Ignorance might be bliss for some situations in life, but not when it comes to guaranteeing effective weed control for profitable corn yields, caution several Midwestern extension weed scientists.

“Good weed control starts by knowing what the predominant weed species problems are for each field,” says Aaron Hager, University of Illinois Extension weed scientist. “If you don’t know what your worst weeds are, then you won’t know how to choose the best product, or mix of products, to control them.”

Learning from last season’s mistakes can be the key to successful control for the future, particularly when it comes to managing herbicide resistance issues, says Mike Owen, Iowa State University Extension weed specialist. “For any hard-to-control weed, using a diversity of weed management tools is very important, but first you need to know the herbicide history of a field and whether any weed populations have developed herbicide resistance,” Owen says. “So scouting, record keeping and advanced planning are the places to start.”

Most major weed and herbicide resistance problems in corn vary by location — across the Corn Belt, within a state, and even within different fields on the same farm, says Lowell Sandell, extension weed scientist, University of Nebraska-Lincoln.

Sandell ranks velvetleaf, waterhemp, horseweed (marestail), green foxtail and giant ragweed as the five worst weeds (the most competitive weeds with corn, with the largest weed populations in fields) for the eastern part of Nebraska. For the western part of the state, he ranks common lambsquarters and kochia as the fourth- and fifth-worst weeds, ahead of foxtail and ragweed.

Neither Owen nor Hager rank velvetleaf among the three worst weeds for their states. For Iowa, Owen ranks giant foxtail first, common waterhemp second, giant ragweed third, horseweed (marestail) fourth and velvetleaf fifth. For Illinois, Hager ranks these weeds from first- to fifth-worst: waterhemp, giant ragweed, foxtail, morningglory, and fall panicum and other annual grasses.

Waterhemp

The worst-weed rankings for corn become more consistent across regions when rated by their difficulty to control. When defined in this manner, all three experts rank common waterhemp as worst.

“In Illinois, waterhemp tops our list of worst weeds, no matter what the crop,” Hager says. “I list it first because of its predominance across the state and because it’s ideally suited to adapt to today’s modern farming practices: It does well in limited tillage situations; it has an extended period of emergence; one female plant can produce one million seeds; and machinery can transport the small seeds from field to field.”

Owen agrees that waterhemp has an uncanny ability to stymie efforts to stop it, compared to other weeds. “It’s come to greater prominence in Iowa recently because of its ability to adapt to whatever product farmers use to control it,” Owen says. “Nationwide, waterhemp has evolved resistance to ALS-inhibitor herbicides, PPO-inhibitor herbicides, glyphosate, HPPD inhibitors, photosystem II inhibitors and now to 2,4-D in Nebraska.”

Any solution to waterhemp has to start with an integrated approach, Hager says. “Farmers will need to use more than just one herbicide to control it,” he says. “They should begin by planting into a weed-free environment, either by using some form of preplant tillage and/or a soil residual herbicide that is applied at the appropriate rate for soil type.”

When using postemergence herbicide products, weed height is an important factor for effective control, Hager emphasizes. “For waterhemp, a timely application should occur at or before a 4-in. weed height,” he says.

Farmers also should scout after treatments to see how well they worked, he adds. “Very few soil residual herbicides have activity that will last the whole season,” he says. “And with waterhemp, very few postemergence herbicides will provide adequate control with just one application.”

Giant ragweed

Hager, Owen and Sandell all rank giant ragweed as the second-hardest weed to control for corn.

“Giant ragweed is a top concern in Nebraska cornfields, due to pockets of glyphosate resistance that have been found in the eastern part of the state,” Sandell says. “We advise farmers to take care of giant ragweed prior to planting with an effective spring burndown.”

In Iowa, pockets of giant ragweed populations have developed resistance to glyphosate, Owen says. “Giant ragweed becomes very difficult to control with most postemergence products, with the exception of 2,4-D and other auxinic herbicides,” he says. “Post HPPD-inhibitor herbicides and triazines will do a nice job if the giant ragweeds are small. If you plan to use glyphosate, do not skimp on the rate and apply sooner rather than later.”

Hager says, in Illinois, giant ragweed is another example of a weed species that has evolved. “Its emergence extends well into June or even to early July, so it’s difficult to control with just one application,” he says. “There are now pockets of giant ragweed in the state with resistance to ALS-inhibitor herbicides, but we have not yet documented instances of giant ragweed that are resistant to glyphosate here as in other states.”

