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CLASSIFICATION AND CHEMISTRY OF HERBICIDES 193 Although bensulide is an organic phosphate, it is relatively nontoxic to small laboratory animals. The acute oral LD 5, is 770 mg per kg for rats. The acute dermal LD <<, is 3,950 mg per kg for albino rabbits. Bensulide has little effect on weeds when applied as a foliage spray but pro- vides pre-emergence control of many grasses and broadleaf weeds. It is em- ployed in lawns for pre-emergence control of crabgrass, annual bluegrass, and other weeds. Bensulide persists for relatively long periods in the soil, although it dis- appears slowly, apparently by microbiological breakdown. No loss of bensu- lide by vaporization or photodecomposition has been detected from the soil.
CHAPTER 11 Interaction of Herbicides with the Knvironment Interaction of a herbicide with the environment begins at the moment of ap- plication and ends with its ultimate dissipation. Interaction occurs in the atmosphere, on and beneath the soil, and within the plant. The critical phase of herbicidal interaction that occurs between plant penetration and arrival of the chemical at a phytotoxic site is discussed in Chapter 13. Most interactions with the environment alter activity and selectivity, and the interrelations of these factors as they affect the expression of plant response to herbicides are complex. It is desirable to distinguish between interactions that occur within and without the plant. Necessarily, there is some overlap, particularly since herbicide placement, plant morphological characteristics, and physiochemical and environmental variables are intimately involved in plant penetration. Knowledge of the environmental factors regulating herbicide performance or limiting herbicide usage is essential to successful control of weeds with chemicals. Research and experience with herbicides over the past 20 years has developed to the point where many herbicide-environment interactions can be described. Nevertheless, our understanding is still incomplete. Con- tinued research will provide answers to questions for which we now have only partial or speculative answers. Environmental factors are known to modify herbicide performance appre- ciably. The same response may be expected time after time from a typical herbicide if it is applied at the same dosage rate in the same environment. In another environment, however, the rate required for a similar level of herbi- cidal activity may differ by 2 to 4 orders of magnitude. The user of a herbi- cide must recognize environmental and soil features that call for higher or lower rates and adjust his applications accordingly. In some situations, 194
INTERACTION OF HERBICIDES WITH THE ENVIRONMENT 195 precautions may be taken to greatly reduce or offset the undesirable effects of certain environmental factors. The conditions to which a herbicide may be subjected are governed by the methods of application and placement. For example, sprays from aerial appli- cation are often exposed to greater wind action than those applied by surface equipment. Proper placement is dependent on certain physiochemical prop- erties of the herbicide and the response of crops and weeds to them. Herbicides applied to plant foliage or broadcast on the soil surface may react to such di- verse factors as wind action, temperature, rainfall or irrigation, humidity, and sunlight. Herbicides that are within the soil profile are influenced by clay and organic colloids, soil moisture, soil temperature, pH, and microbial activity. HERBICIDES IN THE ATMOSPHERE MOVEMENT IN THE ATMOSPHERE When herbicides are applied as sprays, air currents acting on the moving spray droplets may carry them through the atmosphere beyond the intended target. The direction, distance, and amount of spray drift are influenced by the size of droplets, type of spray (oil or water, for example), spray additives, application equipment (such as the type of nozzle), and direction and velocity of wind. Spray drift is usually greatest from aerial applications. Not only is the spray rig on aircraft operated at a greater distance from the surface being sprayed, but turbulent air currents produced by the moving plane accentuate the effects of natural winds on the spray. Attachments or modifications to spray equipment, especially to ground rigs, can often be employed to reduce spray drift. Shielded booms are the attach- ments most frequently used for this purpose. In recent years, new herbicide carriers and additives have been produced that greatly reduce or eliminate spray drift. Invert emulsions (water in oil), particulating agents, and polymeric thickeners, for example, can be used in aerial applications, with significantly reduced drift. Movement of herbicides in the atmosphere may also occur through the diffusion of vapors and by wind action on vapors. Much has been learned about this problem since the late 1940âs when vapors of certain herbicides, particularly the lower alkyl esters of 2,4-D, resulted in damage to sensitive unsprayed crops such as cotton, grapes, and tomatoes. Herbicide vapors in the atmosphere can arise from plant or soil surfaces. Physical and chemical properties of the herbicide and of the surfaces involved and the temperature of the air and surface deposit influence the rate of vapor- ization. Vaporization increases as air temperature and temperature of the
196 WEED CONTROL surface deposits increase. Adsorption of herbicides to plant and soil surfaces and penetration of the herbicide into foliage or soil reduce or eliminate vapor losses. Losses by vaporization from soil surfaces are discussed later in this chapter. Under some environmental conditions, herbicides adsorbed on soil particles may be physically moved from their site of application. Sands are moved shorter distances by wind and water than the smaller clay and organic particles. Concentrations of herbicides in a crop row may be so reduced by severe wind during drought as to reduce or prevent weed control. Distribution of dust particles is far more diffuse than that of sand, and deposition normally occurs over a much broader area. FATE OF HERBICIDES IN THE ATMOSPHERE Herbicide droplets or vapors may be carried various distances through the at- mosphere. Little research has been conducted on the fate of herbicides released into the atmosphere. In recent years, photodecomposition (i.e., radiation- induced decomposition) has attracted considerable attention. Photodecomposi- tion of some herbicides is known to occur in water solutions, on glass surfaces, and on filter paper. The first step in the photodecomposition process is the absorption of solar radiation by herbicide molecules. Absorption of light increases the energy of molecules, specifically the energy level of certain orbital electrons, and sequen- tially increases reactivity. Molecules may lose the excess energy by heat ex- change, reradiation, or fluorescence and revert to their original energy state with or without undergoing reaction. Infrared radiation lacks sufficient energy to induce reaction. Solar radiation reaching the earthâs surface ranges from about 2,600 millimicrons in the infra- red region to about 290 millimicrons in the middle ultraviolet region. Because of their higher energy, reactions are induced by the shorter wavelength com- ponents of visible light. Only limited progress has been made toward identification of photodecom- position products of herbicides. Free radicals are produced during exposure to radiation, and their high reactivity gives rise to a variety of products from ex- posure of a single herbicide to radiation. Such studies are therefore extremely complex. Vapors of herbicides and droplets from spray drift also disappear from the at- mosphere by deposition on plant foliage and soil. The drift of highly phyto- toxic herbicides and their subsequent deposition on sensitive plants can result in plant injury.
