Herbicides in agriculture An economic boon to agriculture, herbicides pose latent problems that are not fully understood It was not until after World War II that weeds were extensively controlled with chemicals. In the past two decades, the use of herbicides in the U.S. has increased dramatically. In 1966, 227 million pounds were applied; in 1981,625 million lbs—more than the total amount of insecticides. During the same 15 years, farm use increased 280%. Herbicides are used to maintain rights of way, to kill brush and plants along roadsides, to control poison ivy and ragweed, to control aquatic weeds, and to maintain rangeland by killing brush and poisonous plants that harm livestock. But by far the greatest proportion of herbicides (three-fourths of the total) is used to kill weeds in farm crops. The benefits of herbicide use in agriculture are relatively well-known. Weed killers make it possible to employ no-till and minimum tillage methods—planting with no plowing, or discing only rather than plowing and discing—and in this way greatly reduce soil erosion and fuel requirements for tractors. Also, herbicides replace the human labor and tractor fuel required for extensive cultivation; consequently they lower farm costs. But there are drawbacks and possible serious problems with herbicide use that have received insufficient attention. It is very difficult to assess the relative importance of these concerns because little research has been done on the possible negative effects. These include: • weed resistance to herbicides, • herbicide-plant disease interactions, • toxic effects on nontarget organisms, and • farm practices that have deleterious effects on the long-term fertility of the soil. Research progress toward elucidating the extent of these concerns and finding suitable ways to deal with them is slow. One major factor that inhibits research on herbicides is that weed scientists in this country number only several hundred; in comparison, ento0013-936X/82/0916-0645A$01.25/0
"Don't stint! Remember our competition—mammoth weeds and voracious insects. " mologists number about 8500. Also, outside of the chemical industry, the funding for herbicide research is minimal. Very few universities have a department of weed science though many have a department of entomology. The U . S . Department of Agriculture ( U S D A ) — t h e government agency whose primary purpose 50 years ago was research—supplies little funding for herbicide research and relatively little support for studies in any area. Total research at U S D A accounts for only 2% of its present budget. Growing popularity Yet, several factors have caused herbicide use to increase almost exponentially in recent years. At a time when fuel prices have risen precipitously, herbicides reduce fuel requirements. Herbicides are petroleum-based chemicals and require petroleum in their manufacture, but the fuel used for plowing, discing, and cultivating that is saved with herbicides more than offsets the petroleum
© 1982 American Chemical Society
needed for their manufacture. Herbicides also replace human labor, which has become more expensive during the last three decades. And the fact that herbicides permit the use of farming methods that greatly reduce soil erosion has also made them highly popular because erosion has become generally recognized as a serious problem in the last decade. Herbicides are almost invariably (over 90%) some type of synthetic organic chemical. In all, there are more than 180 basic types, and this number is growing each year. Despite the great variety of existing weed killers, atrazine and alachlor constituted one-half of the herbicides used in farming in 1976. Large amounts of 2,4-D, trifluralin, and butylate were also employed. In their use patterns, there are three general types of herbicides: preplant (applied before seeds are planted); preemergent (applied before the seedlings appear); and postemergent (applied to the foliage of weeds after the crops come up). In this country, preemergent compounds are used on by far the greatest acreage. A boon to agriculture Wind and soil erosion in the U.S. now causes annual soil losses of five billion tons. The soil losses that can be prevented by using herbicides in combination with no-till and minimum tillage methods (known collectively as conservation tillage) are often dramatic. According to the 1981 Farm Bill, conservation tillage reduces soil erosion by 50-90%. In a study on continuous no-tillage corn production that spanned five years, "erosion losses on a nine percent slope during the growing season decreased from 1761 k g / h a on conventionally tilled watersheds to 27 k g / h a on no-till watersheds." In many experiments, no-till systems reduced soil losses almost to zero. Since conservation tillage decreases erosion greatly, crops can often be grown on slopes that otherwise could not be used for farming or that would have had to be terraced or conEnviron. Sci. Technol., Vol. 16, No. 12, 1982
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Since World War II, herbicide use in U.S. agriculture has increased dramatically
Herbicides account for a growing share of the pesticides used in the U.S.
