Will Rising Temperatures Make Superweeds Even Stronger?
Widely used herbicides are struggling to kill some weeds. Some experts think heat could be part of the problem.
From the digital magazine, Undark • Dec. 7, 2020
By Natasha Gilbert • 12.07.2020
Around 10 years ago, in the fierce heat of the Kansas summer, many of the noxious kochia weeds invading Phillip Stahlman's research fields simply wouldn't budge when sprayed with a mixture of two widely used herbicides, glyphosate and dicamba. Just a few months earlier, in the cooler spring weather, the herbicide mixture had easily triggered the weed's small thin leaves to curl up and turn brown, signaling the plant's demise.
Now the stalwart weed had Stahlman stumped. Stahlman, then a weed scientist at Kansas State University, had never encountered this problem with herbicides before. He initially assumed that he had applied the agrichemicals incorrectly. But year after year, the same thing happened. Stahlman knew something was up. He kept a close watch on the weeds in his fields. He also conferred with local farmers who reported seeing similar problems. “The light didn't come on for a while until the issue kept reoccurring. It was like putting together a puzzle,” says Stahlman.
Eventually, Stahlman, who has since retired, decided that the problem was likely temperature: Something about spraying in high heat was rendering the herbicides less effective.
Stahlman is not alone in making this observation. Today, mounting evidence suggests that temperatures of around 90 degrees Fahrenheit or above can make some herbicide-resistant weeds even more resistant, and cause other weeds to be less sensitive to certain chemicals.
Some farmers say they know high temperatures can mess with some herbicides, so they try to avoid spraying in the heat of the day. “A good rule of thumb is if it's 85 to 90 degrees Fahrenheit, just don't spray,” says Curt Gottschalk, a farm manager in Hays, Kansas.
Not all experts agree that this pattern, largely demonstrated in lab experiments, poses a problem for farmers. And some herbicides appear to work even better at high temperatures. But if heat is boosting many weeds' resistance to major herbicides, the implications could be significant. Left unchecked, weeds can devastate harvests and income — if farmers didn't make an effort to control weeds, they could wipe out around half of all corn and soybean yields across the U.S and Canada, according to the Weed Science Society of America, a nonprofit society of academic and industry scientists. The loss could cost farmers around $43 billion annually.
Farmers are already battling against epidemics of weeds that have developed genetic resistance to multiple herbicides, including glyphosate and dicamba. Stahlman and other weed researchers argue that temperature could be an overlooked second factor strengthening weeds' defenses against herbicides. Although most farmers now understand that temperature affects herbicides, to the untrained eye it could look just like herbicide resistance, Stahlman says.
These researchers also fear the issue could worsen in the future as climate change raises temperatures and extreme weather events, including heat waves, become more frequent.
“We already know that herbicide resistance is the most problematic issue in chemical weed control,” Maor Matzrafi, a weed scientist at Israel's national Agricultural Research Organization, wrote in an email to Undark. “Maybe reduced sensitivity due to climate change is next in line.”
Farmers used to rely less on herbicides, instead controlling weeds using laborious methods such as tilling and manual removal. But, starting in the mid-1990s, biotechnology companies began debuting genetically modified crops that were resistant to common, powerful herbicides. The new seeds allowed farmers to liberally spray their fields with agrochemicals to kill weeds while their GM crop flourished. The technology made most manual weeding unnecessary, and herbicide use surged globally. But weeds evolved in response, and herbicide resistant varieties emerged. That has set off a new battle between farmers and weeds, with farmers increasingly using combinations of chemicals, as well as additional doses, to try to knock off weeds.
After Stahlman's experience with kochia weeds, though, he began to examine whether temperature could be affecting herbicide performance more than was widely recognized. (Stahlman, like many academic weed scientists, has received research funding from agrochemical companies in the past.)
To decipher how heat helps weeds fend off herbicides, Stahlman teamed up with KSU colleagues Mithila Jugulam, a weed physiologist, and Junjun Ou, a research assistant. The team grew seedlings from kochia populations that originated in Kansas, in chambers kept at temperatures ranging from 63.5 to 90.5 degrees Fahrenheit — representative of the state's spring and summer daytime heat. The temperature in the chambers dropped every 12 hours to mimic cooler nighttimes. When the seedlings reached around 4 inches high, the researchers dosed some with glyphosate and others with dicamba. At weekly intervals, the team examined the weeds for signs of injury. After one month, they cut down the weeds and dried and weighed them.