INTERACTION OF HERBICIDES WITH THE ENVIRONMENT 197 HERBICIDES IN THE SOIL Despite the method of application, portions of all sprays reach the soil. Herbi- cide deposited on foliage also may be washed off onto the soil by rainfall. In row crops, herbicides are most commonly applied directly to the soil. The be- havior and fate of herbicides in soils are thus significant aspects of herbicide interaction with the environment. PERSISTENCE Persistence of herbicides in the soil has received considerable attention from researchers. Investigations were carried out in California on the persistence of several inorganic herbicides (arsenates, borates, and chlorates) prior to the de- velopment of 2,4-D. Study of soil persistence has become an integral part of research development for most herbicides. Persistence in soils is an essential feature of herbicides used for pre-emergence weed control. Phytotoxic chemicals that decompose too readily are unsatisfac- tory, since herbicides, generally, will not injure dormant seeds. Phytotoxic res- idues, therefore, must persist for several weeks for pre-emergence sprays to be effective. Many of the uses of soil-sterilant herbicides require residual activity extending over months or years. Various properties of soils and herbicides as well as environmental factors influence persistence through their effects on volatilization, movement in soils, decomposition by microorganisms, adsorption to mineral and organic colloids, and absorption by plants. Interactions of these variables and the physical and chemical processes governing them are discussed in following subsections. A major problem arising from herbicide persistence in soils is associated with phytotoxic residues that remain from one season to the next and injure sensi- tive crops grown in rotation with crops sprayed for weed control. Sporadic problems that vary in seriousness have been encountered in the midwest and Great Plains states from residues of simazine and atrazine. Oats and sugar beets have been killed or injured in some cases. Soybeans have occasionally exhibited temporary chlorosis of leaves from atrazine that was applied the previous season. Several other herbicides in widespread use are hazardous to sensitive crops planted within 6 to 12 months after application. These include dichlobenil, diphenamid, diuron, monuron, and fenac. Several new herbicides in use or in late stages of development will require careful observation and study. Thorough knowledge of the persistence and residual effects of several pesticides that per- sist in soils for 6 to 12 months under some conditions has made it possible to use them safely and effectively.
198 WEED CONTROL Plant injury from soil residues of many pre-emergence herbicides can be re- duced by plowing. The application of wettable powder instead of granular formulations of certain herbicides may reduce soil residue and subsequent sus- ceptible plant injury. Crop damage from atrazine residues was prevalent in the corn belt of the United States in 1962âthe first year after extensive use of this herbicide. Since that time, crop losses from atrazine damage have decreased greatly as farmers have become better acquainted with precise application tech- niques and have regulated their rotational systems to avoid injury. Two other potential problems of herbicide persistence that must be con- sidered are long-term injurious effects on beneficial soil microorganisms and illegal residues in untreated rotational crops. Effects of herbicides on soil microorganisms are discussed later. It will suffice to note here that so far only transitory effects on microorganisms have been observed. Illegal residues, however, might become a problem if a herbicide persisted in a soil in quantities that could be absorbed by tolerant untreated crops and a tolerance level had not been established. Although such problems have not been encountered with herbicides, illegal residues have occurred in the use of persist- ent insecticides. The half-life concept is often employed to compare differences in persis- tence between herbicides. Half-life is a measure of time required for a herbicide to drop to half of the original concentration. Although the concept is useful in comparing the persistences of related herbicides, it assumes that decomposi- tion is logarithmic. Sometimes this is not the case, and comparison must be made on the basis of loss curves rather than half-life. ADSORPTION, DESORPTION, AND SOLUTION Adsorption and associated phenomena exert a profound influence on the bio- logical activity of herbicides applied to the soil. Adsorption affects the move- ment of herbicides and regulates their availability to plants. It also influences and regulates rates of decomposition by soil microorganisms through its effects on availability and uptake. Adsorption is the accumulation of a component of a mixture at an interface. The adsorption of solutes to solids (i.e., on the surface of the solid at the clay colloid-water or organic colloid-water interfaces) is of particular importance in the interactions of herbicides with soils. An example of adsorption would be the adherence of diuron molecules to the surface of montmorillonite clay or activated charcoal. It is important to distinguish between adsorption and absorption. Absorp- tion refers to the movement of solutes from one place to another through an interface of a two-component mixture, including penetration into plant cells,
