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Herbicide resistance may change future of row crop farming in Southeast

In 2004, when a call came in to Macon County Georgia Extension Coordinator Jeremy Kichler, it seemed routine enough — a farmer had problems controlling pigweed in his cotton. What that call has led to is a firestorm of questions about the extent of Palmer amaranth pigweed resistance to glyphosate in the Southeast and what to do about it.

Kichler didn’t have the answer to his grower’s pigweed problems, so like a good Extension agent should do, he sought help in solving the problem. He called Steve “Weedy” Brown, an Extension weed scientist at the University of Georgia’s Tifton campus. Brown didn’t have the full answer either, but knowing the grower’s reputation as a top producer, he also recognized something was different about these pigweeds.

Brown enlisted the help of Stanley Culpepper, an Extension weed scientist and protégé of Alan York, one of the top authorities worldwide in weed control in cotton. Culpepper took the suspect pigweed back to the lab, planted seed from the female plants, grew the plants in the greenhouse, zapped them with high rates of glyphosate, and unfortunately they survived these greenhouse treatments. In the spring of 2005, he went to the same field where the original call came to Kichler to conduct numerous studies in an effort to confirm his greenhouse results.

The 2005 results were more shocking than he expected, there was a high level of resistance in the field.

Though weed scientists throughout the Southeast have put together herbicide resistance management programs, little progress has been made at the grower level in dealing with either existing or potential problems.

Jennifer Ralston, Monsanto Roundup technical manager, makes the following recommendations for growers in areas with glyphosate-resistant Palmer amaranth.

“Start clean with a burndown herbicide program or tillage, then apply a pre-emergence residual herbicide labeled for control of Palmer amaranth such as pendimethalin and fluometuron (Cotoran). For the in-crop application tank-mix Roundup WeatherMax at 22 ounces per acre with metolachlor before Palmer amaranth emerges. Use Roundup WeatherMax in-crop as needed at a minimum rate of 22 ounces per acre to control other weeds,” Ralston explains.

“To prevent additional flushes of Palmer amaranth, make a post-directed application of Roundup WeatherMax tank-mixed with a residual such as diuron (Direx) or flumioxazin (Valor).”

In other areas where farmers do not face glyphosate resistant weeds, Ralston says Monsanto recommends cotton growers planting Roundup Ready and Roundup Ready Flex varieties apply a residual herbicide according to recommended label rates and timing as part of the weed control program. The residual herbicide can be applied pre-emergence or early post. “This recommendation reduces the risk of glyphosate resistant weeds developing and enhances early season weed control,” Ralston says.

Farmers can get more information on managing glyphosate-resistant weeds in Roundup Ready and Roundup Ready crops at

Being the tip of the arrow in glyphosate resistance has been both exhilarating and exasperating for Culpepper. He and his team often spend seven days a week, planting, spraying, counting, and measuring cotton and weeds in their more than 12-acres of small plots located near Garden Valley in which resistant Palmer amaranth was first found in 2004. His intense research program is looking at a wide range of problems that widespread resistance to glyphosate and other popular herbicides would cause for farmers in the Southeast.

The son of a North Carolina farmer, Culpepper fully understands the need for an applied, practical approach to understanding the scope of the problem and for developing the most economically feasible solution.

Perhaps the most telling of Culpepper’s test plots are end-to-end plots. In one plot, glyphosate was sprayed on resistant pigweed three times — at an unbelievable 88 ounces per acre. The results are shocking, or better said, the lack of results are shocking. The Palmer pigweed that received 264 ounces of glyphosate is alive and healthy. The drought-plagued cotton plants are dwarfed by the glyphosate resistant weeds.

In the adjacent plot, a Cadillac treatment of Prowl plus Reflex, applied pre-emergence, followed by glyphosate plus Dual Magnum or glyphosate plus Staple, followed by diuron plus MSMA or diuron plus glyphosate produced a nearly perfect plot of cotton — at least from a weed standpoint. The catch is that the herbicides were carefully timed and placed and in the real world would cost farmers in the neighborhood of $35 per acre — not counting the $10-12 per acre technology fee.

In each of the different test plots weed and grass pressure was minimal, except for Palmer pigweed, which dwarfed the drought-stressed cotton plants in all but the Cadillac treatments.

Pigweed in general is very competitive, and Palmer amaranth, or Palmer pigweed in particular is highly competitive. On a level playing field pigweed will outgrow cotton. Give pigweed a little advantage, and it will be a one-sided game.

