Weed control today is more complex and difficult than it was for past generations. Not only do today’s growers need to prepare to face difficult weeds, an increasing number also face herbicide resistance. In fact, three out of four growers who participated in a recent BASF survey suspect that glyphosate resistance is a cause of tough-to-control weeds. A key element to combatting resistance is the herbicide site of action.

Herbicide-resistant weeds became a problem when growers depended on a single-herbicide program over a long period of time without supplementing any significant cultural practices, such as tillage. As the same herbicide was applied year after year, weeds were progressively selected for resistance because only one herbicide site of action was used.

Many wonder about the difference between site of action and mode of action. The site of action is the location within the plant where the herbicide impacts the development process. The mode of action is the name for the process the herbicide uses to control the weed. So site of action is “where” and mode of action is “how.”

“The site of action is the location in the plant where the herbicide has its primary effect,” said Bryan Young, Ph.D., associate professor of weed science, Purdue University. “Typically, the target is an enzyme used in carrying out a process like amino acid production or photosynthesis. The herbicide targets that enzyme and stops the process.”

Though sites of action can be referred to by the protein function they inhibit, such as acetolactate synthase (ALS) or p-hydroxyphenylpyruvate dioxygenase (HPPD) they are also grouped numerically based on their function. For example, an ALS herbicide that inhibits amino acid synthesis is labeled as Group Two. This group label is always listed on the product label.

Today, resistance exists to glyphosate, ALS and triazine. 

“Because it is the key to a herbicide’s effectiveness, we recommend using a variety of effective sites of action,” said Chad Brommer, biologist, herbicides at BASF. “Utilizing different sites of action is like having multiple tools in your weed control tool belt.”

There are about two dozen sites of action registered today for commercial products. About half of those are available in the United States. Because of the limited number of available sites of action and the spread of weed resistance, it’s important to use overlapping effective sites of action to reduce selection pressure on any single site of action and mitigate the development of further resistant populations.

“One way to combat resistance is through rotation and diversification,” said Young. “Even if crops are rotated from year to year, the goal should be to use a diverse combination of herbicide sites of action each year with minimal overlap of any single site of action between years in the same field.”

For improved weed management, use more than one site of action and rotate herbicide usage to control weeds and prevent resistance.