Production methods need to be highly efficient if they are to meet the ever increasing demand for agricultural goods
for the food, feed and fibre industries.
Therefore, pesticides and fertilisers are applied frequently along with adjuvants
that support the performance of the active agent. Adhesives, surfactants, humectants, stabilisers and drift control
agents (DCA) are often incorporated in spray applications to prevent a loss in efficacy at all stages of delivery.
For example, wind-driven aerial displacement and evaporation can account for a loss of up to 50% in the total amount
of spray the farmer applies to his fields (Pimentel and Burgess, 2012). In addition, drifting agrochemicals can damage
susceptible plants on neighbouring fields and have a devastating impact on adjacent ecosystems. For this reason,
ever stricter regulations on the use of agrochemical sprays are being implemented. These facts emphasise the importance
of adopting both drift control and performance-enhancing measures to ensure efficient target coverage and high
activity on the leaf.
Adjuvants in spray applications for drift control
With regard to spray drift, apart from meteorological conditions such as wind, temperature and relative humidity,
droplet size is a crucial factor for risk management. On a warm day with a light breeze, a fine spray droplet measuring
70 microns is carried for roughly 10 metres after leaving the spray nozzle (Ozkan and Zhu, 2016). Under the same
conditions, an even finer droplet measuring 50 microns will evaporate completely before covering a distance of 50 cm.
Optimising droplet size is therefore highly important in agricultural sprays.
A variety of factors influence droplet size in sprays, such as the application equipment (especially nozzle design), operating
parameters (spray pressure, driving speed) and the physiochemical characteristics of the spray liquid itself. To control
the latter, synthetic or natural polymers are commonly applied as drift control agents (DCAs). These enhance the
elongational viscosity of the liquid, thus preventing breakaway of very fine droplets. This shifts the volumetric mean
diameter (VMD) to higher values and reduces the fraction of droplets below 150 microns, commonly defined as
“driftable fines” (Lewis et al. 2016).
Long chain polyacrylamide, polyvinyl or poly(ethylene oxide) polymers are often used as a synthetic option. However,
their limited mechanical stability is a major drawback. The recirculation system in the spray tank confers high shear
rates, breaking down the polymers and rendering them inefficient during the spraying process (Lewis et al., 2016;
Hilz et al., 2013). Furthermore, their limited biodegradability and the fact that they are produced using non-renewable
resources are important environmental issues. The polysaccharide xanthan gum has been applied as a DCA, being
a natural alternative derived from microbial fermentation of renewable materials. It offers high mechanical stability,
complete biodegradability, stability over a range of physical and chemical conditions and high compatibility with chemical
and biological active agents.
Adjuvants for improved performance on the leaf
Once the spray liquid has been delivered to the target surface successfully, its further properties become relevant.
To reduce bouncing or rolling off the leaf, adjuvants that increase adhesion are commonly added – and this task can
be fulfilled by xanthan gum. Wetting of the hydrophobic leaf surface is commonly improved by adding non-ionic
surfactants, many of which are petrochemical in origin such as ethoxylates (Castro et al., 2013). As an efficient, green
alternative, CITROFOL® AI may be applied. This brand name stands for triethyl citrate, an ester of citric acid, which
has been investigated as an adjuvant in agriculture (Johnson et al., 2002).
The combination of xanthan gum and CITROFOL® AI has the potential to overcome many challenges in the spray
delivery of active agents – from the tank mix, through the nozzle and on to the plant surface. In our trials, we investigated
the influence of these two bio-based adjuvants on relevant properties in agricultural spray, such as droplet size and
range, wettability, adhesion and moisture retention.