HERBICIDE FORMULATION AGROBIOLOGY

Following up on the previous article on Herbicide Mode of Action, this article covers Agrobiological aspects of Herbicide Formulation.

Herbicides are chemicals that are used to control or kill unwanted plants, also known as weeds. They are widely used in agriculture and horticulture to improve crop yields, reduce the need for manual labor, and prevent the spread of invasive species.

Herbicides can be classified into several types based on their mode of action, chemical structure, and selectivity. Formulating herbicides into effective and stable products is a complex process that involves the selection of appropriate active ingredients, adjuvants, and excipients.

In this article, we will discuss the various types of herbicides and their modes of action, and we will explore the factors that are considered when formulating herbicides. We will also discuss the challenges and considerations involved in formulating herbicides for different applications and environments.

An important factor to consider when formulating an herbicide is the solubility of the active ingredient in the herbicide. Highly soluble herbicides are generally easier to formulate, as they can be dissolved in a variety of solvents. However, highly soluble herbicides can also be more prone to leaching, which means they can be washed away by rain or irrigation and may not be as effective in controlling the weeds.

In addition to the active ingredient, herbicide formulations may also contain a variety of other ingredients, including solvents, surfactants, and adjuvants. Solvents are used to dissolve the active ingredient and make it easier to apply, while surfactants help to improve the spreading and wetting of the herbicide on the plant surface. Adjuvants are substances that improve the performance of the herbicide, such as by helping it stick to the plant or by increasing its efficacy.

Herbicide formulations can be applied to plants in a variety of ways, including as a spray, a dust, or a granule. The method of application will depend on the specific needs of the crops or plants being protected and the conditions in which they are grown. For example, a spray may be more effective in moist environments, while a dust or granule may be more effective in dry conditions.

It's important to note that herbicides can be harmful to non-target plants, including crops and desirable plants. Therefore, it's important to carefully follow the instructions on the herbicide label and to use the herbicide in the manner recommended by the manufacturer.

Overall, the formulation of an herbicide is an important consideration in its effectiveness and safety. Careful consideration of the active ingredient, mode of action, solubility, and other ingredients can help to ensure that the herbicide is properly formulated for the specific needs of the crops or plants being protected and the environment in which they are grown.

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Herbicide formulation – grass and broadleaf weeds and crops

Herbicide formulation development options are determined by weed biology, and active ingredient physical-chemical characteristics, including:

• Target/selectivity – grass or broadleaf weeds and crops
• Uptake – foliar, shoots, roots
• Application timing – pre or postemergence
• Solubility – hydrophilic or lipophilic; solubility in water and organic solvents
• Translocation – contact or systemic (xylem, phloem)
• Herbicide target/selectivity – grass or broadleaf weeds and crops

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Flowering plants (crops and weeds) are divided into two main taxonomic groups based on their morphology, anatomy, and physiology:

• Monocotyledons (monocots or grasses) are grass and grass-like flowering crops (rice, wheat, maize) or weeds with one embryonic leaf, or cotyledon.
• Dicotyledons (dicots or broadleaves), crops or weeds with two embryonic leaves, or cotyledons.

For agrochemical formulation retention and coverage, several morphological differences between grasses and broadleaves are relevant:

• Leaf orientation (upright vs horizontal) - due to their upright leaf orientation, grasses can be difficult to reach with herbicide sprays, while the horizontal orientation of broadleaves allows for easier coverage and reduced run-off of the spray droplets.
• Cuticle (more vs. less waxy, typically 1–10 !m) - grasses have a thicker cuticular barrier making them more difficult to wet with herbicidal spray solutions.
• Stomata (ad- + abaxial) - the lower epidermis (broadleaves) or both lower and upper leaf surface (grasses) contain stomata, respiratory openings which provide an uptake route for pesticide spray solutions.
• Meristem (protected vs. exposed) - protected meristems (growing points) at or near the soil surface can make grass growing points challenging to reach with herbicide sprays, while the exposed meristems of broadleaf weeds are more easily reached.
• Roots (fibrous vs. tap) - grasses tend to have fibrous root systems which spread near the soil surface, while broadleaves have taproots that penetrate deeply into the soil, below the herbicidal layer (typically the upper 5cm of soil).

The physical or physiological differences between grasses and broadleaves provide opportunities for developing selective formulations that control broadleaf weeds in grass crops or grass weeds in broadleaf crops.

In addition to these examples of physiological selectivity, metabolic selectivity provides additional benefits and opportunities for selective formulation development.

An example of this is the increased glutathione-s-transferase activity found in some cultivated grass species, which, through the use of safeners, allows for the control of grass weeds in grass crops.

Finally, deep-seeding crops below the herbicide layer protects the germinating crop seeds from herbicidal activity, while the greater robustness of cultivated crops permits them to break through the herbicidal layer with little to no damage.

