Herbicide Drift

If you think we’ve had more “high” wind days this spring you are thinking correctly. In March and April of this year, northwest Iowa has had 27 days (out of 55) with average daily wind speeds over 10 mph. This compares to an average of 23 graph

days over the previous 4 years. Additionally, the area has had more days (9) with average wind speed over 15 mph compared to the last 5 years. Why is this important? March and April is the time when farmers spray herbicides on crop fields to control troublesome weeds. Wind speeds over 10 mph greatly increase the risk of herbicide drift and potential off-target injury. Additionally, pesticide drift complaints registered by the Iowa Department of Agriculture have increased every year over the last 6 years. With the challenge to timely apply pesticides in low wind conditions and the increasing frustration of rural residents with off-target injury problems, several management strategies need to be employed to reduce the risk of off-target injury.

Use nozzles that produce large spray droplets. Droplet size is the major factor influencing herbicide drift. Small droplets take more time to fall to the ground and can thus drift farther. It is desirable to use a nozzle that produces large, uniform droplets. Switching from standard flat-fan nozzles (such as an XR11003) to turbulence-chamber or venturi nozzles increases droplet size and can greatly reduce the amount of drift. Examples of such nozzles are Turbo TeeJet and AI TeeJet nozzles (Spraying Systems Co.), Raindrop Ultra (Delavan), and the TurboDrop (Greenleaf Technologies).

Table 1. Influence of droplet size on potential distance of drift.
Droplet Diameter (microns)	Type of Droplet	Time required to fall 10 ft	Lateral distance droplets travel in falling 10 ft in a 3 mph wind
5	Fog	66 minutes	3 miles
20	Very fine spray	4.2 minutes	1100 feet
100	Fine spray	10 seconds	44 feet
240	Medium spray	6 seconds	28 feet
400	Coarse spray	2 seconds	8.5 feet
1000	Fine rain	1 second	4.7 feet
Source: Klingman (9), Potts (11) and Akesson and Yates (2)

Distance between nozzle and target (boom height)
Less distance between the droplet release point and the target will reduce spray drift. Less distance means less time to travel from nozzle to target and therefore less drift occurs.

Herbicide volatility
All herbicides can drift as spray droplets, but some herbicides are sufficiently volatile to cause plant injury from drift of vapor (fumes). For example, 2,4-D or MCPA esters may produce damaging vapors, while 2,4-D or MCPA amines are essentially non-volatile and can drift only as droplets or dry particles.

Relative humidity and temperature
Low relative humidity and/or high temperature will cause more rapid evaporation of spray droplets between the spray nozzle and the target than will high relative humidity and/or low temperature. Evaporation reduces droplet size, which in turn increases the potential drift of spray droplets.

Wind direction and velocity
Herbicides should not be applied when the wind is blowing toward an adjoining susceptible crop or a crop in a vulnerable stage of growth. The amount of herbicide lost from the target area and the distance the herbicide moves will increase as wind velocity increases, so greater wind velocity generally will cause more drift. However, severe crop injury from drift can occur with low wind velocities, especially under conditions that result in vertically stable air.

Spray pressure
Spray pressure influences the size of droplets formed from the spray solution. The spray solution emerges from the nozzle in a sheet, and droplets form at the edge of the sheet. Increased nozzle pressure causes the sheet to be thinner, and this thinner sheet will break into smaller droplets than from a sheet produced at lower pressure. Also, larger orifice nozzles with high delivery rates produce a thicker sheet of spray solution and larger droplets than smaller nozzles.

Communication
The majority of drift complaints pertain to trees, shrubs and ornamentals. Communicating with non-agricultural rural residents may reduce complaint submissions originating from lack of knowledge of products used and injury symptoms.

All nozzles produce a range of droplet sizes. The small, drift-prone particles cannot be eliminated but can be reduced and kept within reasonable limits. Here are some tips:

· Read and follow the pesticide label. Instructions on the pesticide label are given to ensure the safe and effective use of pesticides with minimal risk to the environment. Each pesticide is registered for use on specific sites or locations. Surveys indicate approximately 65 percent of drift complaints involved application procedures in violation of the label.

· Use spray additives within label guidelines. This will increase the droplet sizes and pesticide effectiveness.

