People involved in weed management for at least 20 years probably remember corn being damaged by glyphosate drift when Roundup Ready soybean were first introduced. The extensive damage was due to the high susceptibility of corn to glyphosate; equipping sprayers with low-drift nozzles and the adoption of Roundup Ready corn hybrids has minimized problems associated with glyphosate drift.
The potential for off-target herbicide injury is affected by both how much herbicide leaves the field and the sensitivity of plants in adjacent areas. The relative sensitivity of ‘highly susceptible’ plants to three herbicides is shown in Figure 1. For comparison purposes, I expressed the dosage in terms of percentage of normal use rate (glyphosate = 0.75 lb/A; dicamba = 0.5 lb/A; 2,4-D = 1.0 lb/A). I searched the literature for studies that evaluated crop response to low doses of the herbicides, and selected the lowest dose that induced significant crop injury (visible injury symptoms, not yield loss).
Soybean were 200 times more sensitive to dicamba as corn was to glyphosate, whereas cotton and soybean were at least 10 times more susceptible. Some people think the risk for off-target movement of dicamba is largely due to volatilization, and that the new formulations will minimize/eliminate those problems. The high sensitivity of soybean to dicamba makes them prone to injury, whether the exposure is due to vapor drift, particle drift or sprayer contamination. A new level of stewardship will be required to minimize problems caused by the large increase in use of dicamba and 2,4-D associated with the new herbicide resistant traits.
Bhatti, M.A. et al. 1996. Wine grape response to repeated exposure of selected sulfonylurea herbicides and 2,4-D. Weed Technol. 10:951-956.
Ellis, J.M. et al. 2003. Rice and corn response to simulated drift of glyphosate and glufosinate. Weed Technol. 17:452-460.
Everitt, J.D. and J.W. Keeling. 2009. Cotton growth and yield response to simulated 2,4-D and dicamba drift. Weed Technol. 23:503-506.
Solomon, C.B. and K.W. Bradley. 2014. Influence of application timings and sublethal rates of synthetic auxin herbicides on soybean. Weed Technol. 454-464.