Glyphosate :Residue and Points of Attention in Direct Seeding
Glyphosate, often described as non-persistent, can actually accumulate in the soil depending on soil type, climate, and agricultural management. In no-till (NT), its degradation is slower due notably to the organic matter on the surface. After treatment, molecules can be released by the roots of still living plants, affecting the following crops. Glyphosate is then reabsorbed, harming root growth and plant nutrition. To limit these effects, a waiting period before sowing and light soil tillage are recommended.
Persistence in the soil
Although glyphosate is frequently described as "non-persistent" or having "minimal residual activity" in the soil, the reality of its persistence is more complex and varies considerably depending on various pedoclimatic factors and soil management. This is a simplification that needs to be nuanced.
Several factors influence this persistence:
- Soil type and organic matter content: Soils rich in clay and organic carbon tend to have stronger adsorption of glyphosate, which can slow its degradation.
- Climatic conditions: Warmer and drier conditions can accelerate glyphosate degradation in soil, while cooler temperatures and high humidity can slow it down.
- Microbial activity: Microbial degradation being the main dissipation pathway, the health and activity of soil microbial communities are critical factors.
Degradation dynamics in the soil
- Glyphosate has a strong affinity for soil particles.
- It adsorbs strongly to iron and aluminium oxides (variable charge minerals).
- In bound form, it becomes almost biologically inactive and its mobility is greatly reduced.
- Its main degradation is microbial, producing AMPA (aminomethylphosphonic acid).
Glyphosate: No-till (NT) vs Tillage (CT)
An Italian study from the University of Padova showed greater persistence of glyphosate in the no-till system. Accumulation in the surface horizon is generally observed but is more pronounced in NT.
Comparison of soil management on glyphosate persistence
As seen previously, the less the soil is tilled, the more glyphosate accumulates, especially on the soil surface. The active ingredient concentrated in the superficial horizon has a systematically greater impact on crop stand losses under NT conditions compared to tilled soil, regardless of the commercial formulation and the waiting period before sowing.
However, it is noted that the time between treatment and sowing mainly influences glyphosate damage on the crop.
Why does glyphosate persist more in NT?
- More organic matter (OM) on the surface ➜ glyphosate binds there and degrades little.
- More compacted soil (during NT transition phase) ➜ reduced microbial activity.
- Presence of living vegetation at application time ➜ secondary release by roots.
Glyphosate release
- Glyphosate is translocated and then accumulated in roots.
- It is then released into the rhizosphere:
- By decomposition of treated plant residues
- By exudation from roots of living plants, including glyphosate-tolerant GMOs
Soil contamination by exudations is often overlooked, independent of runoff or spray drift.[4]
Case of no-till
- Favors emergence of resistance in weeds.
- Favors establishment of weeds, especially perennials:
- Tougher, they require higher lethal doses of herbicides,
- They detoxify and release glyphosate for longer,
- They survive longer than species in tilled systems, generally less well-rooted. Glyphosate thus persists longer in the soil, prolonging its presence and effects.
Glyphosate reabsorption
Once glyphosate is in the soil, it can be reabsorbed by plant roots, whether target crops or not.
Case of no-till
More than 50% of wheat roots in untilled soils follow the same pathways as roots of the previous crop. This increases the risk of glyphosate damage due to contamination by contact (reabsorption) with roots of glyphosate-treated weeds that would have released active molecules into the environment during detoxification.
Effect on the following crop
- Impact on root growth: When glyphosate is reabsorbed, it can inhibit root elongation, lateral root formation, and root biomass production.
- Interaction with plant nutrition: Glyphosate is a chelator of divalent metal cations and can reduce nutrient uptake and translocation in crops. It forms poorly soluble chelated complexes with essential micronutrients such as manganese (Mn), iron (Fe), zinc (Zn), and boron (B), hindering their absorption by roots. This interaction can have consequences on crop health, including disease resistance.
Field feedback
This is a wheat plot sown in autumn 2022. A glyphosate treatment (2l) was applied post-sowing.
Observation: In areas without decomposing plant material, emergence is good and regular. Conversely, in areas where there were living plants destroyed by glyphosate, emergence is sparse and insufficient (red box).
Interpretation: This is very likely the phenomenon of release by roots of weeds that detoxified the active molecule before dying. The action of glyphosate is therefore delayed over time and the crop suffers.
Conclusion
| Decreases persistence | Increases persistence |
|---|---|
| Spraying 10 days before sowing | Spraying shortly before or after sowing |
| Dry soil | Wet soil |
| High OM content | Low OM content |
| Low glyphosate dose | High glyphosate dose |
| High clay content | Light soil |
| Low vegetation cover | High vegetation cover |
When glyphosate treatment is applied, it is recommended to respect a sufficient waiting period before sowing, about 10 days (according to recommendations specific to commercial formulations), especially in the presence of dense vegetation cover and absence of soil tillage, which can accelerate weed destruction and limit release.
If long-term accumulation of glyphosate in no-till penalizes emergence and yields, light surface tillage (such as shallow stubble cultivation) can help remedy this problem.
Sources
This article was written by Jasmin Razongles, agronomy engineering student in apprenticeship at the Centre National d'Agroécologie.
- ↑ Dynamics of Glyphosate and Aminomethylphosphonic Acid in Soil Under Conventional and Conservation Tillage - Laura Carretta, Alessandra Cardinali, Andrea Onofri, Roberta Masin & Giuseppe Zanin - 2021 - https://link.springer.com/article/10.1007/s41742-021-00369-3#Fig4
- ↑ Evidence for glyphosate damage of winter wheat depending on waiting-times after precrop glyphosate application and density of desiccated weed plants under field and experimental conditions - Bott, Sebastian Lebender, Ulrike Yoon, Duck-Joong - 2009 -https://escholarship.org/content/qt25v599pr/qt25v599pr.pdf
- ↑ Dynamics of Glyphosate and Aminomethylphosphonic Acid in Soil Under Conventional and Conservation Tillage - Laura Carretta, Alessandra Cardinali, Andrea Onofri, Roberta Masin & Giuseppe Zanin - 2021 - https://link.springer.com/article/10.1007/s41742-021-00369-3
- ↑ https://pmc.ncbi.nlm.nih.gov/articles/PMC6918143/
- ↑ Watt et al., 2006, Ann. Bot. 97, 839-855
