Understand herbicide groups

In our industry it’s easy for most of us to spout off the names of at least a couple of herbicides and what herbicide group they belong to. However, most of us wouldn’t be able to do a very good job explaining what these herbicide groups mean.

Herbicides are placed into different groups based on the way they kill a target plant. Herbicides will target a specific process within the plant that is essential for plant survival. For the most part, herbicides will stop a plant’s growth by binding to a specific enzyme within the plant. So without getting too technical, here is the classification of some common herbicide groups.

Group 1 – ACCase Inhibitors Grass Herbicides

In a nutshell, this group of chemicals works by stopping the plant from making fatty acids. Fatty acids are used in the plants to make waxes and cell membranes and used to store energy. Without fatty acids the plants cannot survive.

Group 1 activity will first appear in the newest leaves and the crown of the grassy weed where you may see wilting and yellowing on leaves, suppressed growth and leaves that are easily pulled from the sheath.

The group is split into two categories — fops and dims. They have slight chemical differences. The take home message on fops and dims is this: most dims are degraded rapidly in sunlight.

This brings up two important points. When using a dim, spraying at dusk may produce the best results. And, always use the corresponding adjuvant or product failure can occur. You can easily find out if a herbicide is a dim or fop by looking at the last few letters in the active ingredient name.

Group 2 – Amino acid inhibitor/ALS Inhibitors Grass and broadleaf herbicides

Group 2s stop the production of amino acids, which are used in the plant to build protein. Without proteins, plants will die. Plant death with this group is often slow (two to three weeks) but plant growth will stop quite quickly. Symptoms most often show up at the growing points with reddening or yellowing tissue. Some Group 2 chemicals are also soil persistent.

Group 3 – Inhibitors of cell division

Grass and broadleaf herbicides. This one is a little easier to visualize as the herbicide will stop plant cells from dividing, which stops growth. Group 3s will specifically stop growth in the root tips, stop shoot elongation and prevent secondary roots from forming.

Symptoms on target weeds may be a little difficult to identify as many controlled weeds will not visually emerge. Symptoms on weeds that do emerge generally show abnormal roots or poor root development. All of the most common Group 3s are soil applied before weeds emerge.

Group 4 – growth regulators Broadleaf herbicides

In a nutshell, this group causes the plant to grow uncontrolled, which gives the plants a bent, twisted, thickened look. The plant grows faster than it can support itself, and eventually dies. This group is usually a little slower to kill but generally kills a wide range of broadleaf weeds and is part of many herbicide mixes.

Group 10 – glutamine synthase inhibitor

This is an important group in herbicide tolerant crops. This group causes ammonia to rapidly accumulate in the plant during photosynthesis or photorespiration. This accumulation eventually leads to the plant unable to photosynthesize. This means that when using any Group 10’s, sunlight will greatly increase the effectiveness and speed of kill. Typical symptoms include white mottling within a few days leading to yellowing, necrosis and death.

More than one active

Keep in mind that some herbicides have two or more active ingredients, which means they could belong to more than one herbicide group. Mixing active ingredients that belong to different herbicide groups is a great way to help slow down the incidence of herbicide resistant weeds.

Of course there are a few smaller groups we could look at. The take home message is that understanding how the herbicides are placed into these groups can give us some basic insight as to why different herbicides work at different speeds, mixing issues, the need for special adjuvants, why different herbicides should be used in different lighting or temperature conditions and why we have resistance issues in some groups. †

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