Growers who try to control resistant pigweed with more and more glyphosate are actually making the problem much worse. They are taking out all the natural weed and grass competition, but more importantly they are taking out all the pigweed that remain sensitive to glyphosate. The result is a perfect environment for resistant pigweed to grow and reproduce.

In a drought situation, such as conditions that still plague most of the lower Southeast, Palmer pigweed are much more efficient than cotton, or most any other plant outside a cactus. Even with irrigation, growers have not been able to provide adequate moisture to offset long days with temperatures staying above 85 degrees and topping 95 during the day, with 110-112 degree heat indexes. In these conditions many herbicides break down over time, but Palmer pigweed keeps on trucking.

Even with the Cadillac treatment, growers with herbicide resistant pigweed are way out on the edge, according to Culpepper. “If for any one of a thousand reasons, like drought, the soil-applied residual herbicides don’t work, you are in trouble,” the Georgia Extension specialist says matter of factly.

To control resistant pigweed that emerge through the soil-applied herbicides, timing is key, he says. In irrigated cotton, or in fields with good rainfall, Palmer pigweed can grow from 2 inches to five inches in three days or less. That three days can be the difference in good control or no control with alternative postemergence herbicides such as Staple on non-ALS resistant pigweed or with Ignite on pigweed in Liberty Link cotton.

“Most people just don’t understand how big a problem this is, but if they get in a situation like this 12 acre test field, and have to spend hundreds of dollars an acre hoeing pigweed or mowing it down, they will be faced with some tough questions,” Culpepper contends.

“All this is brand new — we didn’t really know for sure that we had a problem until 2005, and we are still in the early stages of sorting all this out. This has been a near record drought year in the Southeast, and our final results may be somewhat different. Despite the growing conditions and the effects of irrigation, the best treatment for cotton in fields where glyphosate resistance is a problem may end up being at least $30 per acre, but it will never be $15-16 per acre again — those days are gone,” he concludes.

Resistance to glyphosate doesn’t stop at the cotton field, it has wide-ranging effects on other crops and other production practices. A great fear shared by researchers and Extension workers throughout the Southeast is that growers will break their dependence on glyphosate by over-using other chemicals.

In areas that are dominated by peanuts and cotton, the overuse of ALS chemicals is already creating some concern that pigweed and other weed pests may be developing some resistance to Cadre, StrongArm and other popular herbicides that come from the same family of chemistry.

Kichler, himself the son of a farmer in Baldwin County, Ala., fully understands the implications of losing both glyphosate and ALS chemistry in Macon County, Ga. “Macon County is a diverse agricultural county, with dairies, peaches, vegetables and row crops. However, losing our two best systems for weed control would be tough for farmers,” he concludes.

On a broader scale, North Carolina State University Professor and Weed Scientist Alan York says,

“I have worked with cotton at NCSU for 28 years and nothing I have seen to date compares with the seriousness of glyhosate-resistant Palmer pigweed. I think it is every bit as serious as the boll weevil, and probably worse.”

York says, “there is more glyphosate resistance to Palmer amaranth than previously thought at this time last year, and I expect it to spread like wildfire unless growers get a lot more serious about it than I anticipate. It will cause a lot of growers to change their management systems, and it will put some out of the cotton business. It will increase input costs and management requirements for those that hang in there,” York adds.

North Carolina scientists first picked up resistance in the summer of 2005, but most growers don't realize they have a problem until a fairly heavy infestation has built up. “We think it was present two, and probably three, years before we detected it in 2005,” York says.

“We already have glyphosate-resistant horseweed in North Carolina, which was first detected in 2003, and it seems to have spread quite a bit across the eastern part of the State. With seed being dispersed by wind, we expect it to continue to spread quickly. To date, we have not detected any in the western half of the state. So far, I have encountered only a few fields with a "disaster" situation. However, it is easy to find scattered spots across a lot of fields. We expect it to get worse,” York says.

While glyphosate-resistant horseweed is, or will become, a serious problem, it is a Sunday school picnic compared to glyhosate-resistant Palmer amaranth. We can handle the horseweed; growers just have to be willing to do so. We also are looking into a couple other weeds also. One is definitely resistant to glyphosate, but we are unsure of the second species at this time,” York concludes.

