Glyphosate-Resistant Waterhemp
How the Palmer Amaranth Crisis Foretells Northern Agriculture's Challenge
The Silent Spread of Superweeds
Across the agricultural landscapes of America, a quiet but relentless invasion is underway. Herbicide-resistant weeds have evolved to defy humanity's most sophisticated chemical defenses, threatening crop yields and farm economies. While much attention has focused on the devastating impact of Palmer amaranth across southern states, a similar threat is now emerging in northern agriculture: glyphosate-resistant waterhemp.
Key Fact
Over 75% of soybean farmers in New York state rely on glyphosate as their primary weed control method 4 . Glyphosate-resistant waterhemp populations show 5 to 12 times more resistance than susceptible varieties 4 .
This article explores how the lessons learned from Palmer amaranth's destruction can help prevent a similar crisis with waterhemp, preserving both crop yields and farming livelihoods.
The Rise of Superweeds
From Miracle Solution to Evolutionary Challenge
Glyphosate, introduced commercially in the 1970s, was initially hailed as a revolutionary herbicide with broad-spectrum effectiveness and relatively low environmental impact. Its widespread adoption accelerated with the development of glyphosate-resistant crops in the 1990s, allowing farmers to apply the herbicide without damaging their crops 6 .
1974
Glyphosate first introduced commercially as a broad-spectrum herbicide
1996
First documented case of glyphosate resistance in rigid ryegrass in Australia 1
2000s
Roundup Ready crops lead to massive increase in glyphosate use
Present
Over 50 weed species globally have developed glyphosate resistance
Mechanisms of Resistance
Target-site mutation
Some plants develop alterations in the EPSPS enzyme, glyphosate's target protein, reducing the herbicide's ability to bind effectively 1 .
Gene amplification
Resistant weeds sometimes produce multiple copies of the EPSPS gene, creating more target enzyme than the herbicide can disable 6 .
Enhanced detoxification
Certain resistant weeds rapidly metabolize glyphosate into inactive compounds before it can act.
Reduced translocation
Some plants prevent glyphosate from moving to its site of action, compartmentalizing the herbicide in less sensitive tissues.
Multiple Resistance Threat: A Minnesota survey found that 91% of waterhemp populations were resistant to at least two herbicides, with nearly 60% resistant to three or more herbicide sites of action .
Palmer Amaranth: A Cautionary Tale
Biological Characteristics and Impact
Palmer amaranth (Amaranthus palmeri) possesses biological traits that make it particularly devastating:
- Rapid growth: Capable of growing 2-3 inches per day under ideal conditions
- Prolific seed production: A single plant can produce up to 1,000,000 seeds
- Extended emergence pattern: Germination throughout the growing season
- High competitiveness: Aggressive resource acquisition that starves crops of nutrients, water, and light
Recent research has quantified Palmer amaranth's devastating impact on furrow-irrigated rice production. Studies showed that even a single Palmer amaranth plant emerging within three to four weeks after rice could negatively impact rice growing within 1.2 square meters 2 .
Palmer amaranth infestation in a soybean field
Economic Impact Assessment
Yield Reduction
Palmer amaranth emerging just one week before rice caused 50% yield reduction when located within 40 centimeters (15 inches) of the crop 2 .
Control Costs Increase
Infested fields require expensive herbicide programs, increased manual labor, and equipment sanitation.
Waterhemp: Northern Agriculture's Emerging Nightmare
Biology and Spread
Waterhemp (Amaranthus tuberculatus) shares many of the concerning traits that made Palmer amaranth so destructive:
- High genetic variability: Promotes rapid adaptation to control methods
- Extended emergence: Germination from early spring through midsummer
- Rapid growth: Can complete its life cycle quickly under favorable conditions
- Prolific seed production: A single plant can produce over 1,000,000 seeds
Waterhemp was first documented in New York in 2014 and has since spread to 23 counties in the state 4 .
