Editor's note: Agricultural scientists have said for years that it was not a matter of whether weeds would become resistant to herbicides applied to genetically modified crops; it was a matter of when. It's happening now across the Sunbelt, in various crops. Palmer amaranth appears to be the most often cited resistant weed but others are likely to develop.
The article below details findings in Georgia that could affect the way farmers across the country deal with weed management decisions.
In the fall of 2004, Stanley Culpepper was alerted to a problem field of Palmer amaranth in central Georgia. Heavy glyphosate treatments barely registered on the field's pigweed and many wondered if, after long expecting it, glyphosate-resistant pigweed had finally arrived in U.S. row-crop land.
It had. Culpepper, a Georgia Extension weed specialist, investigated the field for months with research colleagues before declaring it a “probable” case of glyphosate resistance in the summer of 2005. Following a bevy of lab tests, “probable” was updated to “definite.”
Since then, Culpepper has been studying how best to deal with the resistant pigweed. With suspected glyphosate-tolerant, or resistant, pigweeds now popping up across the South, Culpepper has a head start on research into the problem weed. He is frequently asked to speak on the impact of the resistant weed in Georgia agriculture.
In early November, Culpepper spoke with Farm Press about intriguing plot-work, keeping resistant pigweed in check and how Georgia's experience might translate to the Mid-South. Among his comments:
On current research…
“Glyphosate-resistant Palmer amaranth continues to be a major problem in Georgia.
“We had 12 acres of small-plot research evaluating 168 weed management systems and an additional 75-acre trial determining what efforts were required to completely eliminate Palmer amaranth from a cotton field.
“In the small plots, the amaranth was devastating and very discouraging for us. Most of the tools we have, despite being put out in a timely manner, weren't very effective.
“We were greatly hampered by Mother Nature. There's no doubt that to manage resistant Palmer amaranth, you'll have to rely heavily on residual herbicides. That means the farmer will be depending on rainfall and/or irrigation.
“In the area with the bulk of our resistance currently - which is where the research is on-going - there isn't a lot of irrigation. So when it doesn't rain for 40 or 50 days it causes great problems.
“Of the 168 herbicide systems in small-plot research, we were able to pick about 10. And that doesn't account for the economics. The cost for some of the programs was over $50 per acre. So it's very discouraging.
“What clearly came to the forefront is a warning for growers without resistant Palmer amaranth. They need to adopt preventative programs. Yes, those programs may cost a few dollars. But they don't double, or triple, the input cost to manage weeds - and that's the kind of cost they'll face once this resistant weed is in their fields.
“For the areas already infested with resistant Palmer amaranth, we're facing a massively complicated situation. And much of it is predicated on how timely the grower is, how willing he/she is to spend money and how cooperative Mother Nature is with activating herbicides. Having said that, we know we need timely rains but also know that if we get too much rain it may actually break down the herbicide very quickly thereby reducing the length of residual control.
“Plainly speaking, it's a nightmare. I hope this year has been a positive for us at least in one way: that everyone now realizes resistance is a looming pest and significant issue. If they don't see that, it'll put some folks out of business.
“Coming into 2006, I don't think enough producers understood the resistance threat. We had too many growers that continued to spray only Roundup. They relied only on that wonderful technology. And in our situation, if we continue relying too heavily on Roundup, the Roundup Ready technology won't last.
“Maybe I'm dreaming and hoping, but I think everyone now knows how big this monster is and how much they need to work with residuals and alternative chemistries in their Roundup Ready programs.”
So, for some (Georgia) producers, depending on circumstances - weather and other factors - there will be a need for a new prescription every year?
That's right and I can give you examples. In two of the 10 most effective, economical herbicide systems if you were six days off with a post-emergence application, it meant the difference between picking and not picking the crop.
“In another effective system, if you were off by three days it meant the difference between picking a crop and not picking a crop.
“In this year's research we were plagued by drought. Even so, mere days - not weeks - made all the difference in yields and an economically-feasible crop.
“That means growers, consultants and Extension agents will have to stay on top of the situation. There is no room to float, even for a couple of days. Really, everyone must step up to a whole other level of vigilance.
“There are growers that need to read the signs and say ‘My herbicide is beginning to give out. I'm going to need more residual and I've got to get it out before my glyphosate-resistant pigweed, which is also possibly ALS-resistant, comes up. And I've got to make sure it's activated.'
“That is what the immediate future for those with this pest holds. That's why those who don't have resistant pigweed should do everything to keep it out.”
What about the acreage affected between the time resistant pigweed was discovered to now? Is it making big jumps or staying put in one area?
“We just finished sampling for 2006 so it will likely be a couple of months before we know how much worse it is this year from last. We don't know how quickly it's spreading yet although it's been confirmed in one more county.
