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HERBICIDES

resulting in the development of over 180 herbicides for weed control by the end of the twentieth century.

Herbicides are now primarily developed in the pri- vate sector. Chemists typically synthesize a variety of compounds, which are screened for their ability to con- trol weeds and then modified and formulated for efficient use. Present herbicides tend to have very low mammalian toxicity because they inhibit biochemical pathways that are unique to plants.

There are a number of chemical classes of herbicides and various mechanisms by which herbicides kill plants. Herbicides generally act by inhibiting specific cellular functions, including photosynthesis, plant-specific amino acid biosynthesis, pigment formation, shoot and root growth, cell membranes, cellulose biosynthesis, lipid bio- synthesis, and growth hormone activity.

Herbicides may be applied in many ways. Some her- bicides are applied to the soil and absorbed by the plant root and/or shoot and move to their site of inhibition within the plant. Others are primarily applied to emerged foliage and either have an immediate contact effect on the foliage by burning or desiccation, or are translocated throughout the plant, leading to total plant death (sys- temics). Most soil-applied herbicides kill weed seedlings as they emerge from the soil, while foliage-applied her- bicides control emerged weeds and can kill quite large plants.

Herbicide selectivity, the ability to kill weeds but not crops, can be accomplished either by directed application or through biochemical mechanisms. Placement of the herbicide to avoid contact with the crop is widely used. For example, tree crops with deep roots often do not ab- sorb soil-applied herbicides. While it is an effective her- bicide for killing most broadleaf plants (dicots), 2,4-D is ineffective on most grassy weeds (monocots). This makes it useful in monocot crops, such as grains and turf. Oth- ers selectively kill monocot grasses but not dicots, mak- ing them effective in crops such as soybean. Some crops metabolize an applied herbicide to an inactive form while the weeds cannot, so the weed is killed, but the crop is not harmed. For example, atrazine is metabolized to an inactive form by maize while weeds are killed.

In many weed and crop situations there are no good selectivity mechanisms for herbicides. With the advent of recombinant DNA technology (genetic engineering) certain crop plants, such as soybean, corn, and cotton, have been made resistant to nonselective herbicides such as glyphosate by adding genes that make the crop im- mune to the herbicide. This technology is expected to increase, though its rate of acceptance has been slowed by the reluctance of the food industry to utilize trans- genic crops because of concerns expressed by certain con- sumer advocacy groups.

Modern agriculture in the United States is almost inconceivable without the use of herbicides. Herbicides reduce labor inputs for weed control and make it possi-

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ble to control weeds where cultivation is infeasible. They reduce the need for mechanical cultivation that can in- jure crop plants and lead to soil degradation via structure loss and compaction. Herbicides allow the use of no-till crop production, which reduces the need for plowing, now considered a destructive practice. Efficient weed control improves crop growth by reducing weed compe- tition for nutrients and water, and results in improved harvesting and crop quality.

A Source of Controversy

Despite the obvious advantages of herbicides, their use has raised concerns relating to human health and the en- vironment. Since herbicides are toxic to plants, critics have questioned their toxicity to other organisms exposed di- rectly or indirectly. The persistence of some herbicides in the environment has led to concerns relating to their car- ryover in the soil and effects on subsequent crops as well as their influences, due to drift or volatilization, on non- target plants. Furthermore, through repeated exposure to herbicides, many weeds have become resistant, which re- duces the efficacy of previously effective herbicides.

Other concerns involve herbicide costs, the require- ment for additional equipment for precision application, and questions relating to proper disposal of unused her- bicides.

The advantages and disadvantages of herbicide use are thoroughly evaluated by the U.S. Environmental Pro- tection Agency (EPA) prior to registration and labeling of any new compound. All new pesticides must be granted a registration, permitting their distribution, sale, and use. The EPA assesses a wide variety of potential human health and environmental effects associated with use of the prod- uct, including the particular site or crop on which it is to be used; the amount, frequency and timing of its use; and recommended storage and container disposal practices.

In evaluating a pesticide registration application, the registrant must provide data from tests done according to specific EPA guidelines conducted under recognized “Good Laboratory Practice.” Results of these tests de- termine whether a pesticide has the potential to cause ad- verse effects on humans, wildlife, fish, or plants, including endangered species and non-target organisms, as well as possible contamination of surface water or groundwater from leaching, runoff, and spray drift. The potential hu- man risks evaluated include short-term toxicity and long- term effects, such as cancer and reproductive system disorders. A pesticide will only be registered if it is de- termined that it can be used to perform its intended func- tion without unreasonably adverse effects on applicators, consumers, or the environment. The EPA also must ap- prove the specific language that appears on each pesti- cide label; the product can only be legally used according to label directions. The EPA continually evaluates her- bicides as to their safety, and any compound that is found to cause any adverse effect is immediately removed from the market.

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