Glyphosate is a topical subject, so we thought we'd take a look at how it works, what a 'Bio' formulation actually means and what we, as an industry, can do to ensure that glyphosate is available for use for the foreseeable future. Pitchcare's Technical Manager, John Handley, provides a detailed explanation
How it works - the science
Glyphosate is a non-selective, chemical herbicide that kills plants by inhibiting the shikimate pathway. A pathway in the field of biochemistry is a sequence of reactions, usually controlled and catalysed by enzymes, by which one organic substance is converted to another. Glyphosate is a competitive inhibitor of a plant enzyme - EPSP synthase.
EPSP synthase participates in biosynthesis of the aromatic amino acids phenylalanine, tyrosine and tryptophan. These amino acids are only synthesised in plants and microorganisms. Animals obtain aromatic amino acids by ingesting plants and other organisms, therefore this pathway is a very appropriate target when seeking to focus on controlling species within the plant kingdom and, thereby, creating a formula that has a good toxicological profile.
Glyphosate binds more tightly to EPSP synthase than its natural substrate and inhibits the production of the enzyme and, consequently, shuts down the entire pathway. Plants require the shikimate pathway to produce aromatic amino acids which, in turn, are used to make proteins. By shutting down this pathway, this effectively kills the plant, though the plant must utilise its available store of these amino acids, which is why we see a difference in the time that it takes to kill some plants, though the effect is inevitably the same.
Shikimic acid was first isolated from the plant star anise (Illicium religiosum) in 1885. The name is derived from the Japanese name for this plant, shikimi-no-ki.
So, what makes one glyphosate different to another glyphosate, and which one should we choose?
Glyphosate, in its raw state, is a waxy grey substance that isn't very soluble and would have very little effect if it was rubbed onto the plant. It is converted into a soluble salt and combined with surfactants to enable it to spread, adhere and penetrate the leaf cuticle. A surfactant, also known as surface-active substance, is defined as a substance that, when dissolved in water, lowers the surface tension of the water and increases the solubility of organic compounds. The effectiveness of the kill and speed of translocation is controlled by the types and quality of the salt and surfactants (see tables 1 and 2).
Due to their physicochemical, toxicological and eco-toxicological properties, chemicals can be dangerous for the physical integrity and health of individuals and for the safeguarding of the environment. REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) regulations (2007) dictate that the hazards of the product are stated within Section 2 of the Material Safety Data Sheet (MSDS).
However, there are older formats of MSDS which state the hazards in section 3 or section 15. To comply with the regulations, the classification and label elements should be stated in section 2, with hazardous ingredients and their hazards found in section 3, and other regulatory information in section 15. This information can be used to help us distinguish between the products and determine if they are appropriate for our intended use, though some explanation can help elucidate this (see table 3).
Risks and Hazards
The Chemical (Hazard Information and Packaging for Supply) Regulations, CHIP, provide information on hazardous chemicals in Great Britain.
The statements of risks, R, and hazards, H, (see tables 4 and 5 respectively), which are present on the label, supplement the hazard symbols (see table 3).
The Classification, Labelling and Packaging Regulation, CLP, is a European Union regulation which aligns the European Union system of classification, labelling and packaging chemical substances and mixtures to the Globally Harmonised System (GHS). They are intended to form a set of standardised phrases about the hazards of chemical substances and mixtures that can be translated into different languages.
As such, they serve the same purpose as the well-known risk phrases, which they are intended to replace (see table 5).
We can see that, within the MSDS for Asteroid Pro, the Lethal Concentration (LC) to 50% of a population of Rainbow Trout over a ninety-six hour period is greater than 1,000mg/l. An Effective Concentration (EC - has an effect on) to 50% of a population of Daphnia spp. (a water flea that is considered to be representative of aquatic invertebrates) over a forty-eight hour period is 610mg/l. An Effective Concentration to 50% of Selanastrum spp. (an aquatic algae) over a one hour period is 189mg/l. In table 5, we can see that the high concentrations represent the relative safety of products and can be used to distinguish between the different formulations.
A Bio Formulation is defined as a formulation which, when it has been tested, achieves 'No Classification' which means that it falls outside of the following predetermined limits in respect to causing harm to wild-life or the user.
For classification as Aquatic Chronic 3: H412 (CLP) one of the following criteria must be met:
- 96 hr LC50 (fish): >10 to <100 mg/l
- 48 hr EC50 (crustacea): >10 to <100 mg/l
- 72 or 96 hr ErC50 (algae or other aquatic plants): >10 to <100 mg/l
And, the product must also be shown to be either not rapidly degradable or it must have a potential for bioaccumulation.
A material is considered rapidly degradable if there is data to show that the substance will degrade by >70% of the theoretical maximum in twenty-eight days.
A material is considered to have a potential for bioaccumulation if the experimentally determined bioconcentration factor (BCF) is >500. If data on the BCF is absent, the partition coefficient, log Kow* or log Pow**, is used. If log Kow >4, the material is considered to have the potential to bioaccumulate.
However, if the above parameters for classification as Aquatic Chronic 3 are met, additional tests can be done to ensure the correct classification. These are long-term, or chronic, studies of aquatic toxicity, and they outweigh any classification based on short-term, or acute, data where the L(E)C50 value is >1 mg/l.
The chronic toxicity tests are based on No Observable Effect Concentrations (NOEC). If these values are above 1 mg/l for fish, crustacea and algae then no classification as for chronic toxicity is needed.
*log Kow is a quantitative thermodynamic measure of the hydrophilic/lipophilic balance of an organic compound.
**log Pow is the ratio of concentrations of a compound in a mixture of two immiscible phases at equilibrium.
These coefficients are a measure of the difference in solubility of the compound in these two phases.
For classification as R52/53 (CHIP) one of the following criteria must be met:
- 96 hr LC50 (fish): >10 to <100 mg/l
- 48 hr EC50 (Daphnia): >10 to <100 mg/l
- 72 hr IC50 (algae): >10 to <100 mg/l
And, the product is not readily biodegradable.
A material can be considered readily biodegradable if there is convincing scientific evidence available to show that the substance can be degraded to a level of >70% of the theoretical maximum within a twenty-eight day period.
There are several reasons why selecting the safest pesticide to use is important; the risk assessment states that we need to review the available options and select the product that offers the lowest risk.
Increasingly, pesticides are being reviewed to assess their impact upon wildlife, the environment and the health of the user. If we, as an industry, want pesticides to continue to be available, then we have to regulate and be seen to regulate ourselves. Trying to determine accurately which product fulfils these objectives is not straightforward, and it could be stated that some manufacturers intentionally muddy the water in what is already a complex field.
Consumers are drawn toward clear information and strong brands. Ultimately, the producer that can deliver these objectives with the most effective and safest product will, in time, dominate the marketplace.