"A weed that never germinates, emerges, and produces seed can never develop resistance to a herbicide," said Kevin Carlson, Federated's senior agronomist, "and this is a key point to remember when we talk about the issue of weed resistance in general."

Two definitions are important to remember.

• Herbicide control: The mechanism in the plant that the herbicide detrimentally affects so that the plant succumbs to the herbicide.
• Herbicide resistance: The herbicide no longer works on the mechanism in the plant, and it lives, producing seed for the next generations.

"This is another key point: Once herbicide resistance is in a population of a weed species in a field, it never leaves. Ever. Especially the pigweeds (amaranths, such as waterhemp)," said Carlson.

The development of herbicide resistant crops, such as seen in corn and soybeans, has led to a overdependence on herbicides. It has also led to changes in agronomic practices both good and bad.

Unfortunately, bad practices have led to new problems. For example, using the same herbicide over and over in a cropping system puts pressure on the weeds to overcome the herbicide. Thus, without proper management practices and herbicide stewardship, herbicides exert high selection pressure on weeds. The end result is a shift in weed species, changes in population and density, and the development of herbicide resistance. 

The challenge in today's crop and weed environment is to think differently, according to Dr. Aaron G. Hagar, a weed expert from the University of Illinois. Hagar's message, Carlson observed, was clear: Growers must think differently about amaranths (waterhemp, Palmer amaranth). Carlson added that giant ragweed should be added to that list as it appears to be glyphosate resistant in some of Federated's service area.

Without a change in thinking, more weeds will become resistant to herbicides and the cost of weed control will rise. Growers must learn to "use the management tools available today, and use them correctly," said Carlson.

The next several editions of the Agronomy Update will focus on ways to think differently about weed resistance, and why it matters. Federated's winter grower meetings will also explore this important topic. Talk to your Federated Agronomist about weed resistance -- anytime.