Source: Chemical & Engineering News, Aug. 3,1981, Vol. 59, p. 21.
toured to prevent excessive soil losses. Besides the prevention or minimization of erosion, herbicides present several other advantages. They reduce crop losses to weeds during very wet weather when mechanical cultivation and hand labor are not effective. In the no-till systems made possible by herbicides, crops can be harvested and planted again without plowing; consequently, less time needs to elapse between crops and there is more potential for multicropping. T h e timing of planting can often be improved because the farmer does not have to wait for the proper soil conditions for plowing. Water evaporates less quickly from the soil since it is usually covered by mulch and is not aerated as much. As a result, crops benefit more from the natural rainfall, and less irrigation water is required for crops that are normally irrigated. Herbicide use sometimes allows the grower more choices of crops and crop rotations. Finally, the yield per acre for a given crop often increases when weed killers are applied. Unrecognized problems For many reasons, therefore, herbicides would seem to be an unalloyed benefit from both an economic and environmental point of view. As a consequence, almost no concern is being expressed at the present time about the possible negative effects of herbicides. There are some problems, however, that merit serious study. The first and probably most important drawback is that herbicides can act in two ways to cause weeds to become more of a problem than they were before herbicides were introduced. One way is to induce a simple species displacement. One weed species is killed off by the herbicide. In its place an646A
other weed species proliferates—often from an entirely different family—to which the herbicide is not toxic. This weed may pose an equal or greater problem than the original target weed. To combat the new weed, it is necessary to change herbicides or to use an additional herbicide in combination with the original. As one weed scientist observed, when you eliminate one weed flora, "it is an act in stimulation of a new one." Genetic resistance The other situation is that a weed of a particular species may develop resistance to the herbicide in the same way that an insect can develop resistance to an insecticide. A given variety of weed continues to be a problem, but it is another genotype that has taken over. The vulnerable genotypes of a given species are killed off by the herbicide; the resistant genotype multiplies. This does not happen as often as it does with insects and insecticides because the generation times for weeds are longer than those for insects (see ES&T,Vo\. 16, No. 5, 1982,p.282A). Weed seeds are mobile and persist in the soil for many years, constantly diluting the resistant genes by a continuous input of nonresistant individuals. In the literature, there are several cases of weeds developing resistant strains, however. Four different plant species in three quite different families have shown acquired resistance to the herbicide atrazine, for example. And the weed control literature is replete with documented cases of situations in which weed control is said to have become "difficult" after herbicides are used for a number of growing seasons. In these circumstances, the amount of herbicide applied to a given crop must be increased year after year or a dif-
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ferent herbicide must be used. It is not known what proportion of these cases involve true genetic resistance. A report on pesticides by the National Academy of Sciences ( N A S ) states: "In general, we believe that the potential for resistance to herbicides by weeds has been underestimated and recommend that weed scientists maintain a careful watch for resistance development." There are two ways to combat the problem of weed resistance. One is to rotate herbicides year after year so that weeds have less of an opportunity to develop resistance. Another is to rotate crops. This ensures almost automatically that different herbicides will be used because a weed killer that is suitable for one crop is usually unsuitable for another. Ironically, the phenomenon of herbicide resistance, which is a negative factor for weed control, can be used to help in crop selection. The variation of herbicide resistance among the different crop genotypes is what makes it possible to choose those that are most resistant to the desired weed killer. Insecticide use increased Another serious drawback of herbicide use is that it often amplifies the need for insecticides. When conservation tillage is used in conjunction with herbicides, insecticides are almost invariably applied in greater amounts. This happens as the result of several phenomena that often act simultaneously to promote insect pests. Tilling interferes with the life cycles of soildwelling insects. Without tillage or with reduced tillage, these insects have an opportunity to proliferate. Also, the mulches used in conservation tillage provide a habitat for slugs and for insect pests to lay eggs. And on farms where herbicides kill more weeds than
would be killed with other methods of weed control, the habitat of valuable predators may be destroyed. This is particularly true when herbicides are used to destroy weeds along fence rows or in other areas adjacent to the field where crops are grown. For some crops, a certain number of weeds is necessary to maintain the ecological balance that keeps insect pests in check, as in orchards where damage from insects often escalates dramatically when herbicides are used to kill weeds around the trees. On the other hand, there are cases in which destroying additional weeds with herbicides also destroys part of the food supply of insect pests and consequently decreases their abundance. Two recent studies have revealed a more complicated way in which herbicides can affect the need for insecticides. The microbial activity in soils where conservation tillage is employed is usually much greater than that in plowed soils. After repeated use in certain soils, certain herbicides sensitize the microbes to produce enzymes that break down the insecticides used as well as the herbicides, thereby reducing the effectiveness of both. This is possible because the chemistry of certain herbicides is very similar to the chemistry of the insecticides applied to the same soil. As a consequence, control of soil insects may be diminished or eliminated. Reduced incentive A third disadvantage of herbicides is that they diminish a farmer's incentive to rotate crops. One reason that crops are rotated is to further weed control. If weeds are not a problem, as is often the case when herbicides are Corn, soybean, wheat, and cotton growing consume the greatest amounts of herbicides.