Marestail

“For marestail, glyphosate resistance is again the big issue,” Sandell says. “However, even non-glyphosate-resistant marestail can really reduce corn yields very quickly when in high density.”

Horseweed can be controlled with either a fall or spring burndown, Sandell says. “However, it’s unrealistic to expect residual control to last from fall through the first postemergence application, especially when planting soybeans,” he adds. “Controlling marestail prior to planting is critical, as postemergence control can be challenging if the plants have any size to them.”

Marestail tends to be more of a problem in no-till fields, Owen notes. “For marestail, if you include a little tillage, then that will take care of it, as long as you control it while it is still a rosette and before it bolts and sends up a flower stalk,” he says. “At that stage, it’s much more difficult to control.”

In Iowa, there are fewer issues related to herbicide resistance with marestail than with waterhemp or ragweed, Owen says, “but resistant marestail populations do indeed exist here.”

Sandell agrees that “tillage is another good tool to consider for Nebraska” but advises caution so that tillage doesn’t expose fields to erosion or cause farmers to jeopardize their benefits from government conservation programs.

Glyphosate-resistant marestail populations are also taking root in Illinois. “These resistant populations are more common across the southern third of the state,” Hager says. However, he adds, anecdotal observations during 2011 indicate that glyphosate-resistant horseweed populations have become a greater concern among farmers in central Illinois as well.

Foxtail

Foxtail is the most common grass weed that competes with corn in Illinois, Hager says. “In the past, soil residual herbicides have typically taken care of it,” he says. “However, farmers tend to use lower rates of application now than they once did, and lower rates shorten the duration of weed control.”

Foxtail is also widespread across Iowa “at fairly significant levels of infestation, and it’s been that way since the 1950s,” Owen says. “For now, there is no significant herbicide resistance issue related to foxtail, but the early-season competition from this weed can really reduce corn yields if postemergence applications are less than timely.”

Owen recommends using the full rate of a soil-applied herbicide that has residual activity for foxtail and then following up with either cultivation or a postemergence application. “It’s relatively simple to control foxtail with either Ignite [glufosinate] or glyphosate, but the timing is extremely critical to prevent a drop in corn yields,” Owen says. “So, for Iowa, we recommend an early preplant application with a chloroacetamide herbicide, such as Outlook, Dual Magnum or another product with similar residual activity for foxtail sometime during late March or early April.”

In Nebraska, green foxtail tends to be more prevalent than giant foxtail, Sandell notes. “While glyphosate continues to provide excellent control of green foxtail, a diversified approach that includes an effective soil-applied preemergence herbicide helps producers eliminate early-season competition from foxtail and maintain high yield potential,” he says.

Velvetleaf and morningglory

Sandell and Owen both rank velvetleaf as the fifth-most-difficult weed to control in corn, mostly due to its prevalence, competitiveness and the longevity of seed in the soil.

“Velvetleaf is common in cornfields all across Nebraska,” Sandell says. “It can be difficult to control if an application isn’t timely or is made too early or late in the day. Research has demonstrated lower levels of control with glyphosate are possible when velvetleaf’s photosynthetic rate is slower in the early morning or late day.”

Velvetleaf tends to be less of a problem in Illinois than in states further west, says Hager, who ranks annual morningglory as the fifth-most-difficult weed to control for corn in Illinois, partly due to how swiftly it spreads. “Timing is extremely important with morningglory, because it can get away from you very quickly,” he says.

Control problems also can occur with this weed when relying on glyphosate alone, Hager points out. “Annual morningglory is not as effectively controlled with glyphosate as other weeds are,” he says. “So for glyphosate to be effective, you’ll need to tank mix a product that offers good morningglory control.”

No silver bullet

No matter what the worst weeds are in your cornfields, you should weigh the pros and cons of all weed control tools, Owen says. Strategies to consider include rotating crops, herbicide modes of action, tankmix combinations and tillage practices from season to season. “Farmers tend to think in the short term, but they need to think about implementing a production system for the next five years,” he says.

“The expectation is that the industry will step in with a new silver bullet that will take care of all herbicide-resistant weeds, but that’s not going to happen,” Owen states. “There’s no new product on the horizon that will do that.”

“Farmers got themselves into this bind with herbicide-resistant weeds by getting away from the use of diverse tactics to control weeds,” says Owen, “and [using multiple techniques] is what they need to do again.”