Are we making progress or losing ground on weed resistance management? — that’s the question posed by Ken Smith, technical manager for Cheminova and a former Arkansas Extension weed specialist, at the West Texas Agricultural Chemical Institute (WTACI) annual conference in Lubbock.
The answer, he says, is complicated. “It depends a lot on human nature.” If people don’t perceive a problem, they won’t fix it, he says. The failure in that logic is determining when an inconvenience becomes a problem.
“I can argue that we’re making progress in managing resistant weeds,” Smith says. “We’ve come a long way in the last 10 years — we’re learning things. We have extensive research, educational programs, and experience, both good and bad. We also understand that we may get a lot of bad before we get to the good.”
One can also argue, he says, that progress has been slow and, at times, absent. “We haven’t had a new herbicide mode of action molecule developed in the last 15 or 20 years. That’s a problem. We have new technology that lets us use old technology, but weeds have been exposed to those materials in the past, so they may not last as long.”
He acknowledges that herbicide resistance is not new. “The first confirmed case of herbicide resistance, to atrazine, occurred in 1951. We had a lot of chemical companies back then, and all had bench chemists screening molecules, finding new herbicides. If one ran out, we had another that weed populations had never been exposed to.”
RESISTANT SPECIES INCREASING
Resistant weed numbers have increased. From 1996 through 2005, 168 weed species were entered into a weed resistance database. Of those, 84 were resistant to ALS herbicides and 42 were resistant to ACCase herbicides. “Some had multiple herbicide resistance.”
In 2006, 191 weed species were noted in an international survey of herbicide-resistant weeds. Resistance was documented in 39 states, and 112 species were glyphosate-resistant. Many were resistant to multiple herbicides.
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“When we began to see herbicide resistance in 1996, we were concerned,” Smith says, “but we had other tools coming along. Resistant weeds were an inconvenience — we could control them.”
But, he says, Palmer amaranth and waterhemp are different, “and they are now driving the train.” No county in Arkansas is now free of herbicide-resistant Palmer amaranth. “It happened quickly — and that’s where West Texas will be in two or three years. I hope we can avoid the worst resistance.”
In 2006, Georgia and Arkansas were the only two states with herbicide-resistant Palmer amaranth. Today, Smith says, “almost the entire Sunbelt has populations of herbicide-resistant pigweed. It will be bad here. Palmer amaranth evolved in the Southwest; it’s well-adapted here. Driving across West Texas now, I see the Mid-South of 2007 and 2008.”
PROLIFIC SEED PRODUCTION
That’s part of the challenge, he says. Add to that the prolific nature of Palmer amaranth reproduction, and the task seems near insurmountable. Conservative calculations are that just one escaped pigweed plant produces about 300,000 seed. Even if 95 percent of those seed fail to germinate, 15,000 will survive. If a farmer uses a weed management strategy that controls 99 percent of that population, 150 plants will be left. If 33 percent of those are female, he will have 50 female plants per acre, producing perhaps 150,000 seeds each, resulting in 7.5 million plants. If just 95 percent of those germinate and the farmer again gets 99 percent control, he still will have more than 3,500 plants per acre.
“We have no economic threshold for pigweed or waterhemp,” Smith says. “We have to consider the potential for herbicide resistance. Is zero tolerance possible? I can think of 101 reasons why it’s not. But hoe hands in a field with low pigweed populations can eliminate the weeds for about $5 per acre.”
Farmers have to work together to destroy the weed seed bank in the soil, he says. Then, the answer to the question about whether or not progress is being made against weed resistance becomes “maybe.”
“It depends on us,” Smith says. “Everyone in this room — leaders in agriculture — can change no or maybe to yes.”
Awareness of the problem has improved, Smith says, noting that in 2000 only 5.1 percent of the articles in a leading scientific journal dealt with herbicide-resistant weeds. By 2010, the percentage had doubled, and in 2014 nearly 25 percent of the journal entries were related to herbicide resistance.
Research trials have increased, too.
“We can skip some of the bad experiences if we take advantage of what they have learned in the Mid-South and Southeast,” he says. “We have new technologies —LibertyLink and dicamba and 2,4-D-tolerant cotton. We have new tools, and we can manage this issue if we use available research, education opportunities, and experience from the success stories of others.”
PIGPOSIUM SPOTLIGHTS PROBLEM
Information is essential, Smith says, as is acknowledging the seriousness of the problem. “A meeting in the Mid-South, the ‘Pigposium,’ had an attendance of 800. That shows how bad pigweed had gotten. Now, we’re seeing problems scattered all over the West Texas landscape. It’s serious when 800 people come to a conference to learn how to manage a single weed.”
West Texas has not experienced the calamity level of weed resistance that has occurred in the Southeast and Mid-South, he says, and farmers can tap into that experience to limit explosions of resistant weed populations.
“Experience has taught us that we can’t spray out resistant weeds with postemergence applications,” Smith says. “We must focus on the seed bank. That is one of the most important lessons we’ve learned. If the soil seed bank isn’t constant or decreasing, control is not sustainable. If it’s not sustainable, if weed seed increases every year, it will beat you. Soil residual herbicides are essential, regardless of any new technology or crop option.”
The current condition in West Texas is not sustainable, he says. “We will have more weed seed in the ground this year than we did last year. We have to turn it around. That’s the reality.”
NO SILVER BULLETS
New herbicides that will control all herbicide-resistant weed problems don’t exist, Smith says. “We don’t have any silver bullets. New technologies will offer new tools — but not silver bullets. We need all the tools we can get, but these new ones won’t answer all our problems.”
Dicamba and 2, 4-D, and tolerant varieties, will be good, he says, “but not as good as glyphosate was initially.” Control measures must take into consideration application timing. “Treat small weeds, those less than 3 inches tall.” That’s not always easy, he notes, since weeds may grow 1/2 inch to 3/4 inch per day.
“At 4 inches, Palmer amaranth is too large to control effectively,” Smith says. “We can kill it with dicamba with good coverage — but how many weeds in the field will get full coverage? The weeds close to the crop often escape control.”
And escapes mean worse problems for next season. “If we get 90 percent control, that’s not good enough. Technology will have holes, so it’s a management issue.”
To avoid the horrors of Southeast and Mid-South pigweed resistance, Smith says, West Texas farmers have to be flexible. “If your herbicide program is working, change it. If you don’t, it won’t continue to work. If we had more modes of action, we could talk about diversity of control products.” That’s not the case, so farmers have to rotate chemistry and application methods.
“We also have to work together — producer, industry, Extension, and research. Teamwork is essential. We are making progress, but it takes a change of mindset. No one likes change, but it is essential.”
Farmers will look to industry and Land Grant universities for answers, he says.
He quotes farmer Drew Oliver as putting the issue in perspective: “We are farmers — that’s what we do,” Oliver said. “We aren’t going to quit farming. It’s your job to tell us what to do.”
“We are making progress,” Smith says.