(Source: MIT TR)
One morning in July, I met Aaron Hager, a weed control specialist, outside the old Agronomy Seed House on the University of Illinois South Farm. In the distance is a circular barn built in the early 20th century to withstand storms in the Midwest. A storm hundreds of miles wide swept in, accompanied by gusts of 80 miles per hour, and set off dozens of tornado warnings, reminiscent of Cold War-era bomb drills.
Farmers grow corn and soybeans on about 23 million acres, about two-thirds of the state's area, as well as a small amount of wheat. Aaron Hager, who grew up on a farm in Illinois, says they typically spray herbicides on almost every acre. But while these chemicals allow a plant to grow undisturbed in a wide space, they no longer stop all weeds from growing.
Since the 80s of the 20th century, more and more plants have evolved immunity to the biochemical mechanisms by which herbicides kill them. This herbicide resistance can lead to reduced yields, with wildly growing weeds reducing crop yields by 50% or more, and in extreme cases, potentially ruining entire fields.
In the worst-case scenario, it could even lead to the bankruptcy of farmers, which is the equivalent of "antibiotic resistance" in the agricultural sector, and the situation is getting worse.
As we drove east from the campus of the University of Illinois at Urbana-Champaign, we saw a soybean field overgrown with dark green, spiky plants that grew to chest-height.
"So herein lies the problem. It was full of amaranth, and I guess it had been sprayed at least once, if not more than once. "With these herbicide-resistant weeds, it's only going to get worse." ”
Amaranth can invade almost any crop field, growing an inch or more per day, and females of this species can easily produce hundreds of thousands of seeds. Native to the Midwest, it has bred in large numbers over the past few years because it has developed resistance to seven different classes of herbicides. According to the Purdue University Extension Center, seasonal competition from amaranth can reduce soybean yields by 44 percent and corn yields by 15 percent.
Most farmers are still barely making ends meet. Two different classes of herbicides are generally still effective against amaranth, but there is a growing number of species that are resistant to both methods.
"We're watching defeat come and of course, we're still in a very dangerous situation." Kevin Bradley, a plant scientist at the University of Missouri who studies weed management, said.
Elsewhere, the situation is even more dire.
"We do need to make fundamental changes to weed control, and we need to change quickly because weeds have 'caught up' with us and we have now reached a very critical moment." said Larry Steckel, professor of plant science at the University of Tennessee.
Uptrend
According to Ian Heap, head of the International Herbicide-Resistant Weed Database and an expert in weed control, there have been more than 500 unique cases of the phenomenon out of 273 weed species, and the number is growing. Weeds have evolved resistance to 168 different herbicides and 21 of the 31 known "modes of action" (specific biochemical targets or pathways that chemicals are designed to destroy), and many herbicides share some common modes of action.
One of the most vicious weeds in the South that plagues Larry Steckel and his colleagues is a cousin of a large rhubarb-stemmed amaranth called Amaranthus palmeri, and weed populations have now been found to be ineffective against nine different classes of herbicides. This plant can grow more than two inches per day, reach up to eight feet in height, and "dominate" an entire field. It is native to the deserts of the southwest, has a sturdy root system that is resistant to drought. If it's rainy, then you may have missed the opportunity to control them chemically.
"Amaranth will zero the yield of the crops you grow." Aaron Hager said.
Several other weeds, including Italy ryegrass and a tumbleweed called "Kochia," are causing real pain to farmers in the south and west, especially those who grow wheat and sugar beet fields.
Chemicals are born
Before World War II, it was common for farmers to use tools such as plows and harrows to weed and loosen the soil, or they did it by hand. Just like my mother, she remembers hoeing grass on a farm in Indiana as a child.
This changed with the advent of synthetic pesticides and herbicides, which farmers began to use in the 50s of the 20th century. By the 70s of the 20th century, some of the first examples of resistance appeared. By the early 80s of the 20th century, Ian Heap and his colleague Stephen Powles discovered ryegrass populations that were resistant to the most commonly used herbicides, known as ACC enzyme inhibitors, and were spreading in southern Australia. Within a few years, this species had developed resistance to another class of herbicides called ALS inhibitors.
The problems are just beginning, and it's only going to get worse.
In the mid-to-late 90s of the 20th century, agricultural giant Monsanto (now part of Bayer CropScience) began selling genetically modified crops, including corn and soybeans, that were resistant to the commercial herbicide "Roundup" (whose active ingredient is called glyphosate). Monsanto described these "Roundup resistant" crops, as well as spraying glyphosate throughout the field, as a "panacea" for weed control.