The rope-wick herbicide applicator, used to apply postemergent weed killers to weeds growing above the canopy of row crops, reduces the amount of herbicide needed to control weeds in cotton and soybeans. employed, then the farmer has lost this motivation and may continue to grow the same crop year after year, particularly when this crop is the most profitable and the farmer is in financial difficulty. Crop rotation, however, is almost always desirable for other reasons. It helps in maintaining soil nutrients and preventing plant disease. In some instances, herbicides interfere directly with crop rotation. Certain crops cannot be grown in a field the year following the use of a particular herbicide because it may persist in the soil and kill the crops. For example, the most widely used herbicide, atrazine, commonly applied to corn, persists in the soil for 17 months and prevents broad-leafed rotational crops such as soybeans from being grown the following season. The persistence of atrazine also means that if corn is grown the next year, less herbicide is needed. These two factors provide the farmer with a double incentive not to rotate crops and can make corn growing in consecutive years economical in the short run. Most agricultural experts would agree, however, that the long-term effects on the soil of not rotating crops are inevitably negative. In contrast to atrazine, most herbicides break down in the soil by the end of the growing season. But some of them, such as picloram and monuron, persist for two to three years. The more persistent the herbicide, the more opportunity it has to move out of the crop area and affect nontarget organisms such as other plants, animals, and humans. Effects on plant disease A fourth disadvantage of herbicides is that under specialized sets of conditions, certain herbicides can predis-
pose plants to increased disease. They can accomplish this by several mechanisms. They may cause the crop plant to be more susceptible to pathogens present at low levels in the environment. For example, they may reduce wax formation on leaves, change carbohydrate, nitrogen, or glucoside metabolism, or retard or stimulate plant growth in a way that predisposes the plant to disease. The composition, but not always the total number of microorganisms in the soil, may also be altered by herbicides, and this can affect the crop's susceptibility to disease. An example of an herbicide-plant disease interaction is that between 2,4-D and corn leaf blight. In corn, 2,4-D stimulates the accumulation of protein that may favor the growth of the southern corn leaf blight pathogen. Many cases of decreased susceptibility to disease following herbicide use can also be found in the literature. It is hard to tell from reading a review of the recent literature whether the net effect of herbicides on plant disease is positive or negative. This research area is obviously one where more work is needed. In 1977, weed specialists Jack Altman of Colorado State University and C. Lee Campbell of Pennsylvania State University wrote in the Annual Review of Phytopathology: "Herbicides fill an important role in the production of more than 50 vegetable crops, deciduous tree fruits and nuts, citrus fruits and subtropical fruits, and ornamental plants that are produced commercially in the United States. . . With such extensive use, the potential for an herbicide-pathogen-host interaction is quite apparent, but this potential has, in a multitude of cases, been disregarded or rejected." Synergism Several studies have shown that some herbicides can have a synergistic Environ. Soi. Technol., Vol. 16, No. 12, 1982
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effect with insecticides, making them more toxic. The mathematical number of possible combinations of herbicides and insecticides is astronomical, but the actual number of combinations found in the field is probably less than a few hundred, and the possible syn ergism of some of the most common combinations needs further research. A report on pesticides prepared by Ν AS cautions: "The existence of syn ergism between different chemicals creates a potential and unevaluated hazard to man and his environ ment." One economic and sociological ef fect of herbicides that is usually cited as an advantage is that they decrease the man-hours required to grow a given crop. In an era when it is be coming increasingly difficult to find work for the uneducated and untrained members of society, this may not nec essarily be an advantage for all crops and all regions. Observers often say that no one likes to hoe fields. But this is not always true. In rural areas, for example, certain teenagers are very eager to obtain farm employment and find such work valuable and reward ing. On the other hand, it is often not economically feasible to pay workers minimum wage to do the weeding now taken care of by herbicides. Effects on nontarget organisms The toxic effects of a few herbicides on nontarget plants, humans, and other animals have been highly con troversial for many years. In general, it can be said that humans are almost always the target of some small frac tion of the herbicides used in agricul ture. The N A S report on pesticides states: "Because most herbicides must be released into the environment in order to perform their function, it is inevitable that some portion, however small, of these chemicals will reach the general population via the diet and other routes." For example, from 1965 to 1970, the average daily intake of 2,4-D (2,4-dichlorophenoxy acetic acid) from food bought in five major U.S. cities varied between 0.005 and 0.001 mg. Dioxin contaminant The phenoxy-based herbicides 2,4,5-T (2,4,5-trichlorophenoxy acetic acid), 2,4-D, and chemically related silvex have aroused the most concern because of their possible toxic effects on humans and other animals. Both 2,4,5-T and silvex contain T C D D , commonly known as dioxin, as a con taminant. This is one of the most powerful poisons known and was also present in Agent Orange (a mixture of
