Five of the top weeds impacting global food security
Global food production faces ever-evolving challenges – weeds that can throttle crops and drive down yields. Let’s take a closer look at five of the biggest threats.
For farmers, trying to ensure that every acre of land is as productive as possible is critical for feeding a growing global population. Research shows that weeds reduce the world’s potential harvest by a staggering 34 percent, making good weed management an essential farming practise.
One of the most common and effective tools are herbicides, but even when herbicides control weeds there is always a chance they’ll develop resistance.
This has led to the rise of integrated weed management: a strategy for maximizing the efficacy of herbicides and using multiple tactics to keep weeds under control.
Weeds, like the ryegrass in this corn field, pose a constant challenge to farmers' yields.
Weeds, like the ryegrass in this corn field, pose a constant challenge to farmers' yields.
Andy Heggenstaller is an agronomist and crop strategy lead at Syngenta. He says: “The basic idea behind integrated weed management is being more proactive. Farmers need a strategy using multiple control tactics, so you're not hitting the weed with the same management tactic all the time. Having a strategy like this is vital for delaying the onset of resistance and ensuring that tools remain as effective as possible for as long as possible.”
Some of those other tactics might include crop rotation, or herbicide mixtures that combine multiple modes of action.
Here, Syngenta experts offer their assessment of five different weeds impacting yields across the globe. All can have a serious economic impact on farmers. Crucially, it is the resistant biotypes of these five weeds that pose an increasing challenge to farmers in many parts of the world.
Palmer amaranth
As Joe Wuerffel, an expert with a PhD in weed science, explains, there’s one weed that presents a distinct collection of challenges for farmers around the world and for US farmers in particular – Palmer amaranth.
“What makes Palmer Amaranth unique is that it's one of the most aggressively growing weeds that we deal with in the US,” says Joe. “It can grow up to two to three inches in a single day.”
Around 2005, Palmer amaranth was identified as resistant to glyphosate. Already prevalent across the Southern US, this resistance helped it spread even faster.
Able to germinate throughout the growing season, it is a constant pressure on farmers’ yields. “It is highly prolific and USDA research shows that a single plant produces anywhere from 100,000 to 500,000 seeds,” explains Joe.
And those seeds are tiny, dispersing easily via equipment, birds, or even just the tread of a farmer’s boot.
That’s not all. There is a serious shortage of control tools for Palmer amaranth. Joe says: “There are a lot of weeds out there that have resistance issues, but there's nothing that has quite as many resistance issues as Palmer amaranth.”
The sheer scale of the resistance problem means farmers always need to be vigilant.
Joe says: “It has resistance to every site of action that we have presently in the USA. In certain very isolated populations, it can be resistant to 10 different herbicide sites of action. That's all we have for use on Palmer amaranth. So that's obviously a huge problem for farmers.”
Unsurprisingly, the economic impact can be colossal.
“It’s one of the very few weeds in the USA that can cause almost complete yield loss under the right circumstances. For corn, if you have a really bad infestation in part of the field, that’s 90 percent yield loss. Soybeans are a little bit more adaptable, but 75 percent yield loss isn't out of the question.”
Despite the serious risk that Palmer amaranth poses, practical things can be done to limit its spread. Joe offers two important pieces of advice.
“First, don't let them get out of the ground. While there is resistance confirmed to nearly every site of action, it’s exceedingly rare for all those resistances to be in the same population and even more unlikely for all those resistances to be in the same plant. In other words, we have the herbicide tools today to control multiple-resistant Palmer amaranth, but those tools must be deployed correctly.
“The second thing with Palmer amaranth is that, unfortunately, it's a zero-tolerance strategy that you must follow if they do get out of the ground. If a post-emergence herbicide doesn't kill them, you need to go out there and pull them up. It just takes one plant to produce 500,000 seeds potentially.”
As with many other weeds, integrated management strategies are also critical.
Joe explains: “You can layer in a crop that breaks up the life cycle, maybe it's alfalfa or silage. It has been shown that cover crops also can help. However, if you don't do cover crops right, there are risks. If you don't get a good stand, it can hold moisture in the ground while reducing the amount of residual herbicide that gets to the soil, allowing more Palmer to germinate while not providing enough ground cover to prevent emergence.”
Goosegrass
Some threats to crops are easy to spot. Others might look innocuous. Take the grass species Eleusine, otherwise known as goosegrass. Common across the world, it ranks among the major weeds facing farmers today.
Daniel Nigro, a Global Technical Manager for Herbicides, knows the threat well. “It’s a big issue in my region, especially on the soybean and cotton areas across South America.”
Eleusine, or goosegrass might look small but it spreads rapidly and takes light, moisture and nutrients away from crops.
Eleusine, or goosegrass might look small but it spreads rapidly and takes light, moisture and nutrients away from crops.
In favourable conditions, a single goosegrass plant can produce up to 140,000 seeds that can disperse across a wide area.
As Daniel says: “If you miss just one plant in your fields, it can cause big problems. What makes it so difficult to control is that it’s very growth-stage sensitive. If you miss the right spray timing, it gets really hard to control.”
Because of Brazil’s tropical climate, the timing window for herbicide application can be as short as a matter of days.
