Adjuvants – key enablers of sustainable formulations
Feeding a growing world population with limited natural resources, while climate change advances, is a significant challenge for the 21st century. Therefore, mitigating the effects of climate change is at the heart of the European Green Deal.1 Within this EU program, the Farm to Fork Strategy2 addresses agrochemical sustainability needs and targets, including the reduction in chemical pesticides use and risk by 50% by 2030. Sustainable intensification of agricultural practice is a potential solution, i.e., by reduced application of crop protection products and maximized effect (improved efficacy). Therefore, precise application techniques and adjuvants play a crucial role to achieve the ambitious EU targets. Adjuvants can significantly help to safeguard robust product performance and enable efficient active ingredient (a.i.) delivery. For example, retention agents reduce a.i. losses, spreaders ensure homogeneous a.i. distribution on the leaf and penetrators enable rapid a.i. uptake, even under challenging weather conditions. Moreover, adjuvants and other co-formulants need to fulfill more stringent environmental, health and safety criteria, and should have a sustainable footprint.
The optimum selection of adjuvants is determined by multiple factors, including a.i. physical-chemical properties, a.i. mode of action, targeted crop and pathogen, formulation type, and interaction with other co-formulants. There is no one adjuvant that fulfils all the criteria and it is always chosen on a case-by-case decision. However, detailed adjuvant usage guidance helps formulation chemists to aid optimum selection. In this paper, we discuss several adjuvant testing methodologies using the well-established adjuvant Plurafac® LF 221 as model substance and compare with a market standard, sorbitan monolaurate ethoxylate. Plurafac® LF 221 is an alkoxylated branched alcohol, which can significantly improve retention and spreading, and enhances a.i. uptake.
Retention, spreading, and uptake are vital when considering the delivery chain (from leaving the nozzle until uptake) of the applied spray solution (see Figure 1).
Figure 1: Delivery chain:
Retention – first and important step within delivery chain
Foliar application is the most common method to apply crop protection formulations. Therefore, the correct selection of a retention agent is a crucial step to maximize the amount of a.i. available to the crop. Excellent retention is of especially high importance for spray application on grasses, due to the vertical leaf position, epicuticular wax structure, and low surface area. Herein, adjuvants with low dynamic surface tension (DST) are known to improve retention of the spray solution3.
As shown in Example 1, the retention of an Azoxystrobin SC spray droplet on an oilseed rape can be significantly enhanced with the use of Plurafac® LF 221. High speed videos show that droplets of either a non-adjuvanted Azoxystrobin SC or an Azoxystrobin SC containing sorbitan monolaurate ethoxylate readily ″bounce off″ the leaf surface, whereas the addition of Plurafac® LF 221 enables highly effective ″sticking″ and thus excellent retention and wetting.
Example 1: High speed videos on oilseed rape with Azoxystrobin SC (1%) without and with adjuvant (0.05%); droplet size 20 µl; angle of incidence 45°:
Although laboratory test methods give a good indication for adjuvant performance, such tests can have certain limitations and cannot substitute testing on plants. Therefore, retention trials on wheat (BBCH 12) were conducted in a spray chamber.4 As shown in Example 2, the addition of Plurafac® LF 221 to a non-adjuvanted Azoxystrobin SC can double the retention of the spray on difficult to wet wheat leaves, thus retaining more a.i. on the crop.
Example 2: Retention trials in spray chamber on wheat (BBCH 12). Application parameters: Azoxystrobin SC (1%), adjuvant (0.05%); nozzle ID 120 02 at 3 bar, velocity 2.67 m/s.
Spreading and wetting on the leaf to ensure equal active distribution
Adjuvants can also improve spray droplet leaf spreading via the reduction of the applied droplet contact angle on the leaf surface. Rapid spreading ensures fast drying and thus contributes to rainfastness. Rapid wetting is primarily driven by using an adjuvant with low static surface tension and depends on many variables, notably crop type and growth stage. As shown in Table 1, the use of Plurafac® LF 221 can significantly lower the contact angle and thus ensures good leaf wetting.
Table 1: Contact angle after 1s at room temperature on various substrates
Penetration – key step for systemic actives
Plant cuticles protect leaves from dehydration, but therefore also provide an efficient transport barrier for the penetration of agrochemical formulations into leaves. There are three main pathways for a.i. uptake into leaves (1) hydrophilic (for water soluble actives, such as Glyphosate) (2) hydrophobic (through the waxy cuticle), and (3) stomatal flooding. The rate of uptake depends on crop type, adjuvant selection and its formulation, application conditions, and a.i. properties (e.g., a.i. hydrophobicity is often expressed in logP, molecular weight, and melting point). The influence of adjuvants on the rate of penetration of radiolabelled Epoxiconazole through cherry laurel cuticles was determined over 48 h using ″simulation of foliar penetration″ (SOFP) tests (see Example 3 a).5 Herein, Plurafac® LF 221 was shown to be a highly effective cuticle penetration enhancer at a concentration of 0.1% w/w, enabling the uptake of more than 1/3 after 8 h and almost 95% after 48 h.
Example 3: a) SOFP Penetration rate (%) over 48 h; Test conditions: adjuvant concentration: 0.1%; Test cuticle: Prunus laurocerasus . b) Biological performance (field trial); Test conditions: Infestation of rapeseed with Sclerotinia, SC (Boscalid 200 g/l, Dimoxystrobin 200 g/l), Spray volume 300 l/ha using Air Mix 110-03 at 2.1 bar.
Biological efficacy – that is what counts
In a GEP (good experimental practice) trial, a SC (Boscalid 200 g /l, Dimoxystrobin 200 g/l) was applied in triplicate on rapeseed variety NK Technik at growth stages BBCH 53, 53, and 65 at a the label rate (N = 500 ml/ha) and half rate (N/2= 250ml/ha). As shown in Figure 3 b, the addition of 150 ml Plurafac® LF 221 to N/2 alone in the spray tank mix significantly improved the yield compared to N/2, reaching a similar performance the full label rate. Therefore, the use of Plurafac® LF 221 can enable the reduced dosage of pesticide formulations, while matching the original dose rate performance.
Adjuvants selection for your formulation
Adjuvant selection for a formulation and/or application type can be significantly optimised by an in-depth understanding of adjuvant properties and modes of action. Herein, we show Plurafac® LF 221 to be a highly versatile adjuvant for retention, spreading and penetration enhancement, and can be readily used in most common formulation types, e.g., EC and SC.
To discover BASF´s broad adjuvants portfolio, please use the product recommendations provided under https://care360.basf.com
1. https://ec.europa.eu/info/strategy/priorities-2019-2024/european-green-deal_de, accessed 6 December 2021.
2. https://ec.europa.eu/food/horizontal-topics/farm-fork-strategy_de, accessed 6 December 2021.
3. Wirth, Storp, Jacobsen, Pestic. Sci. 1991, 33, 411-420.
4. H. de Ruiter et al., Weed Science 1990, 38, 567-572.
5. L. Schreiber et al., J. Agric. Food Chem. 2022, 70, 777-784.
This article was published in AgroPages '2023 Formulation & Adjuvant Technology ' magazine published this May.
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