Dicamba and 2,4-D are post-emergence herbicides that have been used for many decades to selectively control broadleaf weeds in corn, hay fields or pastures, small grains such as wheat, and turf. They are also used to destroy existing broadleaf weeds prior to planting agronomic crops.
Starting in 2016, the use of these herbicides changed. Over-dependence on glyphosate has resulted in widespread glyphosate-resistant broadleaf weeds. To improve management of these weeds, new soybean varieties tolerant to over-the-top applications of dicamba (Xtend soybean) or 2,4-D (Enlist soybean) were commercialized in 2016 and 2019, respectively. Widespread adoption of dicamba- and 2,4-D-resistant soybeans has increased use of these herbicides during the months of May, June, and July. (USDA 2019).
Dicamba and 2,4-D have been effective in controlling glyphosate-resistant broadleaf weeds, but their expanded use has increased the risk of drift damage to high-value fruit and vegetable crops, landscape plants, and soybeans that do not carry the trait for dicamba or 2,4-D resistance. In addition to being more phytotoxic than glyphosate for many specialty crops, both herbicides are highly prone to drift. Off-target movement of dicamba, in particular, continues to be well documented in Missouri (Bradley 2017, 2018), Illinois (Illinois DOA), and Indiana (Office of Indiana State Chemist 2019), despite efforts to reduce drift though improved formulations, training, and label restrictions.
While concentrations of herbicides in drift are typically low, several crops have shown extreme sensitivity to dicamba and/or 2,4-D. For example, damage has been documented on grapes exposed to 2,4-D and dicamba concentrations as low as 1/800th of the labeled rate. For high-value sensitive specialty crops, a small amount of drift can result in huge losses. Notable lawsuits from producers of honey, peaches, grapes, and tomatoes have recently brought this issue to the attention of specialty crop growers nationwide.
This publication is a product of the North Central IPM Center working group on Herbicide-Drift Risk Management, with support from the USDA National Institute of Food and Agriculture through agreement 2018-70006-28884.
Reference to any commercial products or trade names implies no discrimination or endorsement by the North Central IPM Center or any of the contributing authors or their universities. Nor does this document constitute legal advice. Always seek legal advice from a professional who is knowledgeable in current agricultural law in your state.
Authors
Cassandra Brown, Horticulture and Crop Science, The Ohio State University
Stephen Meyers, Horticulture & Landscape Architecture, Purdue University
Mary Ann Rose, Pesticide Safety Education Program, The Ohio State University
Douglas Doohan, Horticulture and Crop Science, The Ohio State University
Acknowledgements
The following individuals reviewed part or all of this fact sheet: Bill Johnson, Purdue University; Bryan Young, Purdue University; Aaron B. Wilson, Byrd Polar and Climate Research Center; Cathy Herms, The Ohio State University
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