1.Techniques to study fate of foliar applied agrochemicals

 

1.1  Examination of Rain-fastness

R&B Research have developed robust techniques to allow us to examine the rain-fastness characteristics of a range of natural fungicides (although the technique can be applied to all types of synthetic and natural active ingredients) and how these can be manipulated by formulations technology specifically with respect to the inclusion of very specific surfactants/ adjuvants into the formulation.

The principle of the methodology involves laboratory-scale spray application of in-use dilutions onto a variety of plant surfaces where control of the key variables such as spray pressure, spray height, spray volume allow us to simulate the variations that may occur in the field. Once deposits are dry we can use a similar instrument set-up to allow us to replicate a range of rain-fall events where we can examine the type of rain (heavy or fine mist), the volume of rain-fall and the duration of the event.

Analysis of collected rain-fall from the plant surface allows us to accurately determine the residuality of applied deposits. This technique then allows us to accurately assess the effectiveness of various surfactant systems in improving the degree and rate of active ingredient retention on a plant surface.

The results given below are typical of the type of data generated using these techniques and clearly demonstrate how surfactant/ adjuvant chemistry can be used to enhance the performance of agrochemical active ingredients under typical field conditions.

 

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Fig 1.1 Plot of active ingredient retention on leaf surfaces as a function of formulation type.

The commercial product was a typical system used in the field and the two micro-emulsions contained different surfactant/ adjuvant systems. Whilst both micro-emulsions significant improvement in the retention of active ingredient following a rain event the additional surfactant incorporated into system B results in further enhancement when rain occurs in the early period following spray application.

 

1.2. Plant Up-take Profiling

During the examination of the rain-fastness characteristics a number of anomalies came to our attention which were suggestive of a significant increase in the rate of active ingredient penetration across the leaf surface and this required the development of additional techniques to allow us to study this in more detail.

The basic elements behind the methodology involved the extraction of material remaining on the surface with a series of controls using non-absorbant surfaces treated and stored under identical conditions. A Franz cell technique using isolated wax cuticles can be used as an initial screening process before carrying out more detailed studies on plant surfaces.

Plant surfaces were treated by spray application with a range of active ingredient dilutions and these allowed to stand for various time periods. At each of the time periods the surface of the plant material were washed thoroughly with methanol to remove any active ingredient remaining on the surface and at the same time glass surfaces were treated, stored, and extracted under identical conditions to confirm that there was no decomposition or loss through evaporation of the material. The analytical data produced in such studies allowed us to determine very accurately the amount of active ingredient that was no longer on the surface of the plant material but had penetrated across the wax membrane and cuticle.

By use of these techniques we were able to demonstrate the effectiveness of specific surfactant moieties in improving the rate of penetration of an active ingredient into the sub-surface layers of a leaf following application of in-use dilutions under typical field conditions and this is shown in the data below:

 

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Fig 1.2 Rate of fungicide penetration as a function of formulation type.

The commercial product demonstrates very poor penetration across the wax cuticle even at 24 hours post application. Both micro-emulsion systems show very significant and rapid penetration across this barrier with again formulation B showing some benefits in the early stages. This correlates well with the rain-fastness data and suggests that the improvement in wash-fastness is not purely caused by mechanical binding to the surface but more to a movement across the surface to the point of activity.

 

1.3. Photo-stability of active ingredients

The loss of biological efficacy due to photo-decomposition of active ingredients following foliar application is of major concern and therefore a detailed study of the way in the which formulation additives can improve this aspect of performance is of paramount importance.

At R&B Research we have developed techniques that allow us to accurately determine the rate of photo-decomposition but more importantly we have conducted extensive studies using a range of chemical materials that have demonstrated potential in preventing active ingredient loss by this mechanism.

Active ingredient is spray applied onto inert surfaces which are then exposed to simulated sunlight at a range of intensities and durations. Analysis of these surface deposits following extraction has allowed us to identify and elucidate specific additives for specific active ingredients and typical data is shown below:

photostability ex1

Fig 1.3. Rate of photo-decomposition of a fungicide as a function of formulation type.

The loss of active ingredient in a short time period after exposure to artificial sunlight is significant with the commercial product and certainly extrapolation beyond 3 days would result in very significant losses. The three experimental formulations A, B and C do show that by careful manipulation of the chemistry used this rate of loss can be all but removed.

 

1.4 Analysis of spray fluid coverage and distribution

As part of the on-going studies to examine pesticide fate post application to plant surfaces we were interested in studying the deposition and coverage characteristics as a function of formulation manipulation. A number of techniques were employed in these studies including contact angle and surface tension using a Kruss DSA30 system.

Surface deposition and coverage studies were carried out following spray application under different conditions to mimic a fine or coarse spray. A water soluble dye was incorporated into the spray dilution and this allowed us to examine droplet deposits under a microscope using Motic Advanced Imaging software to calculate percentage coverage and droplet size and distribution. Typical coverage patterns are shown below:

20cm 5 sec h20 parafilm image 10cm 5 sec h20 parafilm image

Fig 1.4. Microscopy images at 10x magnification showing droplet distribution as a function of spray parameters.

 

 2. Techniques to study pesticide fate following soil application

2.1 Soil Movement

R&B Research have developed a range of laboratory techniques to look at the movement of a range of active ingredient types following application to artificially prepared soil columns. Soil types can be prepared where sand content, organic matter content, clay levels, moisture content etc. can be varied widely. The soil material can then be packed into PVC columns using a compression gauge to ensure that packing parameters are maintained within narrow margins.

In-use dilutions can then be applied to the surface of the column followed by water to simulate a variety of rain events. Columns are then stored for a range of time periods and at the end of this the columns are split open and small segments removed and extracted with methanol for HPLC analysis allowing us to profile accurately the positioning of active ingredient within the column.

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Fig 2.1 Soil movement as a function of formulation type.

The aim of this study was to see whether or not we could get even distribution of a herbicide across circa 2-3 cm of soil following spray application to the soil surface. The commercial formulation currently used gives a very high loading only in the 1st cm where the active ingredient is prone to evaporative loss. The formulations B and C do demonstrate however that with careful manipulation with formulation excipients we can get close to the ideal distribution.

In the example below we were looking to achieve good distribution of an insecticide down to a depth of 10-15 cm and clearly whilst the current commercial formulation does not achieve this formulation A gets very close to this ideal distribution.

 

soil movement ex 2

 

 

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