Herbicides & Pesticides

A herbicide is a pesticide that is used to kill weeds. A weed is any plant that is not wanted in a particular area that may be used for growing food or decorative plants. Some herbicides will kill certain “weed” plants while leaving the food or flower crop mostly unharmed. Some of these herbicides/pesticides work by stopping the growth of the weed and are most usually based on certain plant hormones. Herbicides/pesticides that are used to clear large tacts of land or ground are nonselective about the plants that they kill and will decimate all plant life that they come into contact with. There are some trees and plants that produce natural herbicides, such as the walnut tree and the tobacco plant. Herbicides are used in agriculture and in landscaping. They are applied in what is known as a total vegetation control program, which is used for the maintenance of highways and roads, and power lines. Herbicides/pesticides are also used in forestry, pasture systems, and management of areas set aside as wildlife habitat, but are used on a much smaller scale.

Developed by the British during World War II, 2,4-dichlorophenoxyacetic acid, often abbreviated 2,4-D was the first and most widely used herbicide/pesticide. This herbicide/pesticide saw major production and useage in the late 1940s. It is very easy and cheap to make, and kills many of the broadleaf plants and weeds while leaving crops mostly unaffected…although high doses of 2,4-D during the crucial growth periods can harm grass crops such as corn, wheat, barley, or other grains. The low cost of 2,4-D has made it still popular enough to still be in use and it still remains one of the most regularly used herbicides/pesticides in the world today. Like other acid herbicides, current formulas use either an amine salt, which is usually trimethylamine, or one of the many component esters of the basic compound. These are usually much easier to handle than the salt acids.

Unfortunately, 2,4-D has a really poor selectivity, this means that it causes major stress to crop plants. It is also not very effective against some of the “broadleaf” weeds, this includes many of the vine plants, such as the various types of ivys. A herbicide is considered to be selective if it only targets certain types of plants, and nonselective if it kills basically any type of plant. There have been other herbicides have been developed to be able to have a bit more selectivity about them.

The 1970s saw the introduction of atrazine, which is really bad for groundwater contamination. Atrazine does not break down quickly after being applied. Instead it will be washed deep into the soil by rainfall and contaminate the ground water. This can cause major problems with drinking water resources. Atrazine is said to have a high carryover, and this is a very undesirable and unwanted property for any herbicide/pesticide to have.

Glyphosate, which is usually marketed under the name of ‘Roundup’, was introduced to the market in the late 80s for use as a non-selective weed control. It is now used a major herbicide/pesticide for selective weed control in growing crop plants because of the development of crop plants that are very resistant to it. This pairing up of this herbicide with the resistant crop seeds brought about the union of the seed and chemical industry in the late 90s.

Many of the chemical herbicides for agriculture now-a-days are designed to break down and decompose after a short time. This is very desirable and necessary because it allows crops that may be harmed by the herbicide/pesticide to be grown on the land in future seasons. However, herbicides that decompose quickly often do not provide weed control throughout the growing season.