Take Action to Counterattack Herbicide-Resistant Weeds

Lurking just beneath the crop canopy, waiting to rob yield and wreak havoc, are hidden criminals: herbicide-resistant weeds. As technology has advanced, these weeds adapt to control tactics, often faster than we can respond, making them the “most-wanted” weed species. 

Herbicide-resistant weeds are one of the most significant problems facing farmers, costing them time and money, said Isaac Schroeder, division agronomist for Superior Ag, headquartered in southwest Indiana. “Resistance has forced a larger financial investment in a more robust residual program because weeds are a lot easier to control before they emerge,” he said.

According to the Iowa State Estimated Costs of Production, the average cost of herbicides in 2010 was $25 in corn and 

$20 in soybeans. In 2024, this had increased to $45 in corn and $53.50 in soybeans. Several species can be particularly challenging to reign in once they have a foothold. 

Palmer Amaranth

Rap Sheet

According to a 2023 survey by the Weed Science Society of America (WSSA), Palmer amaranth was ranked as the most troublesome weed in cotton and soybeans. Palmer amaranth is dioecious (separate male and female plants), which increases genetic diversity and ability to confer resistance. It also has extended emergence throughout the grow-ing season, and fall-emerging plants can still produce seed. Unruly offspring: 600,000 seeds per plant with adequate moisture. Palmer amaranth seeds are short-lived, with only 15% of the seeds in the top 4 inches of soil remaining viable after three years.

Thwarting Authority

Palmer amaranth has evolved resistance to ALS-inhibitors (Group 2), root-growth inhibitors (Group 3), synthetic auxins (Group 4), PSII-inhibitors (Group 5), EPSP synthase inhibitors (Group 9), glutamine synthetase inhibitors (Group 10), PPO inhibitors (Group 14), long fatty acid inhibitors (Group 15), and HPPD inhibitors (Group 27). Not all populations are resistant to all groups, although multiple resistances are common. 

Waterhemp

Rap Sheet

Waterhemp is listed as the second most problematic weed in soybeans, according to the 2022 WSSA survey. Like Palmer amaranth, waterhemp is dioecious, with extended emergence throughout the growing season, making postemergence control difficult. Seeds are small and easily transported.

Unruly Offspring

Waterhemp can produce 250,000–1 mil-lion seeds per plant. Seeds have a short longevity, with only 1% to 12% of seeds viable in the soil seed bank after four years.

Thwarting Authority

Waterhemp has evolved resistance to several herbicide groups, including ALS inhibitors  (Group 2) synthetic auxins (Group 4), PSII-inhibitors (Group 5), EPSP synthase inhibitors (Group 9), PPO inhibitors (Group 14), long fatty acid inhibitors (Group 15), and HPPD inhibitors (Group 27).

Common Ragweed 

Rap Sheet

Common ragweed is highly competitive, with studies showing that only two common ragweed plants per 3 feet of row can result in soybean yield losses of 40%–76%.

Unruly Offspring

Common ragweed can reach up to 6 feet in height, producing up to 62,000 seeds per plant. Plants that emerge early in spring 3 can produce up to six times as many seeds as those emerging late in the season. Seeds buried in soil can persist for over 30 years. However, those near the soil surface survive only a few years, with 50% losing viability after one year.

Thwarting Authority

Common ragweed has evolved resistance to several herbicide groups, including biotypes with multiple resistance to ALS inhibitors (Group 2), PSII inhibitors (Group 5), EPSP synthase inhibitors (Group 9), and PPO inhibitors (Group 14).

Giant Ragweed

Rap Sheet

Giant ragweed emerges early in the season and, if not controlled shortly after emergence, it can quickly outgrow and shade the crop. Biotypes with extended emergence have evolved, which favors survival in crop fields. Unruly offspring: Giant ragweed can produce up to 10,000 seeds per plant. Giant ragweed has a limited lifespan in the soil, with over 96% being depleted after two years.  