INTERACTION OF HERBICIDES WITH THE ENVIRONMENT 199 microorganisms, and other similar materials. Absorption, therefore, includes the movement of a herbicide from a soil solution into a root or from a leaf surface into the leaf. Adsorption, desorption, and solution are interrelated. Desorption is the re- lease of adsorbed molecules (an example would be release of molecules from the surface of a colloid in water). The concentration of a herbicide in the soil solu- tion is greatly affected by the degree and tenacity of adsorption. Once a herbi- cide is applied to soils, a dynamic equilibrium is established between its adsorbed and solution phases. As herbicide molecules are removed from the aqueous phase by decomposition, volatilization, or plant uptake, desorption occurs in re- sponse to the disrupted equilibrium between the two phases until equilibrium is restored. Desorption has been demonstrated for several herbicides, including 2,4-D, amiben, monuron, dalapon, atrazine, and simazine. Through its control over the soil-solution concentration, and hence uptake by microorganisms, adsorption plays a major role in the soil persistence of her- bicides. Strongly adsorbed herbicides may decompose very slowly, because low concentrations in the soil solution limit the uptake by soil microorganisms. If other conditions and requirements for decomposition are met, decomposition is much more rapid for more freely available herbicides. All herbicides that reach soils are adsorbed to some extent, and their herbi- cidal activity is reduced in proportion to the amount lost from the solution. Although biological activity is reduced by adsorption, persistence may actually be prolonged. In fact, adsorption tends to retard decomposition and thereby increase persistence. Soils are complex mixtures of inorganic particles, nonliving organic constitu- ents, air, water, and living organisms (microbes, plant roots and stems, and macrofauna). The components of a normal soil interact with herbicides de- posited on or in the soil. Adsorption occurs on most surfaces and may be ex- tensive on material of high surface area such as clay. The extent of adsorption depends on several properties of the herbicide and soil, the soil moisture, and temperature. The organic and mineral colloids are capable of absorbing in variable degrees herbicides with a wide range of properties. Adsorption studies are often conducted with isolated single components of natural soils or with synthetic adsorbents. The extent of adsorption to purified montmorillonite, for example, can be predicted under specific conditions. In natural soils, however, adsorption is governed by the interacting effects of all components of the soil system. Experiments have shown an inverse relationship between soil-moisture level and adsorption of EPTC. Adsorbed water on clay surfaces may compete with EPTC for adsorption sites, or layers of water surrounding the clay colloids may physically prevent association of the water-insoluble EPTC molecule with ad- sorption sites on the clay. Increasing soil moisture may increase the thickness of the aqueous layer across which herbicide molecules must migrate.
200 WEED CONTROL Soil pH influences the adsorption of many herbicides. The ionic character- istics of some herbicide molecules change with changes in pH, causing different degrees of adsorption in soils. A few herbicides, particularly diquat and paraquat, take part in cation ex- change. On the other hand, dalaponâa negatively charged moleculeâis appar- ently repelled by negative sites on some soil constituents. The extent of adsorption of herbicides may range from almost complete involvement (and loss of herbicidal activity) to little or none (and activity ap- proaching that in a sand or solution culture). These wide differences in adsorp- tion are accounted for by the complexities and variations of soils and differ- ences in the structures and properties of herbicides. VOLATILIZATION AND CODISTILLATION All herbicides are to some extent volatile, although vaporization rates are often negligible from a practical point of view. Losses to the atmosphere from several pre-emergence sprays that possess a low-order toxicity do not ordinarily create hazards to plants from vapor activity and movement through the atmosphere. Such losses are undesirable, however, because concentrations in the soil may be so reduced that effective weed control is not obtained. Vapor losses of herbicides from soils are usually considered as a surface phe- nomenon. Herbicide molecules mostly escape into the atmosphere from soil-air or water-air interfaces. Sometimes the interface is herbicide-air, as when subli- mation occurs from the surface of insoluble herbicide crystals. The volatilization of a herbicide from soil is dependent on the vapor pressure of the compound, its adsorption to soil, and its solubility in water. Volatiliza- tion is influenced by several soil properties and by factors such as temperature. Generally, volatilization increases with increases in temperatures. Convection near the soil surface may greatly accelerate losses of herbicides by continuously removing the vapors from the air in contact with the surface from which evaporation occurs. Under such conditions, a steep concentration gradient is maintained. The losses of different herbicides become largely de- pendent on diffusion rates from points of concentration below to the areas where air is being continuously exchanged by convection currents. Adsorption of herbicides to soil reduces vapor losses. Losses of simetone decrease with changes in soils that are generally associated with increased her- bicide adsorption. Losses are directly related to the percentage of sand and in- versely related to the percentage of clay and organic matter. Volatile herbi- cides that are strongly absorbed may remain on the surface of clay soils or soils containing large amounts of organic matter. Losses of relatively volatile herbicides, such as trifluralin and dichlobenil, can be essentially prevented by reducing surface exposure through incorporation.
