Pesticide shelf life and storage stability: what the 54 °C test means and how to check stock

Shelf life is the period during which a crop protection product stays fit for use when stored as the label instructs [4]. During storage the active ingredient can degrade, the formulation can change physically, for example by separation, thickening, claying or growth of particles, and the product can interact with its packaging [4]. This guide explains how the FAO/WHO specifications and CIPAC methods test storage stability, what the results mean for buyers, and what to check in the warehouse and when stock arrives.

1. Shelf life and expiry date are not the same

CropLife International distinguishes the shelf life of a product, a period supported by stability data and granted by regulators, from the expiry date of a batch, a point in time set when the batch is released; the time from release to expiry cannot be longer than the shelf life [4]. The FAO/WHO manual states that chemical formulations, unlike biological pesticides, are generally expected to remain satisfactory for at least 2 years from the release date in unopened original containers stored according to the label, and that the label must carry storage stability information if the intended shelf life is shorter [1]. The release date, given as month and year, is the date from which the supplier guarantees quality; the manual prefers it to “formulation date”, which can confuse buyer and seller [1].

2. The accelerated storage test: 14 days at 54 °C

CIPAC method MT 46.4 places a sample in a closed container in an oven at a set temperature for a defined time, to simulate normal long term ageing by heating; it describes only the storage, and the tests afterwards use other methods [2]. Under the FAO/WHO manual, after 14 days at 54 ± 2 °C the formulation must still comply with the clauses for active ingredient content, relevant impurities and relevant physical properties, and the average active ingredient content should not fall below 95% of the content measured before the test [1]. If degradation exceeds 5% or a physical property is affected, the manufacturer must provide more information, such as the identity and amount of the degradation products [1].

Lower temperatures may be chosen when a product is not intended for hot climates and is damaged by very high temperature, and temperatures above 50 °C are likely to be avoided for products in water soluble bags [1]. If no significant chemical, physical or packaging change occurs in the standard test, CropLife expects the product to meet its specification after 2 years at ambient conditions; doubling the storage time, for example to 4 weeks at 54 °C, is proposed to support a shelf life of up to 4 years, although some countries also require real time data [4]. Real time studies, usually for at least 2 years at ambient temperature, do not rely on extrapolation and are always acceptable [4].

Accelerated tests carry uncertainty: higher temperatures can start degradation pathways that do not occur at lower temperatures, and reaction rates are not linear with temperature, so a fluctuating warehouse temperature is not equivalent to a constant one with the same average [1][4]. The manual notes that deviations of up to ±10% from the nominal active ingredient content are generally accepted not to affect biological performance significantly, and that an overage of no more than 10% may be used where degradation is unavoidable [1].

3. Stability at 0 °C

For liquid formulations the FAO/WHO manual uses CIPAC MT 39.3, adopted in 1998 as a combination of two earlier methods for the stability of liquid formulations at 0 °C [1][3]. After 7 days at 0 ± 2 °C, solutions such as EC and SL formulations may separate no more than 0.3 mL of solid or liquid, while suspension concentrates and oil dispersions must still meet their suspensibility or dispersion stability and wet sieve limits [1]. The risks are crystallisation of the active ingredient and separation of liquid phases; the original properties must return on warming and mixing, and for capsule suspensions freeze and thaw cycles may matter [1][4].

4. Why physical properties are rechecked after storage

A formulation can keep its active ingredient and still fail in the sprayer, so the physical clauses protect application. The wet sieve test limits insoluble particles that could block nozzles or filters, to a maximum of 2% retained on a 75 µm sieve; suspensibility keeps the active ingredient evenly dispersed in the spray liquid, normally 60% to 105% after 30 minutes; emulsion stability does the same for EC and EW formulations; and pourability limits the residue left after a standard pouring test to 5% [1]. Suspensibility and emulsion stability are normally measured before and after the elevated temperature test [1].

The manual also advises inspecting SC and OD containers before sampling: on standing they develop a concentration gradient and may show clear liquid on top or sediment at the bottom, so they should be homogenised as the manufacturer instructs, and no sticky, undispersed layer should remain at the bottom [1].

5. Microbial products follow different rules

Microorganisms are sensitive to heat, and the FAO/WHO manual for microbial pesticides states that the 54 °C test used for chemical pesticides is not suitable for them; their labels therefore need an expiry date and storage conditions [5]. The chemical manual likewise notes that biological products based on microorganisms may carry a shorter shelf life on the label [1]. For Huasheng's microbial products, such as Triva™, a Trichoderma harzianum T-22 wettable powder, and Baciva™, a Bacillus amyloliquefaciens D747 suspension concentrate, follow the storage conditions and expiry date printed on the label.

6. How warehouse conditions affect shelf life

FAO's storage manual notes that shelf life is shortened when pesticides are stored badly, for example at high temperatures, and that pesticides deteriorate more slowly in a cooler store, which is one reason why ventilation matters [6][7]. Dust and wettable powder formulations tend to cake under high temperature, high humidity, strong sunlight or compaction, more than liquids in sealed containers, and shelf life declines rapidly once containers are opened and left partly empty [6]. CropLife advises keeping products under a roof, storing weatherproof drums outdoors only temporarily and only if their contents are not sensitive to temperature extremes, and limiting sunlight through windows because it can damage plastic containers and labels [8].

Climate matters when stability is claimed. CropLife gives typical annual average temperatures of 18 to 22 °C for moderate, 23 to 27 °C for hot and 28 to 31 °C for very hot climates, and notes that a formulation stable under the conditions of a hotter zone is suitable for cooler ones [4]. Transit time also counts: stock planning should allow for the time products spend between manufacture and arrival in the store [6].

7. Reading the label: release date, batch and expiry

FAO/WHO labelling guidance asks for each lot or batch to be identified, for the release date (month and year) to be shown, and for an expiry date and storage stability information where appropriate; a shelf life or expiry date should appear when the shelf life is less than 2 years from the release date [9]. National rules add their own requirements:

• China: the Regulations on Pesticide Administration require the production date on the label, and a pesticide past its quality guarantee period is treated as substandard [10].
• Mexico: NOM-232-SSA1-2009 requires labels of agricultural pesticides to show the lot number, covering the manufacturing date, and the expiry date, which may be printed on the container body [11].
• Where no shelf life is given, FAO's storage manual advises assuming 2 years unless the manufacturer or distributor gives better information [6].

A manufacturer may set a new expiry date for a batch only before it expires, on the basis of retesting and stability data, where national rules allow, and the new date must not exceed the expiry date of the packaging [4]. Outdated stock may still be usable, but only analysis by the manufacturer or an independent laboratory can show it; raising the dose by trial and error is not recommended [6].

8. What to check when stock arrives

• Read the release or production date, batch number and any expiry date, and compare the remaining shelf life with the season of use [6][9].
• Order no more than about one year's requirement and issue stock first in, first out [6][8].
• Inspect containers for leaks, seepage, bulging or panelling, corrosion along seams and damaged labels [4][6][8].
• Look for caking of powders, sedimentation or gelling of liquids, and discoloration [6].
• For SC and OD formulations, homogenise before sampling and check for a sticky layer at the bottom [1].
• Agree the specification, including the storage stability clauses, in the contract and compare the certificate of analysis with it [1].
• Store under a roof, out of direct sun, in a ventilated building, and use opened containers first [6][7][8].

Frequently asked questions