Waterhemp infestation in a corn field
Documented Resistance Patterns
| Herbicide Group | Example Herbicides | Resistance Prevalence | Control Effectiveness |
|---|---|---|---|
| Group 2 (ALS inhibitors) | Imazethapyr, Thifensulfuron | Widespread in Midwest | Poor to moderate |
| Group 4 (Synthetic auxins) | 2,4-D, Dicamba | Emerging | Variable |
| Group 5 (Triazines) | Atrazine | Common | Poor in resistant biotypes |
| Group 9 (Glyphosate) | Glyphosate | Confirmed in multiple states | Poor without integration |
| Group 14 (PPO inhibitors) | Fomesafen, Lactofen | Still largely effective | Good to excellent |
Source: 4
Research has documented yield losses of 43% in soybean and up to 74% in corn in states with established waterhemp populations 4 .
Key Experiment: Understanding Resistance Development
Methodology and Approach
A crucial study conducted in Nebraska set out to confirm and quantify glyphosate resistance in common waterhemp populations while evaluating alternative control options 5 . The research team:
- Collected seeds from suspected glyphosate-resistant waterhemp plants across seven eastern Nebraska counties
- Conducted dose-response experiments in greenhouse settings, applying nine different rates of glyphosate (0 to 163 times normal field rates)
- Evaluated post-emergence soybean herbicides for efficacy against the resistant biotypes
- Measured resistance levels by comparing the response of suspected resistant biotypes to known susceptible standards
Results and Analysis
The study revealed striking levels of glyphosate resistance in waterhemp populations. Depending on the biotype and comparison standard, waterhemp exhibited 3- to 39-fold resistance to glyphosate 5 .
| Herbicide | Mode of Action Group | Control Effectiveness (%) | Notes |
|---|---|---|---|
| Glufosinate | 10 | ≥80% | Consistent control |
| Acifluorfen | 14 | ≥80% | Effective with proper timing |
| Fomesafen | 14 | ≥80% | Reliable control |
| Imazamox | 2 | Variable | Poor on resistant biotypes |
| Imazethapyr | 2 | Variable | Widespread resistance |
Source: 5
Multiple Resistance Concern: Most glyphosate-resistant biotypes also showed reduced sensitivity to ALS-inhibiting herbicides, indicating multiple resistance mechanisms 5 .
Lessons Learned and Future Strategies
Integrated Weed Management Principles
The experience with Palmer amaranth in the South provides a roadmap for addressing waterhemp in northern agriculture before it becomes equally devastating.
Diversified Herbicide Programs
Rotating herbicide modes of action and using strategic tank mixes to reduce selection pressure
Cultural Practices
Narrower row spacing, adjusted planting dates, and competitive crop varieties to give crops an advantage
Mechanical Control
Strategic tillage and cultivation where appropriate for your production system
Prevention & Sanitation
Cleaning equipment between fields to prevent seed spread 4
Economic Benefits of Proactive Management
Yield Advantage
Soybeans treated with preemergence herbicides had an 11-bushel yield advantage compared to plots without them .
Early Planting Benefits
Early planting and narrow rows increased soybean yields by up to 12 bushels per acre compared to late-planted, wide-row systems .
Conclusion: Applying Southern Lessons to Northern Challenges
The devastation wrought by Palmer amaranth across southern agriculture offers both a warning and an opportunity for northern farmers facing the rise of glyphosate-resistant waterhemp. The biological similarities between these two species—prolific seed production, extended emergence patterns, and rapid adaptation—suggest that waterhemp has similar potential to disrupt agricultural systems if not managed proactively.
The key lesson from the Palmer amaranth experience is that reliance on any single management tool, however effective initially, inevitably leads to resistance evolution. Sustainable weed management requires diverse approaches that incorporate chemical, cultural, mechanical, and biological strategies tailored to specific farming operations.
As waterhemp continues to spread across northern agricultural landscapes, the time for proactive management is now. By applying the hard lessons learned from Palmer amaranth, farmers can preserve both their yields and their herbicides for future generations.