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Herbicide application timing - pre- and post-emergence

With respect to application, herbicides may be classified as Pre- or Post-Emergent.

Pre-emergent (PRE-EM) herbicides are applied to the soil, killing the weed seedling when it is germinating. Active ingredient PRE-EM uptake is primarily via the roots and shoots, and soil-applied herbicides do not need additional adjuvants as there is no cuticular barrier in the roots or shoots, neither is there a need for optimal leaf retention and coverage.

However, adjuvants allow water to penetrate soil more easily and/or flow through (infiltrate) the soil, providing benefit when soils have become hydrophobic and will not wet easily. Many pre-emergent herbicides rely on mechanical incorporation or rainfall to activate or reach the root zone.

Crop selectivity to soil-applied herbicides can be a combination of seeding depth (crops are deep-seeded while weed seeds tend to populate the upper soil layer), application timing (e.g., pre-crop application), and differential metabolism in crops.

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Post-emergent (PO -EM) herbicides kill weeds after they have emerged from the soil, and may be broken down into peri-emergence application (PERI-EM; when the seedling tips are just visible at the soil surface), early post emergence (EPO-EM; up to the stage when the first two true leaves have developed) and true post-emergent (PO-EM).

Active ingredient foliar uptake requires additional adjuvants to overcome the cuticular barrier and optimal leaf retention and coverage.

One of the factors limiting leaf uptake is the premature drying of droplets on the leaves. The hydrophilic liquid nutrient UAN (urea + ammonium nitrate solution) is known to increase the activity of hydrophilic, foliar-applied herbicides.

Nitrogen-surfactant blends consist of premix combinations of various forms of nitrogen (e.g., ammonium fertilizers) and surfactants and are generally used with herbicides. The surfactants reduce surface tension and improve leaf surface spreading, while the nitrogen compounds neutralize ionic charges, prevent active ingredient precipitation in the tank mix or on the leaf surface, and promote herbicide uptake primarily into broadleaf weeds.

Crop selectivity to foliar herbicides can be a combination of application timing, differences in physiology between grasses and broadleaf weeds and crops, and differential metabolism in crops ((see previous chapters).

Herbicide acids, esters, and amine salts

Many herbicidal active ingredients are weak acids. Acids are neutral when protonated and negatively charged (ionized) when deprotonated. Bases are positively charged (ionized) when protonated and neutral when deprotonated. Depending on the pH of the soil to which they are applied, herbicides may be neutral, positively charged or negatively charged.

A soil's cation exchange capacity (CEC) is a measure of its ability to bind positively charged ions by negatively charged clay or organic matter colloids. The amount of the negative depends on the pH of the surrounding solution.

The pH and CEC of soil determine the solubility and mobility of herbicides, with implications ranging from irreversible binding to soil colloids to leaching to the groundwater.

Herbicide acids are often formulated as either amine salts or esters to alter their solubility and activity. Once the amine or ester enters the plant, it is converted to the herbicidally active acid.

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Characteristics of herbicide ester and amine salts include:

Esters:

• More lipophilic - more soluble when in contact with the cuticle, increased cuticular uptake
• More active
• More volatile (preferred for lower temperatures)

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Amine salts:

• More water soluble (higher loading for water-soluble ai's)
• Less active
• Less volatile (preferred for elevated temperatures)

High electrolyte (salt) herbicide formulations

High-load herbicide salt formulations (such as glyphosate, glufosinate, dicamba), especially when mixed with other salts such as fertilizers are considered "high electrolyte (salt) systems" which pose specific formulation challenges.

Suspension (SC and WG) formulations can experience issues with dispersibility/suspensibility in high electrolyte systems due to reduced surfactant solubility, as the salts reduce the partitioning of many common surfactants between the water phase and active ingredient.

Alternatives to common surfactants include compatibilizers, such as combinations of a hydrotrope (like surfactants but with a much shorter hydrophobic tail) with an amphoteric (zwitterions with dual positive and negative charges) surfactant.

Hydrotropes (sodium alkylbenzene sulfonates, sodium alkylnaphthalene sulfonates, ethanol, urea, and sodium hydroxyalkyl sulfonates) increase the aqueous solubility of slightly lipophilic active ingredients without forming micelles.

Amphoteric surfactants (phosphate esters, betaines, imidazoline derivatives, amino oxides) have dual positive and negative charges and work as compatibilizers to allow different active ingredients to be mixed.

In addition, dispersants may reduce precipitation due to high salt content.

Herbicide Classes and agrobiological groups

Active ingredient physical-chemical characteristics properties may be obtained through various sources, including the Pesticide Properties Database (University of Hertfordshire) and ePesticide Manual (British Crop Protection Council)

It is possible to identify trends and group the key herbicide modes of action based on our understanding of weed biology, herbicide active ingredient physical-chemical characteristics, and target sites.