· Use larger orifice sizes. This will give larger droplets and will increase the number of tank refills, but will improve coverage and effectiveness.

· Avoid high pressure. High pressure creates finer droplets; 45 PSI should be considered maximum for conventional broadcast spraying.

· Use drift-reduction nozzles. They will produce larger droplets when operated at low pressures.

· Drift is minimal when wind velocity is less than 10 mph. Do not spray when wind is greater or blowing towards sensitive crops, gardens, dwellings, and livestock or water sources.

The following information was taken from a paper presented by the US EPA concerning pesticide drift and how EPA regulates pesticides, educates users and handles drift complaints. I have provided only a small portion of the full draft in order to highlight the key points. The full paper can be obtained at the following web address: http://www.epa.gov/pesticides/citizens/spraydrift.htm

United States Environmental Protection Agency
Office of Pesticide Programs

Spray Drift of Pesticides
The drift of spray from pesticide applications can expose people, wildlife and the environment to pesticide residues that can cause health and environmental effects and property damage. For these reasons and because EPA's Office of Pesticide Programs (OPP) is responsible for regulating the use of pesticides in the U.S., OPP has been actively engaged in a number of initiatives to help prevent such problems.

What Is Pesticide Spray Drift?
EPA defines pesticide spray drift as the physical movement of a pesticide through air at the time of application or soon thereafter, to any site other than that intended for application (often referred to as off-target). EPA does not include in its definition the movement of pesticides to off-target sites caused by erosion, migration, volatility, or contaminated soil particles that are windblown after application, unless specifically addressed on a pesticide product label with respect to drift control requirements.

What are the Impacts of Spray Drift?
Off-target spray can affect human health and the environment. Drift can also contaminate a home garden or another farmer's crops, causing illegal pesticide residues and/or plant damage. The proximity of individuals and sensitive sites to the pesticide application, the amounts of pesticide drift, and toxicity of the pesticide are important factors in determining the potential impacts from drift.

How Does EPA View Off-Target Spray Drift?
When labels of pesticide products state that off-target drift is to be avoided or prohibited, our policy is straightforward: pesticide drift from the target site is to be prevented. However, we recognize that some degree of drift of spray particles will occur from nearly all applications. Nevertheless, applicators and other responsible parties must use all available application practices designed to prevent drift that will otherwise occur. Prudent and responsible applicators must consider all factors, including wind speed, direction and other weather conditions, application equipment, the proximity of people and sensitive areas, and product label directions in making their decisions about pesticide applications. A prudent and responsible applicator must refrain from application under conditions that are inconsistent with the goal of drift prevention, or are prohibited by the label requirements. EPA uses its discretion to pursue violations based on the unique facts and circumstances of each drift situation.

Where to Direct Complaints about Spray Drift
Should you believe that you have been exposed to pesticide spray drift and have health-related questions, you should contact your physician, local poison control center, or health department for assistance. You can also contact the National Pesticides Telecommunications Network (see below for specifics). If you suspect that there has been an occurrence of illegal spraying, you should contact your state or tribal pesticide regulatory agency (either the department of agriculture or environmental protection).

Todd Vagts
Iowa State University Extension
Field Crops Specialist
1240 D. Heires Avenue
Carroll, IA 51401
Office: 712-792-2364; Cell: 712-249-6025; Fax: 712-792-2366
Email: vagts@iastate.edu

ISU Extension
IA State Univ.
IA Dept. of Ag
IA DNR

Windy Days and Herbicide Drift

Distance between nozzle and target (boom height)
Less distance between the droplet release point and the target will reduce spray drift. Less distance means less time to travel from nozzle to target and therefore less drift occurs.

ISU Extension IA State Univ. IA Dept. of Ag IA DNR

Windy Days and Herbicide Drift

Distance between nozzle and target (boom height) Less distance between the droplet release point and the target will reduce spray drift. Less distance means less time to travel from nozzle to target and therefore less drift occurs.

ISU Extension
IA State Univ.
IA Dept. of Ag
IA DNR

Distance between nozzle and target (boom height)
Less distance between the droplet release point and the target will reduce spray drift. Less distance means less time to travel from nozzle to target and therefore less drift occurs.