INTERACTION OF HERBICIDES WITH THE ENVIRONMENT 201 Slight changes in chemical structure alter volatility. Within the phenylcarba- mates, IPC with an unsubstituted phenyl] ring is much more volatile than CIPC, which has a single chlorine substitute on the ring. Prometone is more volatile from low adsorptive metal surfaces than prometryne. Prometryne, in turn, is more volatile than propazine. The differences in structure of these herbicides are restricted to substituents at the two position. Prometone is 2-methoxy; prometryne, 2-methylmercapto; and propazine, 2-chloro. Rates of volatilization of esters of dalapon, 2,4-D, and other phenoxy herbi- cides vary with the size and structure of the alcohol moiety. The isopropyl ester of 2,4-D is much more volatile than the propyleneglycolbutylether ester. Most esters are more volatile than acids, and acids are more volatile than salts. The soil-moisture level also affects vapor losses of herbicides. Water is be- lieved to compete with CDAA for adsorption sites on clay. Under high soil- moisture conditions, a temperature rise increases the volatilization of CDAA. Conversely, when soil moisture is low, CDAA losses are reduced. The compe- tition for adsorption sites appears to be restricted to clays and apparently is unimportant in the adsorption of CDAA on organic matter. Conflicting views exist on the codistillation of herbicides with water. Rapid vaporization of such herbicides as EPTC and DNBP at high soil-moisture levels simultaneously with rapid water evaporation has been attributed to codistilla- tion. Disagreement over this phenomenon may result from differences in in- terpretation of data or a misunderstanding of the exact nature of codistillation. Certainly, one may easily visualize water-insoluble herbicides applied with emulsifiers concentrating at water surfaces if soil moisture is high. Whether rapid vaporization is strictly the result of codistillation or promoted by greater diffusion gradients because of concentration at the interface is a matter of opinion. Whatever the explanation, vapor losses of several herbicides are much more rapid at high soil-moisture levels. High soil moisture has an opposite ef- fect on vapor loss for some herbicides. Soil pH affects vapor losses in two ways. First, through its influence on adsorption, pH may affect the vaporization of certain herbicides from soils. Secondly, within the pH range encountered in natural soils, some herbicides vary from an ionized form at one pH value to an un-ionized form at another value. The degree of dissociation may determine the rate of vaporization for some herbicides. In the spring of 1952, when DNBP vapors injured or killed many acres of emerging cotton, it was found that such injuries did not occur on limed soils. At a low pH, DNBP was in the volatile un-ionized form (phenol). When the pH was increased by liming, DNBP ionized to the less volatile salt form. Both research and experience have shown how several rather volatile chemi- cals may be used so that undesirable consequences are reduced and eliminated to achieve effective weed control.
202 WEED CONTROL DECOMPOSITION IN SOILS Herbicide molecules in a soil solution can be absorbed by microorganisms and subjected to metabolic attack. The herbicide provides a carbon source for the microbe, which derives energy from catabolic reactions. Losses of herbicides from soils are enhanced by environmental and soil conditions that favor growth and proliferation of microorganisms. Such conditions include warm, moist soil and a favorable supply of mineral nutrients and organic matter. Herbicides are known to undergo several specific types of reactions in soil microorganisms. Such reactions include dehalogenation, dealkylation, hydroly- sis, hydroxylation, B-oxidation, cleavage of aromatic ethers, conjugation, com- plex formation, and ring cleavage. Conjugation and complex formation lead to inactivation without destroying the basic herbicide structure. The others are usually part of a series of reactions leading to decomposition. In a few cases, hydrolysis and oxidation may actually result in the formation of herbicides from inactive compounds. Sodium 2,4-dichlorophenoxyethylsulfate (sesone) and related esters are herbicidally inactive. Hydrolysis yields the phenoxy- ethanols, which, on oxidation, form phenoxyacetic acid derivatives. In this manner, 2,4-D is produced from sesone. B-Oxidation of the phenoxybutyric acids to yield the phenoxyacetic acids (MCPB£2%!@ation, MmcpA) has been demonstrated in soils. In laboratory experiments, microbial decomposition of many herbicides, including 2,4-D, MCPA, 2,4,5-T, IPC, CIPC, and dalapon, is characterized by three usually distinct phases: a lag phase in which little or no decomposition occurs, a rapid decomposition phase, and a slow decomposition phase which proceeds at an ever-decreasing rate. The lag-phase enrichment has been described as the time required for de- velopment of a population of soil microorganisms that are capable of metabo- lizing a particular herbicide. According to the most popular view, microbes are incapable of metabolizing many herbicides at their time of application. However, in the presence of a new energy source (the herbicide), enzymes that catalyze decomposition of the herbicide are formed from a closely re- lated enzyme already present in the microbe population. Once a population of microorganisms having the adapted enzymes proliferate, rapid decomposi- tion occurs. The duration of each phase varies widely among herbicides. It is also af- fected by environmental factors that govern microbial growth. In a study conducted by L. J. Audus, the lag phase for 2,4-D was found to be 14 days; for MCPA, 70 days; and for 2,4,5-T, 270 days. Under favorable conditions for microbial growth, the rapid decomposition phase of 2,4-D may be only 12 to 24 hours and the third (slower) phase 8 to 96 hours. With some herbicides, such as dichlobenil and simazine, the third phase may be a very slow loss after concentrations in soils have decreased to about
INTERACTION OF HERBICIDES WITH THE ENVIRONMENT 203 10 to 20 percent of the original application. The decomposition rate appears to be dependent on the concentration in the soil solution (its availability to the soil microorganisms) and, hence, indirectly related to the rate of release of adsorbed herbicide molecules. Disappearance curves of herbicides applied to soils under natural conditions in the field do not always exhibit three well-defined phases. Several factors in- fluencing the growth of soil microorganisms interact to varying degrees and alter decomposition rates so that the typical lag and rapid decomposition phases of disappearance curves are not always discernible. Certain herbicides (for example, simazine and 2,3,6-TBA) are metabolized by soil microbes that require no adaptation. One of the ethylamino side chains of the simazine molecule is slowly dealkylated by several common soil fungi, Aspergillus fumigatus, Fusarium roseum, Trichoderma, Pennicillium, and Geo- trichum. Degradation appears to occur without delay once the herbicide is available to the fungi. Fragmentary data now suggest that a few herbicides are absorbed by plants, metabolized in the roots, and the metabolites exuded back into the soil. Per- sistence of atrazine is greater in fallow soil than in soil-supporting sorghum, corn, or johnsongrass. Decomposition of several herbicides appears to occur nonenzymatically. Conversion