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Researcher certain more weeds will be glyphosate-resistant

In the next few years, it is a certainty that weed herbicide resistance in the United States will be far more widespread and a far greater problem than anything seen in the past.

Why is that so? “Because of the fantastic technology of Roundup Ready crops,” said Steve Powles, director of the Western Australian Herbicide Resistance Initiative and leading resistance expert.

Glyphosate-tolerant crops have been a technological advance enthusiastically adopted by U.S. growers. In fact, the adoption of the technology in the United States is the “fastest and most complete of any agricultural innovation. In 1996, there wasn't one acre of Roundup Ready soybeans and then quickly reached in excess of 80 percent — probably closer to 90 percent — of all the soybeans grown in the country.

“Similarly, most of the cotton — roughly 90 percent — grown is also Roundup Ready. Probably around 50 percent of the corn to be grown this year will be as well.”

What that means is nearly every crop field in the U.S. corn, bean, and cotton belts are being treated with the same herbicide. And glyphosate is “simply the world's greatest herbicide. Everyone in the United States is in love with Roundup Ready crops and glyphosate. It makes plenty of sense for everyone. It's cheaper, simpler, and faster than competing technologies and that's why it's been so widely adopted.”

But such widespread acceptance has consequences. A country can't have nearly every crop field intensively treated with glyphosate and not have biological repercussions, said Powles. And the chief repercussion is development of resistant weeds.

View from Down Under

Watching the United States situation develop from Australia, “I could see weeds would develop resistance,” said Powles. “In fact, I had already published (papers on) glyphosate resistance developing with ryegrass in Australia, the first instance in the world of such weed resistance. But even then, in the mid-1990s, I knew glyphosate resistance might be a big deal in Australia but it would be a far bigger deal in the United States.”

So Powles began warning about the “dramatic over-reliance” on glyphosate in the United States. “Of course, (resistance) is now under way. What's clearly happening is the first weed to develop resistance here — horseweed or marestail — is now occupying somewhere around 3 million acres of U.S. cropland and in densities that require another herbicide treatment.”

Marestail seeds can float on the wind for long distances. If there are 3 million acres of it this year, “next year there will be many more. And pretty soon, the entire country will be covered with glyphosate-resistant marestail.

For the last two years, Powles has been invited by Syngenta to speak with U.S. producers about resistance. “(In 2005), I had the privilege of spending weeks traveling across the country meeting and talking to hundreds, if not thousands, of farmers, consultants, those in the chemical industry, a cross-section of U.S. agriculture. I formed the view that the evolution of glyphosate resistance would be coming on big-time. Everyone is so attached to Roundup Ready technology. There are so many commercial imperatives making people use it that resistance is inevitable. It's just a question of how big a problem it will be.”

From the South

On the heels of 2005, when several resistant weeds were discovered, Chris Main believes more discoveries will occur in 2006.

“I want to focus on some of the impacts this problem will bring to producers in the South,” said the South Carolina (Clemson) Extension weed management specialist. “First of all, from a technology/herbicide standpoint, glyphosate is much too important in Southern agriculture's current economic and regulatory climate to lose its utility.

“What I've been trying to promote is for producers to make adjustments that will preserve glyphosate-tolerant technology because, in the near term, there's not an adequate replacement. That goes along with focusing on prevention of weed resistance rather than reacting to a crisis, as we've historically done. That's a hard sell to farmers for many reasons. It would be a shift in (their collective) mindset.”

Focusing on cotton in the South, with glyphosate-tolerant technology, the savings realized from fewer herbicide applications, fewer trips across the field and conservation tillage, are well over $250 million annually. That breaks down to over $60 million savings just in tillage savings including the slowdown of the depreciation of equipment and fuel costs.

At the same time, “we've seen a tremendous reduction of active ingredients applied to the ground,” said Main.

“For cotton, there's been a reduction of about 14 million pounds in herbicide applications due to the glyphosate-tolerant technology.”

Even more astounding, since the introduction of glyphosate-tolerant cotton in 1997, there's been a 371 percent increase in the acreage of conservation tillage, said Main. “That's very important for environmental concerns. With the government going to more payments based on ‘green credits’ and con-till, if we lose this technology we'll lose that basis for farmers to realize an economic advantage.”

Reasons why not

As Main travels, “I always hear why people don't want to manage for resistance. The number one reason they cite is, ‘I'm already charged a tech fee. If you ask me to add another herbicide — or multiple herbicides — to the mix, it'll increase my costs for something I've already paid for in the seed.’

“The (second reason) is, ‘It's just inconvenient.’ A lot of farmers I deal with won't buy a generic glyphosate that doesn't contain surfactant. They say it's too inconvenient to pour two jugs of chemicals into a spray tank. They like the idea of having one tank for all crops.”

Main's next concern is the use of soil-applied products — whether making a pass over the field with a pre-emerge or putting out a preplant incorporated with a disk. “(Producers) see that as taking away from their time and costing more to make that extra trip.

“Finally, one of the biggest concerns we have — particularly in cotton production — is getting farmers to implement some type of crop rotation. That helps not only with weed management but with diseases, nematodes and soil properties in general.”

