How a controversial herbicide can both harm and help one of the world's most important crops
What if one of the world's most controversial herbicides could sometimes help crops grow better rather than harming them? This isn't a theoretical question but a very real phenomenon that sugarcane farmers and scientists have been observing for years.
The relationship between glyphosate—the active ingredient in Roundup—and sugarcane represents one of modern agriculture's most fascinating paradoxes.
The same chemical that can kill weeds is also used as a "ripener" to boost sugar production when applied correctly.
Understanding this delicate balance is crucial for sustainable agriculture, as sugarcane remains a vital global resource—not just for sugar production but increasingly for biofuel and bioproduct development. The complex dance between this powerful chemical and sugarcane's metabolism reveals much about plant physiology, adaptation, and how we can work with natural processes to improve agricultural outcomes.
Glyphosate is far more than a simple weedkiller when it comes to sugarcane. While it's famously known for its use in controlling weeds in agricultural settings, sugarcane farmers have found a very different application—as a chemical ripening agent applied shortly before harvest.
This practice highlights the extraordinary principle that "the dose makes the poison," as the same compound that can damage sugarcane at high concentrations can enhance sugar recovery at precisely controlled low doses.
The key to understanding glyphosate's effects lies in its primary mechanism of action. Glyphosate works by inhibiting a crucial enzyme called EPSPS (5-enolpyruvylshikimate-3-phosphate synthase) in the shikimate pathway 8 .
This pathway is responsible for producing essential aromatic amino acids in plants—phenylalanine, tyrosine, and tryptophan—which are the building blocks for proteins and numerous other compounds 8 .
One of the most important discoveries in sugarcane research is that different varieties respond dramatically differently to glyphosate applications. Agricultural scientists have categorized sugarcane varieties based on their sensitivity levels 1 :
| Highly Responsive Varieties | Moderately Responsive Varieties | Very Sensitive Varieties |
|---|---|---|
| HoCP 96-540 | HoCP 00-950 | HoCP 09-804 |
| L 99-226 | L 01-283 | Ho 12-615 |
| L 01-299 | Ho 07-613 | L 14-267 |
| HoCP 04-838 | HoCP 09-804 |
This variability means farmers must carefully select application rates based on their specific cultivars. For instance, varieties like Ho 12-615 require lower application rates and strict adherence to treatment-to-harvest intervals to prevent damage that could impact subsequent growing seasons 1 .
Perhaps the most fascinating aspect of glyphosate's relationship with sugarcane is the phenomenon of hormesis—where a typically stressful or toxic substance provides beneficial effects when applied at very low doses.
Growth Stimulation
Research has consistently demonstrated that while high doses of glyphosate can damage or kill sugarcane, precisely calibrated low doses can actually stimulate growth and enhance photosynthesis .
In a landmark 2018 study published in Pest Management Science, researchers discovered that ultra-low doses of glyphosate (between 6.2 and 20.2 grams per hectare) led to significant increases in shoot dry weight for both sugarcane and eucalyptus plants . The magnitude of this growth stimulation ranged from 11% to 37% depending on species and timing, a substantial improvement by agricultural standards.
The researchers conducted meticulous experiments to measure not just growth but physiological changes in the plants. They tracked multiple indicators of plant health and function at 40 and 60 days after glyphosate application . The results revealed the mechanisms behind the growth stimulation:
| Glyphosate Dose (g a.e. ha⁻¹) | Shoot Dry Weight Increase | CO₂ Assimilation Enhancement | Stomatal Conductance Change |
|---|---|---|---|
| 6.2 | 11-15% | Moderate increase | Moderate increase |
| 20.2 | 25-37% | Significant increase | Significant increase |
The hormetic response appears to work because the mild stress triggered by low glyphosate doses stimulates the plant's defense and growth systems, much like how moderate exercise strengthens human muscles.
The plants don't just tolerate these low doses—they actually thrive better than untreated plants in some respects.
Recent advances in molecular biology have allowed scientists to peer even deeper into the glyphosate-sugarcane relationship. A 2025 study used molecular docking techniques and gene expression analysis to understand how different sugarcane varieties respond to glyphosate at the genetic level 4 7 .