of simazine to hydroxysimazine and the decomposition of amitrole take place in soils at elevated temperatures under which enzymes would normally be denatured. A few efforts to demonstrate the decomposition of herbicides on soil sur- faces have been only partially successful. The importance of the direct effect of solar radiation on decomposition at the soil surface remains to be determined. A practical difficulty in this endeavor has been the differentiation between losses from volatilization and those from photodecomposition. If photodecom- position is important to the loss of herbicides from soils, it must occur on the soil surface. Any herbicide movement into the soil with irrigation water or rainfall would greatly reduce or eliminate light absorption and, hence, negate the chance for induced reactivity. Most herbicides are decomposed in the soil. Pathways of decomposition by microorganisms have been established in part for 2,4-D, 2,4-DB, dalapon, CIPC, chloroxuron, diuron, monuron, atrazine, simazine, trifluralin, paraquat, and a few other herbicides. Knowledge of the process is far from complete. HERBICIDE MOVEMENT IN SOILS Soil fumigants such as methy! bromide, which exist as gases at prevailing soil and air temperatures during the growing season, diffuse in soils in accordance with the gas laws. Pore space is a prime factor in diffusion, and permeability
204 WEED CONTROL varies with soil texture, the degree of soil compaction, and soil-moisture level. The sorption capacity of a given soil for a specific compound in the vapor phase and the solubility of the compound in soil water are important factors governing the distance through which a gas will diffuse from the point of in- jection. Except for highly volatile soil fumigants, herbicides rarely move as vapors in soils for more than a few millimeters. The only exceptions are herbicides such as vernolate, EPTC, dichlobenil, and trifluralin, which may move a few centimeters in the vapor state. This may account for the slow losses of these herbicides from surface layers of soil after incorporation. Diffusion of herbicides in soil water is also a very slow process. The aqueous pathway is circuitous, and soil particles are physical obstacles to direct diffusion pathways. Except for losses from the surface layer of soils to the atmosphere of such herbicides as trifluralin, the amounts and distances moved by diffusion in the gaseous state or in solution are negligible. Zones of herbicidal activity are relatively unaffected by diffusion. Therefore, diffusion plays an insignifi- cant role in the movement in soils for many herbicides. The movement of herbicides as solutes in gravitational and capillary water is of major importance. The amount, pattern, and distance of movement of herbicides in solution in rain or irrigation water are governed by many factors, including the properties of the soils and herbicides and the amount, intensity, duration, and frequency of irrigation and rainfall. These factors interact di- rectly and indirectly in many combinations with adsorption, desorption, solu- tion, and diffusion of herbicide molecules to cause the different patterns of soil movement. Some insoluble crystals are carried downward through cracks and fissures. Although soils are a much more complex system, the leaching of herbicides through soil profiles may be considered analogous to percolation through chromatographic columns. The same basic principles that apply to chromato- grams are applicable to herbicides in soil columns. The soil matrix is the sta- tionary phase, and water is the developing solvent. Many things tends to delay movement of herbicides through the soil. Ad- sorption to clay and organic matter is the major process influencing leaching. The rate of movement is related directly to the partition coefficient, which is defined as the ratio of the concentration of herbicide in the adsorbed phase divided by the concentration in the solution phase. Some herbicides move so slowly and over such short distances as solutes in water that movement is not measurable by customary methods. Paraquat and diquat are cationic and so tenaciously adsorbed to clay that they remain in the vicinity of application, except possibly in very sandy soils. Other herbicides, such as CIPC, are very insoluble in water and also adsorb to organic matter. The combined effects of water-insolubility and adsorption greatly restrict CIPC
INTERACTION OF HERBICIDES WITH THE ENVIRONMENT 205 movement. This herbicide rarely moves more than 1 inch downward, except in porous, nonadsorptive sands. Movement of uncharged organic-herbicide molecules may be retarded by be- coming associated with organic matter. Depending on the pH of the soil, a num- ber of herbicides may exist as uncharged polar molecules. Dalapon, 2,3,6-TBA, fenac, and similar negatively charged herbicides are not strongly adsorbed to soil colloids. Their movement with percolating water is relatively unrestricted compared with more-insoluble, strongly adsorbed com- pounds. The effects of physical properties of herbicides on movement are exempli- fied by the substituted-urea series (in order of increasing solubility) of neburon, diuron, monuron, and fenuron. Neburon is the least water-soluble and the most strongly adsorbed to soil colloids. Fenuron exhibits the opposite charac- teristics. The others are intermediate, as predictable by their solubility. These differences are reflected in the relative leaching rates. Neburon is leached least and fenuron most readily. As gradients produced by water evaporation from the soil surface cause up- ward flow of water by capillarity, dissolved herbicides are carried upward with water. Equilibrium concentrations of herbicides are approached more closely in slow-moving capillary water because there is more time for solution of the herbicide from solid and adsorbed phases. When equal volumes of water move through the soil, herbicides such as diphenamid and dicamba are therefore transported greater distances and in greater quantities upward than downward. Lateral herbicide movement in capillary water is controlled by the same physical laws and environmental factors as upward movement. In furrow- irrigated crops, lateral movement markedly alters the profile pattern of some incorporated herbicides and affects surface applications to some extent. Down- ward movement also reduces herbicide concentrations in surface layers of soil. If leaching is excessive, weed control may be unsatisfactory. Crop seedlings may be injured in some cases, depending on soil type, the solubility and adsorp- tivity of the herbicide, and crop tolerance. Relative mobilities of herbicides in soils must therefore be carefully considered if weed-control practices involve preplanting or pre-emergence applications. EFFECTS ON SOIL MICROORGANISMS Research on the effects of herbicides on soil microflora has been prompted by the importance of soilborne pathogens to agriculture and the vital contributions of many soil microorganisms to organic-matter decomposition and soil fertility. Although studies of the responses of many species of beneficial and plant- pathogenic soil microbes to all herbicides are by no means complete, sufficient
