Herbicides

Herbicides Lennart Torstensson Swedish University of Agricultural Sciences

I. II. III. IV. V. VI.

History and Classification Appearance in the Environment Persistence Toxicology Side Effects on Flora and Fauna Regulations

Glossary Active ingredient Active part of a formulated her­ bicide product. ADI Acceptable daily intake of an herbicide dur­ ing a lifetime which appears to be without appre­ ciable risk on the basis of all facts known at the time. It is usually expressed as mg of the chemical per kg of body weight. Formulated product Liquid concentrate or solid particles containing the active ingredient of an herbicide as well as other substances which en­ hance its effect. L D 5 0 Dose (given as mg per kg body weight) that kills 50% of the test animals. Persistence Resistence to deomposition. Pesticide Includes different types of pest control products (e.g., herbicides, fungicides, insecti­ cides, rodenticides). Potentiation The possibility that the toxic effect of an herbicide could be enhanced by another com­ ponent in the formulation, by another pesticide with which it is used, or by other chemicals in the environment.

HERBICIDES are chemical substances used to control unwanted vegetation (e.g., weeds) in agri­ cultural and horticultural crops, shrubs in reforesta­ tions, or all plant growth on areas like industrial sites, railway embankments, and garden pathways. The herbicides constitute a large and very diverse group of organic chemicals. It is estimated that 150

or more substances with herbicide activity, ap­ pearing in still more formulated products, are com­ mercially available. The intention is that the herbi­ cide only affects the unwanted plant species (i.e., by design interferes with plant systems) and should be nontoxic to mammals or other organisms. However, herbicides are not always that selective in their ac­ tions and may occasionally cause damage to nontarget organisms. They may also contribute to the general increase in the pollution of air, soil, and water to our planet.

I. History

and

Classification

A. Use of Herbicides The problem of weeds and their control has been present since the early days of agriculture. The sim­ ple methods of the preagricultural revolution era were replaced by the concept of crop rotation and the prophylactic measures embodied in "good hus­ bandry." The development of fertilizers, tractor power, and mechanization further increased the ability to reduce crop/weed competition. It was the discovery of the organic herbicides, however, which truly presented the farmer, horticulturist, and for­ ester with the power to control weeds in crop or noncrop situations. In early stages of development, herbicides had a broad range of effects. Later, it become possible to create substances with selective phytotoxicity. Today, the sophistication is such that it is possible to control broad-leaved weeds in narrow-leaved crops, broad-leaved weeds in broadleaved crops, narrow-leaved weeds in narrowleaved crops, or narrow-leaved weeds in broadleaved crops.

B.

Classification

Herbicides may be classified in a variety of ways, including by chemical group, selectivity, nature of

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351

Herbicides

352 action, or application characteristics. A number of herbicide formulations are common. 1 . Chemical Groups Herbicides can be classified on the basis of their chemical affinities (Table I). Herbicides of the same chemical group tend to have similar physiological characteristics and it may be possible to predict how new compounds of the group may act. However, minor differences in chemical structure may lead to considerable differences in selectivity. 2. Selectivity Total or nonselective herbicides are those which kill all vegetation, whereas selective compounds will control some plants while leaving other species un­ harmed. Nonselective herbicides (e.g., sodium chlo­ rate) are used to eliminate weeds from industrial sites, railway tracks, paths, etc. Selective com­ pounds (e.g., 2,4-D and MCPA) control weeds with­ out adversely affecting the growth of the crop. Se­ lectivity may be due to differences in the retention, uptake, movement, metabolism, or biochemical ac­ tion of the herbicide in crops and weeds. 3. Nature of Action Contact herbicides affect only those parts of the plant to which they are applied, whereas translo­ cated or systemic substances can move within the plant to regions remote from the point of applica­ tion. Contact substances, such as paraquat, tend to be relatively phytotoxic, acting on the membrane systems of the leaf tissues and inhibiting photosynthetic and respiratory metabolism. Conversely, phloem-translocated herbicides (e.g., 2,4-D and MCPA) tend to act relatively slowly and are translo­ cated to the regions of metabolic activity. Normally, root-absorbed compounds are transported in the xylem to the shoots. 4. Application Characteristics Herbicides may be classed as soil- or foliageapplied. Soil-applied compounds are normally ab­ sorbed by the roots or emerging shoots and trans­ ported to the shoot in the transpiration flow. Foliage-applied compounds penetrate the outer waxy cuticule and are absorbed into the leaf tissue, where they may or may not be translocated basipetally in the phloem. Soil-applied herbicides normally are of relatively low water solubility and ideally re­ main in a discrete band at the soil surface. They are normally absorbed by the root or emerging shoots of

