Weeds In Waiting

Giant ragweed

This winter, dealers and growers may be trying to decide if dry weather, herbicide resistance, or poor application timing caused unacceptable weed control last year. No matter the cause, more weeds in the seed bank will mean more plants in 2008, says Jeff Stachler, Extension program specialist with The Ohio State University.

Bill Johnson, weed specialist with Purdue University, says a dry spring in Indiana in 2007 led to erratic and sometimes poor performance of soil-applied herbicides, as they weren’t activated by rainfall. “And it was so hot and dry later that weeds that are a problem to control anyway — including common lambsquarters, giant ragweed, and morningglory — weren’t controlled because they were hardened off by the drought.”

Ohio saw a similar weather and control scenario, reports Stachler. Giant ragweed hit corn hard, while soybean fields also saw common ragweed, common lambsquarters, velvetleaf, and common pokeweed.

Could resistance be playing a role? “I think glyphosate resistance is confusing to growers compared to ALS resistance,” says Johnson. “Glyphosate resistance is difficult to diagnose because we are dealing with low levels of resistance. This means that weeds will be injured by glyphosate, and whereas ALS resistance is clear-cut, weeds show very little injury by a glyphosate herbicide. Growers often think their herbicide failure is weather-related, or a result of a misapplication, and they may be surprised at how much resistance is out there.” Fourteen counties in Indiana now have confirmed glyphosate resistance in giant ragweed, and the problem will only continue to expand if growers do not manage this weed more aggressively, he says.

In addition, it appears that increasing percentages of Indiana’s growers planting glyphosate-tolerant corn were not using residual herbicides or lower rates of residual herbicides.

“I have a great deal of concern about the minor yield losses that are occurring because of early-season weed competition before glyphosate is applied in corn, and the growers may be thrilled with 200 bushels per acre corn when they could have had 210 to 220 with a more aggressive approach to weed management,” he says.

Johnson hopes that because corn prices were high, growers can invest some of that income on these products “to get 10 to 15 more bushels per acre this year.”

Johnson is encouraged that residual herbicide use is, in general, higher in corn than in soybeans. “The problem with corn is that there is such a narrow window to put on postemerge products, because the crops outgrow sprayers or labels,” he says. Plus, weed infestations can be difficult to see from the road.

For 2008, “we’ve had plenty of precipitation, our groundwater has been recharged, and we’re going into spring with normal soil moisture.” That’s good news for soil-applied herbicides.

Ragweed and Volunteer Problems

“Giant ragweed has the greatest potential to be a problem in Ohio because of how much went to seed in corn and soybeans due to improper application timing and herbicide resistance,” says Stachler. He also anticipates a ragweed problem as growers switch back to normal soybean acreage in 2008. “The giant ragweed that went to seed last year in corn will cause more problems in soybeans because some will be resistant,” he explains. In fact, he’s seen more stacked resistance: ALS/glyphosate-resistant giant ragweed, common ragweed, and marestail.

Officially, glyphosate resistance has not been documented in Iowa, but there are plenty of stories throughout the state “that strongly support the supposition that resistant weed populations exist in Iowa,” says Michael Owen, Extension weed management specialist, Iowa State University. Suspects for resistance include giant ragweed, marestail, and common lambsquarters, but recent control problems with common waterhemp should also concern growers.

Use of glyphosate has also caused volunteer crop management issues, Owen says. Spring conditions in Iowa in 2007 were not ideal, calling for replanting in many corn acres. Growers had to make sure to destroy stands that possessed herbicide resistance traits.

“I view volunteer Roundup Ready (RR) corn as a huge problem,” says Kevin Bradley, Extension weed specialist at the University of Missouri. And Dawn Refsell, University of Illinois Extension specialist, says her growers may see more volunteer RR corn in both corn and soybeans.

Waterhemp Woes

The “comer” weed for 2007 in both Illinois and Missouri seemed to be waterhemp. “Our newest problem is glyphosate-resistant waterhemp,” says Refsell. “This weed is going to change how we currently manage our soybean acres. We’ll no longer be able to rely on total postemergence glyphosate applications — growers are going to need the addition of preemergence chemicals and tank mixes.”

Velvetleaf may be showing some ALs resistance, causing trouble for soybean growers.elvetleaf may be showing some ALs resistance, causing trouble for soybean growers.
Velvetleaf may be showing some ALs resistance, causing trouble
for soybean growers.elvetleaf may be showing some
ALs resistance, causing trouble for soybean growers.

Illinois also faces combination resistances in waterhemp in the ALS, triazine, and PPO chemical classes; “our options for controlling waterhemp in soybean are getting limited to preemergence products only,” he explains.

“It would only help our situation if in fact we have less soybean planted next year,” says Missouri’s Bradley. “I’m not sure that’s going to happen. We’re a heavy soybean state, and we had a slight drop in soybean acreage last year but not as much as expected.”

In 2008, growers in Illinois should also be on the lookout for common lambsquarters, giant ragweed, morningglory, and marestail. Bryan Young, professor of weed science at Southern Illinois University, reports these weeds, as well as waterhemp, saw inconsistent foliar control with glyphosate products. He says: “I’m afraid we may have widespread problems in 2008.”

Cotton Concerns

In North Carolina, glyphosate-resistant Palmer amaranth has been identified in at least 11 counties, says Alan York, Extension specialist, North Carolina State University. “Unfortunately, ALS inhibitors in cotton have always been marginal on Palmer, and ALS-resistant Palmer is widespread in the Southeast. That exacerbates glyphosate resistance concerns,” he says.

But York is encouraged that growers did better with the weed in 2007 than 2006, and believes better control came from “a greater awareness of the problem and greater use of other herbicides within Roundup Ready systems – plus rotating out of some bad fields.”

Indeed, North Carolina cotton acreage was cut roughly in half in 2007 as more growers planted corn. York says the switch may have helped, as good control options are available in corn. Cotton acreage will likely drop again for 2008, with wheat being planted in highway medians and “everywhere else this winter,” says York — followed by double-cropped soybeans, also a crop with some good Palmer control choices. “The good — or bad — thing about weeds,” he notes, “is that they will handle whatever weather we have pretty well. That gives guys like me job security.”

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