Thwarting Authority

Giant ragweed has evolved resistance to ALS inhibitors (Group 2) and EPSP synthase inhibitors (Group 9), including biotypes with resistance to both.

Common Lambsquarters

Rap Sheet

At high densities, common lambsquarters can reduce corn yields by over 50%, and soybean yields by up to 25%. The weed exhibits early-season germination and prolific seed production, making it harder to control. Also, lambsquarters can become difficult to control under dry conditions.

Unruly Offspring

Common lambsquarters can produce 70,000–100,000 seeds per plant. Common lambsquarters seeds have a relatively high longevity, with 50% of seeds remaining viable after 12 years. Seeds on a single plant have different dormancy requirements, which contribute to lambsquarters’ soil longevity. 

Thwarting Authority

In 1975, common lambsquarters became one of the first weed species with documented herbicide resistance. Today, common lambsquarters populations have resistance to ALS inhibitors (Group 2) and PSII inhibitors (Group 5).

Marestail

May go under the alias horseweed.

Rap Sheet

Marestail populations can be prolific in no-till fields. Marestail can germinate in spring and fall, and emer-gence windows are prolonged compared with other weed species. Spring germinations are often well established before planting and are highly competitive, leading to significant yield loss.

Unruly Offspring

A single plant can produce up to 200,000 seeds. Seeds are easily dislodged and spread through gentle breezes. Marestail has a relatively short lifespan, with most seeds surviving no longer than three years. Seeds cannot germinate if buried deeper than a fifth of an inch in the soil, so tillage and cover cropping can be effective control practices. 

Thwarting Authority

Marestail has developed resistance to ALS inhibitors (Group 2), PSII inhibitors (Group 5), EPSP synthase inhibitors (Group 9), and PSI electron diverters (Group 22).

Kochia

Rap Sheet

Despite low soil temperatures, kochia emerges in early spring and grows rapidly. Seeds can germinate as early as January and as late as July. Kochia tolerates heat and drought. Seeds are spread through physical dispersal and do not exhibit innate dormancy.

Unruly Offspring

Field-grown kochia plants can produce 2,000–30,000 seeds, with an average of 14,000. Recently, kochia plants in sugarbeet production have produced a reported 100,000 seeds. Kochia has an average lifespan of one to two years in the soil.

Thwarting Authority

U.S. kochia populations have demonstrated resistance to several herbicide groups such as ALS inhibitors (Group 2), synthetic auxins (Group 4), PSII inhibitors (Group 5), and EPSP inhibitors (Group 9). 

Italian Ryegrass

May go under the alias annual ryegrass.

Rap Sheet

Italian ryegrass is primarily problematic in the South, although it is becoming more prevalent in the Mid-west. Yield losses can be 50%–100%, depending on infestation levels. High genetic diversity has allowed the weed to adapt to a wide range of soil and environmental conditions. Italian ryegrass also has a staggered but continuous emergence pattern until shade from the crop canopy suppresses emergence. 

Unruly Offspring

According to the University of Arkansas, individual plants can produce up to 108–173 tillers per plant and 20,500–45,000 seeds per plant. Limited studies have been conducted on longevity in the soil seed bank, although the need to re-seed forage systems suggests seeds are relatively short-lived. 

Thwarting Authority

Italian ryegrass is one of the most herbicide resistant weeds in the U.S. The weed has documented resistance to several herbicide groups: ACCase inhibitors (Group 1), ALS inhibitors (Group 2), EPSP synthase inhibitors (Group 9), glutamine synthetase inhibitors (Group 10), long fatty acid inhibitors (Group 15), and PSI electron diverters (Group 22). 

Giant Foxtail

Rap Sheet

In Michigan, corn yields were reduced about 14% from three giant foxtail plants per foot of row. Giant foxtail seeds have a lengthy period of emergence, and mature giant foxtail plants can cause problems for grazing livestock.