Glyphosate quickly became one of the most widely used agrochemicals, and it still remains so today. In fact, it has been so successful that it has led to the depletion of research and development of other new herbicides: it seems that none of the major commercial herbicides may soon enter the market that could help solve the problem of herbicide resistance on a large scale.
Monsanto claims that glyphosate-resistant weed resistance is "highly unlikely" to become a problem. Of course, there are also those who correctly predict that this scenario is inevitable, including Jonathan Gressel, professor emeritus at the Rehovot Weizmann Institute of Science in Israel, who has been working on herbicides since the 60s of the 20th century.
Stanley Culpepper, an expert in weed control at the University of Georgia, confirmed the first case of Roundup resistance in Amaranth in 2004, and the resistance spread rapidly. Both Amaranth and Amaranth produce both male and female plants, with the former producing pollen that can travel long distances in the wind to pollinate the latter. This also gives plants a great deal of genetic diversity, allowing them to evolve more quickly and more favorably for the development and spread of herbicide resistance. These superweeds have sown chaos across the state.
"It broke us completely." Stanley Culpepper says he recalls a particularly difficult period from 2008 to 2012, "We were busy mowing the lawn. ”
"Stay Alive"
Patrick Tranel, a leader in molecular weed science at the University of Illinois, explains that herbicide resistance is a predictable result of evolution, and his lab is just a few miles from the southern farm.
"What does it do when you try to kill a certain creature? It tries not to be killed. Patrick Tranel said.
Weeds have developed surprising ways to bypass chemical control. A 2009 study published in PNAS showed that mutations in the genome of Miscanthus longifolia allowed the plant to replicate more than 150 copies of the gene targeted by glyphosate. Franck Dayan, a weed control expert at Colorado State University, said, "This gene amplification has never been reported in plants before. ”
Another strange way in which this species develops resistance is through a structure called extrachromosomal circular DNA, a chain of genetic material that includes glyphosate gene targets that are present outside the nuclear chromosome. This gene can be transferred from plants with this adaptive capacity by wind pollen.
But scientists are increasingly discovering that weeds are metabolically resistant, and plants have evolved mechanisms to break down almost any foreign substance, including a range of herbicides.
Assuming that a herbicide has a year-long effect on the Amaranth brown population, if any plant "escapes" or survives and produces seeds, their offspring may be metabolically resistant to the herbicide used.
There is evidence of resistance to two chemicals that replace or mix with Roundup to kill this weed: a herbicide called glufosinate-ammonium and two substances called 2,4-D and dicamba. These two substances also typically kill many crops, but now that millions of acres of corn and soybeans are genetically modified, the basic response is to use more chemicals to solve the problem.
"A herbicide was effective last year, and if it is metabolically resistant, there is no guarantee that it will still be effective this year." Aaron Hager noted.
Nathan Donley, director of environmental health science at the Center for Biodiversity in Tucson, Arizona, said many herbicides harm the environment and have the potential to harm human health. For example, paraquat is a neurotoxic chemical that has been banned in more than 60 countries (it has been linked to diseases such as Parkinson's), but its use is increasing in the United States. 2,4-D, one of the active ingredients in Agent Orange, is a potential endocrine disruptor and exposure to it can lead to an increased risk of cancer. Glyphosate is listed as a probable human carcinogen by an agency of the World Health Organization and has been the subject of tens of thousands of lawsuits worth tens of billions of dollars. Atrazine can stay in groundwater for years and can shrink the testicles and reduce sperm count in certain fish, amphibians, reptiles, and mammals.
"Replacing glyphosate with herbicides such as 2,4-D and dicamba, which are often more toxic, is definitely a step in the wrong direction." Nathan Donley said.
Find a solution
Not just chemicals, weeds can become resistant to any type of control method. In a typical example in China, a weed called barnyard grass evolved over the centuries to resemble rice, thus avoiding manual weeding.
Due to the relatively rapid rate at which weeds evolve, researchers have recommended a variety of control strategies. Patrick Tranel says, "Mixing herbicides with two different modes of action sometimes works, even though it's not the best for the environment or the farmer's wallet. Crops planted in rotation can help, as can planting winter cover crops, and most importantly, don't use the same herbicide in the same way every year. ”
"Fundamentally, the solution is not to focus solely on herbicides for weed management. This is a very big problem for farmers and the current situation on farms in United States. Micheal Owen, weed scientist and professor emeritus at Iowa State University, said.
Over the past few decades, farms have expanded due to rural exodus, rising labor costs, and the advent of chemicals and genetically modified crops, allowing farmers to rapidly apply herbicides over large areas to control weeds, which has led to a "sinister simplification" of crop diversity, weed control practices, and weeds have adapted.
