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Plan Your Weed Control Strategy

Underneath the snow, buried in the frozen ground, a season-long battle is taking shape—a running fight against a steadily spreading population of herbicide-resistant weeds. At the start of 2012, the U.S. was home to 139 different herbicide-resistant weed biotypes, many of them resistant to more than one family of herbicides. Maintaining control is going to require planning, constant surveillance and decisive action. Most important, the process has to start now so you’re not stuck trying to play catch-up with a dwindling set of weapons.

Assume the Worst

Scientists are unsure how many U.S. cropland acres are already infested with herbicide-resistant weeds—conservative estimates run around 12 million acres, while a survey last year by Stratus Agri-Marketing concluded that more than 61 million acres were host to glyphosate-resistant weeds. To maintain the upper hand, you have to assume your fields are harboring resistant weeds, too.

“Start like you’ve got a resistance problem before you ever see the first resistant weed,” advises Ford Baldwin, former University of Arkansas Extension weed scientist and founder of Practical Weed Consultants in Austin, Ark.

The first step in planning your 2014 weed control program is assessing what happened last year. Bryan Young, professor of weed science at Southern Illinois University, points out that tracking your rates of glyphosate is a useful—and maybe sobering—clue about your weed populations.

“If you’re applying anything higher than 22 ounces [of glyphosate per acre], you probably have resistance and you’re just not willing to admit it,” Young told farmers at a Respect the Rotation field day in Collinsville, Ill., in 2012. “If you’ve had to jack up your rates already, it’s started.”

Sketch It Out

While you’re checking your history of glyphosate use, take note of the other herbicides you’ve applied over the past two to three years. Look for red flags that could indicate an increased risk of selecting for resistant weeds, including repeated use of the same products or an over-reliance on a postemergence herbicide program, suggests Lisa Behnken, University of Minnesota regional Extension educator in Rochester, Minn. Prevent future red flags by writing out a herbicide plan for each of your fields for the next two years, she recommends.

As you plan for the next couple of years, pay attention to the fundamental strategies in the war against weed resistance:

  • Target the worst weeds in each field.
  • List the herbicides that are effective on those weeds.
  • From that list, be sure to employ at least two to three different sites of action (SOA) over the course of the season, and avoiding the use of the same SOA twice in a row. (For a comprehensive SOA chart of herbicides and premixes—an indispensible planning tool—visit http://bit.ly/1aIeCIj.)
  • Include contact, preemergence and a variety of postemergence herbicides in your program—avoid relying on a total-post strategy. Young recommends “overlapping residuals,” adding a residual herbicide to a postemergence application to provide a long-lasting boost to your preplant residual. Just be sure that the residuals aren’t from the same SOA group.  
  • Plan carefully to prevent potential problems like antagonism, which can occur when you tank mix ALS inhibitors with PPO inhibitors, for instance.  
  • Rotate among Roundup Ready, Liberty Link and conventional varieties. 
  • Consider non-chemical control strategies, too, like tillage rotation, cultivation, narrow rows and other cultural approaches to combating weeds.  
  • Rotate crops to maximize the variety of weed control tools at your disposal.   
  • Give your crops every possible advantage to maintain an edge over weeds, from selecting the most appropriate varieties for the site to providing the population, fertilizer and crop protection needed for a healthy stand.

Most important, notes Iowa State University weed scientist Micheal Owen, don’t treat each field alike. “Growers are used to diversifying hybrid choices and fertility treatments to mitigate risk,” he points out. “The same approach for weeds means knowing what you’ve done historically, knowing if you’re seeing issues in specific fields, scouting and managing weeds for as small a unit as possible.”

Keep On Fighting

The old proverb, “one year’s seeding is seven years’ weeding,” has never been more threatening. In the fight against weeds like pigweed, Palmer amaranth and marestail, which can each produce thousands of long-lasting seeds per plant, just a few escapes can become a nightmare. Even cultivation isn’t fail-safe—Young points out that a field cultivated three times in a research trial still had 3% of its Palmer amaranth survive.

The key is to scout fields carefully after each weed control pass—chemical or mechanical—to check for escapes. Do they indicate a herbicide failure? Is a new flush coming up? Whatever the reason for weeds in the field, make sure you don’t let them go to seed. Plan another herbicide application or cultivation, or dig out an old bean hook from the back of the barn. That’s especially important in the South, where early harvests and long autumns can enable weeds to cycle all the way to maturity well after most farmers are accustomed to watching their fields. However, few fields are truly safe—tough species like Palmer amaranth can go from seedling to seedhead in about three short weeks.

Act Now

The bottom line is that resistance management has to start today, while the ground is still cold and the sprayer is still parked. If you’ve got a resistance problem, muster all your forces to control those weeds with other tools, and prevent the problem from getting worse. If glyphosate, glufosinate or other great tools still work for you, count your blessings. Then plan to back them up with other SOA groups and cultural tactics to help ensure that they’re still effective in the years to come.

Losing Control, Losing Money

The weed control stakes have never been higher. Southern farmers battling resistant weeds such as Palmer amaranth are racking up huge herbicide bills. “Most of my growers have at least three residuals on their soybeans, at least,” University of Tennessee Extension weed scientist Larry Steckel told farmers at a Respect the Rotation field day in Illinois. “Most of my growers have $60 to $70 worth of herbicides on their soybeans, and they still have weeds—and we’re still hiring chopping crews.” Timeliness is vital, especially in corn. Studies in Minnesota and Wisconsin documented a yield loss of 3 bu/A per day in corn battling four-inch-tall weeds in its V3-V4 stage of growth, according to Jeff Gunsolus at the University of Minnesota, racking up losses of 12 to 13 bu/A in the first week and 27 to 29 bu/A in the second week. In Iowa, researchers charted a yield loss of 7 bu/A in corn from just one foxtail plant per foot of row. Soybeans tend to recover fairly well from early season competition, but corn suffers dramatically when weeds steal nitrogen and block sunlight, creating uneven stands.

Keep Your Sights on Site of Action

Managing herbicides by their sites of action is absolutely critical to battling resistance. The site of action (SOA) system categorizes herbicides by the biochemical site the product affects, which is sometimes a more precise description than the “mode of action” classification commonly used in the past.

Consult an SOA chart to avoid applying related herbicides to a resistant population—a waste of time and money, as well as a way to continue selecting for even bigger resistance problems in the future—and to ensure that you are deploying a variety of different approaches to controlling weeds.

Premixes and tankmixes with multiple SOAs are great tools for adding diversity to your weed control program if—and this is a big “if”—your weed population isn’t resistant to any of the components of the blend. “Even if you’re applying a herbicide with multiple numbers, you aren’t diversified if the weeds already are resistant to some of those sites of action,” notes Micheal Owen at Iowa State University.

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