Herbicide


A herbicide is a pesticide used to kill unwanted plants. Selective
herbicides kill certain targets while leaving the desired crop relatively
unharmed. Some of these act by interfering with the growth of the weed and
are often based on plant hormones. Herbicides used to clear waste ground are
nonselective and kill every plant with which they come into contact.

History

The first widely used herbicide was 2,4 dichlorophenoxyacetic acid, often
abbreviated 2,4-D. It first saw widespread production and use in the late
1940s. It is easy and inexpensive to manufacture, and kills many broadleaf
plants while leaving grasses unaffected. Its low cost has led to continued
usage today. Like other acid herbicides, current formulations utilize either
an amine salt (usually trimethyl amine) or one of many esters (ester) of the

base compound. These are easier to handle than the acid.

There are earlier examples of cultural controls, such as altering soil pH,
salinity, or fertility levels to control weeds.

2,4-D exhibits relatively poor selectivity, meaning that it causes stress to
non-target plants. It is also less effective against some broadleaf weeds,
including sedges and many vinous plants. Many other herbicides have been
developed to address these limitations.

The 1970s saw the introduction of atrazine, which has the dubious
distinction of being the herbicide of greatest concern for groundwater
contamination.

Glyphosate, frequently sold under the brand name Roundup, was introduced in
the late 1980s for non-selective weed control. It is now a major herbicide
in selective weed control in growing crop plants due to the development of
crop plants that are resistant to it. The pairing of the herbicide with the
resistant seed led to the consolidation of the seed and chemistry industry
in the late 1990s.

Uses

Herbicides are widely used in management of landscape turf and in
agriculture. They are used in total vegetation control [tvc] programs for
maintenance of way for highways and railroads. Relatively smaller quantities
are used in forestry, pasture systems, and management of set-aside areas for
wildlife habitat.

Classification of herbicides

Herbicides can be grouped by chemical family, mode of action, and type of
vegetation controlled.

They are also classified by their activity:

   * Contact herbicides destroy only that plant tissue in contact with the
     chemical spray. Generally, these are the fastest acting herbicides.
     They are ineffective on perennial plants that are able to re-grow from
     roots or tubers.

   * Systemic herbicides are foliar-applied and are translocated through the
     plant and destroy a greater amount of the plant tissue.

   * Soil-borne herbicides are applied to the soil and are taken up by the
     roots of the target plant.

   * Pre-emergent herbicides are applied to the soil and prevent germination
     or early growth of weed seeds.

Application

Most herbicides are applied as water-based sprays using ground equipment.
Ground equipment varies in design, but the greatest number of acres is
sprayed with self-propelled sprayers equipped with a long boom (often 60-80
feet) with flat fan nozzles spaced about every 20". Towed, handheld, and
even horse-drawn sprayers are also used.

Herbicides can also be applied aerially using helicopters or airplanes, and
can be applied through irrigation systems (chemigation).

Terminology

   * Control is the destruction of unwanted weeds, or the damage of them to
     the point where they are no longer competitive with the crop
   * Suppression is incomplete control still providing some economic
     benefit, such as reduced competition with the crop
   * Crop Safety, for selective herbicides, is the relative absence of
     damage or stress to the crop. Most selective herbicides cause some
     visible stress to crop plants.

Some major herbicides in use today

   * Glyphosate, a systemic nonselective herbicide used in no-till burndown
     and for weed control in crops that are genetically modified to resist
     its effects
   * Paraquat, a nonselective contact herbicide used for no-till burndown
     and in aerial destruction of marijuana and coca plantings. More acutely
     toxic to people than any other herbicide in widespread commercial use.
   * 2,4-D, a broadleaf herbicide in the phenoxy group used in turf and in
     no-till field crop production. Now mainly used in a blend with other
     herbicides that act as synergists.
   * clopyralid, is a broadleaf herbicide in the pyridine group, used mainly
     in turf, rangeland, and for control of noxious thistles. Notorious for
     its ability to persist in compost.
   * metoalachlor, a pre-emergent herbicide widely used for control of
     annual grasses in corn and sorghum; has largely replaced atrazine for
     these uses
   * dicamba, a persistent broadleaf herbicide active in the soil, used in
     turf and field corn
   * picloram, a pyridine herbicide mainly used to control unwanted trees in
     pastures and edges of fields.
   * atrazine, a triazine herbicide used in corn and sorghum for control of
     broadleaf weeds and grasses. Still used because of its low cost and
     because it works as a synergist when used with other herbicides.

Other herbicides of historical interest

   * 2,4,5-Trichlorophenoxyacetic acid (2,4,5-T) was a widely used broadleaf
     herbicide until being phased out starting in the late 1970s. While
     2,4,5-T itself is of only moderate toxicity, the manufacturing process
     for 2,4,5T contaminates this chemical with trace amounts of
     2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). TCDD is extremely toxic to
     people. With proper temperature control during production of 2,4,5-T,
     TCDD levels can be held to about .005 ppm. Before the TCDD risk was
     well understood, early production facilities lacked proper temperature
     controls. Individual batches tested later were found to have as much as
     60 ppm of TCDD.

2,4,5-T was withdrawn from use in the USA 1983, at a time of heightened
public sensitivity about chemical hazards in the environment. Public concern
about dioxins was high, and production and use of other (non-herbicide)
chemicals potentially containing TCDD contamination was also withdrawn.
These included pentachlorophenol (a wood preservative) and PCBs (mainly used
as a stabilizing agent in transformer oil).

Some feel that the 2,4,5-T withdrawl was not based on sound science. 2,4,5-T
has largely been replaced by dicamba and triclopyr.

   * Agent Orange was a herbicide blend used by the U.S. military in Viet
     Nam between January 1965 and April 1970 as a defoliant. It was a
     mixture of 2,4,5-T, 2,4-D, and picloram. Because of TCDD contamination
     in the 2,4,5-T component, it caused serious illnesses in many veterans
     who were exposed to it. Agent Orange often had much higher levels of
     TCDD than 2,4 5 T used in the US. The name Agent Orange is derived from
     the orange color-coded stripe used by the Army on barrels containing
     the product.