Integrated practices

Main is currently keying on implementing integrated weed management practices, “kind of like IPM (integrated pest management) principles. We need to utilize all control methods we have in an economically sustainable manner.

“With the advent of Roundup Ready crops, weed scientists got away from the mindset we've had throughout our history. We (once looked at) tillage, herbicides, cultural practices, and the time of year crops were planted.

“We completely abandoned those and said weed control was instead based on the ability to use glyphosate — kill all the weeds and… focus on having a field with no weeds present when you drove by and had a look.”

Main said economic control levels need to be looked at again, particularly in cotton. A lot of times, producers allow weeds to canopy over the cotton crop prior to the first glyphosate application. “They want as much activity out of that first spraying as possible. However, that approach reduces the crop's yield potential tremendously from the very beginning.

“Something else I see is farmers going out late and spraying very large weeds over-the-top in rescue situations. They clean up the field and make it look better — what I call a ‘revenge killing.’ They aren't realizing any economic benefit from that spraying because the weeds have already competed. By removing them at that late date, no yield is being added because it's already been set.”

Regarding glyphosate-resistant Palmer amaranth (pigweed), “in South Carolina, where I'm from, we're not concerned there was (any) Palmer with resistance in 2005. I couldn't find any fields with resistance. But I predict we'll find it in 2006 and all along the Cotton Belt — especially in the Southeast Coastal Plain — it'll be found in high abundance.”

Mains' PhD dissertation work was on resistant marestail in Tennessee. There, in order to gain adequate control of the weed, many producers have abandoned low-till practices that had become prevalent.

“In west Tennessee, (the increased tillage) is very problematic with their soil (composition). Fifteen tons to 17 tons of soils can be lost annually to water erosion. From a conservation standpoint, controlling weeds with tillage isn't the best option there.”

Recently, Main has pressed several general principles on how to stave off glyphosate resistance. Among them:

  • Start clean

    Whether using reduced-, no- or conventional-till, make sure there are no weeds present when the crop goes in. “If that takes chemical methods such as adding products to glyphosate or using mechanical methods, you need to start clean. That could mean multiple passes with equipment or multiple herbicide chemistry.”

  • Stay clean

    “I stress the use of pre-emergence herbicides. The banner I carry is ‘anything you use as a pre-emerge is better than using nothing.’ I don't make a strong recommendation for one product over another. Just use something other than glyphosate to buy time.”

  • Post-emergence herbicides

    If you have to use a post-emergence herbicide to stay clean, use timely applications based on weed science, not the crop's development. “Too often I see weeds in soybean and cotton fields covering the crop up before an application is made. That isn't what's needed because there will be weeds that are off-label at that point.”

Another concern in the South is weed shift. To combat such shifts, “we ask producers to diversify their weed management practices,” said Main. “We do that by not relying on a single herbicide like glyphosate for all control needs. We want to rotate our crops and the herbicides put into a field to control particular weeds. Something as simple as adding additional products to a glyphosate tank can provide resistance management benefits.

“We also need to look at an integrated weed management program that includes not only herbicides but cultural practices such as tillage and cultivation. We must also look at the biological area, how and when weeds emerge and compete and when they're actually economically damaging the crop.”

With the threat of glyphosate-resistant pigweed, it is also important producers sanitize equipment before moving to another area, said Main. “We don't want to carry seed from a known infected area to another part of the county or farm where there isn't a problem.”

Most importantly, producers need to monitor fields for escapes. If any are found, producers should immediately get in touch with Extension agents, crop advisors or other professionals.

“An area of great frustration for me is most farmers are farming for 2006,” said Mains. “They're hoping to produce enough, to be profitable enough, to farm again in 2007. To effectively manage herbicide resistance, they need to plan for the next three to five years. In that time span, there's nothing new in the pipeline from a technological or chemical perspective to solve glyphosate resistance in certain weeds.

“We're taught to be good stewards of the land, to pass on the resources to the next generation. This is very true of herbicide-tolerant seed traits. We must manage now so that trait has value 10 or 20 years in the future.”

Not an easy sell

Both men acknowledge the difficulty in convincing farmers — already facing financial jeopardy and skyrocketing input costs — to use less glyphosate.

“It will be difficult to persuade many farmers, perhaps not all,” said Powles. “I feel the real action will be around solutions for this problem because it's coming big-time. That's the honest answer.

“At the same time… we should do everything we can to alert growers to the situation because they'll be worse off when glyphosate no longer works. There's a longer-term perspective. We need to be planning for post-glyphosate days. But at the same time, we should try to (push) that as far into the future as possible.”

Having expressed pessimism that glyphosate resistance can be kept from developing in pigweed, what is Mains' take on Roundup Ready Flex cotton which allows producers to spray glyphosate a week prior to picking?

“It's of great concern,” said Main. “A lot of the research across the Cotton Belt shows three timely applications of glyphosate post-emergence in Roundup Ready Flex cotton gives complete weed control in a non-resistance world.”

Instead of providing Mains comfort, “that scares me to death. In the Southeast, with this Palmer amaranth problem, I'm afraid producers will adopt that model — make three over-the-top applications — and not be able to control Palmer amaranth.”


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