable last year,” notes Aaron Hager, University of Illinois Extension weed specialist. “One thing I am concerned about this year is producers might say they didn't see the level of control from those products last year, so they'll forget about them this year. That is the worst thing they can do.”

That's because even though drought crunched many crops last year, weeds thrived. Their seeds will spawn more weeds in fields this spring.

“The challenge will be to control weed populations you have,” says Hager. “The need for residual herbicides will be even greater this year.” 

Effective modes

Multiple modes of action is a phrase that rolls off the tongues of herbicide company officials who tout weed resistance management efforts.

It's true that applying multiple action modes through preemergence and post-emergence herbicides can help forestall herbicide-resistant weeds. Still, there's a key word missing from the multiple mode moniker: effective.

In Iowa, for example, weed scientists in 2011 confirmed a waterhemp biotype that resists glyphosate, ALS inhibitors (like Pursuit), and HPPD inhibitors (like Callisto). A herbicide lineup that contains these three modes of action may not control this waterhemp biotype.

Learn more (Click on Integrating Residual Herbicides into Corn and Soybean Weed Management Plans)

“Consider effective modes of action,” says Mike Owen, Iowa State University Extension weed specialist. “You have to recognize that multiple herbicide resistance can exist in waterhemp populations and the ways weed populations will respond to various herbicides.”

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