Resistance issues are becoming increasingly serious.“If you look back five, six years, resistance was not as big a deal as it is today.” Daniel explains. “The growth has been exponential. Six or seven seasons ago, around 30 percent of the area was affected. Today, above 70 percent of the growing areas have at least one weed resistant to an existing mode of action.”
Inevitably, goosegrass can mean that harvests suffer. “We have robust data from Brazil gathered over the last two years. It depends on the crop and the density of the weed population, but goosegrass is aggressive and losses can reach over 60 percent.”
Like many weeds, goosegrass populations are developing resistance to many of the most widely used control methods.
Like many weeds, goosegrass populations are developing resistance to many of the most widely used control methods.
So, what can farmers do? Daniel says: “For effective weed management there's one big strategy: integrated weed management. Combining chemical and non-chemical measures. That means rotating modes of action, use pre-emergent herbicides, use cover crops, rotate crops and no-till if that suits your conditions.”
But farmers have some positive news with the introduction of VIRESTINA™ technology, the first new selective herbicide to control resistant grass weeds in soybean and cotton in a very long time.
Daniel says: “VIRESTINA™ technology is very exciting for the whole region. It’s not just good news for Brazil, but if you look at our neighbors in Argentina, they also have a huge problem with another grass weed, Lolium, which is also resistant to glyphosate and ACCase inhibitors.”
Thanks to its unique design, this technology represents a step change in grass weed control. “We have a new tool that works on every single known resistance mutation for these grasses,” says Daniel.
A new technology doesn’t mean the resistance problem is solved. Daniel is quick to point out the importance of delaying its onset: “We have something amazing on our hands, and we need to protect this technology. Integrated weed management is a key success factor for this groundbreaking innovation, ensuring farmers can benefit from it for several years.”
Barnyard grass
Rice feeds half the planet, and across Asia millions of farmers depend on keeping their fields free of weeds to protect their harvest.
Echinochloa crus-galli, commonly known as barnyard grass can be found around the globe, but it’s a particular problem for farmers in this part of the world.
Ben Wu, AMEA & JANZ Product Biology Lead for Weed Control, explains: “In Asia, rice is the most important crop, and this weed is the most important weed for paddy fields. As a farmer, if you don’t control it appropriately, research shows it can cost you up to 50 percent of your yield, maybe more depending on the weed pressure.”
The threat is year-round. “Asia has a tropical climate with a consistent temperature range. It’s perfect for rice, but also ideal growing conditions for this weed, you’ll see it growing at every time of year,” says Ben.
Like several of the most challenging weeds, Barnyard grass has evolved resistance to many of the methods farmers depend on to keep it controlled.
“Resistance is an issue for many countries, especially areas that have used the same modes of action for a long time. In Japan, Korea, Vietnam, Thailand, Turkey and China, the resistant biotypes are very common. For these countries, they don’t rely on a single control measure anymore and must use mixtures: two, three, sometimes even four different modes of action. This shows how difficult this weed is to control.”
Even so, farmers still have options. Ben offers some tips:
“Using herbicides with alternative modes of action is essential to mitigate the risk of resistance. Proper timing and applying the recommended dose rates of herbicides as early as possible are also critical. Some areas are shifting to applying on the same day that plants are transplanted or sowing in the field because if we wait until the weeds germinate, they become more established and their tolerance increases.”
When it comes to finding ways of tackling weeds, the solution doesn’t rest solely on innovative control products, as new technologies also have a part to play.
Ben says: “By using GPS-based temperature forecasts, we can predict weed growth stages, particularly for barnyard grass. Since temperature strongly correlates with its development, the tool can identify the optimal timing for effective herbicide application. We are not just providing products; we are integrating technology to empower farmers to make better decisions.”
Horseweed
Vast acres of soybeans thrive in the agricultural heartlands of Brazil.
While the country’s tropical climate is ideal for crops like soybean, it also provides perfect conditions for weeds to flourish.
One of the most common is known as horseweed – and there are over 150 different species that can impact farmers around the world. Most common across South America are the species known as Conyza bonariensis and Conyza canadensis. These pose one of the biggest threats to a soybean farmer’s bottom line, quickly spreading over large areas.
A single plant can produce 200,000 seeds, making controlling this weed a consistent challenge.
A single plant can produce 200,000 seeds, making controlling this weed a consistent challenge.
Bruno Giovanelli is a Herbicide Researcher specializing in horseweed. He explains what makes horseweed such a problem: “Researchers have found that a single plant can produce around 200,000 seeds that are spread by the wind.”
Emerging when temperatures drop, horseweed can be a big issue for Brazilian farmers throughout June, July and August. It t spreads quickly and grows aggressively, outcompeting crops for valuable nutrients, limiting a farmer’s yield.
“Just a single horseweed plant per square meter in your soybean fields can result in yield losses of 34 percent, with studies showing it can impact yields by as much as 68 percent,” says Bruno.
An additional complication is that horseweed, like all weeds, can adapt to the methods farmers use to control it.
Left unchecked, horseweed can cause a serious infestation as seen in this field in Paraná State, Brazil. Picture by Bruno Giovanelli.