206 WEED CONTROL observations have been made of most herbicides to indicate that field applica- tions are not creating any serious hazards. Several herbicides have no known effects on any species of soil microflora at concentrations that result from normal rates of application. At rates nor- mally used for weed control, a few herbicides have transitory effects, either stimulatory or inhibitory, on soil microorganisms. No herbicides have had a lasting effect. In laboratory surveys of microbial responses, the concentrations of herbi- cides employed often far exceed concentrations that would be encountered as a normal result of weed-control practices. In many cases, greatly excessive concentrations have no adverse effects on numbers of soil microorganisms or on such microbial processes as nitrification, nitrogen fixation, and cellulose decomposition. Dalapon apparently does not affect the general soil population of micro- flora at rates of 5 to 10 pounds per acre. Indeed, rates of application several times greater stimulate the growth of some species. Concentrations of 2,4-D at 100 to 200 times the amounts normally used for weed control usually have no appreciable effect on the soil population of bacteria, fungi, and actinomycetes. Reduced bacterial counts have been ob- served with 2,4-D concentrations as low as 100 ppm, but in several experi- ments 500 ppm have not altered bacterial counts. Concentrations of 100 ppm and up cause response in one or more species of bacteria. More is known about the effects of 2,4-D on soil microflora than about any other herbicide, and some interesting interactions have been observed. The herbicide is more toxic to microorganisms in acid than in alkaline soils and most toxic to aerobes and facultative anaerobes. Spore-forming bacteria appear to be more sensitive than nonsporeformers to 2,4-D. Bacteria are more sensitive than fungi to the herbicide. Even closely related species differ in response to 2,4-D. Azotobacter agilis is more resistant than A. chroococcum. In summary, different groups of soil microorganisms differ greatly in their re- sponses to herbicides. Several common herbicides inhibit growth of plant pathogens in soils, al- though concentrations required to inhibit pathogen growth are usually much greater than those that would be employed in field use. Concentrations that inhibit growth of plant pathogens are sometimes less than would be required for growth inhibition of beneficial species of bacteria. Certain herbicidesâCIPC, EPTC, amitrole, PCP, and DNPBâare toxic to some specific groups of soil microbes at normal rates of application, but the effects are short-lived. Within a few weeks after application, following herbi- cide decomposition, the microflora regain their original numbers. Upon re- covery, numbers often increase far above counts obtained before herbicide application. The high numbers, however, do not remain for long.
INTERACTION OF HERBICIDES WITH THE ENVIRONMENT 207 Recent studies suggest that there are interesting and involved interactions be- tween herbicides and soil microorganisms. Simazine and atrazine stimulate one or more genera of fungi that are antagonistic to some root organisms (Fusarium spp.). Thus, applications of these herbicides may produce beneficial effects on the soil beyond those of weed control. Differential responses of groups of soil microbes impose limits on interpreta- tion of data on oxygen consumption, carbon dioxide evolution, and counts of total microorganisms or separate counts of total bacteria, fungi, and actinomy- cetes. Difficulties may arise when an inhibition of one or more species is masked by its small contribution to total counts or total soil respiration or by the herbicide stimulating one or more other species. Nevertheless, it is often necessary to measure gross effects or responses. Such measurements greatly aid our understanding of herbicide interactions with soil microflora and prove valuable in the development of safe chemical weed-control practices. The interactions between herbicides and soil microorganisms have scarcely been explored. General surveys of the effects of several new herbicides, and even some older ones, on beneficial soil microbes have yet to be conducted. Opportunities for studying interactions among soil factors, herbicides, and species of microflora are almost unlimited. HERBICIDE MOVEMENT TO UNSPRAYED SOILS Movement of herbicides through the atmosphere as vapors and drift and as in- soluble deposits or adsorbed molecules on windblown sand and dust particles has been discussed earlier. Herbicides may also be carried from their point of application to unsprayed areas in surface-water drainage. When excess rainfall or irrigation water drains from fields, herbicide molecules may be carried in solution or in the adsorbed phase on suspended soil particles and become de- posited along the path of the flowing water. In the southeastern United States, where intense rain showers occur, movement of atrazine over short distances from sprayed fields of corn to lower elevations has injured sensitive crops, particularly tobacco, peanuts, and soybeans. Similar problems have developed from the use of soil sterilants along fence lines, utility rights-of-way, and high- way guardrails. Such problems have occurred because inadequate consideration was given to the potential hazard of herbicides to untreated plants and to the soil and weather factors that influence herbicide movement and persistence. In the southeastern United States, the movement of 2,4-D and atrazine has been measured in surface wash-off that occurs when intense rainfall follows herbicide application. An ester formulation of 2,4-D moved in greater quanti- ties than an amine salt. Losses in surface wash-off were greatest immediately after application and decreased with time. Light showers soon after application
208 WEED CONTROL and prior to heavy, erosive rainfall moved surface-applied herbicides short dis- tances into the soil. Herbicide losses to surface wash-off were thereby reduced in the subsequent erosive rains. Although the complete facts are unknown, losses in surface runoff are probably negligible except under the unusual cir- cumstances of rapid runoff without prior incorporation of herbicide or move- ment into the soil by rains. HERBICIDES IN WATER PERSISTENCE AND DECOMPOSITION Increasing interest in the control of aquatic weeds during the past decade has stimulated research on the persistence of herbicides in water and submerged soils. Our knowledge of herbicidal persistence and decomposition in normal agricultural soils is not necessarily applicable to aquatic environments. Microbial decomposition, a major factor responsible for loss of herbicides from normal soils, may be much reduced, or the metabolic pathways may be altered under submerged conditions. The herbicides that have been investi- gated have proved more persistent in water than in well-aerated soil. Oxygen levels are lower and microbial populations are therefore different. Anaerobic species are favored in aquatic environments, whereas aerobes usually predomi- nate in agricultural soils, particularly in the surface-soil layers. In the natural environment, the disappearance of herbicides from water is the sum of losses from decomposition and adsorption by soil in contact with the water. Losses to the soil through adsorption and penetration greatly ac- celerate losses from the water, especially for some herbicides, such as para- quat, that take part in cation exchange or for others that are strongly adsorbed. A few herbicides persist for relatively long periods in aquatic environments. Amitrole and dichlobenil have been found in significant amounts in aquatic sites more than one year after application. Significant residual activity of some herbicides, lasting for several days or a few weeks, is essential for maxi- mum control of some species of aquatic plants. Most microorganisms that effectively decompose herbicides are aerobic. Anaerobic decomposition of herbicides has not been thoroughly investigated. However, disappearance rates from aquatic soils and associated water suggest that anaerobic microbes are involved. Information on pathways, mechanisms, and products of anaerobic microbial metabolism is almost completely lacking. Reactions that lead to alteration of specific herbicides in aquatic environments may be different, at least in degree, from those that occur in normal agricul- tural soils. The extended persistence of herbicides in water poses two possible hazards. In sufficient concentrations, persisting phytotoxic chemicals in irrigation waters