On the one hand, farmers often resort to the cheapest way to control weeds, which is understandable. But Katie Dentzman, a rural sociologist at Iowa State University, says that "barriers are a medium- to long-term problem that conflicts with short-term thinking and incentive systems." ”
Her research shows that farmers are often informed and concerned about herbicide resistance, but are limited by a variety of factors that prevent them from actually preventing herbicide resistance. Some farmers say the farm is too big to control weeds more economically without one-time spraying; Others lack labor, capital, or time.
According to Micheal Owen, agriculture requires a diverse approach to weed control, but this is easier said than done.
Steven Fennimore, a weed control expert at the University of California, Davis, said, "We were too narrow and focused on herbicides as a solution. ”
Steven Fennimore focused on vegetables and had few herbicide options and even fewer herbicide options for organic growers, so innovation was necessary. He developed a prototype that could inject steam into the ground, killing weeds within a few inches of the entry point. This has proven to be about 90% effective, and he has used it in fields where lettuce, carrots and onions are grown. But it's too slow: it takes about two to three days to process a 10-acre piece of land.
Many other non-chemical controls are becoming increasingly popular in vegetables and other high-value crops. Eventually, if the economy and logistics go well, these could become popular among inter-row crops, which can be grown in rows and can be mechanically cultivated.
For example, a company called Carbon Robotics has produced an AI-driven system called "LaserWeeder" that, as the name suggests, uses lasers to get rid of weeds. It is designed to automatically move up and down in the crop rows, identifying unwanted plants and "vaporizing" them with one of 30 lasers. According to the company, LaserWeeders is currently available in at least 17 states.
You can also use electric shock weed, and this electric shock device is available in the United States and Europe. A typical design involves the use of a height-adjustable copper boom to get rid of the weeds it comes into contact with. The most obvious disadvantage of this method is that weeds usually have to be taller than crops, but when weeds grow that tall, they may have already caused crop yields to drop.
Weed seed destroyers are another promising option, and these devices are commonly used in Australia and are also becoming popular in places like the Pacific Northwest to grind and kill weed seeds when wheat is harvested.
An Israel company called WeedOut has incubated a system that irradiates and disinfects the pollen of the Amaranth plant and then releases it into the field, so that the female plant receives sterile pollen and cannot produce viable seeds.
"I'm excited about this because it's a long-term way to reduce seed banks and manage these weeds without the need to spray herbicides." Micheal Owen says.
WeedOut is currently testing its method in corn, soybean and sugar beet fields in the United States and working toward approval by the United States Environmental Protection Agency. It recently received $8 million in funding to scale up.
Stephen Duke, who studies herbicides at the University of Mississippi, said AI-driven equipment and precision spraying could ultimately reduce herbicide use. "I expect that eventually we'll see robotic weeding and AI-driven spraying equipment take over." But he expects it will take some time for crops such as soybeans and corn to materialize, as it is very difficult financially to invest large sums of money to care for such "low-value" crops grown over such a large area.
There are also a handful of startups that are developing new herbicides that are based on natural products found in fungi or natural products that plants use to compete with each other. But none of this will be on the market anytime soon.
Farm Day
Some of the most successful tools for preventing drug resistance are not entirely high-tech. Cornell's presentation at Aurora Farm Field Day, organized north of its campus in Ithaca, New York, made this clear.
For example, Lynn Sosnoskie, an assistant professor at Cornell University and an expert in weed control, said that one of the most important things farmers can do to prevent the spread of weed seeds is to clean up their combine harvesters after harvest, especially if they buy or use equipment from another state.
"Combine harvesters have introduced amaranth to the state, and now there are at least five populations in New York," she said. ”
Another classic method is crop rotation, which is switching between crops with different life cycles, management practices, and growth patterns, which is a planting method that helps prevent weed habits; Another option is to plant winter cover crops, which can help prevent weeds from growing.
"We're not going to solve the weed problem with chemicals alone, which means we have to start pursuing something simple." Lynn Sosnoskie said.
This is especially important in places like New York, where the problem has not yet become a top concern, in part because the state is not as monocropped as the Midwest and has more diverse land use.
But this is not immune to this problem. Vipan Kumar, an expert in weed control at Cornell University, said resistance had arrived and could "break out."
"We have to do everything we can to prevent this from happening, and my job is to educate people, it's coming, and we have to be prepared." Vipan Kumar says.
The author of this article, Douglas Main, is a journalist who has served as a senior editor and writer for National Geographic magazine.
Original link:
https://www.technologyreview.com/2024/10/10/1105034/weeds-climate-change-genetic-engineering-superweeds-food/