In 2005 North Carolina researchers collected Palmer amaranth seed from about 280 fields from the Virginia border to the South Carolina border in the fall of 2005. They are screening those seed in the greenhouse for resistance and are almost done with the first run. It appears that most of the problem is in the south, central part of the state (Hoke, Robeson, Scotland, Cumberland, Duplin, Wayne, Lenoir, and Johnston counties). However, there is one population in the northeastern part of the state that York says is highly suspect. And there are probably others that have not been brought to the attention of North Carolina State Extension and research personnel, he says.

The resistant Palmer pigweed is in both cotton and soybeans. In corn atrazine and other herbicides apparently hold it in check. “I was riding through the areas mentioned above the second week in July, and the problem appears to be rampant. It is easy to see where glyphosate has been used, and it is easy to see from a few to thousands of survivors in field after field. Though just a wild guess, I expect we have 100,000 acres with some level of infestation,” York says.

“I don't think we will "lose" glyphosate. I anticipate growers continuing to use Roundup Ready technology. The big impact will be that it will no longer be a stand alone treatment. Growers will have to integrate other chemistry into their management systems, which means more money spent. The days of glyphosate-only systems are over,” he says.

“We tried hard in all our various meetings last winter to impress upon growers just how serious this problem is. I think we raised the level of awareness and did cause a few growers to make a few changes in their practices. But, from what I am seeing this summer, growers must take this problem much more seriously and implement resistance management strategies now if we have any chance of getting a handle on it. Some are moving in that direction; most are not. Those that are not will be the ones calling me in a year or two and demanding that I do something,” the North Carolina weed scientist says.

Cultivation can be an important component of a resistance management strategy. However, about 50 percent of North Carolina cotton is no-till or strip-till. And, among growers planting conventionally, almost none are cultivating. The reason is fuel, equipment, and labor costs. It is going to be very hard to convince growers to start cultivating cotton even though that would likely help with the Palmer situation, according to York.

What growers have left to work with is basically use of multiple modes of action (i.e, other herbicides in place of or in addition to glyphosate). In conservation-tillage systems, that means something other than just glyphosate for burn down. It means pre-emergence herbicides. It means tank-mixes with glyphosate over-the-top. And, it means tank-mixes with glyphosate or non-glyphosate programs for layby, he adds.

Crop rotation will become more important than ever. There are good options to control Palmer in corn. Unfortunately, Palmer is commonly found on land that is better suited for pine trees than for corn. But, there are some good options in soybeans. The big issue here is that management (i.e., timeliness) becomes critical if growers control Palmer with non-glyphosate programs in soybeans. Many growers have become so spoiled by Roundup Ready soybeans that they will have to re-learn the importance of timely application.

Among the questions researchers are trying to document related to glyphosate resistant pigweed is how many weeds per foot of row cause problems with quality and harvesting of cotton. Even if fully grown Palmer pigweed are dessicated by using defoliants, one 2-3 inch diameter stalk could knock the head off a picker. Likewise, one healthy, green Palmer pigweed every few feet of row could cause significant staining and subsequent quality loss.

Another critical concern is the impact on no-till and other conservation practices. Many farmers are enrolled in long-term programs and going back to conventional-tillage to control pigweed would be costly at best, and in many cases, impossible for farmers who lack the equipment and labor to go back to conventional methods.

Culpepper, who grew up on a conservation-tillage farm in North Carolina and remains a staunch supporter of conservation-till cover crops and other soil conservation practices, says he first thought losing glyphosate would mean the end to conservation farming in cotton.

However, his 2006 tests at the Garden Valley sight offer good hope for coping with the problem in conservation-tillage systems.

The key, he says, is to get enough cover to suppress weeds without preventing pre-emergence or at planting herbicides from getting to the ground. In test plots at the Garden Valley sight, the same Cadillac treatment that worked in conventional-tillage has so far kept test plots clean in no-till plots planted into wheat stubble.

He points out that the area of the 12-acre test sight where no-till cotton was planted does not have as high percentage (40-45 percent) of resistant pigweed as the field with conventional-tillage systems, which is over 75 percent glyphosate resistant weeds. No-till farming without glyphosate is possible, but it greatly increases the risk, he points out.

As is the case with so many of the parameters being tested in Georgia, more will be known after the 2006 crop is harvested and analyzed, but suffice to say Culpepper, Kichler, York and others working with glyphosate resistant pigweed are certain it will be a big problem in the future.

“I knew it was bad, but I didn’t think it would be this bad,” Culpepper laments. Right now we only know for sure that we have glyphosate resistance in three counties, but we strongly suspect we have it in fields hours of driving time away from the Garden Valley fields, he says.