INTERACTION OF HERBICIDES WITH THE ENVIRONMENT 209 may injure crop plants. In lakes, rivers, and estuaries, herbicide contamination could possibly injure aquatic life. EFFECTS ON AQUATIC ANIMALS Only a few herbicides are quite toxic to fish. Concentrations used for aquatic- weed control are carefully regulated to prevent injury to these and other aquatic animals. There is no evidence to suggest that any herbicide now in use on agri- cultural Iand poses a hazard to aquatic organisms. POSSIBLE CONTAMINATION OF GROUNDWATER The low water solubility of many herbicides and their adsorption to soil restrict downward movement so that decomposition occurs before significant amounts of most herbicides move beyond the upper foot of soil. However, monuron, atrazine, and simazine, usually considered relatively immobile in soils, have been detected and measured at depths of 1 to 2 feet. Herbicides having the greatest solubilities in water would be expected to leach to the greatest depths. Although 2,4-D in the salt form fits this category, its rapid loss through microbial decomposition apparently prevents significant con- tamination of groundwater. Fenac, 2,3,6-TBA, and picloram are not strongly adsorbed in most soils, are relatively soluble in water, and are very persistent under many environmental and soil conditions. Fenac, 2,3,6-TBA, or their phytotoxic decomposition products have been detected in significant quantities at depths of 4 to 6 feet in Kansas. Limited use of these herbicides in areas where leaching occurs eliminates soil-water contamination as a major problem. Although the possibility of isolated instances of low-level contamination of groundwater from certain benzoic, phenylacetic, and picolinic acid herbicides cannot be ruled out, the hazard is believed to be so low that it can be con- sidered unlikely under present patterns of herbicide use.
CHAPTER 2 Safety Kactors in Herbicide Use Development of new, more-effective herbicides over the past decade has prompted increasingly widespread adoption of chemical weed control to raise farm-production efficiency. In 1965, in the United States alone, about 100 million acres of agricultural land and over 30 million acres of other lands were treated with herbicides, at a cost of more than $300 million. The expanding use of herbicides and other pesticides has caused public con- cern about their possible effects on foodstuffs and the influence of toxic ma- terials on human, animal, and plant environments. Public attention focused on such problems soon after the Food and Drug Administration (FDA) of the U.S. Department of Health, Education, and Welfare discovered residues of amitrole in cranberries during the winter of 1959 and removed contaminated stocks from the market. Publication of Rachel Carsonâs controversial book, Silent Spring (1962), also magnified public concern. Although public appre- hension about pesticides has often been clouded by misinformation and con- fusion, it also has had the effect of emphasizing that safety is a matter of universal concern in the use of agricultural chemicals. Recent years have seen a thorough reexamination of application practices, the review and strengthening of regulatory procedures, increased study of means of reducing toxic residues, and intensified monitoring of the environ- ment, especially in regions of high pesticide use. There is growing emphasis on developing highly selective formulations of herbicides with short-lived residues that are metabolized by plants or biodegraded in the environment. Much remains to be learned about the useful and deleterious aspects of pesticides. Agriculture generally has established an outstanding safety record in large-scale use of the agricultural chemicals so urgently needed to maintain 210
SAFETY FACTORS IN HERBICIDE USE 211 high-level productivity in a world of population explosion. Several countries, particularly in North America and Europe, have established high standards and stringent criteria to assure the efficiency and safety of pesticides. Comprehen- sive laws and enforcement procedures regulate pesticide use for the protection of man and his over-all environment. While some nations have lagged behind in implementing pesticide safety measures, the United Nations and numerous scientific organizations are playing an active role in promoting safe and efficient use of pesticides on a worldwide basis. Fortunately, taken as a group, most organic herbicides used today probably have lower mammalian toxicity than chemicals taken at random from the shelves of a chemical storehouse. Many common herbicides are in a mammalian toxicity range similar to ordinary substances such as table salt. While most herbicides present no practical hazard when handled with reason- able care, they may prove harmful if swallowed or otherwise misused. Excessive skin exposure through careless handling may have no effect with one product and prove quite dangerous with another. Herbicide users should always recog- nize that care in the handling of all pesticidal materials is as important to public and personal safety as habits of careful driving. SAFETY RESEARCH ON HERBICIDES After the herbicidal potential of a new product is discovered, the manufacturer must conduct intensive research to determine its possible hazards. These safety studies increase in depth as the product is developed. Evaluation of safety in- formation during a research and development program requires the combined efforts of toxicologists, organic chemists, analytical chemists, wildlife specialists, and other biologists. Initial safety studies by a manufacturer are designed to determine acute toxicity and define possible hazards to workers in manufacturing and formula- tion plants. Herbicide toxicity hazards are relative to concentration and length of exposure. This is evident when we consider that many natural components of the foods we eat are harmful if ingested in large amounts. Alkaloids in coffee are toxic if taken in large enough doses, but probably have no general adverse effect following repeated low exposure. Herbicides or other pest- control compounds may enter the body by inhalation, ingestion, or through contact with the skin. All avenues of bodily entry are studied by the manu- facturer, and special tests are required on the potential hazard to the eyes. A recognized procedure for the first phase of a safety investigation with a new compound is measurement of oral toxicity of a single dose to laboratory animals. Such studies precede experimental field evaluation of a prospective herbicide.
