6 months ago from DTN AgDay
Glyphosate now rolls off some of plants like water
The waterhemp seedlings germinating in the University of Missouri greenhouse don't carry a kryptonite gene.
They just act as if they do. Glyphosate, once the world's most popular weed killer, now rolls off some of these plants like water.
Kevin Bradley, a University of Missouri weed specialist, and his graduate students pulled 200 waterhemp samples from weed populations growing in Missouri soybean fields in 2012. The scientists grew the progeny out in the greenhouse during the spring and summer of 2013.
"We wanted to see if the multiple-herbicide resistance in waterhemp being found in other states was true for Missouri," Bradley said. Five different classes of herbicides proved problematic. Some individual weeds within these populations were able to resist as many as three or four herbicides at one time. There was one waterhemp population that showed resistance to all five classes of herbicides.
The dwindling number of effective herbicides has scientists sounding an alarm across the industry. Although several new trait technologies await regulatory approvals, they are based on existing chemistries, such as 2,4-D, dicamba and HPPD inhibitors, that already have a few chinks in the armor.
Ford Baldwin, an independent weed scientist based in Arkansas, knows farmers want to believe another answer to weed resistance is coming in a jug. "Complacency is killing us," Baldwin said. "We've been managing weed resistance for nearly 50 years. But we've never lost a herbicide like Roundup at a time when the last new mode of action discovered is more than 30 years old." HPPD inhibitors were the last big discovery in 1982.
An average of 11 cases of weed resistance emerge around the world each year, explained Ian Heap, an Oregon-based weed scientist who tracks the statistics through the International Survey of Herbicide Resistant Weeds (www.weedscience.org).
As of January 2014, there were 416 cases (weed species multiplied by site of action) of herbicide-resistant weeds globally among 222 weed species (129 dicots and 93 monocots).
Weeds have evolved resistance to 21 of the 25 known herbicide sites of action and to 150 different herbicides. Herbicide-resistant weeds have been reported in 74 crops in 63 countries.
The Palmer-amaranth-ravaged South often gets the resistance spotlight, but Michigan and California have the actual bragging rights with 24 different resistant weed species.
Iowa State University weed specialist Mike Owen said denial is just as prevalent as weeds in his state, although waterhemp with resistance to five sites of action has been confirmed in several weed populations, and the state has 17 different species of resistant weeds. "There are approximately 8 million acres of Iowa that still just get glyphosate. It's truly a potential tragedy of the commons if it continues.
"Somehow, we are not affecting change," Owen said. "Convenience, simplicity and out-front cost are clearly problems -- even though we can demonstrate return on investment favors using best-management practices. Everyone is talking about diversity, but it's not happening, and I fear if it doesn't happen soon, we will not be able to get out from underneath it," he stressed.
IN THE LAB
Bradley's studies showed 58% of the waterhemp samples could tolerate glyphosate. "We found much higher triazine resistance than expected -- about 30% of the populations screened were resistant," he said. Ninety-nine percent of the waterhemp sampled was resistant to ALS herbicides, such as Classic and Pursuit.
"We did the study because farmers were calling to say they couldn't control waterhemp with PPO inhibitors, such as Cobra and Flexstar," Bradley said. "But only 10% of the populations tested resistant.
"The good news to me was that we can still use those herbicides for waterhemp control if we use them appropriately," he continued. "Everyone just thinks the waterhemp are resistant to these PPO inhibitors because they don't get control, but in reality, what is really happening is that they are spraying weeds that are too big. There's no way those herbicides are going to kill waterhemp if it has any size to it."
Perhaps the biggest surprise in Bradley's survey was that 2% of the samples were resistant to HPPD inhibitors such as Balance, Callisto and Command. Previous cases of resistance in these bleacher-type herbicides have been found only in corn and almost exclusively related to seed production.
IN THE MODE
"It wasn't that long ago that we could look at resistance and line up the weed, and give farmers an (herbicide) option," said Jerry Green, who recently left DuPont Crop Protection to start Green Ways Consulting.
"Today, we can't do that -- it has to be part of a system and a program. There are no silver-bullet herbicide offerings. There are just too many herbicide-resistant weeds out there," Green said.
While discovery tools and science continue to improve, there remains a disturbing drought of novel new chemistry. Hermann Stuebler, Bayer CropScience head of weed control, noted that in 1990, the industry filed 250 active-ingredient (ai) patents and introduced approximately 80 new herbicide compounds in that decade. Since 2000, 50 to 75 new ai patents have been filed yearly, and about 20 new compounds came to market from 2000 to 2010.
Monsanto recently moved a new concept into its pipeline called BioDirect weed control. One part of the BioDirect platform uses RNAi (ribonucleic acid interference) to silence the genetic mechanism the weed uses to resist the herbicide. The first product under development is a spray farmers could use with herbicides to make Palmer amaranth and waterhemp as sensitive to glyphosate as it was before it developed resistance.
"We have our own research and development efforts around RNAi," said Markus Heldt, president of BASF Crop Protection. "It is very promising technology, but we are really talking next decade, and we also need to better understand the regulatory profile."
BACK TO SCHOOL
Meanwhile, many older farm chemicals are making a comeback. "We are very fortunate that in the heyday of glyphosate in our major crops, when companies were barely selling anything, they kept those registrations active," Green said.
The next thing for growers is to go back to basics, Bradley said. "We could solve many of our weed problems if we understand the biology of the weed present. What is the weakness of the weed, and how can we exploit it?"
For example, giant ragweed germinates early in Missouri. "If we get those killed early with a burndown, we don't have problems. Try to get by with Roundup or maybe a light rate of Roundup and 2,4-D, and you're going to get burned," he pointed out.
Once a weed evolves resistance, that tool is lost, said Jason Norsworthy, University of Arkansas weed scientist. "We have ALS-resistant weeds in fields that haven't seen that chemistry in 15 years.
"If you lose a technology, it puts a tremendous amount of pressure on the next technology to come," he said.