The challenge. Now that soil temperatures have begun warming, in many fields weed seed germination and plant establishment has begun.

Herbicide resistant weeds are complicating weed management throughout the Midwest. Throughout the past 40 years, more than 140 unique cases of herbicide resistant weeds have been documented throughout the U.S. (Figure). A unique case is the first documented occurrence of a population of a specific weed species exhibiting resistance to a specific herbicide mode of action.

Populations of some weed species (including common waterhemp and marestail) have evolved herbicide resistance to multiple modes of action. Since he moved to the center in 2011, my colleague Robert Bellm has worked to develop an effective herbicide program at the Brownstown Agronomy Research Center. His efforts are complicated by waterhemp populations that are resistant to three modes of action: EPSP synthase inhibitor (ex: glyphosate), ALS inhibitors (ex: imazethapyr), and PPO inhibitors (ex: lactofen).

Over time, individual herbicide active ingredients may become less effective if multiple weed species within a field evolve populations that are resistant. World-wide, populations of 66 weed species have evolved resistance to atrazine (Figure).

Additionally, Palmer amaranth, a very aggressive, invasive relative of common waterhemp, has recently moved into Illinois. This weed has the potential to cause very significant yield losses if a 'zero tolerance' policy is not adopted. University of Illinois Extension Weed Specialist, Dr. Aaron Hager, recently wrote a Bulletin article detailing recommendations for Palmer amaranth management in Illinois.

Management strategies. Producers are encouraged to select and apply effective herbicides and diversify the modes of action to which weeds are exposed. Applying premixes or tank mixing herbicides with different modes of action can achieve this. Producers are also encouraged to overlap residual herbicides when appropriate and to apply herbicides at the full rate and in a timely fashion. Diversifying herbicide and non-herbicide weed management strategies will reduce the speed at which weed populations will shift to become herbicide resistant.

Online tools. In collaboration with university extension personnel and private partners, the United Soybean Board (USB) has funded a large research and extension project aimed at increasing awareness of and providing potential solutions to the management challenges associated with herbicide resistant weed populations.

One resource is an Herbicide class chart. The left side of this chart groups herbicides by their mode of action. On the right side of the chart, viewers will see herbicide premixes listed alphabetically. Active ingredients, trade names and herbicide sites of action are listed for each premix component.

Another resource is called "Weed Out Resistance". This document lists 11 common weed species that have had herbicide resistant populations confirmed. As of this time, these weeds are considered to have the highest potential to complicate weed management as populations have exhibited resistance to one or more herbicide modes of action – sometimes in the same plant.