Unruly Offspring

One giant foxtail seed head produces 500–2,000 seeds, with an average of 900. Michigan State University reports seed banks are reduced by 50% after five years, and 99% after about 30 years. However, seed viability is limited, with little emergence occurring after two years in the soil.

Thwarting Authority

Giant foxtail resistance has been reported for ACCase inhibitors (Group 1), ALS inhibitors (Group 2), and PSII inhibitors (Group 5).

Morningglory Species

Rap Sheet

Morningglory’s large seed size and impenetrable seed coats can make control with preemergence herbicides difficult. Additionally, morningglory can emerge late into the season, when residual herbicides have worn off. Due to their vining growth habits, morningglory can climb up the plant stems and often pull them to the ground, making harvest challenging. Also, it can be difficult to separate the pods and seeds from grain.

Unruly Offspring

Morningglory can produce 5,000–15,000 seeds, depending on the species. Morningglory seeds can persist in the soil due to their long seed coats. According to Sustainable Agriculture Research and Education (SARE), morningglory seeds can remain viable for 17 years at burial depths of up to 6 inches. 

Thwarting Authority

Although there are no confirmed cases of herbicide resistance, these weeds consistently show up on the WSSA’s most problematic weed species survey, and a tolerance to glyphosate has been noted.

Resistance Management

The major problematic weed species can cause 60% to 95% yield loss if left completely uncontrolled, say Tommy Butts, weed science specialist at Purdue Extension. Even with weed control measures, he estimated up to 10% yield loss annually. “So it’s pretty significant on what weeds cost us across our cropping systems,” he said. 

When it comes to resistance management, the take-home message is: “residuals, residuals, residuals,” Butts said. 

In corn, Butts recommends a two-pass program, as it allows for diversifying modes of action. Also, early residual herbicides help minimize weed emergence, which reduces selection pressure on postemergence products. “Long-term, the way to battle resistance is mixing multiple strategies and products, so weeds never get out of the ground,” he said.

When it comes to the amaranth species in the South, “There’s really only two herbicides that are working postemergence on our most resistant location: pyroxasulfone and paraquat, and that’s pretty much it,” said Larry Steckel, weed science specialist at the University of Tennessee.  

In soybeans, Steckel recommends a two- or three-way herbicide premix preemergence, with an overlay of pyroxasulfone. A postemergence application of 2,4-D and Liberty is an option for Enlist soybeans. “Even though Liberty and 2,4-D alone are not working that well, we’re getting a lot better control if we mix the two together,” Steckel explained. 

There is some good news on the amaranths, though. “The seeds are fairly short-lived in the soil,” Steckel said. “If you can keep them down for three years—and that’s the trick—you can get rid of 99% of the seed bank.”

New Modes of Action

There are no silver bullets coming to the rescue anytime soon, Steckel said. “There are some newer herbicide modes of action that are starting to be developed,” he noted. “But with the regulatory environment right now, you’re talking decades. We don’t have decades. The best option is to work with the tools we have now.”

More Than Just Chemistry

There’s no spraying our way out of some of these resistance issues, Butts warned; an integrated approach with multiple strategies is needed. Cover crops, planting date, tillage, and weed seed destroyers on combines can be effective tools. “There’s a whole host of different things we can implement to help the crop be more competitive and suppress weed emergence and then incorporate herbicide strategies,” Butts said. 

Joseph Kern, who raises corn and soybeans in the southern Indiana community of St. Meinrad, has used cover crops to help combat weed resistance. He allows the cover crop to grow and plants to green, terminating after planting. “It makes such a difference in the number of weeds we have,” Kern shared. 

“Cover crops can be a great way to help battle some of these weed problems,” Butts agreed. “It’s not 100%, but if we can reduce the number that are popping through by 60%, 75%, 80%, that is a much smaller population we’re dealing with and can be a really good option.” 

Steckel emphasized that resistant weed species are here to stay. “The number one thing I’m recommending is overlapping residuals,” he said, “doing things with cover crops, weed seed destructors, and other things that don’t come out of a jug. That’s going to be absolutely essential.”