By differentiating between hydrophilic and lipophilic active ingredients, foliar uptake and uptake through roots and shoots, it is possible to predict the translocation of active ingredients within the plant and understand where the relevant target sites are to be found, identifying barriers for herbicide active ingredient uptake, translocation and target site binding which may be addressed by formulation and adjuvant technologies.

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Predictive model for herbicide formulation

Predictive models are valuable tools to help screen initial formulation parameters and focus research on leading prototypes.

The Bromilow model was developed and presented by RH Bromilow et al. in the early 1990s and considers both the partition coefficient (logP) and the charge of the active ingredient molecules. logP and pKa are plotted against each other, and the resulting diagram is a useful model for predicting pesticide mobility in plants at physiological pH.

By placing herbicide active ingredients into the Bromilow model based on their lipophilicity (logP) and partition coefficient (pKa), we can determine and visualize pesticide mobility as well as predict their suitability for pre-emergence (soil) application or post-emergence (foliar) application and uptake.

Using this model, we see that many common herbicides tend towards xylem and phloem mobility; they are systemic and are therefore typically applied as post-emergent (Post em.) foliar treatments, while others are non-systemic and are applied as a pre-emergence (or early post-emergence) soil applications.

Soil-applied herbicides thus tend to be lipophilic and are sprayed on the soil surface, where they function as a waxy barrier to prevent the germination of weed seedlings or are washed into the soil to facilitate root uptake.

Foliar or leaf-applied herbicides pesticides tend to be hydrophilic, and – depending on the logP and pKa of the active ingredient – translocation of systemic herbicides may take place via the phloem or xylem transport tissues in vascular plants.

The expanded Bromilow model can predict appropriate formulations and adjuvants for specific active ingredients. Because herbicides and surfactants differ in solubility, it is important to match each solubility to maximize their performance as a combination.

Hydrophilic surfactants include oil-in-water emulsifying agents, detergents and solubilizing agents and are suited for foliar-applied hydrophilic active ingredients, such as herbicides.

Most surfactants used with post-emergence herbicides have HLB values of twelve or greater.

Lipophilic surfactants include penetrators (oils) and are suited for foliar-applied lipophilic active ingredients. As a rule, however, lipophilic herbicide active ingredients are soil applied for pre-emergence use and do not need additional adjuvants.

Herbicides in the hydrophilic-lipophilic balance (HLB) range from 1 to 10 and are more soluble in oil than in water. Those in the HLB range from 10 to 20 are more soluble in water than in oil.

Likewise, surfactants in the HLB range from 1 to 10 work best with an oil-soluble herbicide, and those in the HLB range from 10 to 20 work best with a water-soluble one.

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Adjuvant options to optimize pre-emergence root uptake

Surfactants - wetters

Allows water to penetrate soil more easily and/or flow through (infiltrate) the soil. These materials are valuable when soil becomes hydrophobic and will not wet easily.

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Humectants

Maintain solubilization of the active ingredient for the plant to absorb. Once out of solution, active ingredients may bind irreversibly to soil colloids. Some humectants may bind water too tightly, not releasing it to the plant, and others may compete with the plant for soil moisture. Organic humectants may be subject to microbial degradation in the soil.

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Adjuvant options to optimize post-emergence foliar uptake

Surfactants - wetters

Surfactants with a low HLB are absorbed into the cuticle and increase the fluidity of waxes, increasing the permeance of lipophilic herbicides in the cuticle (crop phytotoxicity). Surfactants with a high HLB (longer EO chains) enhance the hydration of the cuticle and increase the permeance of hydrophilic herbicides. Nitrogen-surfactant blends consist of combinations of nitrogen (e.g., ammonium fertilizers) and surfactants, and are generally used with herbicides. The surfactants reduce surface tension and improve leaf surface spreading, while the nitrogen compounds neutralize ionic charges and prevent the formation of precipitates in the tank mix or on the leaf surface. They are used primarily with broadleaf herbicides.

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Humectants

Enhance the hydration of the cuticle and the permeance of hydrophilic herbicides into the cuticle.

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Penetrators

COCs enhance spreading and penetration and are used primarily with grass-specific herbicides.

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Alkalizers

Basic pH blend adjuvants (blends of non-ionic surfactants, fertilizer, and basic pH enhancer) increase pH to increase charge and water solubility of some herbicides (but can increase selective herbicide phytotoxicity).

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Acidifiers

Acidifiers reduce pH to reduce charge (increase lipophilicity) of herbicides to increase cuticular uptake

Water Conditioners

Eliminate or reduce the interaction of ions in the spray solution with the active ingredient. Chelating agents, citric acids, and nitrogen (fertilizer) salts are used as water conditioners.

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