Left unchecked, horseweed can cause a serious infestation as seen in this field in Paraná State, Brazil. Picture by Bruno Giovanelli.
Bruno says: “When horseweed first started, growers began using glyphosate. But this kind of intense selection pressure meant resistance developed, not just here in Brazil but in other countries like Argentina too.”
This means that farmers need new ways to tackle horseweed. They must also ensure that effective tools aren’t overused, hastening the development of resistance.
“With horseweed in soybean, you need to control it before the soybean emerges because you don't have many tools for controlling it once the soybean has emerged,” he explains.
Ryegrass
Annual ryegrass (Lolium rigidum) might originally be from the Mediterranean, but it’s become the most economically damaging weed in all of Australia.
Sarah Wilson, a herbicide expert and trialling lead for Syngenta, explains how this happened: “It was brought to Australia in the 1800s as a pasture grass for sheep. That turned out to be one of agriculture's great own goals, because once Australia shifted to large-scale grain, canola, and pulse cropping in the second half of the 20th century, annual ryegrass went from being a useful pasture plant to being a real threat.”
Ryegrass now infests over eight million hectares of Australian crops. “It's not just a problem here in Australia either - it has spread to South Africa, South America (Chile, Argentina), the Pacific coast of North America, New Zealand, and is well established across the Mediterranean,” says Sarah.
Rye grass may look innocuous, but there are several factors that make it such a serious issue.
Sarah explains: “It's an aggressive early-season competitor. Emerging at the same time as the crop, it immediately competes for water, light, and nutrients at exactly the moment the crop is most vulnerable. Cereals like wheat and barley might handle the competition reasonably well - but canola and pulses are less able to fight back in early growth, which makes them particularly susceptible.”
The yield losses can be severe: even for wheat and barley, losses can be more than 80 percent in high-density areas. Crops like field peas can approach near total loss due to severe early-season competition.
“Ryegrass sucks up moisture. In Australian cropping regions, where average rainfall is only 250-450 mm/year and evaporation runs at 3-5 times that, every millimeter of soil moisture is a production input. A heavy ryegrass infestation can be the difference between a crop making it to harvest and a crop that fails in a dry finish,” says Sarah.
The financial cost on Australian farmers is severe. According to industry research it costs farmers more than AUD $1.9 billion ($1.4 billion USD) per year. To complicate this further, ninety percent of Australian broadacre farmers practice no-till farming.
This is because Australia farms some of the oldest and most fragile soils on earth. Millions of years of weathering have left them thin, low in organic matter, and highly prone to erosion.
Sarah says: “Tillage would destroy what little soil structure exists, strip surface cover, and leave soil exposed to the wind and water erosion that are a normal part of the Australian climate. The practical consequence is that Australian farmers are structurally dependent on herbicides as their primary weed control tool.”
This means selection pressure on weeds is intense and resistance has grown rapidly.
Sarah says: “Herbicide resistance is a massive issue. It's why annual ryegrass has gone from a manageable weed to a crisis. In Australia, more than 60 percent of ryegrass populations are resistant to at least one mode of action. Many are resistant to several simultaneously.”
The only effective approach is using every tool available. Sarah outlines some of the tools farmers need to employ.
“Destroying ryegrass seeds at harvest. Technologies like chaff carts, narrow windrow burning, and seed destructors have been gamechangers in Australia. Crop rotation lets farmers use different control options too. Farm hygiene protocols really matter.”
As Sarah points out, good management can keep the threat in check: “The good news is that the ryegrass seed bank is short-lived - most seeds die within 16-18 months. But one missed season resets the clock entirely. Consistency is everything.”
The future of weed control
From around 1950 to 2000 researchers had huge success using synthetic chemistry to develop a host of new modes of action to tackle weeds.
Andy says: “It's now a fact that weeds are developing broad-scale resistance to glyphosate and other popular control tools. We have to adapt to new challenges.”
But even as established chemistry faces increasing resistance there are still new breakthroughs, and Andy is optimistic about the future of chemistry in controlling weeds.
“VIRESTINA™ is the world’s first selective herbicide to control resistant grass weeds in soybean and cotton in nearly 40 years. I think the next decade is going to even more exciting for new herbicide technology than the last 25 years were. Syngenta has several very exciting new things in the pipeline that we just didn't have in our pipeline a decade a prior.”
Find out more about how VIRESTINA™ technology works to tackle the problem of resistance in grass weeds.
But it isn’t only in chemistry that innovation is coming. Andy highlights two more areas that show how the future of weed control could develop.
“We will continue to see new chemistry developed. We will also be able to use biotechnology to create plant crops that are resistant to new broad-spectrum and selective herbicides that kill weeds that have developed resistance to one or more existing herbicide modes of action. A second new development is what I would call selective herbicide application: sensors on sprayers that will detect where the weed is and don't spray the chemistry everywhere.”
Weeds are always going to be a complex problem requiring multiple complimentary tactics to keep them under control and fields clear. With new technology, new chemistry and biotechnology working together, the future of weed control depends on innovative breakthroughs alongside integrated weed management strategies that help farmers achieve higher yields, with lower impact.
