Herbicide-Resistant Weeds: Understanding the Challenge and a Path Forward

By Vipin Kumar and Mandeep Singh et.al

Herbicide-resistant weeds are a growing threat to the sustainability and profitability of agronomic cropping systems in Nebraska, from corn and soybean production fields in eastern Nebraska to sugarbeet in Nebraska Panhandle. With increased herbicide use — particularly in simplified weed control programs — more weed species are evolving resistance to herbicides, creating challenges for farmers. 

Herbicide-Resistant Weeds in Nebraska and in the Midwest

In Nebraska and across the Midwest, herbicide-resistant Palmer amaranth, waterhemp, kochia, common ragweed and marestail are common. Many of these species are resistant to widely used herbicides, including atrazine (Group 5), glyphosate (Group 9), protoporphyrinogen oxidase (PPO)-inhibitors (Group 14), and acetolactate synthase (ALS)-inhibitors (Group 2). In fact, nine weed species in Nebraska have confirmed resistance to one or more herbicides: common ragweed, giant ragweed, Johnsongrass, kochia, marestail, Palmer amaranth, redroot pigweed, shattercane, and waterhemp (Jhala 2017). Some populations, particularly Palmer amaranth and waterhemp are resistant to five or more herbicides with varying mode of action (MOA), including groups 2, 5, 6, 9, 14 and 27. This significantly reduces post-emergence herbicide options and forces farmers to rely on alternate herbicide programs. 

The evolution of resistance is accelerated by repeated use of the same herbicide(s) with the same mode of action, especially when they are used without incorporating cultural or mechanical weed control tactics. The overuse of glyphosate in glyphosate-resistant crops has been a major driver of the evolution of glyphosate-resistant weeds. For example, six glyphosate-resistant weeds — including common ragweed, giant ragweed, kochia, marestail, Palmer amaranth and waterhemp — have been confirmed in Nebraska (Jhala 2017).

The evolution of multiple herbicide-resistant weeds has become one of the most pressing concerns. Certain weeds survive herbicides belonging to two or more MOAs, making chemical control alone ineffective in many cases.