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Time to take action on herbicide resistance

By Jean Caspers-Simmet

Date Modified: 02/06/2014 12:12 PM

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MASON CITY, Iowa —If farmers use herbicides, they are at risk for having resistance problems, Iowa State University Extension weed specialist Bob Hartzler said at the June 9 Crop Advantage Conference at North Iowa Area Community College in Mason City.

"There is not a single farmer here who does not have resistant weeds on their fields," Hartzler said. "We are in an era where we need to change our perspective of resistance. In the past if you were going to get resistant weeds, it probably was based on the fact that you used herbicides in a way that selected the resistance for weeds already in your fields. Especially with glyphosate, we know that resistance occurs at a very low frequency in the weed population. Because you've done a good job of controlling weeds in the past, you have relatively low weed populations in your field and there may not be any resistant individuals present in the fields you manage. That was 10 years ago. You may still manage your weeds well and you may not have any resistant weeds in your fields, but your neighbors most likely do, and those weeds will try to move into your fields either by seed moved from one field to another or pollen."

If farmers use diversified weed management programs, it is unlikely that herbicide resistant weeds will get established, Hartzler said. If farmers rely on using the same herbicides it's very likely those weeds will get established.

"Resistance is so widespread that we are fighting weeds that can evolve resistance to every herbicide that we have available," Hartzler said. "If we're not careful, we'll run out of options to conveniently control weeds."

The Iowa Soybean Association is supporting a survey to determine the extent of herbicide resistance in Iowa, Hartzler said. Approximately 700 waterhemp, marestail and giant ragweed populations were sampled across the state.

In 2011 waterhemp resistance to full rate herbicides ranged from nearly 100 percent to Herbicide Group 2, or ALS inhibitors, to 16 percent to Herbicide Group 14, or PPO inhibitors. The study found 88 percent of waterhemp populations have resistance to more than one herbicide group and 10 percent of the populations were resistant to all five herbicide groups evaluated.

"Not only are herbicide resistant waterhemp populations commonplace across the state, but resistance is evolving to most herbicide groups used in corn and soybean production," Hartzler said. "Not surprisingly, glyphosate resistance evolved in weeds due to it being used repeatedly on nearly all of the crop acres, but the appearance of resistance to Herbicide Group 14, and Herbicide Group 27, or HPPD inhibitors, should be a wake-up call since these products have not been used as widely as glyphosate, and they are almost always used in combination with other herbicides."

Rotating herbicide groups is a simple method of reducing selection pressure but by itself it is unlikely to prevent resistance, Hartzler said. Rotation must be used in combination with other strategies.

Including multiple herbicide groups in a program is not as simple as it sounds since the effectiveness of each component must be evaluated against the individual weeds that would pose a resistance threat, Hartzler said. To effectively manage waterhemp, marestail and giant ragweed, Iowa's primary resistant weeds, each herbicide must be used in a matter that would allow that herbicide to provide effective control if it was used as the sole component of the herbicide program. The active ingredients are often used at too low of a rate to provide a high level of control of target species, Hartzler said.

To manage resistance, postemergence herbicides must be used in a manner that extends activity late into the season.

Farmers need to go beyond improving how herbicides are used, Hartzler said.

"One of the most effective tools at managing weeds is a competitive crop canopy," Hartzler said. "Planting in narrow rows is the simplest approach to providing the crop a competitive edge over weeds. The contribution of seeding rate, planting date, cover crops, fertilization and other factors that influence crop growth are easily overlooked as tools."

Tillage is another option with post plant tillage an effective control tactic.

"Few farmers have time or resources to cultivate all acres, but selectively cultivating fields with problem weed infestations may be something to consider," Hartzler said.

New herbicide resistant crops may provide a short-term fix but if these crops and their respective herbicides are simply inserted into current management programs, weeds will quickly overcome this solution since resistance to these herbicides already exists, Hartzler said.

There are no new herbicide options on the horizon.

"While there are a number of new herbicide premixes now available, these represent combinations of older products," Hartzler said.

Palmer amaranth was identified in five southern Iowa counties in 2013 for the first time with more infestations suspected. A close relative of waterhemp, palmer amaranth is much more competitive and more damaging to crop yields, although Hartzler doesn't anticipate it will become as big of a threat as it is in the South.