Backgrounder
Glyphosate and Drift
September 2002
Monsanto Company
Monsanto鈥檚 glyphosate herbicides, such as Roundup Pro庐 and Roundup UltraMAX庐, are non-
selective herbicides, meaning that they have the potential to affect many species of plants.
Therefore, it is very important that non-target plants are not exposed to direct spray or to spray drift
when these herbicides are applied.
Glyphosate has extremely low volatility, which means that it will not evaporate from surfaces such as
soil or treated plants and move through the air after application to injure non-target vegetation.
However, it is possible, as with any sprayed substance, that spray droplets could drift off-target during
application. Research has demonstrated that application procedures and equipment can be
optimized to significantly reduce spray drift in most circumstances. Monsanto鈥檚 product labels state
that the applicator should avoid drift to desirable plants and crops.
Pesticides, including herbicides, can be applied using various equipment and techniques as specified
on the product label, including aerial application equipment, broadcast boom sprayers, backpack
sprayers, and hand-held applicators. Spray drift from any of these techniques can be minimized by
taking into account spray droplet size, wind speed, other environmental factors, and application
equipment design. When drift does occur, there is a rapid decline in surface deposition with
increasing distance from the target site for both ground and aerial applications. An extensive study of
spray drift of ground applications has been conducted in Europe, in which pesticide drift at varying
distances from the edge of a treated field was measured (Ganzelmeier et al., 1995; Ganzelmeier and
Rautmann 2000). For field crops, these authors report that at 5 meters from the edge of a treated
area, less than 1 percent of the application rate drift was measured (using data from 50 trials). As the
distance from the edge of the field increased, the deposition rate declined to 0.10 percent at 30
meters.
Studies have also demonstrated that aerial applications can be made with significantly reduced drift
to off-target areas when label instructions are followed. There are a variety of techniques available to
aerial applicators to reduce off-target drift, such as swath adjustment, nozzle orientation, and boom
height and length. In a forestry application, it has been reported that deposition was typically less
than 10% and 1% of the application rate at 25 meters and 75 meters, respectively (Payne et al.
1990). It has also been reported that spray deposition decreased to less than 10 percent of the
application rate in the first 30 meters downwind, and was less than 5 percent at a distance of 200
meters (Riley et al. 1991). An extensive study of drift from aerial, broadcast ground, and orchard
applications has been conducted by the Spray Drift Task Force in the United States, and a computer
model (AgDrift庐) has been developed to predict drift under varying environmental conditions and
application equipment configurations (Teske et al., 2002).
Anti-pesticide activists sometimes cite a published study which reported that glyphosate drifted up to
400 meters during ground applications and up to 800 meters during aerial applications (Yates et al.,
1978). These tests were conducted 25 years ago to learn which application methods would minimize
drift. Several nozzle and pressure settings were tested, including some that would intentionally lead
to off-target drift. Activists have cited results of tested application methods that were not acceptable
then or now. Yates et al. includes the statement: 鈥淪ignificant reduction in drift losses were obtained
Backgrounder: Glyphosate and Drift. 2002. Page 1 of 2
with proper application equipment.鈥? Activists have also cited another study (Payne and Thompson
1992) to imply problems with aerial drift, but in truth, the authors concluded that, with proper
precautions, the herbicides 鈥渃ould be applied in a wider range of wind speeds than that currently used
without causing increased environmental impact in sensitive areas.鈥?
The word 鈥渟prayer鈥? is actually a misnomer for most aerial herbicide application systems. A better
term is 鈥渦niform droplet control system.鈥? Today鈥檚 application equipment employs devices similar to
hypodermic needles, which create single droplets in rapid succession. The larger the needle, the
larger the droplets.
Precision is very important to aerial applicators, and good applicators adhere to guidelines to suspend
spraying operations under gusty wind conditions. Professional aerial applicator associations have
developed sophisticated training procedures for their members. In situations where desirable
vegetation borders a targeted site, aerial applicators typically establish a buffer zone (a distance
between the site of application and area to be protected) to minimize effects to non-target vegetation.
References
Ganzelmeier H, Rautmann D, Spangenberg R, Streloke M, Herrmann M, Wenzelburger H-J, Walter
H-F (1995) Studies on the spray drift of plant protection products; results of a test program
carried out throughout the Federal Republic of Germany. Blackwell Wissenschafts - Verlag
BmbH, Berlin.
Ganzelmeier H, Rautmann D (2000) Drift, drift reducing sprayers and sprayer testing. Aspects of
Applied Biology 57: 1-10.
Payne NJ, Thompson DG (1992) Off-target glyphosate deposits from Aerial silvicultural applications
under various meteorological conditions. Pestic Science 34: 53-59.
Payne NJ, Feng JC, Reynolds PE (1990) Off-target deposits and buffer zones required around
water for aerial glyphosate applications. Pestic Science 30: 183-198.
Riley CM, Wisener CJ, Sexsmith, WA (1991) Estimating off-target spray deposition on the ground
following the aerial application of glyphosate for conifer release in New Brunswick. J Environ Sci
Health B26(2): 185-208.
Spray Drift Task Force. URL: http://www.agdrift.com
Teske ME, Bird SL, Esterly DM, Curbishley TB, Ray SL, Perry SG (2002) AgDRIFT庐: a model for
estimating near-field spray drift from aerial applications. Environmental Toxicology and Chemistry
21(3): 659鈥?671.
Yates WE, Akesson NB, Bayer DE (1978) Drift of Glyphosate Sprays Applied with Aerial and
Ground Equipment. Weed Science 26(6): 597-604.
Backgrounder: Glyphosate and Drift. 2002. Page 2 of 2
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