212 WEED CONTROL Acute oral toxicity is often indicated by the term âLD,,.â This value represents the lethal dose in milligrams of a compound per kilogram of body weight required to kill 50 percent of the test animals. The LD <, value for a given chemical is different for various species and even various strains of ani- mals. The LD, values of a herbicide for male and female animals of the same species also often vary considerably. Single-dose oral-toxicity tests have great value as indicators of the relative hazard from accidental ingestion, but taken alone such results can be misleading. An expression of the oral LD, for one or even several test species does not indicate the magnitude of hazard from skin absorption, nor does it establish whether repeated, long-term low-level ingestion or contact poses a hazard. Some chemicals of relatively high acute oral toxicity are not readily absorbed through the skin. Many compounds are metabolized harmlessly in the body and present no practical hazard from re- peated low-level contact. Trace quantities of others are safe because they are excreted rapidly without accumulation. After completion of preliminary toxicity tests, more extensive investiga- tions must be conducted to uncover other possible hazards. The ultimate fate of herbicide molecules is of critical significance. Possible toxic residues that might accumulate in food must be identified if they are present. Safety levels for such residues must be determined as a guide to the establishment of tol- erances. Many laboratories conduct the following types of detailed toxicity studies on herbicides: . Skin, eye, and inhalation toxicity tests . Chronic (continuous) feeding tests of two-year duration on rats and dogs Chronic feeding studies on chickens and quail Reproduction studies on rats and quail Feeding tests on farm animals Toxicity tests on fish and aquatic organisms A A A WN Toxicologists with specialized knowledge interpret results of such safety experiments. Pathologists examine tissues under microscopes for possible histological effects of a compound. Biologists, agronomists, horticulturists, wildlife specialists, and other scientists evaluate the potential effects of use of a herbicide on the environment. Clues to human susceptibilities are provided by medical examination of persons exposed to chemicals during their manu- facture. In assessing potential dangers of herbicide residues, chemists have developed increasingly sensitive techniques for detection of small quantities of an original chemical or its metabolites. Each herbicide has different properties and must be studied individually. Because generalizations can be misleading, experiments
SAFETY FACTORS IN HERBICIDE USE 213 are designed to give specific answers to questions such as the following: Is the herbicide relatively stable or does it degrade? If stable, does it leach in the soil or wash off in surface water? Does it concentrate in living organisms or dilute progressively in the environment? Is it decomposed by hydrolysis, oxidation, ultraviolet light, or soil microorganisms? Such study of the changes that a herbicide undergoes after application can be very complex. In this important phase of work, researchers rely heavily on radioactive tracers in experiments designed to simulate field practices. A âtaggedââ radioactive herbicide is applied and studied over the same course of progression anticipated in the field. After a crop containing the suspected resi- due is harvested, radioactive tracer and other modern biochemical techniques are used to analyze and identify residues in food or feed. The residue may be unaltered herbicide or a derivative changed by the gross environment or bio- chemical processes in the plant. If the residue differs in structure from the original herbicide, it then becomes necessary to investigate the derivative toxi- cologically and determine if more studies are required. Once a residue is identified, an analytical method is developed for the measurement of minute residue traces in raw agricultural commodities of food or feed crops. Accurate determinations must be made of the amount of residue that exists after a herbicide is used, so that any hazard from consumption of treated food or feed may be determined. Collection of field samples for actual analysis of residue is carried out after development of a sensitive analytical technique. Field tests usually involve a wide variety of crops, and the tests are repeated over several years. Often, both representatives of public agencies and industry cooperate in this stage of the effort. Interpretation of the data ob- tained then makes it possible to establish tolerance levels with at least 100-fold margins of safety. As a result of exhaustive safety research by specialists and a careful review of their work by competent authorities, handlers of herbicides and the public are assured that every effort has been made to establish effective health safe- guards. The extensive safety studies that are conducted are expensive, and manufacturers may spend more than $500,000 to develop a new herbicide through the steps necessary to comply with registration and tolerance estab- lishment procedures. GOVERNMENT REGULATION OF HERBICIDES Many countries now regulate the sale and use of herbicides through registration procedures that require proof of efficacy and safety. Although regulatory laws and enforcement procedures vary from country to country, there is a trend to- ward greater uniformity. A discussion of some of the regulatory requirements
