Dicamba-resistant soybean varieties were first available for planting by farmers in 2017. The trait that conferred dicamba resistance in soybeans — Roundup Ready 2 Xtend (RR2X) — and the herbicide products that were used with the trait — Engenia and Xtendimax — were welcome additions to manage waterhemp. Application flexibility and crop safety were also features of dicamba-resistant soybeans that were welcomed by farmers and dealers.
However, issues with off-target herbicide movement began to occur in the summer of 2017. Large areas of non-dicamba soybean fields showed the classic dicamba drift injury symptoms — cupped leaves and stunting. There was a full spectrum of reactions to this dicamba off-target movement, including “We need this technology to manage waterhemp” to “This kind of off target movement is unacceptable.”
EPA’s reaction to this problem was to classify Xtendimax and Engenia as restricted-use products. Also, additional restrictions on application parameters and dicamba specific training was required for applicators who intended to apply dicamba products to Roundup Ready 2 Xtend soybean varieties.
Tolerant bean varieties
The 2018 and 2019 growing seasons saw issues with dicamba off-target movement in Iowa but not at the level that occurred in 2017. The reduction was credited to:
- increased market share of RR2X soybean varieties
- more awareness of how to properly apply dicamba products to soybeans
- results of additional product-use training
The Iowa Department of Agriculture’s Pesticide Bureau reported 249 pesticide-related investigations in 2019, which was similar to 2017 and 2018. Not all of these investigations were related to dicamba, but the number of complaints reported to the IDALS Pesticide Bureau has increased from a level of about 100 per year before 2017. The official complaint number, however, isn’t a good indicator of the number of problems that occur. A significant number of drift injury problems are unreported as neighboring farmers work it out among themselves.
Also noteworthy is that 2020 is the second year of a two-year probationary label, as EPA is re-evaluating the use of this dicamba trait technology. When these dicamba labels expire in December, EPA will have to make some decisions.
Currently, the RR2X herbicide tolerance trait is very popular in Iowa. Educational efforts by ISU Extension and the industry has increased the level of understanding of dicamba off-target movement and crop injury. Thus, Iowa has not added additional application restrictions on dicamba products for soybeans.
Neighboring states like Minnesota, Illinois, South Dakota and Missouri have added additional restrictions. For example, Illinois has enacted a June 20 dicamba application cutoff date for 2020. Illinois is also prohibiting application of dicamba if temperatures at application exceed 85 degrees F or if the National Weather Service forecasted high temperature for the nearest location on application day exceeds 85 degrees.
How to prevent dicamba drift
Continued management to reduce the risk of herbicide drift is needed. It’s likely that as new products are introduced and labels of existing products are revised, the application restrictions related to drift will be more restrictive.
Iowa will likely see increased acres of soybeans with the Enlist trait. You can use 2,4-D on Enlist varieties. You can use dicamba on varieties with the RR2X trait. Dicamba and 2,4-D are both Group 4 herbicides. Thus, the Enlist trait is similar to RR2X in that Enlist confers resistance to a Group 4 (growth regulator) herbicide. However, the RR2X and Enlist traits do not provide cross resistance to the two herbicides. Enlist bean varieties are susceptible to dicamba, and RR2X soybean varieties are susceptible to 2,4-D.
LibertyLink soybean varieties are also susceptible to injury from dicamba products. Soybean varieties with tolerance to Liberty herbicide have increased in availability and planted acreage in recent years and will likely increase in 2020.
Requirements for applicator training and the label restrictions for these products continue to evolve to address the frequency of off-site herbicide injury. The problem isn’t limited to drift from soybean fields, as more acres of corn are also being treated with dicamba to help control herbicide-resistant and other broadleaf weeds.
It is more important than ever to read, understand and follow the label directions if you are using dicamba on dicamba-tolerant soybeans. There are stricter recordkeeping requirements and restrictions regarding application of these products.
What to consider
Only dicamba products specifically labeled for use on RR2X soybeans can be used postemergence on these varieties. These products have lower vapor drift potential than dicamba products you might use to control broadleaf weeds in corn or pastures.
The four dicamba herbicide formulations now labeled for use on dicamba-resistant soybeans are:
- ExtendiMax with VaporGrip Technology (Bayer Crop Science)
- Engenia (BASF)
- FeXapan Plus VaporGrip Technology (Corteva AgriScience)
- Tavium Plus VaporGrip Technology (Syngenta)
The new dicamba labels have the most restrictions regarding application of any agricultural herbicide. Only approved spray nozzles can be used to apply products. Information on nozzles and the parameters they can be used are found on an accompanying website. There is a minimum gallons per acre of total spray solution when using these products. The maximum height of the sprayer boom over the crop canopy is also on the label directions.
Follow the label directions on wind speed for application and whether the wind is blowing toward or away from sensitive crops. Note the air temperature and time of day, which can cause temperature inversions to occur.
There are untreated downwind buffers you must leave when making an application adjacent to susceptible vegetation. The width of the buffers will vary with the rate of herbicide you are applying. Remember, you need to use something different for weed control in these buffers. The product can’t be applied if wind is blowing toward adjacent sensitive crops, including non-RR2X soybean varieties.
There are restrictions on what you can and can’t add to the spray mix, as some of these products may reduce the drift-reducing properties of the herbicide formulation. Approved tank-mix partners are found on the label’s supplemental website. Do not tank-mix with any pesticide or adjuvant not found on the website. Most notably, do not add ammonium-containing additives such as ammonium sulfate since this will significantly increase volatility.
Check with your neighbors on their herbicide technology when your soybean field adjoins another farmer’s soybean field. Use this information to prioritize applications if the herbicide tolerance traits they are using are different than yours. Also, use current weather information to minimize any impact of off-target movement of your herbicide application.
There is a steep learning curve that comes with adopting this herbicide technology to prevent injury to non-tolerant crops in your own farming operation as well as your neighbors’ fields. For more information, visit ISU’s integrated crop management website. Or contact your area ISU Extension field agronomist with questions.
Kassel is an ISU Extension field agronomist at Spencer in northwest Iowa. Contact email@example.com.