Iowa State University agronomists list alternative for managing herbicide-resistant waterhemp in wake of dicamba ban

Date:06-11-2020

Numerous herbicides registered for postemergence use on Xtend soybean have excellent activity on important weeds of Iowa's soybean fields. Unfortunately, herbicide resistance in waterhemp is widespread to the herbicide groups active on this weed: Group 2 (ALS inhibitors), 9 (glyphosate) and 14 (PPO inhibitors).

It is difficult to track the percentage of fields with waterhemp resistant to these herbicides, but our estimates would be: Group 2: >95%; Group 9: >75%; Group 14: >50%. Thus, the repeal of the dicamba label leaves many farmers no effective postemergence herbicides for multiple-resistant waterhemp in Xtend soybean fields.

So what are the most effective options to manage waterhemp? At this time, Tavium plus Vapor Grip Technology (dicamba + S-metolachlor) is not affected by the court ruling since it received registration after the lawsuit was filed. While we are not aware of the supply of Tavium, we suspect Syngenta only has enough product for a fraction of Xtend acres. Most growers will probably be unable to get product for all their fields.

Of the alternatives available, we believe a Group 14 herbicide (acifluorfen, fomesafen, lactofen) has a better chance of controlling waterhemp than glyphosate due to the greater prevalence of glyphosate-resistant waterhemp. Group 14 herbicides should be applied as soon as waterhemp is found in a field, and a Group 15 herbicide (acetochlor, dimethenamid, pyroxasulfone, S-metolachlor) should be included to provide residual control after the POST application. Glyphosate or other appropriate tank mix partners should be included in the mix to broaden the spectrum of weeds controlled.

Preemergence herbicides appear to be providing effective control in most soybean fields at this time, but timeliness of application of the Group 14 herbicide will be critical. Spraying waterhemp between 0.5 and 1.5 inches in height is ideal. Follow all recommendations on the Group 14 label to maximize effectiveness, including carrier volume, nozzle type, spray pressure, spray additives, and sprayer speed.

Mechanical weed control is not a desirable option for most, but farmers with Xtend planted in 30" rows should consider the feasibility of interrow cultivation. Large farms probably can't cover all their acres with a cultivator, but fields could be prioritized based on weediness.

Unfortunately, this court ruling came at a bad time for Iowa soybean farmers. There is no simple solution; hoping the decision will be reversed in time to use a dicamba product is not an effective strategy. Now is the time to determine what products are available and begin scouting fields to prioritize which fields need treated earliest. We have stressed the need for integrated weed management systems for several years, this is a shocking wakeup call showing how fragile our herbicide-based production system is.