The team found that, at high temperatures, they needed more than twice the amount of glyphosate and dicamba to control weeds. They published their results in 2016.
“A good rule of thumb is if it's 85 to 90 degrees Fahrenheit, just don't spray,” says Curt Gottschalk, a farm manager in Hays, Kansas.
To understand why heat reduced sensitivity, the researchers tracked the herbicides' paths through the weeds using mildly radioactive versions of glyphosate and dicamba. The former previously manufactured by Monsanto, and the latter produced by BASF Corp. (BASF Corp. provided a graduate student assistantship to Ou for a different project.)
The team found that the leaves absorbed less glyphosate at higher temperatures. They're not sure why, but Jugulam thinks the heat may encourage the kochia to develop thicker cuticles — a protective layer on the leaf surface — which then boost the weeds' defenses against the herbicide. The team discovered a different process at work when the weeds encountered dicamba. Temperature did not affect the amount of dicamba that the weeds' absorbed, but it did hinder the herbicide's movement through the plant so that less reached its target — the tissue developing at the tips of new shoots and leaves.
In another study, published last year, Jugulam turned her attention to the herbicide 2,4-D, one of the ingredients of agent orange, an infamous defoliant used in the Vietnam War. Today, it's one of the most widely used herbicides. Jugulam tested how temperature affected the herbicide's ability to control common waterhemp, a broadleaf weed found encroaching on Midwestern corn and soybean fields.
In the tests, Jugulam examined some common waterhemp that had developed genetic resistance to 2,4-D, and some that had not. She found that it took more than three time as much herbicide to kill the resistant weed under hot, dry conditions than at cooler temperatures. She found a similar but smaller effect in the susceptible weeds.
Herbicide-resistant waterhemp survives exposure to 2,4-D by quickly breaking the chemical down into nontoxic substances before it can reach its targets at the tips of roots, stems, and leaves. When the temperature is high, Jugulam found, waterhemp breaks down those molecules faster.
Jugulam also noted that under high temperatures, some herbicide resistant weeds appear to become more resistant and some susceptible weeds may require more herbicide for their control.
Not all weeds and herbicides respond to temperature the same way. For example, Jugulam has also found that 2,4-D and glyphosate work better at higher temperatures against common and giant ragweed, two other weeds common in U.S. farmers' fields. Jugulam also says that, even in cases where heat does hinder herbicides, the impacts appear to be mostly limited to dry conditions. Areas with high humidity and rainfall might not see the same effects.
But some experts say evidence is growing in a variety of species and agrochemicals that temperature, and in some cases high carbon dioxide levels, affect weed control, at least in the laboratory.
In tests published in 2016, Matzrafi found that at high temperatures four different species of grass weeds stood up against diclofop-methyl, an ingredient in an herbicide manufactured by Bayer, significantly better than they did at lower temperatures. Matzrafi also found that high temperatures made another herbicide, pinoxaden, less able to curb growth of the invasive grass false brome. Moreover, the grass thrived even when it was switched from cooler conditions to a hotter environment up to two days after the herbicide treatment.(The research was partially funded by ADAMA Agricultural Solutions, an agrochemical company based in Israel.)
“Our findings, and many other studies since the ‘90s, suggest that post-application environmental conditions may also affect herbicide sensitivity,” explained Matzrafi in an email. Even if farmers spray during cooler temperatures, that might not be enough to avoid the effects of heat.
Those conditions, experts fear, will worsen under climate change. Already, many U.S. states important for agriculture, as well as other major food producing regions around the world, regularly experience temperatures topping 90 degrees Fahrenheit during growing seasons. Some researchers say that problems with heat and herbicide performance are coming to the fore now partly because of more frequent episodes of extreme heat over the past few decades.
However, it is hard to pin the effects seen today on recent climatic changes, Lewis Ziska, a plant physiologist at Columbia University in New York, wrote in an email to Undark. But, noting that weeds are “the greatest constraint for food production,” Ziska warns that “they will be a formidable challenge for farmers in a more extreme environment.”
