Effective Low Applications

HANGAR CONVERSATIONS Dr. Diego Martin Oliva diegomartinoliva@gmail.com Agrônomo

During a period prior to the fungicide applications in the Republic of Argentina, a group of application companies I usually work with entrusted me with the task of analyzing the minimum liters per hectare can be applied without losing application efficiency? We know the economic result of an aerial application is directly related to the swath, the ferry distance and the liters per hectare applied. It is very interesting to analyze in terms of application efficiency what is the minimum limit with which you can work in low volumes using oil as a carrier.

In the beginning of the analysis, it cannot be ignored that the configuration and position of the spray boom is decisive for how many droplets will reach the target. This can be seen in the adjacent graphic. if the vortexes of the wings are loaded, droplets rise and evaporate or drift off target.

In earlier years, this vortex effect was intentionally sought, often due to the vapor phase of some chemicals. Now, we know that if droplets enter the vortexes, most are lost going off target with a very noticeable effect; less droplets find their way to the target with these low volume applications.

In the same way, we must also consider what type of boom hanger we are using. Depending on the relative position of the hanger, it will have a drag effect that can be seen in this photo.

All these aerodynamic effects achieved from an adequate configuration / calibration of the spray equipment are absolutely critical in reducing the flow of application since the coverage can be seriously affected. If we assume that structurally the spraying equipment is in perfect condition and we start analyzing in theoretical terms how many drops are produced according to their size and the liters per hectare, we can arrive at an initial approximation of what is the lower limit without losing efficiency.

For this we will apply the volumetric formula:

For example, if we apply five liters of total product per hectare (0.5 gal/ac), we have a droplet spectrum that will equate to 68% of the droplets produced will be 100 microns. We know the theoretical result corresponds to 95 droplets / cm2 should reach the target.

So, why do a lot less droplets arrive? How much efficiency are we losing due to small droplets of low average volumetric diameter for applications with an oil carrier?

Is it possible to achieve good results with low application volumes? The answer will be directly related to two analytical variables. In theory, the production of drops and their orientation are assured considering the parameters described above. Atmospheric conditions will limit the stability and permanence of each drop produced. The air mass between the formation of droplets will also have an impact on the outcome. We will analyze both conditions.

To analyze the atmosphere, we must know the affects of both temperature and relative humidity; the latter being much more limiting in terms of the longevity of droplets produced. We know lower relative humidity has a higher evaporation rate, causing droplets to have a shorter life.

Studies carried out a few years ago determined the lifetime of a droplet of 100 microns (typical diameter of an aerial application) is directly related to atmospheric conditions and the density of the spray mix. However, the tank mix density can only be modified with a suitable adjuvant.

The graph shows the evaporation time for 100-micron droplets with different volume concentrations by the addition of adjuvants in a controlled atmosphere. That is to say, the type of evaporation reducing adjuvant, used in aerial applications, is limited because it is initially emulsified as a first step and then the bulk density of the tank mix must be modified to preserve the greatest amount of drops from atmospheric demand, e.g. evaporation.

In order to analyze the atmosphere between the droplet forming and the target, it is important to consider the different scenarios that may arise for each treatment. It is not the same to achieve good coverage with little or no crop canopy (fungicides in cereals), since only the flag leaf needs coverage. It is much more complicated to achieve the same coverage with a denser crop canopy (fungicides in soybeans or corn). (treatment without umbrella biomass

In conclusion: Ultra low volume sprays (5 liters/ ha or 0.5 gl/acre) are perfectly possible and highly efficient (70 drops/cm2), if the following parameters are considered.

• Configuration and calibration of the application equipment.

• Atmosphere’s % of relative humidity.

• Tank mix chemistry, or evaporative reducing capacity.

• Analysis of the environmental criteria of the application.