When University of Georgia Weed Scientist Stanley Culpepper, and his mentor North Carolina State Weed Scientist Alan York, first documented glyphosate resistant pigweed in cotton and soybeans, one of their biggest fears was multiple herbicide resistance in the same weed.
If not in a field in the Southeast, it’s coming. In southern Illinois, a scant 800 miles from the heart of North Carolina’s Cotton Belt, quad-resistant water hemp has been documented.
Water hemp is a first cousin to Palmer amaranth and among several pigweed species worldwide that has shown the capability of developing resistance to multiple herbicides, if crop use and herbicide use patterns are optimum.
In southern Illinois, pigweeds are resistant to ALS inhibitors, glyphosate, PPO inhibitors and triazines. Four distinct modes of action and water hemp is resistant to all four in 23 counties in southern Illinois.
“In Missouri we only have ‘triple-stacked’ weeds, quipped University of Missouri Weed Scientist Kevin Bradley.
Speaking at a recent Syngenta-sponsored Media Summit, Bradley says the occurrence of resistance to glyphosate, PPO-inhibitor, and ALS-inhibitor herbicides in one weed is rare throughout the Midwest. However, the implications are dire for farmers, if this level of resistance spreads rapidly through soybean-producing areas of the country.
In the Southeast multiple-resistant pigweed would up the ante considerably, because growers depend heavily on PPO inhibitors for weed control in peanuts and other crops considered to be niche crops in which glyphosate is not labeled for use.
The use of atrazine-containing herbicides in corn for residual weed and grass control and to break up glyphosate resistance is common. Losing both PPO-inhibiting and atrazine-containing herbicides would be tough for Southeastern farmers to overcome.
In short, if grain farmers in the Southeast develop the kind of resistance that is beginning to occur in the Midwest, there would be serious concerns as to what to do next.
How closely related are water hemp and Palmer amaranth? “Unless you know what you are looking for and look at the two weeds on a regular basis, they look just alike,” says Bradley.
From a technical standpoint, water hemp is Amaranthus rudis from the pigweed family (Amaranthaceae). It is highly variable in size, ranging from 2-8 feet tall. Larger plants branch frequently and are broader at the base than at the top. The hairless stems are either round or ridged, and green or pinkish red. The hairless alternate leaves are up to 6 inches long and 1.5 inches across.
Sound like a Palmer pigweed? Technically a Palmer Amaranth (Amaranthus palmeri) is a dicot weed in the Amaranthaceae family — just like water hemp. Farmers don’t need a technical description to know it grows five to six feet tall, significantly reduces crop yields and absolutely makes harvesting grain crops or picking cotton miserable.
For farmers who think multiple herbicide resistance is a Midwest problem consider use patterns and pollen.
The reason Midwest grain farmers are facing such dire options with water hemp is largely because they farm large acreages, have abandoned tillage on a wide scale, spray post-emergence with glyphosate year after year, and in recent years have added consistent postemergence-applied applications of PPO-inhibiting herbicides to the mix to control resistant pigweed and weeds not controlled by glyphosate.
Conservatively, a million acres of cropland in North and South Carolina, Virginia, Georgia and Tennessee are already infested with glyphosate and/or ALS resistant pigweed. To the west and north another million acres are infested in Kentucky, Alabama, and southern Illinois and Indiana.
To say growers in the Southeast don’t need more resistant pigweed is a classic understatement. Whether triple and quad-resistant pigweeds are already here is open to debate.
Add to the current population of glyphosate resistant pigweed the capability of spreading that resistance via pollen. A grain of resistant pigweed pollen is roughly 1/100 the size of a similar grain of corn pollen. On even a mildly windy day in the growing season this lighter pollen can easily be spread 20-30 miles. In a growing season, it could conceivably move hundreds of miles.
Despite the mobile capabilities of resistant pigweed, by far the bigger threat comes from the natural selection process that takes place in every field planted to a crop.
In any field a weed with a natural mutation can occur. If this mutation gives a weed resistance to a particular chemical reaction, or mode of action, and it’s sprayed with a herbicide containing that particular mode of action — nothing happens to that particular weed. All the other weeds with no mutation are killed, leaving the mutated weed to thrive and reproduce.
On the other hand, if the weed with a mutation that protects it from one chemical reaction is sprayed with a different chemical, it’s vulnerable and dies, taking its mutation with it. Hence the need to spray with multiple modes of action to prevent the mutated weed from having an opportunity to produce seed carrying the same mutation.
As Bradley says, in Missouri, farmers with triple-resistant water hemp in their fields have no post-applied herbicide options. Once those resistant weeds come up, the only options are cultivation or abandonment.
The best option is a carefully planned and precisely timed pre-plant herbicide combination that gives the grower the best option for controlling a broad spectrum of weeds, including pigweed, before these weeds have a chance to grow and spread.
Chuck Foresman, who is manager of weed resistant strategies for Syngenta and an Illinois grain farmer, says he has come full circle on the best strategy for managing glyphosate resistant weeds.
“I used to think multiple modes of action applied post emergence were the best way to go. Now, I am sure that’s the wrong strategy,” Foresman says.
Using a predictive model to determine the odds for developing glyphosate resistance, Foresman says five years out the chances are comparable — slightly higher for a post-emergence, residual herbicide plus glyphosate versus a pre-emergence, residual herbicide plus glyphosate.
Ten years out in the model, the odds of resistance occurring are double using the post-applied herbicide plus glyphosate. Taking the model out 20 years shows the odds tripling.
“Taking out pigweed, resistant or not, plus other weeds early reduces early-season competition and reduces yield loss. Plus, it is the best option for glyphosate resistance management,” Foresman concludes.
For farmers already coping with skyrocketing input costs, adding another $10-$20 per acre for pre-emergence weed control may seem like too big a risk to take to reduce the odds of getting multiple herbicide resistant weeds in their fields.
While rolling the dice can sometimes come up big for the gambler, the house always wins. In the case of pigweed and herbicides, this is one gamble farmers can’t afford to lose.
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