In the Midwest, for example, temperatures could rise by an average of 8.5 degrees Fahrenheit by the end of the century, with longer and more frequent stretches of extreme heat, according to federal government projections. And in South Asia, including India — a globally important region for producing rice, pulses, nuts, and cotton — the Intergovernmental Panel on Climate Change projects that average annual temperatures will rise by nearly 6 degrees Fahrenheit by 2100.
In the first experiment of its kind, the results of which were reported last year, Matzrafi studied the joint effect of heat and raised carbon dioxide levels on two different weed species and found that the combination boosts weeds' herbicide defenses beyond that by either factor alone.
It's not clear whether herbicide manufacturers are prepared for the coming challenges of a warming planet. Many do not recommend optimal spraying temperatures to ensure efficacy in the guidelines they distribute to farmers.
In a written statement Clark Ouzts, a spokesperson for Sygenta, the manufacturer of pinoxaden, says the company has not studied the potential effects of climate change on the herbicide's activity, but that “field research and commercial applications have not shown temperature to have a significant impact on the activity of Pinoxaden.”
Charla Lord, a spokesperson for Bayer, wrote in a statement that the company's herbicides are “extensively tested to meet all regulators' requirements” and “labeled so applicators know how to apply for them for optimal control and success.” The company did not respond to specific questions regarding the efficacy of their products under high temperatures, although the company has posted about the challenges of high-temperature spraying on its website.
Corteva, which makes herbicides incorporating 2,4-D, did not respond to requests for comment on how high temperatures affect the herbicide's performance.
Not everyone is convinced that these experimental findings spell trouble for farmers. Some researchers and weed experts say that laboratory conditions differ radically from the field, making the results less pertinent. “I don't think we could say for sure that this is having an impact at the real-world scale,” wrote Brad Hanson, a weed expert at the University of California, Davis, in an email to Undark. Hanson worked with Matzrafi on the research published last year.
Hanson, who works with California farmers as a researcher and extension specialist, also wrote that farmers typically use enough herbicide to overcome any small changes in weed sensitivity brought on by heat.
Kassim Al-Khatib, a crop physiologist at the University of California, Davis who has investigated how herbicides perform under heat and humidity, wrote in an email to Undark that the studies are carried out under carefully controlled laboratory conditions that farmers would never find in their fields.
“What happens under controlled conditions does not generally support what happens in field conditions,” he wrote.
It's not clear whether herbicide manufacturers are prepared for the coming challenges of a warming planet.
In his own research, Al-Khatib has studied the effect of temperature and humidity on a small number of herbicide types and weed species. In the cases he has studied, he says, herbicide efficacy generally improves at higher temperature and humidity, unless temperatures exceed 100 degrees Fahrenheit.
Matzrafi and Jugulam agree that the lab does not reflect the more complex farm environment.
“But I don't think it lessens the importance of the results,” Matzrafi wrote. He, Stahlman, and others say they have seen the effect in the field for themselves.
“I think we are seeing something. Over the next 10 years I think we will see more. It's going to be a major problem,” says Chuck Otte, a K-State Research and Extension agent in Geary County, Kansas, who primarily works with farmers.
The research findings are trying news for farmers who rely on herbicides to keep increasingly weaponized weeds at bay. For now, some farmers are striving to stave off some of the effects from heat by spraying herbicides during cooler periods of the day. In the heart of summer, Carie Moore says she sometimes wakes before dawn to spray her 650-acre farm in North Dakota before the heat climbs too high, occasionally topping 100 degrees Fahrenheit. But as the globe warms, farmers will face narrower windows during which temperatures are cool enough to spray, says Ziska. And Matzrafi's research suggests that spraying during cooler periods doesn't guarantee that herbicide efficacy isn't affected by bursts of heat that come later in the day or even the following day.
And sometimes farmers can't avoid spraying herbicides in the heat, particularly on huge farms with several thousand acres.
Otte says that skeptics might not realize their herbicides are not working because heat has given weeds a helping hand. They might first assume that an array of other common factors are at fault, such as herbicide resistance, lack of rainfall, or too little herbicide. “There are so many confounding factors it is difficult to sort it all out,” he says.
In the long-run, farmers will need to lessen their reliance on herbicides and instead use other techniques more often used in organic farming today to keep weeds at bay, many experts say. For example, planting cover crops such as spring oats or crimson clover helps stop weeds taking root in bare soil and growing a variety of crops in rotation can wrong-foot weeds helping to suppress populations. Moore is already moving in this direction. She rotates soybeans with wheat, barley, and a couple of other crops and plants a variety of additional cover crops including rye and peas.