weeds and may be translocated in the xylem to the shoots; many of them, such as urea and triazine compounds, act on the light reactions of photo­ synthesis. Soil-applied herbicides may be subject to a number of degradation mechanisms in the soil and only a portion of the applied dose may be available for absorption by the plant. Crops and weeds have usually both emerged at the time of spraying a foliage-applied herbicide and its efficiency and selectivity thus depend on the ef­ ficiency of spray retention, cuticule penetration, tis­ sue absorption, translocation, and metabolism or fixation/binding at nonactive sites. The timing of application is normally described in relation to the stage of crop development, such as presowing, preemergence, or postemergence treat­ ment. Selectivity of an herbicide may depend partly on differences in the rooting depth of the crop and weed. It may also involve differences in herbicide retention, leaf absorption, translocation, or metabo­ lism; differences in the suceptibility of the ultimate enzyme target sites may also play a role. 5. Herbicide Formulations The active part of an herbicide, known as the active ingredient (a.i.), is generally formulated by a manu­ facturer as a liquid concentrate or in solid particles. The former has to be diluted by the user, generally with water, and many herbicides are formulated in such a way as to be readily dispersible in water. Compounds which are water-insoluble are generally formulated as emulsifiable concentrates or wetable or flowable powders (Table II). Many herbicide formulations contain adjuvants which enhance their leaf retention and cuticule pen­ etration. Surfactants or surface-active agents are used extensively for this purpose. There are three classes of surfactants—cationic, anionic, and nonionic, compounds belonging to the last two groups are most commonly used as herbicide adjuvants.

II. Appearance in the Environment To use herbicides in the most efficient manner (e.g., in effective economic weed removal), it is necessary to have detailed knowledge of the appearance and presistence of the substances in the environment. This knowledge is also urgently needed to avoid the spread of pollution outside the treated areas, as well as to prevent damage to the environment. The con-

353

Herbicides Table I

Classification of Herbicides According to Chemical Group Chemical group

Examples

Amides

Diphenamide (N,7V-dimethyldiphenylacetamide) Propyzamide (3,5-dichloro-N-(l,l-dimethyl-propynyl)benzamide)

Anilides

Flamprop-isopropyl (isopropyl-7V-benzoyl-7V-(3-chloro-4-fluorophenyl)-D,L-alaninate) Metazachlor (2-chloro-7V-(pyrazol-l-ylmethyl) acet-2\6'-xylidide) Propachlor (2-chloro-N-isopropylacetanilide) Propanil (3' ,4' -dichloropropionanilide)

Aromatic acids

Dicamba (3,6-dichloro-o-anisic acid) 2,3,6-TBA (2,3,6-trichlorobenzoic acid)

Carbamates

Asulam (methyl-4-aminophenylsulfonyl-carbamate) Phenmedipham (methyl-3-(3-methylcarbaniloyloxy)carbanilate) Propham (isopropyl phenylcarbamate)

Haloalkanoic acids

Dalapon (2,2-dichloropropionic acid) TCA (CCl3COONa)

Heterocyclic nitrogen compounds Triazines

Atrazine (2-chloro-4-methylamino-6-isopropylamino-l,3,5-triazine) Metamitron (4-amino-4,5-dihydro-3-methyl-6-phenyl-l,2,4-triazine-5-one) Metribuzin (4-amino-6-/-butyl-4,5-dihydro-3-methylthio-l,2,4-triazine-5-one) Simazine (2-chloro-4,6-bis(ethylamino)-l ,3,5-triazine) Terbutylazine (2-/-butylamino-4-chloro-6-ethylamino-1,3,5-triazine)

Pyridazines

Chloridazone (5-amino-4-chloro-2-phenyl-pyridazin-3(2//)one)

