Waterhemp has a cunning way of adapting that includes emerging all summer long and producing a large number of seeds. The survivability of the weed was demonstrated in a University of Nebraska, Lincoln (UNL) evaluation of 2,4-D herbicide resistance. The evaluation involved waterhemp from a grass field where treatments of 2,4-D were applied up to twice a year for 15 years. This resulted in a case of herbicide resistance.

The greenhouse trials determined that the waterhemp populations developed 2,4-D (auxin herbicide) resistance because of the tremendous pressure exerted on a single mode of action over a period of many years. The report also concluded that the auxin-resistant waterhemp population is isolated.

“In looking at this specific field, the importance of multiple modes of action is very clear,” says Greg Kruger, cropping systems specialist, West Central Research & Extension Center, UNL. “It is unrealistic to expect any herbicide that is used exclusively for an extended period of time to not come under some pressure.”

Multiple-mode treatment

“If you can control a weed with two or three mechanisms of action, the likelihood of resistance occurring to all the mechanisms used is greatly reduced,” said Steve Weller, horticulture professor, Purdue University.

Dow AgroSciences reports that its new Enlist Weed Control System, under development, was designed with resistance management in mind. The new system takes into account that nature will always adapt and effective weed control technology must be ready to handle the many changes.

Pending regulatory approval, the Enlist system combines herbicide tolerance traits that will enable the use of glyphosate, glufosinate (in soybeans and cotton), and FOP chemistries (in corn) as well as new 2,4-D choline to combat a wide range of weed pressures. Enlist will provide tolerance to Enlist Duoherbicide, which includes glyphosate and new 2,4-D choline. The system also was designed to be used with a foundation herbicide treatment such as SureStart in corn and Sonicherbicide in soybeans for a total of four modes of action.

“It’s known that plants adapt, and weeds are no different,” says Mark Peterson, global biology leader, Enlist Weed Control System. “Years of research in weed science have demonstrated that continuous use of a single mode of action will ultimately result in the development of a resistant population. If a single weed management practice is used continuously, resistant weed biotypes are likely to arise.”

Peterson adds that the Nebraska evaluation reinforces the need for rotating chemistries and using multiple modes of action.