2,4,5-T and 2,4-D), the herbicide used in Vietnam. However, according to Dow Chemical Company, the leading producer of 2,4,5-T, Agent Orange contained much higher levels of dioxin—1.91 ppm compared to the 0.01-0.02 ppm level of dioxin in the 2,4,5-T sold today. The degree of concern about 2,4,5-T has been so great that all uses of this herbicide have been banned in the U.S. except to control weeds on rice and rangeland. The effects of Agent Or ange remain highly controversial. A long-term study will be carried out by the National Centers for Disease Control (CDC) to determine the pos sible harmful effects of this material on the men who were exposed to it during the Vietnam War. Originally, the study was to be carried out by the Veterans Administration (VA). But consensus has now emerged that it should be taken over by an indepen dent body. At press time, negotiations were under way between the VA and the C D C to create an interagency agreement under which control of the study will be transferred to the CDC. There is laboratory evidence that the dioxin contaminant of 2,4,5-T and silvex shows embryo toxicity, repro ductive effects, teratogenic effects, and carcinogenic effects in animals. There is also epidemiologic evidence that 2,4,5-T caused spontaneous abortions on Oregon women living near forests that had been sprayed with 2,4,5-T to control weeds and brush. This evidence is not conclusive, however. Some sci entists such as Robert M. Devlin, professor of agriculture at the Uni versity of Massachusetts, believe that dioxin residues in areas where 2,4,5-T has been sprayed are not high enough to represent a health hazard.
Herbicides used on major crops in 1976 Herbicide
Quantity (millions of pounds)
Percent of total
Atrazine Alachlor 2,4-D Trifluralin Butytate Propachlor EPTC Linurort All others
90.3 88.5 38.4 28.3 24.4 11.0 8.6 8.4 76.0
24.1 23.7 10.3 7.6 6.5 2.9 2.3 2.2 20.3
Source: Adapted from "Farmers' Use of Pesti cides in 1976," by T. R. Etchers, P.A. Andrilenas, and T.W. Anderson, U.S. Department of Agri culture, 1978.
Whatever its effects on humans, dioxin, particularly the isomer 2,3,7,8-TCDD, ls highly persistent in the environment though it is usually encountered at very low levels. It has invariably been found in the eggs of the Great Lakes herring gull at levels of 9-90 parts per trillion (ppt). One source of the T C D D in the eggs is be lieved to be 2,4,5-T used as an herbi cide, and the other sources are the manufacture of this herbicide and of 2,4,5-trichlorophenol. Also, ppt levels of 2,3,7,8-TCDD were observed in 21 of 62 commercial fish samples (rep resenting nine species) from the Great Lakes. A recent study demonstrates the high toxicity of extremely low concentrations of 2,3,7,8-TCDD. When rainbow trout and pike fish eggs were exposed to 2,3,7,8-TCDD for 96 h, developmental and growth re tardation resulted even at a low level of 0.1 ppt. A novel class of carcinogens At the September meeting of the American Chemical Society in Kansas City, Mo., Kaija Linnainmaa and Harri Vainio, of the Institute of Oc cupational Health, Helsinki, Finland, presented a paper suggesting that the carcinogenic potential of the phenoxy-based herbicides may not be caused primarily by the dioxin con taminant but by the phenoxy acid compounds themselves. According to bacterial mutagenesis assays, these phenoxy compounds are not muta genic, Linnainmaa and Vainio said, but they may belong to a novel class of carcinogens that "act via excessive production of H 2 0 2 and cause prolif eration of peroxisomes in the liver," which in turn damage D N A and lead to cancer. Therefore, although dioxin is known to be highly toxic, the dioxin content of an herbicide may not be the only important determinant of its toxicity. For many herbicides, the toxicity to nontarget organisms has been studied very little. David Pimentel pointed out the need for additional research in his book, "Ecological Effects of Pesticides on Non-Target Species": "Overall, I am less than pleased with the infor mation available, and the review clearly points out the desperate need for intensive investigation concerning the ecological effects of pesticides on nontarget species." He said that much less is known about herbicides than about insecticides and yet more pounds of herbicides than insecticides are ap plied to crops each year. Integrated approach needed With our present knowledge, it is impossible to weigh the relative risks Environ. Sci. Technol., Vol. 16, No. 12, 1982
