One aspect of producing a high quality and high yielding crop is to protect that crop against pests such as weeds, insects and diseases.  While there are a variety of options to accomplish that goal, one widely used tool for pest management is the sprayer.  With any spray application, the applicator needs to balance coverage with minimizing drift and preventing off-target movement of the pesticide.  In today’s column, I will cover spray droplet size as one best management practice that spray applicators need to consider.          

Droplet size relates to both drift potential and coverage.  Smaller droplets can result in better coverage, provided those droplets actually make it to the plant leaf target.  As droplet size decreases, the potential for drift and off-target movement increases.  Jason Deveau, a sprayer technology specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs recently posted an interview on sprayer best management practices on the sprayers 101 web site and here is what he had to say regarding droplet size:

“Larger droplets have more mass, which means they are more likely to fall rather than be carried away. But, for a given rate, the number of droplets a nozzle produces decreases as average droplet size increases. It’s the same amount of pie no matter how many slices.

Fewer droplets might compromise spray coverage, particularly when targeting small weeds or when using a contact pesticide in a dense canopy. The answer is to use more volume to bring the droplet count back up, but that means more refills for the sprayer operator, which is time consuming. Traditionally, a grower would choose a nozzle based on the desired rate for a given pressure. As the sprayer changed speed, this would lead to over — or under — application. So, for convenience and consistency, most growers use rate controllers that monitor speed and auto-adjust the rate using pressure.  However, pressure also changes droplet size and spray pattern. Patterns can collapse at lower pressures (say <30 psi) and average droplet size decreases as pressures increase.  Pulse-width systems have changed this, but they are still few and far between.”

Next week I will cover boom height and weather conditions as best management practices.