What does rainfast mean in crop protection? An expert Q&A

What is driving grower interest in rainfastness solutions? 

Ryan: The key drivers behind interest in rainfastness solutions vary between regions, but the overarching trends are the same across many agricultural markets: to do more with less and to meet the requirements of ever-tightening regulations. Anywhere that rainfall can wash away plant protection products, forcing growers to choose between lower yields and costly reapplications, presents an opportunity for these solutions to shine. 

Rainfastness additives are key to formulation performance in many regions where frequent rainfall is an inevitable part of the growing season. South America and Asia-Pacific are key markets, where heavy rainfall and high humidity create ideal conditions for weed and fungal outbreaks while presenting challenges for formulations. Many parts of North America also experience unpredictable summer storm seasons, where formulations must maintain efficacy throughout key growing windows. The European market has slightly different driving forces, with environmental regulations requiring farmers and formulators to address the challenge of reducing agrochemical runoff into waterways. 

Alongside these regional differences are the increasingly unpredictable effects of climate change. This means that traditionally more predictable regions increasingly need adjuvants that can act as a “safety net” in the event of untimely weather. Target crops, as you may expect, vary by region but cover everything from broad-acre crops to high-value horticulture, as rainfastness adjuvants have use cases in herbicides, fungicides and insecticides. 

A broad formulation fit

Atplus™ Rainfast Pro uses a sticker mode of action to reduce wash-off of dried deposits following rainfall or irrigation. It has been evaluated in suspension-concentrate formulations containing representative herbicide, fungicide and insecticide active ingredients, covering contact and systemic modes of action.

Is rainfastness becoming a competitive differentiator in crop protection?

Ryan: Differentiation through adjuvancy has been a staple in plant protection products for a long time, especially for off-patent actives, where manufacturers frequently face a choice between differentiation and a race to the bottom on price. Rainfastness adjuvants can increase performance in adverse conditions and provide broader effective application windows, an important comparison point when choosing between similar products on the shelf. 

The increased durability offered by rain-resistant products can provide a strong marketing angle for manufacturers trying to convince growers who are already operating on very tight margins. For manufacturers facing costly reformulation due to regulatory restrictions on products entering waterways, improved rainfastness supports more sustainable agriculture while helping protect the long-term viability of their active ingredients. 

Evidence beyond a single claim

Atplus Rainfast Pro was assessed through six complementary analytical and biological approaches.

• Deposit-retention analysis quantified resistance to wash-off under simulated rainfall.
• MALDI-MSI provided qualitative evidence that active ingredient remained associated with washed spray deposits.
• Leaf-surface observations assessed wetting, sticking and run-off immediately after application, alongside visible biological response after seven days.
• SEM imaging showed the continuous protective film remained associated with the treated leaf area after sequential washing.
• Glasshouse testing assessed whether deposit and active-ingredient retention translated into biological efficacy.
• Colony-forming unit testing assessed compatibility with common microbial active ingredients.

Could better rainfastness improve confidence in biological crop protection? 

Ryan: Biologicals are an interesting growth market with their own unique challenges. They often have more sensitive performance and can benefit even more than chemical actives from well-matched adjuvant systems, which can maximise their potential while minimising variability. 

Many biological additives benefit from being hydrated to increase their efficacy, making hydration and durability a delicate balance to maintain. A super-wetter may encourage fast drying to secure the formulation to the leaf, but it may increase run-off and reduce availability or efficacy as a result of drying out. Improving the rain resistance of the formulation offers the opportunity for multiple efficacy windows from a single treatment while minimising wash-off. 

Biological additives often work through a contact mode of action and therefore benefit disproportionately from being more strongly adhered to the target crop. Increasing contact time and retention on the surface will often boost efficacy, with or without a rain event.

Microbial compatibility

Colony-forming unit tests assessed Atplus Rainfast Pro with Bacillus thuringiensis, Beauveria bassiana and Clonostachys rosea. Counts remained broadly comparable with untreated controls across the assessed inclusion levels, indicating no reduction in microbial viability in these tests. 

