Weeds infest wheat fields, compete with crops and can impact yield and crop quality. Growers turn to herbicides to minimize these threats and to protect their wheat. But sometimes, even when everything has seemingly been done to protect wheat fields, growers still find areas where weeds have not been controlled. This could potentially mean a larger issue is developing: herbicide resistance.

What is Herbicide Resistance?

According to North Dakota State University, herbicide resistance occurs with repeated use of a specific herbicide or a combination of herbicides in an effort to control a population of weeds that contain some plants with resistant genes.

According to The International Survey of Herbicide Resistant Weeds, there are 13 herbicide-resistant broadleaf and grass weed species found today in U.S. wheat crops. In particular, wheat growers have observed widespread occurrences of herbicide-resistant kochia and wild oat.

Types of Resistance

There are 2 broad categories of herbicide resistance – target site and metabolic. Target site resistance is more common and occurs when the weed population has changed and will no longer allow the herbicide to attach to the target site, making the chemical ineffective regardless of the rate used. Target site resistance is specific to a particular site of action, and one plant can carry multiple resistances. For example, a wild oat plant that has developed Group 1 target site resistance can also develop Group 2 target site resistance.

Metabolic resistance occurs when the plant produces an enzyme that breaks down the chemical before it can kill the weed. When products are used below label rates, the risk of developing metabolic resistance will increase. For example, some ryegrass can be resistant to ACCase, ALS and photosystem II inhibitors.

It’s extremely important to develop a management program to stay ahead of resistance. Always apply herbicides at the full, labeled rate and at the correct growth stage. This ensures the most effective control. Additionally, sound resistance management practices help to keep resistance at bay while maintaining yields and minimizing costs.

Submitted by Nathan Popiel, Syngenta agronomic service representative in North Dakota

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