In the spring of 2000, a Lauderdale County, Tenn., farmer noticed that some horseweed he sprayed with a quart rate of glyphosate wasn't dying after he applied the burndown herbicide.

So, he sprayed it with two quarts of glyphosate. A few days later, he noticed a number of the plants were still thriving. That was the first reported case of glyphosate-tolerant horseweed in the Mid-South, according to university weed scientists.

“We went back to that field in 2001 and conducted tests to try to verify whether we had resistance,” said Bob Hayes, professor with the University of Tennessee's West Tennessee Experiment Station. “We tried a number of different formulations and spray volumes and surfactants and none gave over 65 percent control in that field.

“Since then, we have had reports of similar control failures with glyphosate on horseweed in Lauderdale, Haywood, Crockett and Gibson counties in west Tennessee.”

Hayes, one of the first weed scientists to work with no-till cotton at the Milan, Tenn., Experiment Station in the early 1980s, says farmers must control horseweed and other existing vegetation before, at or immediately after planting to be successful with no-till.

Confirmation of resistance to glyphosate in a problem weed like horseweed makes it more important than ever before that no-till farmers know the weed species in their fields and plan their weed control strategies accordingly, he says.

“Prior to the introduction of Roundup Ready cotton, most producers used a one-two punch of early preplant glyphosate followed by Gramoxone Max plus Cotoran, Caparol or Bladex at or near planting,” he said. “Many producers are now omitting this latter treatment in lieu of an early postemergence glyphosate application.”

Speaking at the National Conservation Tillage Conference in Tunica, Miss., Hayes said that in some instances, conservation tillage cotton is being planted without complete control of winter and spring vegetation in anticipation that this early postemergence glyphosate treatment will take those weeds out.

“Unfortunately, this has not always been the case,” he says. “Instead of complete burndown, this creates a situation where the landlord, producers or lenders are hot enough to ‘burn up.’”

In the case of the cotton planted in the fields with the glyphosate-resistant horseweed, subsequent applications of glyphosate after the Roundup Ready cotton emerged also failed to provide control, Hayes notes.

“Staple postemergence stunted the horseweed to the point that post-directed diuron (Direx, Karmex) plus MSMA was able to provide acceptable control in most situations,” he said. “MSMA applied over-the-top suppressed the horseweed but did not provide complete control.”

A tank mix of Gramoxone Max and diuron (Direx, Karmex) could help control the glyphosate-resistant horseweed when applied near planting, he said. An early burndown application of dicamba (Clarity) provided nearly 100 percent control of the resistant horseweed.

Perhaps the most important steps in controlling any winter or spring vegetation in a no-till system, says Hayes, are proper identification of the weeds present and selection of the most effective strategy for control, including herbicides, rates, timing of application, spray gallonage, adjuvants, proper sprayer calibration and using recommended nozzles for best coverage and minimizing off-target movement or drift.

“The one-two punch of a glyphosate herbicide early followed by a tank mix of Gramoxone Max and Cotoran or Caparol at or near planting is still the most reliable approach to control of a broad spectrum of grass and broadleaf weeds,” he notes.

“Unfortunately, it can be rather costly, and given the current economic situation, producers are searching for ways to reduce production costs.”

One of the oldest herbicides, 2,4-D, is relatively inexpensive and effective on many winter annual broadleaf weeds. But, Hayes says, only a few 2,4-D products are labeled for application prior to planting cotton, and then there are disclaimers concerning the potential for crop injury. At least one label permits application up to 30 days before planting cotton.

Several studies have demonstrated that there is little risk to cotton when application is made more than 30 days before planting cotton, he says.

The key appears to be the soil degradation of 2,4-D that begins following the first rainfall after application.

“Unfortunately, the disadvantages are sprayer contamination — it is virtually impossible to sanitize a sprayer sufficiently for subsequent use in cotton — and drift to sensitive plants.

“Research is under way comparing the amine and ester formulations of 2,4-D and to possibly shorten the interval before planting.”

A light disking will also eliminate any horseweed, but it will defeat the purpose of planting no-till or reduced-till and is not recommended for highly erodible soils.

Some no-till farmers apply Harmony Extra followed by glyphosate herbicide or paraquat (Gramoxone Max) to provide a broader spectrum of control with their burndown herbicide applications.

“Harmony Extra followed by paraquat or glyphosate improves control of Carolina geranium, curly dock and cutleaf eveningprimrose,” he said. “Application may be made from late fall through early spring, but no later than 45 days prior to planting although research indicates no cotton injury or yield reduction when applied 15 or more days before planting.”

Hayes also cautions growers to avoid tank-mixing burndown herbicides that cause leaf burn with glyphosate herbicides because the combination may interfere or antagonize the uptake and translocation of the systemic glyphosate.

“Most wettable powder formulations will also tend to reduce activity, especially at the lowest use rates,” he noted. “In contrast, photosynthetic inhibitors such as Direx, Karmex and Cotoran may synergize paraquat by inhibiting two photosynthetic processes.”

When it's all said and done, producers must insure that they completely control horseweed, cutleaf eveningprimrose, ryegrass and dock before cotton begins to emerge. “Otherwise, achieving in-crop control may be very difficult and costly at best, if not impossible.”


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