She also occasionally tills the soil to help stifle weed growth. “The less we have to spray chemicals the better,” she says.
Yet, to keep up yields, farming can't entirely abandon herbicides, says Ziska. With very few new chemicals on offer, farmers will need to be more careful with the herbicides they have now to ensure they continue to work in the future. Manufacturers need to provide farmers with better advice on when and how to use herbicides, adds Matzrafi.
“Farmers can no longer just spray and pray,” says Otte.
Natasha Gilbert is a freelance reporter in Washington, D.C., who has spent over a decade covering the environment, agriculture, and sustainability.
Original source: Undark • Dec. 7, 2020/h5>/h5>/h3>/h2>
"A Virtual Conference to Discuss Real-World Weeds".
That's a great tag-line for the California Weed Science Society conference and a sign of the times.
Even though our in-person interactions as weed researchers, managers, and regulators has been put on hold this year, weed control goes on and so does the need for the education and updates provided by the CWSS.
Please remember to register for the conference this year to keep up your education and training and get the latest information as well as supporting YOUR California Weed Science Society. The program committee has put together another high-quality program that touches on the many aspects of weed science in California.
Here's the link to the registration page:
While there is a lot of value in our normal in-person 2.5 day conference format, there are a few benefits of the online format.
- You can view the sessions at your convenience since they are prerecorded. In a state like California, there's no "off-season" for everyone and in this format you don't have to dedicate 3 days to get this great content.
- We have several registration options if you don't want to participate in the whole event. Of course it's a better deal (per credit) if you register for the full program, but there are several options if you just want the Laws and Regs, or just the breakout sessions, etc.
- No travel expense! If you've been CWSS-curious but have not been able to justify the time or expense of the travel in the past, this is your opportunity!
- Lastly, this may be a one-time opportunity to see weed science experts from around California and the US delivering cutting-edge research updates while wearing sweatpants and broadcasting from the spare bedroom in their house while their dog barks at the mailman - what an opportunity! (just kidding, we actually have high quality professional video and audio and on-demand retrieval of the presentations).
The content will be available online from January 25 through February 26. Register early before the price goes up!
Here's how to get to the registration page:
Here's the current program agenda:/span>
Convolvulus can be an ornamental plant. There is a reason it is called MORNING Glory. It's why people plant it. It's commonly sold in nurseries. There's a large purple flowered variety that is growing throughout my Ventura city neighborhood. It's quite charming highlighting the orange fruit high in the canopies of citrus and along backyard fences. There are also pink stripped varieties. They also seem to grow well, and when you decide it's time that they had taken over too much of the garden, it's really hard to get rid of them The seeds are everywhere and they lie dormant, ready to emerge to recolonize that orange tree you had just cleaned out.
Listen to Weedologist Lynn Sosnoskie's advise on how to deal with this beautiful menace in the orchards. This podcast is prepared by Phoebe Gordon of UC's "Growing the Valley".
- Author: Rebecca Ozeran
A few months ago, I was asked about the toxicity of various plants in a horse pasture after the death of a miniature horse using that pasture. While many of the identified plants were chemically harmless (such as filaree [Erodium spp] and some native clovers), the pasture did have fiddleneck (Amsinckia spp) and popcorn flower (Plagiobothrys spp), two native forbs with potentially toxic chemistry.
Popcorn flower (above) has small white flowers. Fiddleneck (below) has slightly larger yellow flowers. Both plants have similar overall shapes: slender flowering stems, relatively small leaves, and hairs on all parts except the flowers themselves.
Fiddleneck is a known alkaloid accumulator and popcorn flower is similarly suspected to accumulate alkaloids. There are no cases that I have found where popcorn flower was identified as a cause of toxicity, however. Most research on popcorn flower chemistry focuses on insect herbivores which like to eat plants with alkaloids, to protect themselves against predation (e.g. Hartmann et al. 2004) – but that's a topic for another blog!
