by Eng. Tim Sander (Micron Sprayers Ltd, UK) Agrs. Pedro Daniel Leiva and Mariano Luna (INTA Pergamino, RA)
Introduction
When crop protection products are applied from the air under conditions of low humidity and high ambient temperature, evaporation of the spray droplets can result in poor efficacy of the treatment.
Factors affecting the rate of evaporation of a spray droplet are: the initial droplet size, the volatility of the spray liquid, the atmospheric conditions (relative humidity and ambient temperature), the wind and the release height above the canopy. In Argentina, insecticide and fungicide applications for soybeans are carried out during the summer, when conditions for aerial spraying can be critical due to low humidity and high temperature. Water based spray formulations are volatile and droplets evaporate quickly under these conditions. For this reason, oil and other adjuvants are added to the spray mix to reduce the rate of evaporation.
The size of spray droplets is critical in determining their ability to penetrate foliage and to reach the lower part of the crop canopy, where the initial infection of plant diseases can occur and where insects may shelter from intense heat. Small spray droplets have a higher probability of reaching the lower part of the canopy, whilst larger ones are likely to impact on the upper part of the plant.
A characteristic that differentiates rotary atomizers from traditional hydraulic nozzles is their ability to produce smaller and more uniformly sized droplets. However, these smaller droplets fall more slowly and evaporate more rapidly than the larger ones, partly due to their reduced weight, but also due to having a larger surface relative to their volume.
It is, therefore, important to consider the factors that can reduce the evaporation of spray droplets. The most important consideration is to reduce the volatility of the spray liquid and this is the reason for the addition of anti-evaporant oils and adjuvants under conditions of high temperature or relative humidity of less than 60%.
This report summarizes a series of field trials carried out to assess the effects of anti-evaporant adjuvants on the size and distribution of spray droplets deposited on water sensitive cards.
Objectives
1- To analyze the effect of adding anti-evaporant adjuvants to spray mixtures applied at a rate of 10 litres/ha. (one gal/ac.)
2- To compare the effects of different anti-evaporant products added to spray mixtures applied at 10 litres/hectare.
3- To compare the effect of the same amount of anti-evaporant when added to spray mixtures applied at 10 litres/ha and 4 litres/ha. (.4 gal/ac)
Materials and Methods
The trials were carried out at Estancia “La Noria” of the Moreno Brothers on August 18 and 19, 2007.
The aircraft used was an Air Tractor 502 equipped with nine Micronair AU5000 rotary atomizers in a 4+5 configuration. The outboard atomizers were mounted at 66% of the wingspan with the remainder on an equidistant distribution. The height of the atomizers over the targets was four meters and all applications were carried out with a crosswind.
Water sensitive cards were used to assess the spray deposit. Forty-six of these collectors were used, located at 30 cm above the ground. 33 cards were horizontal and 13 were vertical, facing the wind. All cards were placed in a single line, 66 meters long and parallel to the wind direction. The distance between horizontal collectors was two m., whereas vertical collectors were placed 4 m apart; the vertical collectors were placed at the downwind end of the line, starting 26 meters from the upwind end (which corresponds to horizontal collector N°13).
For each trial, the aircraft made only one spray pass, with the aircraft passing over card N° 11. On this basis, the collector line was divided into two sectors, 22 meters upwind and 44 meters downwind.
All trials were made with water sensitive cards (25 x 76 mm Water Sensitive Paper, Syngenta Agro).
The spray mixtures and volumes sprayed were as follows:
A- 10 litres/ha (one gal/ac)
1- Water at 10 l/ha
2- Water 8 l + Excet (vegetable oil) 2 l
3- Water 8 l + Natural Oleo (vegetable oil) 2 l
4- Water 8 l + Nutrifol (synthetic anti-evaporant) 2 l
B- 4 lit/he (.4 gal/ac)
5- Water 2 l + Nutrifol (synthetic anti-evaporant) 2 l
6- Water 2 l + Natural Oleo (vegetable oil) 2 l
Excet is a commercial brand name of Laboratorios Nova, Natural Oleo is of Stoller and Nutrifol belongs to Laboratorio Quimeco SRL. Nutrifol is a mixture of foliar fertilizer and Xilonen, anti-evaporant.
The AU5000 atomizers were adjusted as follows:
10 litre/ha applications:
VRU – Position 9
Fan blade angle: 65º
4 litre/ha applications:
VRU – Position 7
Fan blade angle: 55º
The reduction in fan blade angle (increase in rotational speed) for the 4 l/ha applications was necessary to obtain a smaller droplet size to compensate the lower application volume.
The number and size of the stains left by the droplets on the sensitive cards were analyzed with a computer program (StainMaster release 1.0.8). Two hundred seventy-six cards were used for the trials and the images of the stains were digitized using a flat bed scanner (BenQ 4600) at 600 dpi using JPG image format. The parameters assessed were: number of droplets/ cm2, Volume Median Diameter (VMD) in _m and calculated application rate in l/ha.
This same computer program was used to calculate the theoretical deposit based upon a simulation of superposed spray passes, This was based on the data corresponding to a single pass superimposed in a carrousel flight pattern. The parameters used to calculate the coverage with a superposed pattern were: track spacing of 25 meters for application at 10 l/ha and 30 meters for application at 4 l/ha.
A wider track spacing was used for the 4 l/ha simulation to allow for the greater downwind displacement of the smaller spray droplets used for the lower application rate.
Results and discussion
The average meteorological conditions during the trials were:
Relative humidity—51.7% (max. 60, min. 43), Temperature:—7.5º C (5.0 – 10.0º C), Wind speed:—13 km/hr (9 – 14 km/hr), Atmospheric pressure: 1033 millibars.
