HERBICIDES. Weeds have been deemed undesirable during much of human history for their negative influence on crop production, their unsightly appearance in the landscape, and in some cases their toxic properties and negative effects on human and animal health. Consequently, weed control is as old as the discovery of agriculture, eight to ten thousand years ago. Techniques for weed control have progressed from the employment of intensive human labor to complex systems involving mechanical, chemical, and biological methods. The earliest methods to eliminate weeds involved physical removal by grubbing or hoeing, followed by cultivation practices using first draft animals and then tractors. Since 1945, the use of chemical herbicides has become the predominant weed control technique in many parts of the world.

Chemicals have been suggested for weed control since antiquity. Theophrastus (372–287 B.C.E.) mentions killing trees by pouring olive oil over their roots. Cato (234–149 B.C.E.) advocated the use of amurca (the watery residue left after the oil is drained from crushed olives) for weed control. Other chemicals include sodium chloride, sulfuric acid, sodium arsenite, copper sulfate, iron sulfate, carbon bisulfate, arsenic trichloride, and petroleum oils. The first synthetic herbicide, 2-methyl-4,6-dinitrophenol (dinitro) was developed in France in 1932 for selective weed control in beans. In 1940 ammonium sulfamate was introduced for control of woody plants.

The chemical herbicide age began in 1941 when R. Pokorny first synthesized 2,4-dichlorophenoxy acetic acid (2,4-D) and reported that it had growth-regulating effects on plants. E. J. Krause of the University of Chicago later suggested that 2,4-D might be used to kill weeds, which stimulated research to test this and other newly synthesized chemicals for weed control in the field. These herbicides proved effective, and in 1945 the American Chemical Paint Company was awarded a patent for 2,4-D as a weed killer. The great potential of synthetic herbicides to control weeds and reduce human labor stimulated the birth of the herbicide chemical industry, 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 private sector. Chemists typically synthesize a variety of compounds, which are screened for their ability to control 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 biosynthesis, and growth hormone activity.

Herbicides may be applied in many ways. Some herbicides 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 (systemics). Most soil-applied herbicides kill weed seedlings as they emerge from the soil, while foliage-applied herbicides 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 absorb soil-applied herbicides. While it is an effective herbicide 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. Others selectively kill monocot grasses but not dicots, making 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 immune 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 transgenic crops because of concerns expressed by certain consumer 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 possible to control weeds where cultivation is infeasible. They reduce the need for mechanical cultivation that can injure 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 competition 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 environment. Since herbicides are toxic to plants, critics have questioned their toxicity to other organisms exposed directly or indirectly. The persistence of some herbicides in the environment has led to concerns relating to their carryover 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 reduces the efficacy of previously effective herbicides.

Other concerns involve herbicide costs, the requirement for additional equipment for precision application, and questions relating to proper disposal of unused herbicides.

The advantages and disadvantages of herbicide use are thoroughly evaluated by the U.S. Environmental Protection 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 product, 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 determine whether a pesticide has the potential to cause adverse 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 human 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 determined that it can be used to perform its intended function without unreasonably adverse effects on applicators, consumers, or the environment. The EPA also must approve the specific language that appears on each pesticide label; the product can only be legally used according to label directions. The EPA continually evaluates herbicides as to their safety, and any compound that is found to cause any adverse effect is immediately removed from the market.

At the present time herbicides provide consistent, broad-spectrum, and effective weed management in an economical manner. In the future, herbicides will be required to pass even more stringent tests related to their safety. While new-generation herbicides will likely be applied at even lower doses with less environmental persistence and exceedingly low toxicity to non-target organisms, herbicides are now recognized as only one factor in efficient weed control. Weed management is an everevolving system that will continue to use an integrated approach, combining cultural, mechanical, chemical, and biological techniques. In this process, however, herbicides will remain an essential component for weed control to help insure a sustainable food production system that reduces unacceptable risks to the environment while producing an abundant and safe food supply.

See also Agricultural Research; ; Ecology and Food; Government Agencies; Pesticides; Safety, Food; Toxins, Unnatural, and Food Safety.

Stephen C. Weller