What do formulators need to understand about rainfastness? 

Ryan: Growers have all experienced first-hand the challenges of unpredictable weather and the effect it can have on product efficacy, as well as the resulting impact on final yield. Discussing additives that can be used as insurance against those eventualities is therefore important. 

Rainfastness can be a confusing topic because there are multiple effects that can be marketed as “rainfastness”, with different use cases. In short, any additive that improves the durability of a formulation’s effect in the face of rain can be described as a rainfastness additive. This can include uptake enhancers, stickers and drying-time accelerators. 

If a grower is using a sticker, which physically binds the formulation to the target, but expects it to work through a shorter drying time, they will be disappointed. Discussing the mode of action and why it produces the resulting increase in efficacy is important. This helps ensure that the correct additive type is used with the correct formulation and that performance is not incorrectly judged using unsuitable metrics. 

Sticker mode of action 

Atplus Rainfast Pro is a film-forming sticker mode of action. It is designed to improve adhesion and resistance to wash-off of a dried deposit, rather than faster drying or uptake enhancement.

How can companies demonstrate the value of rainfastness technologies? 

Ryan: The returns on investment for rainfastness technologies are spread across a range of metrics, including cost, efficiency, yield and sustainability. Increasing the durability of a formulation reduces the costs associated with reapplications and can enable expanded treatment windows or reduced application rates. 

It can also help to mitigate environmental impact by keeping active ingredients where they are useful rather than allowing them to wash into areas where they may be harmful. 

The most straightforward measure of return on investment is the increased yield of the final product despite adverse conditions, which is the most important metric for manufacturers and growers alike.

Measured performance after simulated rainfall

Across the evaluated active-ingredient chemistries, Atplus Rainfast Pro enhanced deposit retention compared with formulations containing no rainfastness adjuvant.

More than 75% of the original phenmedipham and azoxystrobin deposits and more than 65% of the imidacloprid deposit remained after 300 seconds of washing. In the phenmedipham glasshouse study, biological efficacy remained statistically comparable before and after simulated rainfall. 

What innovations could shape rainfastness over the next five years?

Ryan: Innovation is happening across all aspects of the agriculture sector, covering the chemical, technological and biological sides of plant protection. As a chemical manufacturer, our developments are focused on providing more sustainable ways to deliver the performance that farmers rely on. This means delivering equal or improved performance compared with current market-leading technologies, without the need to rely on petrochemical-derived microplastic materials. 

Alongside these innovations, we are also working on delivery systems that allow more versatile use of existing active ingredients through technologies such as encapsulation and nanoemulsions. The wider industry is also innovating in precision agriculture and drone delivery, allowing ultra-low volumes of active ingredients to be delivered exactly where they are needed, when they are needed. 

Predicting where a market will be in five years will always be uncertain, however, with opportunities for game-changing innovations that could subvert those predictions, such as ongoing work on RNA-based active ingredients. The agriculture market has a strong track record of successful innovation in the face of increasing environmental and regulatory challenges, and this will continue to shape plant protection developments over the next five years.

Designed for future-ready formulations

Atplus Rainfast Pro is microplastic-free. It is primarily intended for built-in use in suspension concentrates at 3% w/w and can also be formulated as a component of a tank-mix adjuvant solution; performance has been seen at 0.03% v/v in the final spray tank-mix. 

From expert insight to formulation evidence 

Rainfast crop protection is more than a property or a single test. The most useful formulation decisions connect the required mode of action with multiple lines of analytical and biological evidence. This helps formulators understand not only whether a deposit remains after rainfall, but whether the active ingredient remains available and biological performance is maintained. 

Atplus Rainfast Pro is our film-forming rainfastness adjuvant for reducing dried-deposit wash-off following rainfall or irrigation. 

Explore the applications datasheet, review the product overview or contact us to discuss formulation support, samples and regional availability.