Herbicides are rather remarkable. We apply them at rates as low as a fraction of an ounce per acre and even at these low doses, they are capable of killing weeds. We generally take this for granted. At the same time, we take for granted the fact that herbicides do not kill the targeted crop. This characteristic of a herbicide to kill one species and not harm another is referred to as selectivity.

 

Why is it that crops are not harmed by herbicides? The simple explanation is that some plants are able to metabolize (or detoxify) the herbicide into non-toxic chemicals before they damage the plant. However, before the discussion continues about metabolism, a quick review about herbicide mode of action is justified. Herbicides kill weeds or damage crops through a four step process that is described as the herbicide’s mode of action (i.e. how a herbicide works).

 

1. Contact or Retention - contact of the herbicide with the roots or retention of spray droplets on a leaf.

2. Absorption – uptake of the herbicide molecules into the leaf, shoot, or root.

3. Translocation – movement of the herbicide from the site of uptake to the location in the plant where it can cause damage such as translocation the roots to the leaves or from sprayed leaves to growing points or roots (contact herbicides don’t need to translocate).

4. Inhibition at the site of action – typically, the herbicide molecule binds to a specific enzyme, blocking the production of essential products such as amino acids or blocking photosynthesis. 

 

In most cases, the key to selectivity is a plant’s ability to rapidly metabolize the herbicide before it causes damage at the site of action. In these cases, the plant has other enzymes that can cleave off a side chain from the herbicide molecule. This often reduces the toxicity of the herbicide molecule because herbicide molecule no longer has the right configuration or shape to bind tightly at the site of action. The next step in herbicide metabolism is often binding of sugars or amino acids to the molecule, which further detoxifies it. Safeners are added to some herbicides like Dual II Magnum, Harness, Option, and Laudis. These safeners trigger the crop to produce more enzymes to detoxify the herbicides, which increases crop safety. Under normal weather conditions, a crop plant may be able to metabolize a majority of the herbicide in several hours. On the other hand, a sensitive weed will only slowly metabolize the herbicide. During this time, the herbicide is able to bind to the site of action and kill the weed. 

 

Herbicide selectivity is based on metabolism in most herbicide-crop combinations such as with Accent, Dual, Callisto, atrazine, or Banvel in corn or Valor, Pursuit, Cobra, or Sencor in soybeans. The exception to metabolism-based selectivity is when the crop has an “insensitive” site of action, which means the target enzyme in the crop has a slightly different shape so the herbicide does not bind to it. This is the case with Assure, Select, and Poast Plus on broadleaf crops like soybean and alfalfa. Since these herbicides cannot bind to the site of action, they do not damage these crops. 

 

Roundup Ready corn and soybean are also resistant to glyphosate based on an insensitive site of action. The gene that was added to these crops produces an altered or insensitive enzyme that glyphosate cannot bind to. Therefore, the enzyme is not blocked and it keeps producing amino acids for the corn or soybeans. The Optimum GAT corn and soybean being developed by Pioneer uses a metabolism-based glyphosate resistance. In this case, a gene (GAT) was added to the crops, which produces a special enzyme to rapidly detoxify glyphosate before it damages the crops. Liberty Link corn and soybean also use metabolism-based resistance where the PAT gene was inserted into the crops, which produces an enzyme to rapidly detoxify Liberty.