“The ability of this resistant pest to spread may depend heavily on pollen movement. We just harvested a pollen trial that will help us understand how far the resistance trait can move through pollen. The question isn't how far the pollen can move but rather how long the pollen is viable while on the move.
“Right now, resistant Palmer amaranth is in four counties. It's very likely it's in several more. We're not naming those suspect counties until we're sure, though.
“As far as acreage affected by this, I've been unwilling to guess that. But I can report on a test done in three resistance-confirmed counties (Macon, Dooley and Taylor) last year. Those counties have over 120,000 row-crop acres. In 104 completely random samples in those counties - and I mean random, not ‘Hey, I've got a problem in my field. Come check.' — 47 percent were resistant.
This is speculation, but do you believe this strictly an excessive glyphosate-spraying issue or was the resistance triggered by other factors?
“There are a lot of opinions on that. But one thing that is surely consistent is the sheer selection pressure we're putting on these weeds because of our dependence on glyphosate. That is a dominate factor.
“Tillage and conservation practices, along with a bunch of other things, are also brought up. But go pull the numbers on how much glyphosate is being applied now versus just 10 years ago. The amount is astronomical.
“The excellence of glyphosate has definitely contributed to the situation we're in. If we don't stop riding the glyphosate train so hard, it'll cause Georgia's problems to explode across the United States. Trust me, you don't want that”
Because Georgia was the first to face this, I know many of the chemical/research companies are working with you. Are you hearing anything about new technologies or products coming to deal with resistant pigweed?
“Absolutely not. Quote me on that. There is nothing coming for a minimum of five years. Something may be repackaged and renamed. But as far as new chemistries, forget it.”
You recently visited Arkansas. Having seen the lay of the land here, does it appear similar to Georgia before the resistance problem blew up?
“In Georgia, we can spray a 4X or 5X rate on a half-inch weed and never see it slow down. Based on what I saw in
Arkansas, the weeds may yellow up and they'll lose an occasional growing point.
“Bottom line, though, is if you can't kill it with glyphosate it's a big problem. The physiology of the plant or reasons for resistance doesn't matter to the grower. All that matters is that weed is hurting the crop.
“So, we're all in the same boat. That's the case even though we may have different physiological mechanisms or tolerance levels.”
What about methyl bromide? What's going on in Georgia with that?
“Hopefully everyone knows that bromide is being phased out. Many states were effective in writing ‘critical use exemption packages.' Those, in effect, provide permission to use bromide in specific, documented situations where we've scientifically proven we can't survive without it.
“Regardless of the exemption packages, the price of methyl bromide continues to rise. It will continue to become scarcer and will eventually go away.
“We must find a replacement. In my area, I have some growers willing to work with me on large-acreage replicated alternatives. Some of those look promising.
“But dealing with fumigants is tricky. It isn't like studying herbicides. I can't go out and put out 12 tests in three weeks. If I can get a couple of fumigant tests out every year, it's still a struggle. It takes a lot of time.
“The EPA and the (Georgia Department of Agriculture) have been great supporters on this research. We couldn't have done this without them. It's rare that someone says they appreciate the EPA. But with this, they've done a great job allowing us to try and find alternatives rather than yank it away.
But the curtain is definitely falling?
“That's right — we've got a couple of years left.
“As methyl bromide is being phased out, we're taking the opportunity to study the best alternatives and how to adopt them. This is just an estimate, but by 2009/2010 we'll likely have to be off bromide. Whether you can find it by then or not is irrelevant because it won't be affordable in Georgia. Now, in some parts of Florida and California that may not be true. But the economic viability of methyl bromide in the South is in trouble.”
What are some of the methyl bromide alternatives?
“Methyl iodide is one. Currently, this one is cost-prohibitive. But a decade ago, the cost of Roundup was also cost-prohibitive in the minds of some. So our hope is methyl bromide becomes an economical alternative in the very near future.
“Dimethyl disulfide mixed with Choropicrin is another potential option. This is an experimental compound that shows promise. In the next couple of years, hopefully we'll get it in more fields to check it more.
“A systems approach using Telone II, Chloropicrin and Vapam also has potential. Some of our growers are already using this one since it's already labeled. So far, it looks very good. The application procedure, as with many things, makes or breaks the system. It isn't as forgiving as methyl bromide but it is as economical. Application of this approach is much more challenging to deal with than methyl bromide.”
“Here's another thing that cotton growers need to keep in mind: Dec. 13 is the cut off date for comment on MSMA to the EPA.
“MSMA is in trouble and if we don't do something it won't be around next year. When you look at resistant weeds in cotton, MSMA mixtures at lay-by are an alternative to glyphosate so it's an option we need. Without it, we're tied to using more and more glyphosate. We certainly don't need to go in that direction.”