What's behind grower complaints that glyphosate has lost some of its “punch” over the past five or six years?

Many growers who have used it alone for weed control have reported a fall-off in control of certain weed species, including new weeds, says Stevan Knezevic, University of Nebraska Extension weed specialist. The list includes: marestail, morningglory, wild buckwheat, Pennsylvania smartweed, lady's thumb, Venice mallow, yellow sweetclover, field bindweed, waterhemp, kochia, Russian thistle, primrose species and volunteer Roundup Ready corn, according to Knezevic.

So, is weed tolerance — or even weed resistance — to glyphosate on the rise?

University of Nebraska weed specialists announced last fall that it has confirmed the first glyphosate-resistant weed species in the state: marestail. (A weed species is considered resistant if it can withstand four times the labeled rate for control, Martin says.)

Indiana and Illinois are among states where glyphosate-resistance has been documented. Glyphosate-resistant marestail has been confirmed in 29 Indiana counties, according to Bill Johnson at Purdue University. Resistant marestail, sometimes called horseweed, has also been documented in Illinois, says Bryan Young at Southern Illinois University. The problem “kind of exploded” during the past year, he adds.

He and Johnson say other species are getting special attention for glyphosate resistance in their states. Those include lambsquarters, giant ragweed and waterhemp.

“We're seeing a lot of escapes of redroot pigweed,” Johnson says, along with control concerns involving morningglory, common ragweed, pricklysida and burcucumber.

Many of the poor control complaints probably center around weed shifts: weed species that previously hadn't been prevalent but are now rearing up as serious control problems in a field, according to Martin and Knezevic.

Glyphosate offers broad-spectrum weed control. But, no herbicide effectively controls all weeds, these weed specialists say. For example, some broadleaf species are more tolerant to glyphosate than others. And if you use glyphosate as the sole weapon in your weed arsenal year after year, weed species less susceptible to glyphosate become opportunistic in the wake of less competition from species that are more readily controlled by the herbicide. You end up with a weed shift, which may seem like a decline in glyphosate's effectiveness, Martin and Knezevic say.

Other factors could be at work as well, Martin says. In some cases, application may have been after weeds had grown too big for best results. Another explanation may be that, in some cases, because of glyphosate's dramatic success in its initial use, expectations for its performance keep going up, Martin says.

Young at Southern Illinois makes a similar assessment. He says he's heard some growers and retailers blame environmental conditions for less-than-expected weed control. But he doesn't think that's always the case. “Application timing (weeds too big) and rate (too low) have a lot to do with our problems,” he says.

Whatever the case, weed specialists emphasize the importance of not relying solely on an herbicide-tolerant crop (HTC) for weed control — whether it's Roundup Ready, LibertyLink, Clearfield or STS.

About nine of every 10 acres of soybeans planted nationally in 2006 was an HTC variety, according to National Agricultural Statistical Service (NASS) surveys. One in five corn acres is an HTC hybrid, according to NASS.

It would probably be best not to use glyphosate every year, Martin says. But for many growers it simplifies weed control and carries little risk of crop injury.

Growers should consider combining it with a preplant or pre-emerge herbicide that offers good control of weeds that are less glyphosate-susceptible, Martin and Knezevic say.

In Roundup Ready corn and soybean production, reduce glyphosate-selection pressure by using a “solid residual herbicide program,” Young at Southern Illinois University advises.

Be sure you start with a clean seedbed, adds Purdue's Johnson. In no-till, that may require a burndown herbicide application that includes 2,4-D, he and Young say.

Knezevic and Martin believe a preplant or pre-emerge herbicide for early weed control followed by a glyphosate application for late-emerging weeds can be beneficial beyond reduced risk of weed shifts or resistance.

Protection from weed competition during the first 20 days following crop emergence “probably buys some yield increase,” Martin says. “We suggest using a pre-emerge and following that with glyphosate.” That's preferable to a glyphosate application for early weed control and a second application of it for late control.

Tankmixing glyphosate with a postemergence herbicide that's effective against weed species that glyphosate doesn't readily control is one way of avoiding weed shifts and potential for resistant species. But it leaves you without protection from early weed competition. And, there's the question of whether to use a full rate of both products in the mix or using one-half or two-thirds rate, Martin says.

While glyphosate poses little crop injury risk, injury risk from certain other products in the mix at full rate should be considered. Partial rates can reduce the injury risk, but probably won't cut cost significantly and may not deliver as much weed control as a full rate, Martin says.

Knezevic and Martin encourage an integrated weed management (IWM) program to preserve glyphosate technology. That means drawing on a combination of preventive, cultural, mechanical and chemical tools. Ideally, a grower would not rely on glyphosate every year.

“The thing I tell growers is that glyphosate is great technology, but it's no longer simple to use,” Johnson says. You need to combine its use with more residual herbicides on both corn and soybeans to sustain its usefulness.

“Glyphosate is a valuable herbicide, the kind of product that gets discovered once in 100 years, and should be preserved for future generations,” Knezevic says.