Alkaloids are secondary organic compounds produced by many plants. Different types of alkaloids have different interactions with animal biology, some of which are benign or beneficial, and others which are harmful. Some alkaloids you may have heard of include morphine, nicotine, and quinine. Pyrrolidizine alkaloids, the type found in fiddleneck and popcorn flower, have harmful effects. Toxicity often occurs when animals eat feed or hay contaminated with fiddleneck seeds, and some cases have been documented from animals grazing the plant in a pasture. Fiddleneck alkaloids can cause liver disease and death of horses, cattle, and pigs, but sheep seem to be less vulnerable (Craig et al. 1985).
Both fiddleneck and popcorn flower may also accumulate nitrates. Nitrates convert into nitrites once the animal eats the plant. Nitrites then react with hemoglobin in the blood and make it unable to carry oxygen. This oxygen deficiency can cause death in a matter of hours depending on the concentration of nitrates in the animal's diet. Sheep, pigs, and horses seem more resistant to nitrate poisoning while cattle are most vulnerable (Tucker et al. 1961).
How to avoid livestock poisoning by fiddleneck and popcorn flower
The best way to prevent livestock poisoning by these forbs is to make sure there is plenty of good forage available. Livestock don't typically seek out fiddleneck or popcorn flower. Fiddleneck and popcorn flower have more stem than leaf, so they aren't very palatable, and they are densely covered in hairs that tend to discourage grazing. In a pasture with plenty of grasses and desirable forbs, then, animals will easily avoid these harmful plants.
The risk of poisoning arises when there is little else for the animals to eat. As a pasture that has been overgrazed or a pasture experiencing a drought therefore might have too few plants for the animals to be able to avoid popcorn flower and fiddleneck. When possible, having more than one pasture can help keep animals safe. Animals should be moved out of pastures where the only available plants may be toxic.
Body size is also a factor in many cases of toxicity. A fully grown 1,000-lb animal may be unaffected by a small amount of these toxins in their diet (such as this horse, pictured above, who ate a mouthful of grass plus a single fiddleneck plant right in front of me), while a young or small animal might become seriously ill after eating just a few plants. Whether an animal develops clinical signs of toxicity or poisoning depends on the concentration of toxin in the forage, the quantity of forage consumed, and the animal's size. However, any amount could be harmful and if you notice your animals are consuming toxic plants, please contact your veterinarian.
Can fiddleneck and popcorn flower be controlled?
You are not likely to eradicate them, but there are ways to control these plants if you are concerned about them.
Both fiddleneck and popcorn flower can be hand-pulled – gloves recommended to protect against the hairs – if present in small patches. On small acreages, mowing an infested pasture before the plants produce seeds in the spring can help reduce the population. Some herbicides can also help kill these plants; generally speaking, you will need to apply herbicides when the plants are young and small, to prevent seed production for the year. Contact your local UCCE office for more specifics if you want to consider chemical treatments.
Long-term, re-seeding bare patches in pastures and ensuring moderate grazing can also help outcompete these species. Because they have relatively small leaves, popcorn flower and fiddleneck rely on plenty of open space and sunshine to grow. As a result, they are less common in pastures that are densely populated by desirable forages that shade smaller plants.
For more on establishing a healthy pasture, even if you have dryland pastures or pastures with animals other than horses, the free guide here is a great place to start: Establishing and Managing Irrigated Pasture for Horses.
If you are concerned that your animals may have been exposed to toxic plants, contact your veterinarian. If you have concerns about plants in your pastures, feel free to contact your local UCCE office for assistance with plant ID.
Check out CalFlora to see the geographic distribution of these and many other plant species in California
Craig, A.M., L.L. Blythe, E.D. Lassen, and M.L. Slizeski. 1985. Resistance of sheep to pyrrolizidine alkaloids. Israel Journal of Veterinary Medicine 42:376-384.
Hartmann, T., C. Theuring, T. Beuerle, L. Ernst, M.S. Singer, and E.A. Bernays. 2004. Acquired and partially de novo synthesized pyrrolizidine alkaloids in two polyphagous arctiids and the alkaloid profiles of their larval food-plants. Journal of Chemical Ecology 30(2):229-254.
Tucker, J.M., D.R. Cordy, L.J. Berry, W.A. Harvey, and T.C. Fuller. 1961. Nitrate Poisoning in Livestock. California Agricultural Experiment Station. Circular 506, 12p.
If you didnt hear or you want to hear again
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CA Avocado Society/CA Avocado Commission/UCCE
August Seminar/Webinar Topic
Life without Glyphosate, Weed ID, and A Review of Microbial Amendments
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