A Weed That Exhibits Resistance to an Herbicide

The humble dandelion, a weed that's been a nuisance in gardens and lawns for centuries. It's a hardy plant that can grow almost anywhere, and it's also quite resilient.

One of the most interesting things about dandelions is their ability to exhibit resistance to herbicides. This is because they have a unique genetic makeup that allows them to survive even the most potent weed killers.

In fact, studies have shown that dandelions can develop resistance to certain herbicides in as little as three generations. This means that if a gardener uses the same herbicide repeatedly, the dandelions may become resistant to it over time.

This is a major problem for gardeners and farmers who rely on herbicides to control weeds. It's a constant battle to stay one step ahead of the weeds, and it's not always easy.

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This weed, smooth pigweed, is a common sight in Ohio, found throughout the state.

It's resistant to aceto-lactase synthase-inhibitor herbicides, also known as ALS-inhibitors.

These herbicides work by preventing the plant from producing essential amino acids needed for growth and development.

Smooth pigweed shows resistance to some commonly applied ALS-inhibitors, such as Harmony GT and Python.

Even at twice the recommended rate, these herbicides only provided 20 and 4 percent control of the smooth pigweed population, respectively.

Smooth pigweed joins a growing list of Ohio weeds that have developed resistance to ALS-inhibitors.

These include shattercane, giant ragweed, common ragweed, marestail, Powell amaranth, common cocklebur, and waterhemp.

However, smooth pigweed is not as big of a problem for most farmers as some of these other weeds.

It's those growers who rely heavily on ALS-inhibitors that will likely be affected by smooth pigweed's resistance.

Smooth pigweed exhibits partial cross-resistance, meaning it's still sensitive to some ALS-inhibitors like Pursuit, Raptor, Lightning, and Scepter.

This is good news for growers, as it gives them some options for controlling smooth pigweed with ALS-inhibitors.

Herbicide resistance is a growing concern for farmers and agricultural experts. It occurs when a weed population develops the ability to survive an herbicide application, often due to genetic mutations or changes in its metabolic processes.

Weeds can develop resistance to herbicides through various mechanisms, including target site resistance and non-target site resistance. Target site resistance occurs when a weed population develops a mutation at the target site of the herbicide, making it ineffective. Non-target site resistance, on the other hand, involves changes in the weed's metabolic processes that allow it to detoxify or metabolize the herbicide.

Some weeds, such as waterhemp and Italian ryegrass, have developed resistance to multiple herbicide groups, making them difficult to control. Waterhemp, for example, has evolved resistance to eight different herbicide groups, including ALS inhibitors, synthetic auxins, and PPO inhibitors. Italian ryegrass, meanwhile, has developed resistance to six different herbicide groups, including ACCase inhibitors, ALS inhibitors, and EPSP synthase inhibitors.

Expand your knowledge: When to Treat Lawn for Weeds

Herbicide resistance can be caused by a combination of factors, including overuse of a particular herbicide, poor weed management practices, and the presence of resistant weed seeds in the soil. To manage herbicide resistance, farmers can use integrated pest management strategies, such as rotating herbicides, using multiple herbicides, and controlling weeds through cultural and mechanical means.

Here are some common types of herbicide resistance:

Multiple resistance is a complex issue that can arise in weed populations. It's a term used to describe weed populations that exhibit more than one resistance mechanism, allowing the plant to withstand herbicides from different subgroups.

Some populations of resistant annual ryegrass possess both target and non-target site resistance to more than one mode of action (MOA). This means that the weeds can survive herbicides from multiple groups, making them even more challenging to control.

Multiple resistance can arise through various mechanisms, including reduced herbicide uptake, reduced translocation, and enhanced herbicide detoxification. These mechanisms can be caused by genetic mutations or changes in the weed's physiology.

Weeds with multiple resistance can have a significant impact on crop yields and agricultural productivity. For example, Italian ryegrass is one of the most herbicide-resistant weeds in the U.S. and can cause yield losses of 50% to 100% depending on infestation levels.

Here are some examples of weeds with multiple resistance:

As you can see, multiple resistance is a serious issue that requires careful management and monitoring. By understanding the mechanisms of resistance and the weeds that possess them, we can develop more effective strategies for controlling these pests and protecting our crops.

Morningglory Species can be particularly challenging to control due to their large seed size and impenetrable seed coats, making it difficult to use preemergence herbicides effectively.

These vining plants can emerge late in the season, when residual herbicides have worn off, adding to the control issues.

Their vining growth habits allow them to climb up plant stems and often pull them to the ground, making harvest a challenge.

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Separating the pods and seeds from grain can also be difficult.

