You know the villains – waterhemp, marestail, pigweed and other pilferers of water and soil nutrients. They’re the plants you spend $50/acre to kill, yet zestfully return, especially if these prolific weed-seed producers like waterhemp are allowed to spread 300,000 herbicide-resistant seeds per plant at harvest.

Aaron Hager, University of Illinois Extension weed specialist, says no-till, strip-till, minimum-till and conventional-till systems all face weed-resistance problems. And it’s not just to glyphosate. Here are some common weeds that give growers fits:

Waterhemp can drown out any profit potential for corn or soybeans if it gets out of hand. It can nullify no-till profits or good returns from strip-tillage.

“Waterhemp pretty much grows wherever it wants,” Hager says. “It has resistance to six different herbicide families. That includes glyphosate, triazine, ALS inhibitors, PPO inhibitors, HPPD inhibitors and most recently, synthetic auxins in Nebraska.”

He says determining which herbicide chemistries waterhemp is resistant to can be a chore. “The big problem is waterhemp’s ‘stacked’ resistance to two or more of the herbicide families,” he says. “Recent reports indicate it is showing resistance to 2,4-D in the western Corn Belt. If you intend to plant a conventional or Roundup Ready variety, waterhemp can be very hard to control once it comes out of the ground.”

The Nebraska HPPD waterhemp resistance was announced in December. Earlier in the year the 2,4-D resistance was discovered, according to Stevan Knezevic, University of Nebraska Extension weed specialist.

Mike Owen, Iowa State University (ISU) Extension weed specialist, says that just like Illinois, “we have everything but the 2,4-D resistance that Nebraska has reported. We have the stacked resistance for waterhemp just like Illinois.” (To learn more about herbicide families and weed resistance, check out ISU’s 2012 herbicide guide http://bit.ly/yryV29.)

Marestail/Horseweed can be as difficult to control as waterhemp and has growers juggling herbicides. “Horseweed may be more challenging in no-till systems, especially if it’s not adequately controlled before planting,” Hager says.

With the increasing prevalence of horseweed, fall herbicide applications may prove more efficient than spring applications. “Glyphosate alone may not provide adequate control when applied in either fall or spring, but a fall application timing provides an opportunity to utilize higher application rates of products like 2,4-D than are feasible to use in spring,” Hager says.

Velvetleaf: With its larger seed, no-till growers have a better chance of dodging resistant velvetleaf problems. But more conventional corn and bean producers better watch out.

“The two selective forces, tillage or lack thereof, have as big an impact on the weed as the herbicide,” Owen says. “But the herbicide has to be adapted to the tillage system. So you can throw velvetleaf out of the mix on no-till or strip-till. Waterhemp and marestail are more in that environment.

Knezevic says velvetleaf is a big problem for Nebraska corn production and that its seed has added longevity in the soil.

 Pigweed: Palmer amaranth is one of several pigweeds that give farmers fits. Its resistance to glyphosate is certainly more prevalent in the mid-South and Southeast. But the Mason-Dixon line hasn’t stopped resistance from spreading northward. “We’ve (Illinois) had Palmer amaranth populations for decades,” Hager says. “In the last 12-18 months we’ve had at least one, if not more than one population confirmed with glyphosate resistance.”

Many believe it’s just a matter of time before major Palmer amaranth resistance is seen in the heart of the Corn Belt, as well as east and west.

Giant ragweed: This trigger of hay fever is nothing to sneeze when it comes to hurting corn and soybean production. And glyphosate resistance is joining ALS-resistant lines in the eastern Corn Belt, and it’s moving westward, say Purdue and Ohio State university weed scientists.

Fortunately, no-till and strip-till operations can escape more heavy giant ragweed damage. But conventional fields may see serious damage because of the weed’s ability to germinate and emerge from deep in fields. And like other weeds, it steals nitrogen that could go to corn.

Kochia: This semi-arid weed has seen ALS-inhibitor resistance for several years across several states. The last few years have seen kochia resistant to glyphosate emerge in western Kansas. There is also some resistance to triazines, 2,4-D and dicamba.
Like many other weeds, it can thrive in no-till situations. However, tillage can help control kochia. “Since kochia germinates very early in the season in Illinois, a tillage operation prior to corn or soybean planting can sometimes eliminate most kochia for the remainder of the season,” Hager says.

Many other weeds are on the hit list for corn and soybean farmers, namely common ragweed, giant foxtail, lambsquarters, Canada thistle, johnsongrass and others. Resistance will likely become more of a problem for them. Weed scientists encourage growers to study weed IDs (see accompanying photos) to distinguish one of the pesky plants from another.


                                              
SITE-SPECIFIC CULTIVATION

Repeated use of the same herbicide can easily result in the evolution of weed resistance, regardless of the type of herbicide used,” says Stevan Knezevic, University of Nebraska Extension weed specialist.

“We have forgotten the time of atrazine and ALS resistance 15-30 years ago,” he says. “With the recent developments of glyphosate resistance, Mother Nature is reminding us that weeds can develop resistance to any type of chemical we develop.”

More modes of action (MOA) are called for by virtually any weed scientist. “We’ve tried to emphasize more integration in weed management for years,” says Aaron Hager, University of Illinois weed scientist. “We first saw glyphosate resistance five years ago; but it’s just not limited to glyphosate.”

He says that MOA of several herbicides can broaden the weed-control spectrum. “Combining 2,4-D and/or glyphosate with soil-residual products can improve control of emerged species and help control biennial or perennial species,” Hager says.

Various chemical manufacturers offer a diverse range of herbicides that can be mixed with glyphosate, LibertyLink or other herbicide systems. But there may also be room for cultivation.

“We talk about rotating herbicides, but also need to talk about the fact that the only way to manage emerged waterhemp or other weeds may be through mechanical tillage,” Hager says.

“We try to convince farmers that if resistance is in the early stages, you can remove many of these weeds mechanically.”

Mike Owen, Iowa State University weed scientist, says cultivating likely won’t involve an entire crop. “Growers need to address the benefits of mechanical weed control against the risks of not including it,” he says. “There is excellent equipment that allows for mechanical weed control for no-till and conventional till without disrupting the benefits of no-till.


 

“One thing people don’t realize is that when we say ‘you need to think about cultivating,’ they think they have to cultivate 2,000 acres. But what we’re saying is it is more site-specific. Maybe only in one field, with specific areas in a field.”