Pyridines

Diquat (l,r-ethylene-2,2'-bipyridyldiylium dibromide) Paraquat (l,r-dimethyl-4-4'-bipyridylium dichloride) Picloram (4-amino-3,5,6-trichloropyridine-2-carboxylic acid) Triclopyr (3,5,6-trichloro-2-pyridyloxyacetic acid)

Pyrimidines (uracils)

Bromacil (5-bromo-3-/-butyl-6-methyl-uracil)

Unclassified

Aminotriazole (l//-l,2,4-triazol-3-ylamine) Benazolin (4-chloro-2-oxobenzothiazolin-3-ylacetic acid) Bentazon (3-isopropyl-l//-benzo-2,1,3-thiadiazin-4-one 2,2-dioxide) Endothal (7-oxabicyclo(2.2.1)heptane-2,3-dicarboxylic acid) Ethofumesate ((±)-2-ethoxy-2,3-dihydro-3,3-dimethylbenzofuran-5-yl methanesulfanate

Inorganic compounds

Sodium chlorate (NaC10 3)

Nitriles

Bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) Dichlobenil (2,6-dichlorobenzonitrile) Ioxynil (4-hydroxy-3,5-iodobenzonitrile)

Nitroanilines

Trifluralin (a,a,a-trifluoromethyl-2,6-dinitro-7V,N-dipropyl-p-toluidine) Pendimethalin (N-(l-ethylpropyl)-2,6-dinitro-3,4-xylidine))

Nitrophenols

Dinoseb (2-sec-butyl-4,6-dinitrophenol) DNOC (4,6-dinitro-o-cresol)

Nitrophenyl ethers

Nitrofen (2,4-dichlorophenyl-4-nitrophenyl ether) Oxyfluorfen (2-chloro-a,a,a-trifluro-/?-toluyl-3-ethoxy-4-nitrophenyl ether) Cacodylic acid (dimethylarsinic acid)

Organoarsenic compounds Organophosphorus compounds

Phenoxyalkanoic acids Phenoxyacetic acids

Phenoxybutyric acids

Fosamin-ammonium (ammonium ethylcarbamoylphosphonate) Glufosinate (ammonium D,L-homoalamin-4-yl methylphosphinate) Glyphosate (Af-(phosphonomethyl)glycine) 2,4-D ((2,4-dichlorophenoxy)acetic acid) MCPA ((4-chloro-2-methylphenoxy)acetic acid) 2,4,5-T ((2,4,5-trichlorophenoxy)acetic acid) 2,4-DB (4-(2,4-dichloφhenoxy)butyric acid) MCPB (4-(4-chloro-ö-toluyloxy)butyric acid) {continues)

354 Table I

Herbicides (Continued) Chemical group

Phenoxypropionic acid

Examples Dichlorprop (2-(2,4-dichlorophenoxy)propionic acid) Fluazifop-butyl (butyl 2-(4-(5-trifluoromethyl-2-pyridyloxy)phenoxy)propionate) Mecoprop (2-(4-chloro-o-tolyloxy)propionic acid) Napropamid (J/V,N-diethyl-2-(l-napthyloxy) propionamide)

Phenyl ureas

Diuron (3-(3,4-dichlorophenyl)-1,1 -dimethylurea Isoproturon (3-(4-isopropylphenyl)-l, 1-dimethylurea Linuron (3-(3,4-dichlorophenyl)-l methoxy-l-methylurea)

Sulfonyl ureas

Chlorsulfuron (l-(2-chlorophenylsulfonyl)-3-(4-methoxy-6-methyl-l,3,5-triazin-2-yl) urea) Sulfometuron methyl (methyl 2-(3-(4,6-dimethylpyrimidin-2-yl)ureidosulfonyl) benzoate)

Thiocarbamates

Diallate (5-2,3-dichloroally diisopropylthiocarbamate) EPTC (5-ethyl dipropylthiocarbamate) Triallate (S-2,3,3-trichloroallyl diisopropylthiocarbamate)

tamination by herbicides and other pesticides of sur­ face and groundwaters is discussed below as is the transport routes of herbicides in the environment.