The specific conditions for each treatment are detailed in Table 1.
When the number of droplets with water at 10 lit/ha was evaluated, we can see in Chart 1 that the number of droplets on vertical collectors is double that obtained on horizontal collectors, 80 and 38 droplets/cm2, respectively. The maximum values are displaced downwind from the centre of the flight pass by 6 meters for horizontal collectors and 16 meters for vertical collectors. This would appear to indicate that the smaller spray droplets are impacting on the vertical collectors and that these are travelling further downwind. Furthermore, as can be seen in Chart 1, there are no impacts on the first 11 collectors because of the downwind displacement of the spray swath.
Analysis of the results with 2 l of Natural Oleo (in the same volume of total mixture) (Chart 2), shows that droplets start appearing on the collectors after Nº 9, with maximum deposits of 55 and 118 droplets/cm2, on horizontal and vertical collectors respectively. The increase in coverage resulting from the use of anti-evaporant was an average of 46% in both positions of the collectors. Also, the offset of the maximum values with respect to the flight path was of four and eight m, for horizontal and vertical collectors respectively.
From this data, we can conclude that the addition of anti-evaporant to the spray mix results in more droplets impacting, with the smaller droplets drifting 50% less (compared against water alone). We deduct this from the fact that the majority of the droplets were recovered at half the distance (8 vs. 16 m), comparing water and the mixtures with oil respectively.
In practical terms, when we simulate a typical field application by superimposing flight lines on a carrousel pattern. This gives the results shown in Chart 3.
When we compare the first treatments at 10 l/ha (in which anti-evaporants were used at 2 l) with the treatment with water at the same rate, the number of droplets increases an average of 80%, namely 30 vs. 16 droplets/cm2. Furthermore, as can be seen in Chart 3, all three anti-evaporants tested gave a similar performance. These results demonstrate the importance of the use of anti-evaporant for conditions of low relative humidity (below 60%), even with low ambient temperature (7.5ºC average/45°F) and moderate wind (13 km/h average/8 mph).
When we analyze the results with a spray volume of four l/ha and a concentration of anti-evaporant of 50% (2 l water + 2 l anti-evaporant), we can see significant differences in the stains on the water sensitive cards between those left with oil based adjuvants (Natural Oleo and other vegetable oils) and with water-soluble products (Nutrifol/Xilonen).
In order to explain these differences, it is necessary to understand two limitations of water sensitive cards: (i) they do not produce reliable stains with droplets with a diameter of less than 50 µm and the (ii) stains are not reliable when the droplets contain less than 75% water. Both factors (small size droplets and high concentration of oil) help to explain why the mixture of Natural Oleo at 4 l/ha gave 30% less droplets per cm2 compared with water at 10 l/ha (11.5 vs. 16.3 droplets/cm2 respectively). Examination of the same cards using reflected light indicated many visible Impacts that did not cause a blue stain on the cards.
Commercially available oil sensitive cards (Oil Sensitive Paper CF-1, Syngenta Agro) do not mark reliably with vegetable oil, especially with small droplet sizes. These were not, therefore, a reliable alternative means for evaluation.
The mixture containing Nutrifol at 4 lit/ha., produced almost four times the number of impacts/cm2 when compared with water at 10 lit/ha, and two times the number with the same total volume and 20% of anti-evaporant. This unusual result shows that when using a low volume it is very important to protect the small droplets with an anti-evaporant. Also, it is important to use a combination of spray mix and sensitive papers that allow the deposit to be quantified in a trial.
When we analyze the size of the droplets with water at 10 lit/ha, (Chart 4), we observe that: (i) the size of the droplets reduces towards the downwind end of the line of collectors (250 to 150 µm) and (ii) the size of the droplets collected on the vertical surfaces is significantly smaller compared with the those on the horizontal collectors, (220 vs. 300 µm).
The size of the droplets collected with the blend with Natural Oleo at 20% (Chart 5), shows the following trends: (i) the initial size of the droplets produced is smaller – compared with water – this being caused by the emulsifiers contained in the oil (200 and 270 µm) respectively; (ii) few differences in size are observed when comparing droplets on both types of collectors; (iii) the reduction in droplet diameter towards the downwind end of the collector line is less when the spray mix contains oil, whilst with water the size reduction is more significant (Chart 4). All along the 44 m the water droplets loose some 100 µm in diameter (270 to 170 µm) over the 44 m length of the collector line, whilst the reduction in diameter over the same distance with the addition of oil is only 40 µm (200 to 160 µ).
Chart 6 illustrates the change in droplet diameter. Water produces a larger initial droplet size but has a higher evaporation rate whereas the spray mixes containing oil produce a smaller initial droplet size but the size is reduced at a slower rate as the droplets move downwind
Conclusions and recommendations
1. The addition of oil or other anti-evaporant adjuvants to the spray mix significantly reduces evaporation, especially under conditions of low relative humidity and/or high temperature.
2. The addition of oil reduces the initial size of the droplet generated by a rotary atomizer.
3. The combination of a lower rate of evaporation and smaller droplet size results in better droplet coverage on the crop canopy.
4. Care must be taken when using sensitive papers with low application volumes (especially with high concentrations of oil) as the stains on the cards may not be representative of the actual deposit on the crop canopy
Acknowledgements
To Pablo and Alexander Moreno of “La Noria” for supplying the Air Tractor AT-502
To the Lic. Esteban Ricagno for the reading of 350 test cards with the StainMaster Program release 1.08
To Mr. Eduard Siri, of Siper Aviation, for sponsoring Eng. Tim Sander of Micronair.