Defoliation Considerations for 2021: On-Target Applications Are Critical!!!

Camp Hand and Stanley Culpepper

It is that time of the season where people are beginning to think about defoliation. Traveling across the state recently, I observed bolls opening in some of our earlier planted crop. It has been a tough year for many, but we are beginning to see the light at the end of the tunnel.

It is definitely time to think about defoliation, and one topic that should be front and center for everyone is making on-target defoliation applications. Georgia farmers and their applicators have reduced pesticide drift complaints to the Cooperative Extension Service over 78% since 2014; this is simply remarkable and an achievement that should make us all proud. However, cotton defoliation drift remains a significant concern and one that we must collectively address if our hope is to maintain the practical use of these important products.

There are multiple factors that influence the movement of pesticides out of the treated area; several of those factors are discussed below.

  1. What is around my field?

As defoliation approaches, it is important to know what is around your fields. Pay attention to surrounding crops (i.e. late planted cotton, fall vegetables) and neighboring areas (i.e. homeowners). Knowing this information can assist in multiple defoliation decisions such as when to defoliate (a day with low drift potential) and application method.

  1. Wind speed and direction

Wind speed and direction are the two most important weather factors influencing spray drift. High wind speeds will move spray droplets off-target in the direction that the wind is blowing. Optimum wind speeds for any pesticide application will be between 3 to 10 miles per hour, with the optimum direction being away from a sensitive area. Examples of sensitive areas where wind speed and direction should be influential in applications are listed above.

  1. Boom Height

Boom height is one of the most important components to reducing spray drift thereby improving on-target applications. As boom height increases, potential particle drift increases. The ideal boom height for ground applications is 24 inches above the target (cotton plant). Of course, make sure your nozzle spacing provides proper spray pattern overlap!

  1. Nozzle Type / Droplet Size

Nozzle type influences droplet size, which in turn can affect the likelihood of those spray droplets to drift. If you will remember from your UPW training, smaller droplets remain in the air for a longer period of time making them more vulnerable to move off-target. Although larger droplets are less likely to drift, many growers have questioned the efficacy of these larger droplets as it pertains to defoliation (along with other applications). A study conducted across the cotton belt demonstrated that sprayer output was far more important than nozzle type, meaning that regardless of nozzle type higher sprayer output resulted in greater defoliation. Dr. Guy Collins at North Carolina State University shared the data in the graph below, which shows defoliation three weeks after treatment as impacted by droplet size (small vs. large) and sprayer output. In this situation, Prep, Folex, and Dropp were the defoliants applied. A nozzle that produces small droplets would be a hollow cone nozzle, with larger droplets being produced from an air induction nozzle (i.e. auxin nozzles).

Thus, effective defoliation can occur with larger droplets, but sprayer output must be in the 15 to 20 GPA range! Although applying more water per acre may take a little more time, if it helps us make on-target defoliant applications, particularly in sensitive areas, then it will be worth it in regards to long-term farm sustainability.

Since the auxin technologies were commercialized in cotton and soybean, Georgia growers have consistently proven to be some of the best in the country at making on-target herbicide applications. Let’s take the lessons we have learned over the past few years, use them when applying defoliants, and reduce the number of drift complaints related to cotton defoliation. As always, if anyone has any questions related to this or anything else, please contact your local UGA Extension agent. They, along with myself and the other specialists, are here to help.