/I. Surface and Groundwaters A number of different pesticides has been found as contaminants in both surface and groundwaters. In surface waters, like creeks and streams, a total of 18 pesticides—11 herbicides, 2 fungicides, and 5 insec­ ticides, have been identified in agricultural areas of Sweden. The most frequently found substances were the herbicides atrazine, bentazone, dichlor­ prop, MCPA, and mecoprop. The largest amounts of the phenoxy acids were found during the spraying season in amounts peaking at 25 μg/l. Around 50 compounds have been found in groundwater in more than 20 states of the United States. Of these, about one-fifth are herbicides like

atrazine, bentazon, bromacil, cyanazine, 2,4-D, dinoseb, metolachlor, metribuzine, and simazine. Contamination of waters by herbicides and other pesticides has been frequently reported from many countries during recent years. The amounts found have generally not exceeded toxic levels, but are an illustration of the serious problem caused by general chemical contamination of our environment.

β. Transport

Routes

A number of mechanisms and factors influence the transport of herbicides in the environment. From the place of application, a substance may move above the ground, in the growth zone and further into the atmosphere, on the soil surface, or within the ground, both in the nonsaturated and the saturated zone. In all zones, the mobility of water is of great importance for the transport of chemicals. In addi-

Table II Classification of Herbicide Formulation Formulation Dry preparation

Characteristics Pelleted, granulated, or microgranulated goods, powder, smoke, and gaseous preparations, etc.

Fluid preparation

Solvent, emulsifiable and flowable concentrate, aerosols, etc.

Emulsifiable concentrate (EC)

A fluent oil solution giving a stable milky homogeneous fluid, emulsion.

Soluble powder (SP)

Easily gives a clear solution.

Wettable powder (WP)

Holds the herbicide in a "filler" such as clay and a dispersing agent which produces a fine suspension of solid particles when water is added.

Flowable powder (FP)

Gives a fine-grained, turbid suspension of particles.

Flowable or suspension concentrate (FWC, SC)

Homogeneous, fluent dredge of a very fine (micronized) flowable powder.

Water dispersible granule (GW)

Granules dredged and dispersed in water.

Aerosol

A solvent in a pressurized pack, which gives a cloud of very small droplets.

355

Herbicides

tion to transport, loss of herbicides from a certain zone is also dependent on decomposition processes as well as incorportion into soil organic matter. Transport of herbicides in the environment occurs through diffusion, through the influence of wind and water, or through mobility of solid material. During transport, the herbicide may occur as a gas or in solid phase, be dissolved in water, or absorbed to particles of different kinds. The rate of the transport, as well as that of different decomposition processes, is influenced by a great number of factors (Fig. 1). A survey of possible transport routes that may result in the appearance of herbicides in surface and ground­ water is given in Figure 2. 1 . Transport by Wind a. Evaporation Evaporation of an herbicide from the soil surface depends on the vapor pressure of the substance and varies with temperature, water solubility, and ad­ sorption. As the vapor pressure is an equilibrium property yet an equilibrium with the atmosphere is impossible, the rate of evaporation is determined by the wind and the surface area where the substance is distributed. At rising temperatures, the vapor pres­ sure increases. After a spraying operation, at least part of the herbicide is dissolved in available water. Because of this, the distribution of the compound between wa-

ter and air is also of interest. It is particularly impor­ tant for substances with both a low vapor pressure and a low water solubility. Examples of herbicides known to evaporate after application on bare soil are trifluralin (80% within 2-3 days) and chlorpropham (49% within 50 days). b. Wind Drift Wind drift of herbicides in connection with spraying operations is a well-known problem. It has caused damage to sensitive plants on other fields, in green­ houses, and in gardens. It also contributes to the general pollution of the agricultural landscape. Wind drift (even slight winds of 1-3 m/s) may lead to minor amounts of the herbicide (often 0.1% or less) detected hundreds of meters from the sprayed field. In stronger winds, the wind drift may become more obvious. 2. Transport on the Ground Surface Transport of herbicides on the ground surface may occur in flowing water or by the wind. In water, the slope and ground porosity, as well as the intensity and/or continuity of the water supply through pre­ cipitation or irrigation, is of importance. The ad­ sorption properties of the herbicide are also of great importance. With low adsorption, the substance easily penetrates the ground and the risk of loss by surface run-off is little, With strong adsorption, a major part becomes bound to particles at the soil surface. Those particles may then be transported by wind or water over short or long distances. The losses by surface water run-off vary between less than 0.1 to 10% or more, depending upon the slope of the area and precipitation. Soil particles with a diameter of more than 0.06 mm are normally transported only a few me­ ters by the wind. Indeed, even particles in the range of 0.002 mm fall down relatively soon after the wind has enveloped them. However, smaller particles, including adsorbed herbicides, may be transported by wind over very long distances. 3. Transport within the Soil Profile a. Diffusion

Figure 1 The appearance and persistence of an herbicide in soil depends on its transport and decomposition, mechanisms influ­ enced by a great number of factors.

