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The Impact of Glyphosate Drift on Corn Yields

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 used 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 and photos of crop response are shown in figures 1 to 5.

*Summary of 2 field trials. Dr. Clarence Swanton. 2001-2002. Weed Research Annual Report. University of Guelph

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.

*Adapted from Roeth et al. (1997)

Again, yield loss was highly correlated to visual crop injury. Therefore the severity of crop injury is a good indicator for estimating the severity of yield loss and can be used for estimating economic impact and next steps for dealing with the susceptible crop.

Figure 1. Corn response to 100% glyphosate drift (based on 0.67 L/ac rate)

Figure 2. Corn response to 10% glyphosate drift (based on 0.67 L/ac rate)

Figure 3. Corn response to 1% glyphosate drift (based on 0.67 L/ac rate)

Figure 4. Corn response to 0.1% glyphosate drift (based on 0.67 L/ac rate)

Figure 5. Corn response to no drift

How to minimize the impact of off-target glyphosate drift.

Mississippi State University Engineers looked at over 100 field studies and identified three factors that had 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 nozzles that will put out larger droplet sizes.
• Adhere to buffer zones as specified on the product label.
  
• In high-risk areas, consider planting Roundup Ready corn around the headlands