Drift control: Agriculture's new normal

Published online: Oct 27, 2013
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Developing and practicing mitigation strategies for glyphosate-tolerant systems is the new normal for agriculture.

Terms such as sub 105 microns, auxin-responsive, rheology, atomization, humectancy and laser refraction and are now commonplace alongside DRT (drift reduction technology), A.I. (active ingredient) efficacy, wind tunnels, driftable fines, invert suspension and polymers.

New or not, all these terms are related to the same issues: Herbicide-resistant weeds and the off-target drift of herbicide droplets from glyphosate, dicamba and 2,4-D. Additionally, new EPA rules on reducing driftable fines are quickly moving forward.

It's important to recognize the scope of the problem we're dealing with. According to the 2011 USDA NASS report, glyphosate-tolerant acres represented 94% of soybeans, 72% corn and 74% cotton. Farms expanded in the last 10 years, becoming more efficient with less spending on herbicides and more spending on land, spray equipment, aerial applications and harvesting. Farms with 15,000 acres are more common, and herbicide-resistant weeds thrive across the U.S.

Redesigning the sciences behind 2,4-D and dicamba is a significant goal to reduce the incidence of these herbicide-tolerant weeds. Mitigation chemistries, based on auxin-responsive research, include a new 2,4-D formulation and crop-tolerant system from Dow AgroSciences that is pending registration and new dicamba formulations with crop-tolerant systems from BASF and The Monsanto Co., which are also pending registrations.

Dealing With The New Normal
Because resistant weeds must be managed with sustainable practices, the biggest challenge for growers and applicators right now is making sense of it all. Applicator training based on newer research is an ever-changing process and nearly all row-crop growers struggle with these issues. Some growers only know glyphosate-tolerant systems. Other growers, accustomed to pre-genetically-modified crop practices, want to maintain the efficiencies gained with their glyphosate-tolerant systems.

It is important in this discussion to not focus solely on the reduction in driftable fines (generally measured in 105 microns or less). Wilbur-Ellis research has focused on the reduction of driftable fines and the management of VMD (volume meter diameters), along with the subsequent efficacy of products following the inclusion of a DRT adjuvant in the tank mix. Many DRT adjuvants are only designed to reduce drift. The best also ensure that the A.I. reaches the target and provides maximum A.I. performance.

Additionally, most research has focused on ground application. Significant differences exist between DRT efficacies when considering aerial application. First, the higher wind speed creates much higher shear forces on the spray solution as it leaves the nozzle. Second, the gallons of spray solution per acre are drastically reduced, which makes the deposition pattern even more critical to manage.

Blowing In The Wind Tunnel
DRT spray adjuvants are being modified and tested in the field and in wind tunnels. Currently there are three classifications of spray adjuvant chemistries for DRT: Invert suspension, polymer-based and oil-based chemistries. Let's review all three. 

Invert Suspension. This unique group is an oil sub-category providing either a suspension of water encapsulated within an oil shell or water surrounded by an oil coating used to minimize the creation of driftable fines (sub 105 microns) after being sprayed through a nozzle tip. Research confirmed by University of Nebraska Wind Tunnel testing shows this adjuvant chemistry to be best in class.

Polymers. These DRT adjuvants are formulated with either synthetic or natural polymers (guar gum) that act to increase the viscosity of the spray solution and affect the rheological profile by producing larger spray particles. Polymer-based adjuvants increase the possibility of spray particles shattering, increasing drift. They are not effective for applications of glyphosate. Wilbur-Ellis does not recommend the use of polymer-based DRT adjuvants for aerial applications of glyphosate or any other A.I.

Oils. This group includes COC (Crop Oil Concentrate) and MSO (Methylated Seed Oil) technologies. They act as humectants to move the A.I. droplets through the spray nozzle and reconfigure the droplets on the outside to keep the A.I. from evaporating.

Wilbur-Ellis research has reinforced the best practices for DRT require the right mix of spray nozzles (air induction/venturi recommended), spray adjuvants (invert suspension recommended for both air and ground), spray parameters (pressure and speed for air or ground), optimum spray volumes (two to five gallons per acre for air, 10 to 20 gallons per acre for ground) and buffer zones. Wilbur-Ellis encourages applicators, retailers and growers to take the time to become familiar with this new normal for agriculture.

Keywords For DRT
Making sense of the new normal involving DRT and glyphosate-mitigation strategies requires an understanding of the terms involved. Here are some definitions:

Auxin Herbicides refers to the chemical classification of herbicides that mimic auxin, plant growth hormones, and include 2,4-D and dicamba technologies.

Auxin-Responsive Genetic Herbicides are the new 2,4-D herbicide formulation and tolerant crops developed by Dow AgroSciences, and new dicamba herbicide products and crop systems from BASF and The Monsanto Co., all with registrations pending.

Atomization Profile or Droplet Spectra refers to the process of reducing spray solution into tiny particles.

DRT (Drift Reduction Technology) refers to the practice of mitigating pesticide drift to off-target areas, improving the accuracy of applications and minimizing volatilization. Technologies and application practices supporting DRT include spray adjuvants, spray nozzles, application speeds and increased flow rates. Adjuvant products within the DRT spectrum are known as deposition agents, drift control agents, retention agents and drift retardants. DRT is also the name for the EPA-lead initiative to "achieve improved environmental and human health protection through drift reduction by accelerating the acceptance and use of improved and cost-effective application technologies."

Humectancy refers to spray particles ability to retain moisture and maximum active ingredient (A.I.) performance when hitting the target; humectants are frequently used in oil-based spray adjuvants.

Laser Defraction refers to the instrumentation inside wind tunnels that allows researchers to measure actual droplet sizes and develop a profile that will tell the EPA how and where droplets are dispersed.

Maintaining A.I. Efficacy refers to the positive deposition of the active ingredient and its performance within the recommended application rates in the practice and science of drift reduction. Maintaining A.I. efficacy is a key objective for Wilbur-Ellis research for spray adjuvants within a DRT system.

Microns is a unit of length, equivalent to a millionth of a meter; a droplet smaller than 105 microns is considered driftable.

Nozzle Overhaul is the process of upgrading spray nozzles by switching to the latest air induction or venturi nozzles that produce the appropriate droplet size to minimize spray drift.

Rheology or Rheological Profile refers to the study of liquid flow. Newer definitions suggest that rheology is the study of liquid materials that behave or flow in an unusual manner. Spray adjuvants for DRT are studied for their rheological tendencies and positive impact on reducing or eliminating drift and maintaining A.I. efficacy.

Simulated Sprays refers to wind tunnel testing of A.I. and adjuvants. It is important that simulated sprays include both the active ingredient and DRT spray adjuvants. Not all wind tunnels are constructed to allow testing of A.Is.

Spray Parameters refers to limits in setting application speed and wind pressure in a DRT program so that A.I. efficacy is not compromised.

Viscosity refers to the thickness of a fluid and its resistance to flow. In DRT the higher the viscosity of the fluid, the likelier the droplets will shatter when exposed to wind shear.

VMD (Volume Median Diameter) refers to the mean droplet size produced from a nozzle tip, in which one half of the spray volume consists of droplets larger than the mean, and one-half consists of smaller droplets, smaller than the mean.

Wind Tunnel refers to the science of bringing the wind inside to evaluate its power on drift, and includes study of wind velocity, application speeds, nozzle selection and carrier volumes through ground or air application. These are also known as drift tunnels with the recent focus on DRT. University of Nebraska, North Platte is the only commercially-available wind tunnel that is authorized to submit measurement for a DRT star ranking by EPA.

Source: www.croplife.com