Diffusion is a process by which a chemical sub­ stance (i.e., an herbicide) tends to obtain an equal distribution in a given space. It implies a transport from areas of high concentrations to those of lower concentration. The rate of diffusion depends on the herbicide's concentration and adsorbtion properties

356

Herbicides Deposition of aerially transported herbicides. Spillage, washing end disposal of containers. ATMOSPHERE

Volatilization Photo decomposition

Wind-drift

HERBICIDE APPLICATION Wind-blown soil particles

Adsorption to mineral/organic surfaces Τ

1 *

Transport in the soil

1

ι Desorption and diffusion in soil solution

Chemical trans­ formations

Microbial trans­ formations

Leaching Figure 2 Entry of herbicides into the soil environment. Movements of herbicides, —agencies affecting the herbicides - - > . [From Torstensson, L. (1988). Microbial decomposition of herbicides in the soil. In "Outlook on Agriculture," Vol. 17. pp. 120-124. Pergamon, Oxford. Reproduced with permission.]

as well as the soil's temperature, porosity, and water content. Diffusion in the gaseous phase is 10,000 times faster that in the water phase.

Exudated herbicides can, therefore (in some cases) 4 be taken up by 'untreated" plants which have their roots in the vicinity.

b. Transport of Particles Herbicides adsorbed to soil particles move with them and may thus very rapidly penetrate cracks and channels in the soil. As a result, immobile herbi­ cides may thus, in minute amounts, penetrate deep into the soil profile. c. Root Exudates Herbicides taken up by plant foliage can be trans­ ported through the roots and thereby be exudated. This transport is quite rapid and exudation may oc­ cur within hours of application. Herbicides are transported through the topsoil (that is normally a barrier to immobile substances) down to the subsoil.

d. Transport by Water The most important route of herbicide transport in the soil is with the soil water. The water solubility and adsorption properties of the herbicide, together with the amounts of mobile water, determine the extent of the transport. The transport is generally downward or in some cases occurs horizontally, during dry conditions it can be upward. It is of particular interest to be able to predict the mobility in soil of an herbicide as a function of ad­ sorption and water flow through the soil. Both TCA and glyphosate are soluble in water. TCA is weakly adsorbed to soil while glyphosate is strongly ad-

357

Herbicides

sorbed. Therefire, TCA is very mobile in the soil, while glyphosate is nearly immobile. Thus, the potential mobility of herbicides can be compared by determining their adsorption to soil. Soil, water, and the herbicide are mixed in a test tube and allowed to equilibrate. Then the amounts of herbicide adsorbed to the soil and in solution are determined. The results are evaluated by the Freundlich's equation: log xlm = log Kf + η log c, where xlm is the amount of herbicide adsorbed to the soil (/Ag/g soil), c is the amount in water (/xg/ml water), K{ is the adsorption coefficient (the amount of herbicide adsorbed per gram of soil in equili­ bration with 1 μ% per ml water), and η is the slope of the line. Examples of Kf values for linuron in some soils are given in Table III. The different Kf values indicate that linuron is adsorbed differently; if the values are < 2 , there is a great risk of mobility of linuron in that soil. The main adsorbents of herbicides in soil are clay minerals, organic matter, iron, and aluminum hy­ droxides. For many herbicides, there is a strong

Table III K{ and Koc Values for Linuron in Some Soils Organic C

Clay

(%)

(%)

^o

^oc

1.39 1.86

4 10

9.0 11

648 591

2.49

25

16

643

3.65

32

25

685

7

73

629

11.6 0.83

5

2.9

349

1.28

7

4.9

381

1.97

9

6.3

320

4.64

5

21

453

8.62

33

50

580

6.35

15

36

567

0.59

ND"

3.3

559

0.31

ND

1.8

581

0.24

ND

1.3

542

0.07

ND

0.4

571 X = 540 ± 21%

a

ND, not determined.

