Herbicides and How They Work

What is a Herbicide?
Herbicides are substances that are used for killing all types of plants, especially noxious weeds.  Herbicides can be put into 8 different groups including one group that is "ungrouped."  Herbicides are chemicals that kill plants or inhibit their growth.  Herbicides doings are varied and theoretically as numerous as the processes essential to plant life.  Herbicides are most often and most effectively used together with good cultural practices in a turf weed management program.  The choice of the best specific combination varies with agronomic, ecological and economic factors.

How Do Herbicides Really Work?
They work in many different ways.  In order to be effective, herbicides must first be applied to a plant.  After the herbicide is applied, it must then be retained on the leaf's surface.  Then it must move through the water filled space that is around the cell.  Once the herbicide has moved through the water filled cell or space, it must then enter a new cell while passing through a lipid like membrane.  When it has passed through that cell the herbicide will usually reach its target.  The target is usually an enzyme.  Finally when the herbicide reaches the target it will bind to and inhibit the target enzyme.  The herbicide must take a different approach.  It must first target the weed and not the plant or crop.  Then the crop and not the weed itself must break it down.

Why Do Herbicides Kill Plants?
They kill the plants by causing a build up of toxic substances where the toxic compounds usually stay at low levels.  By inhabiting the target site the herbicide causes substances to build up and then damage the plant.  This is how herbicide glyphosate works.  In other cases the death of the target plants seems to occur from deregulation of the very carefully controlled process of cell growth.  This is how herbicides such as 2, 4-D work; the plants just grow them selves to death.  Herbicide resistant plants may help a farmer reduce expenses indirectly.  Although seeds are a small part of total farm costs a farmer may save money if the new seeds are paired with herbicides that are cheaper because their patents have expired and have to compete with generic versions.  The biggest potential lies with designer seeds matched with new more efficient proprietary chemicals, according to Charles Benbrook, executive director of the Board of agriculture at the National Academy of Sciences.  Herbicides resistant seeds may also help to solve the problem of “carryover” in which the chemical lingers in the soil and is toxic to the next crop to be planted.  The new plants will almost certainly be designed with multiple resistance's to give the grower even more choices to match his needs.  For chemical companies herbicide resistance is a smart way to protect their current markets or expand into new ones.  Most of the major herbicide manufactures are engaged in some research to develop herbicide resistant plants.  The volume of herbicide use in the U.S. is actually declining because the most recent weed killers are more potent.  Some types of herbicides are soil applied herbicides.

Herbicide Use in Forestry
The herbicide uses in forests are in some instances, a cost effective tool for the control of vegetation.  Landowners use them in several different ways to increase the forest productivity.  Forestry herbicides can help prepare sites for tree planting, reduce unwanted vegetation, and provide conditions for prescribed fires.  They will also reduce competition form herbaceous weeds so that newly planted trees are given a boost for survival and reproduction.  They may be applied to improve the growth rates in established stands by selective removal of non crop trees.  The vast majority of herbicides are used for growing pines although some are applied for herbaceous control and timber stand improvement in hardwood forest.

Many people have the misconception that all compounds whose name ends in “ cide” such as insecticide, rodenticide, or fungicide can be lumped together as danger highly toxic chemicals, and unsafe at any application level.  This is simply not the vast majority of agriculture pesticides and is certainly not true of forestry herbicides.  Table 1 provides the acute toxicity of the active ingredient in several forestry herbicides for comparison some other common chemicals.  The table lists the LD50, which is a rating system for chemical toxicity.  A low LD50 indicates that a small amount of chemical is toxic and is a more dangerous substance.  On the other hand, the larger the LD50 the less toxic the chemical is.  All of the forestry herbicides have active ingredients that are less toxic than caffeine.  And, the active ingredient is diluted to make the herbicide product sold on the market.  All over the counter formulations of the products listed in table 1 have LD50 above 1700 mg/kg and so are therefore less toxic than aspirin.
Trade Name
Active Ingredient
LD50 of Active Ingredient mg/kg
Sulfomenturon methyl
2, 4-D
For Comparison
table salt




Literature Cited
McNabb, Ken, Environmental Satey of Forestry Herbicides, March, 1997.

Alberta Agriculture, Food and Rural Development, How Herbicides Work, Mechanisms of Action, copy write 1996, 56 pgs.


By: Chris Henderson  2/8/2006   

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