Morningglory can produce a staggering 5,000-15,000 seeds per plant, depending on the species.

These seeds can persist in the soil due to their long seed coats, remaining viable for up to 17 years at burial depths of up to 6 inches.

There are several ways to test for herbicide resistance in weeds. One option is to use a commercial seed testing service, which requires collecting suspect weed seed and submitting it to a testing service.

Approximately 3000 seeds of each weed are needed for a multiple resistance test, which is equivalent to about one cup of annual ryegrass seed and 6 cups of wild radish pods.

You can also use the Syngenta herbicide resistance Quick-Test, which uses whole plants collected from a paddock and provides a resistance status result within 4 to 6 weeks.

For the Quick-Test, 50 plants are required for each herbicide to be tested, and trimming the plants prior to herbicide application is recommended to ensure accurate results.

The Quick-Test is performed under controlled conditions, so it is not affected by adverse weather conditions, and testing includes both known sensitive and resistant biotypes for comparison.

Here's a summary of the testing options:

Herbicides are classified into groups based on their mode of action (MOA), which is the specific process they target in plants. This information is crucial for identifying the risk of weed populations becoming resistant.

In Australia, all herbicides are classified into specific groups based on their MOA, which can be found on their labels. These groups are ranked according to the risk of weed populations becoming resistant.

Groups 1 and 2 are considered high-risk, while the remaining groups are moderate-risk. This means that herbicides in these groups are more likely to lead to weed resistance.

Newer herbicide modes of action are being developed, but it's a slow process due to regulatory environments.

Herbicide seed tests are a crucial step in identifying herbicide-resistant weeds. Approximately 3000 seeds of each weed are required for a multiple resistance test, which is equivalent to about one cup of annual ryegrass seed and 6 cups of wild radish pods.

For another approach, see: Herbicide for Lawn Weeds

There are two commercial seed testing services in Australia, with Peter Boutsalis, Plant Science Consulting being one of them.

You'll need to collect suspect weed seed from the paddock at the end of the season and submit it to a testing service. This requires some planning ahead, but it's worth it to get accurate results.

A commercial testing service will analyze the seeds to determine if they have developed resistance to herbicides.

The weed in question has been found to exhibit resistance to an herbicide in various regions, with one study detecting the resistant trait in 25% of the weed population.

Researchers have been working to understand the genetic basis of this resistance, with some studies suggesting that it may be linked to a specific mutation in the weed's DNA.

The resistant weed has been found to grow and thrive in areas where the herbicide is commonly used, making it a significant concern for farmers and agricultural professionals.

A set of 16 L. multiflorum populations from agricultural fields in the Willamette Valley in Oregon were identified for this study.

These populations were collected in 2017-2018 as part of a broader survey of herbicide resistance.

From each field, seeds from 25-30 mature plants were collected and later pooled in approximately equal amounts.

A cultivated, public variety of annual ryegrass known as "Gulf" was included in the study, as it has been used widely as a reference susceptible population for L. multiflorum herbicide resistance characterization.

A previously characterized multiple herbicide-resistant L. multiflorum population called PRHC from California was also included, which exhibits resistance to four different herbicide mechanisms of action.

A cultivated variety of perennial ryegrass (L. perenne L.) was used as an outgroup in the study.

Populations susceptible to all herbicides tested were also included in the study, but the focus was on glyphosate due to its importance in agricultural and noncropping areas.

Glyphosate resistance mechanisms are largely unknown, which is why this study focused on it.

Curious to learn more? Check out: Annual Ryegrass Weed

Copy-number variation of EPSPS has been identified to confer glyphosate resistance in several weed populations, including L. multiflorum from Arkansas (Salas et al., 2012).

In fact, researchers have found that L. multiflorum populations from Oregon have little variation in the number of EPSPS copies across surveyed populations relative to the housekeeping gene ALS.

The mean and median EPSPS copy numbers across Oregon populations relative to Gulf were 1.13 and 0.99, respectively, with no statistically significant differences between resistant and susceptible populations observed.

Genetic analysis also revealed that the ABC transporter ABCC8 is not differently expressed in resistant and susceptible plants in the Oregon L. multiflorum populations, with mean and median ABCC8 expression of 0.93 and 0.81 relative to Gulf.

Researchers used qRT-PCR to analyze the gene expression of ABCC8, finding that it was not constitutively up-regulated in the resistant populations.

The experimental runs were pooled into one dataset based on a Levene test of homogeneity of variance, and copy-number variation and gene expression were quantified using the 2-ΔΔCt method.

Multiple comparisons were performed using Tukey's contrasts with a Bonferroni correction considering 10 populations, revealing that ABCC8 gene expression from resistant populations was not significantly different from Gulf.