The research focused on the rbcL gene, which codes for a key enzyme involved in chlorophyll metabolism and photosynthesis. Scientists examined two distinct sugarcane varieties—G84-47 and GT54-9—treating them with varying glyphosate concentrations (0.2, 0.4, and 0.8 mg L⁻¹) 4 7 .
The molecular study found that the GT54-9 variety demonstrated significantly higher herbicide tolerance than G84-47 4 7 . When examining the genetic response, researchers discovered that rbcL expression remained relatively stable in G84-47 but was significantly upregulated in GT54-9 under high herbicide stress 4 7 .
rbcL expression: Relatively stable
rbcL expression: Significantly upregulated
This suggests that the more tolerant varieties have developed mechanisms to boost photosynthesis-related genes in response to glyphosate exposure, essentially compensating for the herbicide's stress by enhancing their energy production capabilities. This finding provides crucial insights for breeding programs aiming to develop more resilient sugarcane varieties for future agricultural challenges.
The transition from scientific understanding to practical agriculture requires precise management. University recommendations emphasize that glyphosate should only be applied to stubble sugarcane crops, not plant cane, and must be timed according to strict treatment-to-harvest intervals 1 .
Applying too early fails to maximize recoverable sugar.
Optimal window for glyphosate application.
Risks reducing yield potential in current and subsequent crops.
This careful timing ensures that the glyphosate's ripening effects peak exactly when the cane is harvested.
Successful glyphosate use in sugarcane requires attention to several practical factors 1 :
Applications work best on erect cane. If cane has recently lodged (fallen over), farmers should wait 7-10 days for it to naturally erect itself before spraying.
Wind speeds between 3-10 mph are ideal to prevent drift, and applications should avoid temperature inversions that can cause off-target movement.
Rainfall within six hours may reduce effectiveness. If rain is likely, adding a compatible nonionic surfactant (0.25% by volume) can help.
| Factor | Recommendation | Rationale |
|---|---|---|
| Crop Type | Stubble cane only | Plant cane is more susceptible to damage |
| Rate | 5.0-7.0 oz/acre Roundup PowerMAX 3 | Balance between efficacy and crop safety |
| Timing | 35-49 days before harvest | Optimizes sugar accumulation |
| Environment | Wind 3-10 mph, no inversion | Minimizes drift to non-target areas |
| Cane Position | Erect cane | Ensures even coverage and absorption |
The relationship between glyphosate and sugarcane represents a remarkable case study in how modern agriculture must balance multiple competing factors—maximizing yield while minimizing chemical inputs, understanding both immediate effects and long-term consequences, and applying laboratory science to field conditions.
What makes this story particularly compelling is that it continues to evolve. As research advances, scientists are developing increasingly sophisticated understanding of how different sugarcane varieties respond to glyphosate at molecular levels, which may lead to more targeted approaches in the future.
The discovery of genes and enzymes involved in glyphosate response opens possibilities for breeding more resilient varieties or developing even more precise application techniques.
The glyphosate-sugarcane story ultimately reminds us that agriculture is both a science and an art, requiring us to understand natural systems well enough to work with them rather than simply imposing our will upon them. As research continues, this delicate balance of harnessing chemical tools while respecting biological systems will likely become even more important in creating sustainable agricultural systems for our future.
| Tool | Function | Application Example |
|---|---|---|
| Glyphosate Formulations | Herbicide and ripening agent | Roundup PowerMAX 3 used at 5.0-7.0 oz/acre as ripener 1 |
| Refractometer | Measures juice Brix (sugar content) | Determining optimal harvest timing after glyphosate application 1 |
| Molecular Docking | Analyzes interactions at molecular level | Studying glyphosate binding with rbcL enzyme 4 7 |
| Gene Expression Analysis | Measures genetic response to treatment | Quantifying rbcL expression changes post-glyphosate application 4 7 |
| Nonionic Surfactant | Enhances herbicide absorption | Added (0.25% by volume) when rainfall is likely within 6 hours 1 |
| Shikimate Assay | Measures pathway disruption | Quantifying glyphosate's effect on aromatic amino acid production 8 |
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