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7 steps to better weed control

Getting your crop off to a quick start hinges on your ability to nix weed competition. (After all, you want a nice clean field at the start and not a waterhemp-choked one like this one.) Here are seven factors to keep in mind as you plan next year's weed-control program this year.

1. Don't shave herbicide rates

This can enable some weeds to survive and to compete with crops for sunlight, moisture, and nutrients.

“Any surviving weeds are competing for moisture and sunlight,” says Jeff Springsteen, marketing manager for selective corn herbicides for Bayer CropScience. “You want to take out that competition as soon as possible.”

It's also a bad idea from a herbicide-resistance standpoint. Herbicide-resistant biotypes exist in every weed population. It may be one in 1 billion, perhaps one in 10 billion. However, a below-label rate gives these biotypes a chance to survive and to multiply to where they can be a severe problem down the road.

“We see problems with resistant weed populations when people shave rates,” says Carroll Moseley, herbicide brand manager for Syngenta Crop Protection. “That's why we are so adamant that end users (of herbicides) go by rates on the label.”

2. Mix up modes of action

Using multiple herbicide modes of action is another way to keep resistant biotypes from developing. Weed resistance to glyphosate and other herbicides developed from using the same herbicide over and over again.

“I think the days of stand-alone glyphosate in corn are few,” says Moseley. “There are some areas of the country where straight glyphosate works well. But I don't think we will continue to go down that path. People are getting the message about switching modes of action.”

3. Consider preemergence herbicides

They can nix early weed competition and reduce pressure on later postemergence applications.

Lack of rainfall for activation is one concern for preemergence herbicide. However, some of the newer herbicide technologies enable herbicides to last longer without precipitation.

“We have had Corvus on the ground for over two weeks without a rainfall, and it has worked its way into roots and shoots of weeds,” says Springsteen.

Bear in mind, though, that factors other than rainfall, such as herbicide placement and herbicide rate, can influence activation, says Aaron Hager, University of Illinois Extension weeds specialist. Full rates have a greater likelihood of longer residual control than lower rates, he adds.

4. Watch your speed

One perk regarding modern herbicides is that they require less water as a carrier. “Corvus, Balance Flexx, Laudis, and Capreno work well at 10 gallons per acre,” says Springsteen.

Low water requirements enable applicators to spray more acres before they have to refill.

However, less water also translates into slower speeds to ensure that optimum application occurs. In the chemicals listed above, 5 mph to 7 mph is the recommended speed,” says Springsteen.

5. Select Adjuvants carefully

Adjuvants are a key component for herbicide performance and for enhancing weed control. Still, select adjuvants carefully. Not all adjuvants work for all herbicides.

“Before we recommend them, we test them first, especially on postapplied products,” says Springsteen. That's because some adjuvants work better than others.

6. Know what you're buying

In many cases, generic and post-patent herbicide products perform as well as branded herbicide products for less money. However, there have been a number of cases where herbicide quality is diminished.

“Know what you are buying” advises Dave Feist, northern development and technical manager for MANA. Ask for testing data, particularly from third parties, he says.

7. Preserve existing moa

The next time you hear companies tout a new herbicide, remember this: Companies have marketed no new row-crop herbicide modes of action (MOA) for well over a decade.

Several reasons exist. Glyphosate devalued the market for years, thereby, nixing any investment in new chemistry. New chemistry also must clear stringent government environmental and toxicology reviews.

However, glyphosate resistance has spurred new herbicides with existing modes of action. That, in turn, has spurred several companies to research new modes of action. A dream new mode of action would be an inexpensive nonselective herbicide like glyphosate with long residual, notes Springsteen.

Until then, it's important to take steps to preserve existing modes of action herbicides through resistance management steps, Springsteen says.

Plan to Forestall Resistance

Even if you've escaped glyphosate-resistant weeds so far, you'll likely have to contend with them in the future.

If you're battling waterhemp, for example, a single female waterhemp plant can produce more than 1 million seeds. If that single plant happens to be glyphosate-resistant, it sets the stage for future resistance.

Compounding this are weeds resistant to multiple modes of action. A 2010 University of Illinois survey of 122 waterhemp plants from 24 fields in Illinois, Kentucky, and Iowa found 66% resisted glyphosate, 59% resisted ALS inhibitors, 13% resisted PPO inhibitors, and 7% resisted all three of these herbicide modes of action.

So what should you do? Aaron Hager, University of Illinois Extension weed specialist, gives this advice for managing multiple herbicide resistance in soybeans.

● Apply a full rate of preemergence herbicide no sooner than seven days prior to planting and no later than three days after planting.

● Apply an initial postemergence herbicide before waterhemp exceeds 3 to 5 inches in height. This works well if the waterhemp isn't resistant to a postemergence herbicide like glyphosate. If it's resistant, though, waterhemp will be resistant to glyphosate regardless of size.

● Scout fields seven to 10 days later following the initial postemergence treatment.

“If waterhemp control is inadequate, apply a PPO inhibitor (like Cobra),” Hager says. It's important to stick with this time parameter, since waterhemp can grow an inch per day. PPO-inhibitor effectiveness decreases the bigger a plant gets.”

● Rescout fields 10 to 14 days afterward if you spray with a PPO inhibitor. “If waterhemp is still growing, it might be too big or even PPO-resistant,” Hager says. That's when walking the soybeans is recommended.

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