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To determine potential loci involved in glyphosate resistance, researchers annotated the genomic contigs containing outlier loci. These loci were found in common in the top 1% of the most differentiated SNPs between a resistant population and each of the susceptible populations.

Genomic contigs were extracted from the L. perenne draft genome, which was previously created by Byrne et al. (2015). The entire contig containing that site was extracted.

Predicted genes within these contigs were made using an Arabidopsis thaliana trained dataset from Stanke et al. (2006). Augustus was used to predict these genes.

Blast2GO 5 was used to annotate predicted genes with an E-value cutoff of 10. This was done using the nr database from NCBI.

The study found that there's little genetic structuring of resistant and susceptible populations, which means that the genetic differences between these groups are not significant.

On average, 407 million reads per lane were retained after processing, and the average number of reads per sample was around 1.8 billion.

The mean coverage was 12.5X, indicating that each base in the genome was covered about 12.5 times.

The first principal component mainly reveals differentiation between two different species: annual and perennial ryegrass.

PC2 largely separates the geographically distant PRHC population from the Oregon populations.

There's little separation between the resistant and susceptible samples along the first two principal components, suggesting a recent common ancestor of these individuals and/or ongoing gene flow between them.

In the Oregon populations, PCA explains little of the genetic variation present, and resistant and susceptible samples are not differentiated from each other.

The average pairwise FST indicates little differentiation among populations in Oregon, with a mean of 0.09 and a median of 0.09.

Glyphosate resistance is widespread in L. multiflorum, a weed that exhibits resistance to this herbicide. The resistance is so pronounced that a dose of 1456 g e.a. ha killed all individuals from a known susceptible population, while 100% survival was observed in a known glyphosate-resistant population.

Glyphosate resistance is characterized by low accumulation of shikimate, a compound that builds up in susceptible plants when exposed to the herbicide. Susceptible plants consistently accumulated >50 µg g FW of shikimate, whereas resistant plants accumulated <10 µg g FW.

There is a simple genetic basis for glyphosate resistance in L. multiflorum. The qualitative nature of shikimate accumulation between resistant and susceptible populations suggests that resistance is determined by a single gene or a small number of genes.

No known resistance mechanisms were found in Oregon populations of L. multiflorum. Sequence analysis of the EPSPS gene, which is often associated with glyphosate resistance, revealed no mutations at positions 102 or 106.

The lack of known resistance mechanisms suggests that a novel mechanism is controlling glyphosate resistance in L. multiflorum populations from Oregon. This is a concerning finding, as it implies that the resistance is not based on a single, easily identifiable genetic mutation.

There is little genetic structuring of resistant and susceptible populations of the weed.

The researchers found that after filtering out low-quality reads, they retained an average of 2,193 SNPs for population genetic analyses.

Patterns of genetic variation among individuals and populations are structured primarily according to geography and species, rather than their resistance phenotype.

The first principal component mainly reveals differentiation between the two different species: the annual and perennial ryegrass.

PC2 largely separates the geographically distant PRHC population from the Oregon populations.

There is little separation between the resistant and susceptible samples along the first two principal components, suggesting a recent common ancestor of these individuals and/or ongoing gene flow between them.

Among the Oregon populations only, PCA explains little of the genetic variation present, and resistant and susceptible samples are not differentiated from each other.

The average pairwise FST indicates little differentiation among populations in Oregon, with a mean of 0.09 and a median of 0.09.

Resistance management is key to controlling weeds that exhibit resistance to herbicides. The take-home message is to use residuals, residuals, residuals.

A two-pass program in corn allows for diversifying modes of action and minimizes weed emergence, reducing selection pressure on postemergence products.

In the South, the amaranth species is resistant to most herbicides, with only pyroxasulfone and paraquat working postemergence.

In soybeans, a two- or three-way herbicide premix preemergence with an overlay of pyroxasulfone is recommended.

Mixing 2,4-D and Liberty together can provide better control of resistant weeds in Enlist soybeans.

Smooth pigweed in Ohio has developed resistance to aceto-lactase synthase-inhibitor herbicides, commonly referred to as ALS-inhibitors.

Applying ALS-inhibitors like Harmony GT and Python post-emergence at two times the labeled rate only provided 20 and 4 percent control of smooth pigweed, respectively.

Smooth pigweed is not a big problem for most farmers compared to other resistant weeds, but growers who use ALS-herbicides every year or every other year will be affected.

This population of smooth pigweed only exhibits partial cross-resistance, meaning it is still sensitive to ALS-inhibitors like Pursuit, Raptor, Lightning, and Scepter.