How herbicides work.

Today’s wide spectrum of selective herbicides offers farmers many effective and reliable weed-control options. Along with crop rotation and other cultural management practices, herbicides are now key components of most weed control programs.

Unfortunately, however, these powerful crop protection tools are at risk of becoming less effective. Some weeds are now showing resistance to herbicides or families of herbicides to which they were once susceptible. This makes it doubly important for farmers to understand how herbicides work.

"Mode of action" is the term used to describe how a herbicide kills target weeds or disrupts their growth. Among the more than 200 herbicide active ingredients, there are more than 20 different modes of action classified by their ability to inhibit one or more metabolic processes essential for plant growth and survival.

Common modes of action include inhibition of photosynthesis, pigment synthesis, amino acid synthesis, fatty acid synthesis, and cell division. Active ingredients with the same mode of action are identified as members of numbered groups.

As just one example, active ingredients in the Group 1 category all attack the ACCase enzyme which is involved in fatty acid synthesis. Without fatty acids, plants cannot grow. Well-known products such as Assure, Poast and Select are all Group 1 herbicides and thus all affect target weeds the same way. And significantly, a weed that’s resistant to one of these products is probably resistant to the others.

The key implication here is that it pays to understand those grouped modes of action, and switch from one mode to another in successive growing seasons. That slows the emergence of resistant weed populations. Attacking this problem from another direction, herbicide suppliers are increasingly offering premixes containing ingredients from more than one group. Weeds resistant to one mode of action can then be caught by the second.

As well as having a handle on modes of action, it’s important to know how herbicides get into plants and move around inside them. Most commonly in western Canada, entry is through leaves and other top growth. But in some cases the primary path is via the root system.

Having entered a weed, some herbicides move around quite rapidly to all parts of the plant. They are known as systemic products. Others don’t move around at all, and are called contact herbicides. Still others are intermediate, showing limited in-plant movement. Understanding what’s happening on or in target weeds is helpful in figuring out reasons for poor herbicide performance; deciding on the best nozzles, pressures and water volumes; judging how soon before a rain it’s safe to spray; and so on.

You can get much of this critical information from product labels and provincial weed control guides, or by talking to herbicide company reps, government weed specialists or consulting agronomists. Spend a few hours checking these sources whenever you use an unfamiliar herbicide. Learning how the product works, how it behaves in-crop, and exactly where it fits into your broader weed management strategies is time well spent.