Kichler adds that resistance had likely been in the Garden Valley area for some time prior to its discovery in 2004, but had not been diagnosed. In any area, where large acreages of cotton are grown and in which there is high, if not total, dependence on glyphosate for weed control, he says there is a good chance growers have some degree of resistance.

Culpepper says that up to 30-35 percent resistance in spots throughout a field likely goes unnoticed by most farmers. They know there is a problem, but there are thousands of other reasons unrelated to resistance that allow pigweed to escape control.

York has called Palmer amaranth the biggest, baddest plant in the field. Female Palmer pigweed can produce 500,000 seed — resistant or not resistant. Researchers are not sure exactly how resistance spreads. If it spreads solely by seed dispersion, spread of the problem may be relatively slow. If it spreads on pollen, on the other hand, it could spread miles at a time.

Montezuma, Ga., grower Johnny Sanders grows 3,000 acres of cotton and 1,000 acres of peanuts scant miles from Culpepper’s Garden Valley test site. How resistance spreads is a critical issue for him.

Recognized as one of the top growers in the area, Sanders never bought into the idea of total use of glyphosate. He has regularly used pre-emergence herbicides and non-glyphosate directed sprays in cotton and alternates use of ALS herbicides on peanuts. He is as confident as he can be, being only a few miles from documented cases of resistance, that he doesn’t have glyphosate resistant pigweed on his farm.

Despite doing everything right, if researchers determine that resistance can be spread via pollen, he could inherit the problem from not-so-nearby farmers who haven’t been so diligent in breaking up the glyphosate monoculture that is so common throughout the Southeast.

“We can’t just ‘not’ grow peanuts or cotton because of pigweed resistance. We have too much money invested in these crops and too much time invested in learning how to grow them effectively. We have tried canola and other specialty crops and the more popular row crops, but cotton and peanuts is the best fit for our soil and our operation,” he says.

Fortunately, farmers do have a number of good alternative tools to manage pigweeds, glyphosate resistant or not. Though glyphosate has dominated the herbicide market, most companies have maintained label clearance for pre-Roundup Ready products. These products will become the primary tools for growers who either want to avoid resistance problems or who want to manage fields in which resistant pigweed are known to occur.

Chuck Foresman, herbicide resistance manager for Syngenta, says his company has followed a strategy of continuing to invest in development of their line of cotton herbicides, based on resistance research from around the world. These fields in which glyphosate resistance has been documented are just a mess — something that every cotton farmer should see. It’s sad to see, but it makes me glad our company and other herbicide-manufacturing companies maintained the herbicide products that were largely displaced by Roundup Ready technology, he adds

It is encouraging to see for myself that cotton can be grown in fields with glyphosate resistance. And, hopefully farmers will heed the warning of Culpepper, York and others involved in finding solutions to this problem, Foresman concludes.

Paraquat, sold under many trade names, including Gramoxone, is an old herbicide that may be a key player in growing glyphosate resistant cotton in no-till systems. Using paraquat, to burn down winter cover crops will provide some break in the use of glyphosates.

Liberty Link cotton is another option, though current varieties have not been as competitive yield-wise with Roundup Ready varieties. The chemistry is different from glyphosate, and if new, higher yielding varieties become available, this would be an obvious option for cotton growers.

Though the original problem with glyphosate resistance was documented in cotton, it is likewise a problem in soybean production systems that are heavily dependent on the use of these herbicides for weed and grass control.

Considering a vast majority of cotton, some experts contend in excess of 95 percent, grown in the Southeast being Roundup Ready varieties, the potential scope of the resistance problem is enormous — just for cotton. Add in 2.5 million acres, or so, of soybeans, albeit with a lower percentage being Roundup Ready varieties, and there is a recipe for disaster for Southern row crops.

The reality is that Roundup Ready technology probably kept a lot of farmers in business. It lowered overall weed management costs by billions of dollars, and eased the fear of some environmentalists by dramatically reducing the total volume of herbicide used in the Southeast. It is one of a handful of technical developments in the past half century that has helped U.S. farmers become the most productive in the World.

Despite its many advantages, the overuse of glyphosate on Roundup Ready cotton and soybeans will likely take some farmers out of business. Unless the warning of scientists like York and Culpepper are heeded, the loss of control of one weed could have a devastating effect on Southern row crop culture.


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