Depending on a single chemistry for crop protection risks resistant weeds and weed species shift.

What's that old saying? Familiarity breeds contempt?

Something like that appears to happen with weeds and herbicides. The better a weed-killer works, the more growers use it. The more they use it, the more opportunity weeds have to develop resistance to the chemical. That's happening now with one of the more effective herbicides to ever come on the market: glyphosate (Roundup).

"We have glyphosate-resistant marestail in the Bootheel now," said Andy Kendig, University of Missouri weed specialist at the Delta Center in southeast Missouri. "We haven't been able to formally prove it in the greenhouse yet, but when we spray glyphosate on marestail in the field, the weed turns yellow and stops growing for a while, then shakes off the chemical and recovers."

In a way, glyphosate has been the victim of its own success. Because the weed killer has been so universally effective and relatively easy to use, some growers take shortcuts.

"The vast majority of problems we're seeing are management related," said Paul Tracy, MFA staff agronomist. "Good weed control depends on several things: the herbicide and its use rate, weed species and size, spray volume, timing, environmental conditions all can affect the outcome. But because glyphosate works extremely well when properly used, some of us have gotten complacent in our management-especially in the timing of sprays. Weeds are easier to control when they are small and immature. With glyphosate or any other weed-control, the best results come from using a herbicide early at labeled rates, then plan for a second application if it is needed."

And, because glyphosate chemistry is so simple, many farmers who had been hiring custom application have dusted off their old spray equipment and have done the job themselves, Tracy added. If the sprayer isn't applying the herbicide evenly, at the proper pressure and the proper rate, it's not realistic to blame the herbicide for lapses in weed control.

"If we treated glyphosate the same way we do other chemicals with respect to timing and careful application, we'd be in a lot better shape where weed control is concerned," Tracy said.

There have been rumors and suspicions of glyphosate-resistant weeds virtually since the advent of Roundup Ready soybeans. Several states have reported some strange waterhemp; the jury is still out on whether the weed is actually resistant or not. But resistant marestail (alias horseweed, fleabane, muletail and others) is a confirmed fact, first noticed three years ago in Maryland and Tennessee.

"In 2002, we saw some evidence of resistance in marestail in southeastern Missouri," said Kendig. "This past year, we didn't have any trouble finding resistant weeds, especially along the eastern side of the Bootheel."

In that same time frame, Arkansas growers were noticing glyphosate-resistant marestail more and more, especially in no-till and minimum-till fields. Both Kendig and Ken Smith, an Arkansas weed scientist, believe resistant marestail hitchhiked on a southeasterly breeze across the Mississippi River from Tennessee.

If you're lucky enough to not have seen marestail in your fields, it's not that difficult to recognize. It's a tall-growing annual, with leaves alternating on the stem. Lower leaves may grow to 4 inches long and are broadest near the tip. If allowed to mature, each plant can produce upwards of 50,000 seeds. Marestail is native to virtually all humid regions of North America and typically germinates in late winter.

"But we're seeing marestail sprout later in the season, at times after the crop is up," said Paul Tracy. "We may be seeing a shift in the weed's biotype-one that germinates later."

For many farmers, especially no-tillers, "disk" is a four-letter word. But marestail is easily controlled with cultivation, which probably explains why the weed is more prevalent in conservation-tillage fields.

"An early disking gives very good control," said Kendig. "The fact that we still use tillage from time to time in the Bootheel may be why this problem hit Tennessee a couple of years before we saw it here."

Options
If you have resistant marestail-or if you get it-what's the best strategy for managing it?

"Resistance management is a poor term," said Kendig. "Management means it's too late, if you've already got resistant weeds. Resistance prevention, not management, should be our goal.

"But if you already have the problem, managing it is a matter of spraying something else to control the marestail," he continued. "Traditionally, our No. 1 problem with no-till weed control has been the failure to make an effective early burndown application. Now that we have resistant marestail, guess what? Our No. 1 no-till problem is failing to make that special burndown application in March.

"You might want to look at Clarity in cotton burndown applications, and something like FirstRate or Amplify in soybean burndowns," said Kendig. "Clarity is probably the product of choice for best control of resistant marestail. We've also used a burndown with 2,4-D where we need to control cutleaf evening primrose, and 2,4-D also does an OK job on marestail.

"But if you have marestail problems in cotton fields, I'd probably go with Clarity, which can be applied a week closer to cotton planting than can 2,4-D. Whatever you use, that early burndown is critical for weed control in no-till."

Note: One potential problem with 2,4-D is that the compound can contaminate a sprayer tank and plumbing, possibly damaging crops sprayed later. In that regard, a thorough cleaning of spray equipment is prudent, whatever the herbicide used.

With growers planting more than one Roundup Ready crop (soybeans and corn, for example), it's possible-even likely-that more glyphosate resistance in more weeds will show up in the future. It's a case of weeds having more familiarity with the product.

"Even if we find another weed or two that is resistant to glyphosate, it's not the end of the world," said Paul Tracy. "The chemical still works well on a lot of weeds. We simply need to be more careful how we use it."