THE
IMPACT OF GLYPHOSATE DRIFT ON CORN YIELDS
by Mike Cowbrough and Greg Stewart, OMAF
Clarence Swanton and Kevin Chandler, University of Guelph
The increase in Roundup Ready soybean acreage has also increased the likelihood of glyphosate drifting onto non-target susceptible crops, specifically corn. Understanding the impact of glyphosate drift will allow one to identify the need for re-planting injured areas during the spring, or to determine what preventative measures should be put in place to minimize the impact of off target drift.
Field research conducted at the Elora Research Station examined the yield impact on corn sprayed with sub-lethal levels of glyphosate. Results are presented in Table 1.
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Figure
1. Visual Crop injury of corn 12 days after application of sub-lethal
glyphosate rates. |
Yield Loss Is Strongly
Correlated to % Visual Injury
In the above study, yield loss was highly
correlated to visual crop injury with substantial yield losses occurring at 27%
visual crop injury and greater. The University of Nebraska (Table 2) conducted
a similar study in 1997. Again, yield loss was highly correlated to visual crop
injury. Therefore, the severity of crop injury is a good indicator for estimating
the amount of yield loss and can be used for estimating economic impact and next
steps for dealing with the affected crop.
What Does 27% Visual
Crop Injury Look Like?
Since yield loss appears to be highly correlated
with the level of crop injury, it would be useful to have an idea of what various
levels of crop injury look like. A growth room study was conducted in 2004 which
characterized different levels of crop injury occurring 12 days after applications
of sub-lethal glyphosate rates (Figure 1).
Other Factors That
Will Affect The Amount Of Yield Loss From Glyphosate Drift
- Temperature: Cold snaps or severe
heat can cause additional stress to plants and increase glyphosate injury.
- Soil moisture: Drought or waterlogged
conditions may predispose plants to additional glyphosate damage.
- Health
of corn plants: Glyphosate applied to plants that are already stressed will
magnify the initial injury. The ability to out grow very small doses of glyphosate
is therefore diminished.
- Maturity of the plant: Younger, more aggressively
growing corn plants with their growing point below the surface of the soil
will have a better chance of recovering from glyphosate injury.
How Much Yield Loss
Would Warrant Re-planting?
Obviously this will
depend on numerous factors, but for simplicity let us consider only the price
of seed, the expected yield and market value of the corn crop. Assuming that
our seed costs are $60/ac and that we have forward contracted corn at $3.50/bu.,
it would take a yield loss of 17 bu/ac before the cost of the seed alone would
be covered (Figure 2). If we expect to yield 150 bu/ac, this would be equivalent
to 11% yield loss. This simplistic calculation does not factor in the time and
cost associated with re-planting, nor the lower yield potential associated with
a later planting date. Nonetheless, conservatively, a 25% visual crop injury
should occur before re-planting becomes a consideration (based on yield loss
estimates from Figure 3). 
How To Minimize The
Impact Of Off-Target Glyphosate Drift
Mississippi State University Engineers looked
at over 100 field studies evaluating drift and identified three factors that
have the greatest impact on the level of herbicide drift:
- Wind Speed - when wind speed doubled,
there was a 700% increase in drift. In Ontario, it is recommended that herbicides
be sprayed when winds are light to moderate, or when wind speeds are at 11
km/hr or less.
- Boom Height - when boom height was increased
from 45 to 90 cm, drift increased by 350%.
- Wind Direction - avoid situations where
the wind is moving towards the non-target crop.
Other Ways To Reduce
The Impact Of Glyphosate Drift
- Use air-induction nozzles.
- Use the lowest possible rate that will
be effective on the given spectrum of weeds.
- Adhere to buffer zones as specified on
the product label.
- In high-risk areas, consider planting
Roundup Ready corn around the headlands.