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and benefits of herbicide use. Most weed scientists are very enthusiastic about herbicides because of their po tential for preventing soil erosion and for saving fuel. But just as entomolo gists 30 years ago saw few problems associated with insecticides, it may be that herbicides pose latent problems that have not yet fully materialized because weed killers have been used in great quantities and in great variety for only a few years. The possible problems with herbi cides suggest that just as integrated pest management is the best way to control insects, an integrated approach to weed management may be the most desirable technique for controlling weeds (see ES &T, Vol. 16, No. 5, 1982, p. 282A). Among other things, integrated weed management involves using a variety of control measures including in some cases a limited ap plication of herbicides rather than in discriminately applying large quan tities. Several techniques may be used in combination as a total or partial sub stitute for herbicides, such as me chanical cultivation, proper spacing of crop rows, hoeing, burning, proper timing of crop plantings, and biological methods. For a number of crops, it may be more desirable to cultivate mechanically between crop rows and use herbicides only in the row where the crop is grown. In certain situations less herbicide can be used if a postemergent type is applied to the foliage of the weeds after they appear and the farmer knows the kinds of weeds he has to contend with. Crop rotation has been traditionally and remains one of the most effective methods of weed control. To combat soil erosion, a va riety of techniques can be employed in addition to no-till and minimum till age. These include crop rotation, con touring, terracing, strip cropping (the growing of a cultivated crop in strips alternating with sod-forming crop), and intercropping (growing two or more crops simultaneously on the same plot). More research in weed control is definitely needed. If the risks are not clearly understood, the enthusiastic use of herbicides could alter the soil and create weed problems that our tech nology is not yet able to deal with. Overall, what Β. Ε. Day wrote in the book "Pest Control Strategies" is probably true: "Prolonged dependence upon a single chemical or cultural practice or repetitive combination of factors, regardless of how successful initially, will fail over the long term." —Bette Hileman 650A
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Biggs moves on Her top-notch capabilities and unremitting efforts have given much of the ACS chemical literature the superb quality for which it is world famous. E S & T wishes Katherine Biggs a long, healthy, happy retirement In a performance evaluation early in her career, Kay Biggs was said to be like the Rock of Gibraltar. For it is on Kay's shoulders that the ACS peerreviewed evaluation of technical pa pers for a half-dozen publications—a monthly magazine, one bimonthly, and four quarterlies including three engi neering quarterlies—has rested for more than 20 years. In fact, Kay saw the beginnings of the half-dozen pub lications for which she has had the re sponsibility for peer review of technical manuscripts. Her name is the only name on the ES& Τ masthead that has been there from the first issue (Janu ary 1967) to this issue! Retiring this month after 33 years of ACS service, Kay has seen editors come and go, yet has kept a "hum ming" office throughout all the changes. The number of manuscripts she has handled in her career is mind boggling. (Kay describes her ES&T experiences in this month's editorial, p. 636A). She notes that Jerome Seiner of PPG is the third editor of I & EC Product R&D; Bruno Zwolinski is the third editor of the Journal of Chemical and Engineering Data; and Irvin Liener became the editor of the Journal of Agricultural and Food Chemistry this July. The editors of the other two publications have not changed (see photo caption). The statistics of Kay's career are easy to cite, but more difficult to ex press in words is the warm, heartfelt appreciation that is extended to Kay for the way she has conducted ACS business. Kay's ACS career goes back to 1949. There are fewer ACS em ployees than you can count on the fin gers of two hands who can claim a similar work record. Kay has gone to more ACS meet ings than many would like to remem ber. She went to her first in 1950 (the fall meeting in Chicago), and her at tendance at the Kansas City meeting this fall was her 51st. Modestly she says that she has missed a few. She
says her presence there is useful for meetings of the editorial advisory boards and for meetings with editors and authors of manuscripts. A native Washingtonian One of six children, four brothers and a sister, Kay was born in Wash ington, D.C., grew up there, graduated from American University, and now lives in the Maryland suburban area. Kay joined the ACS as an editorial assistant in 1949 and moved up the ladder to manager of the manuscript reviewing operation in 1962. She looks forward to retirement, and serving as a consultant to the ACS on matters relating to peer review of manu scripts. Although not a "workaholic," Kay
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© 1982 American Chemical Society