IV Expected Mobility for Some Herbicides Based on Their Koc Values

Table

Koc value 0-50

50-150

Expected mobility

Examples

Very high

Dalapon, dicamba, TBA, TCA, picloram, hexazinone, chloramben, chlorsulfuron, sulfometuron methyl

High

Atrazine, chlorthiamide, 2,4-D, MCPA

150-500

Medium

EPTC, simazine

500-2000

Low

Diuron, linuron

2000-5000

Very low

Chloroxuron, trifluralin

>5000

Immobile

Paraquat

correlation between K{ and content of organic car­ bon. This has been used to compare the adsorbtion strength of different herbicides. The Kf value is di­ vided by the content of organic carbon in each soil and multiplied by 100. The adsorb tion constant (Koc) for linuron on the basis of data presented in Table III is 540 ± 21%. Table IV gives examples of herbicides within dif­ ferent Koc intervals that are expressions for ex­ pected mobility. However, as with the linuron ex­ ample, if the substance is applied on soils with a low content of adsorbing materials, the risk of mobility may nevertheless be high. This is also the case for other normally strongly adsorbed substances used on areas low in adsorbents.

III.

Persistence

Persistence of an herbicide is a reflection of its resis­ tance to decomposition. The persistence time is the time required for the compound to be degraded to below the detection limit of chemical analysis; the "chemical persistence" time. Persistence time can also be defined as the time required for a sensitive organism to no longer react to the chemical. Often, cultivated plants are used to determine the persis­ tence time for herbicides used in agriculture (i.e., the phytotoxic persistence time). This is of practical importance in farming where it defines the answers the period of time after usage of a certain herbicide before a new sensitive crop can be sown. For many herbicides, there is a considerable difference be­ tween the chemical persistence time and the phy­ totoxic persistence time (see Table V).

Herbicides

358 Table V Chemical and Phytotoxic Persistences for Some Herbicides Herbicide

Chemical persistence

Phytotoxic persistence

MCPA

1-4 weeks

1-4 weeks

TCA

6-12 months

6-12 months

Glyphosate

6-18 months

1-2 weeks

Diquat

Years

< 1 week in a mineral soil

A number of factors influences the persistence time for a particular herbicide (see Fig. 1). Transport discussed previously. Decomposition may be of abiotic and/or biotic nature.

A. Abiotic

Decomposition

1 . Photochemical

The rates of the reactions are highly influenced by pH. This is demonstrated for the sulfonyl ureas that hydrolyze rapidly at pH < 6 but very slowly at pH > 7 . The rate of the reactions may sometimes in­ crease if the herbicide is adsorbed to clay (e.g., for the triazines). Free radical reactions may also degrade herbi­ cides. The biota in the soil may partly be responsible for the generation of these free radicals (e.g., via hydrogen peroxide produced by microbial extracel­ lular oxidase enzymes). Chemical decomposition, as well as photochemical decomposition, thus lead to products that may be further degraded by biologi­ cal reactions.

β. Biotic

Decomposition

1 . Organisms Involved

Sunlight consists of several types of radiation, among them ultra violet radiation or UV light. Pho­ tochemical decomposition of the majority of herbi­ cides is mediated by UV light under experimental conditions, the energy of UV radiation is capable of breaking the bonds in herbicide molecules. There is sufficient evidence to suggest that photodecomposition also occurs under field conditions and that a considerable proportion of some herbi­ cides may be transformed by this mechanism. Since UV light is incapable of penetrating soil, this type of decomposition only occurs when sunlight directly hits the molecules, (e.g., on the soil surface, on plant surfaces, and in the atmosphere). A number of herbicides are known to be partly degraded by photochemical reactions (e.g., phe­ noxy acids, triazines and phenyl ureas). However, the decomposition is not complete but gives prod­ ucts similar to those occurring in other types of de­ composition.

Decomposition of the main part of applied herbi­ cides is carried out by the soil microorganisms. De­ composition also occurs in plants and animals. Apart from decomposition in soil mediated by cellbound enzymes, there are also enzymes bound to soil particles. These enzymes are able to catalyze the breakage of bonds in herbicide molecules. The enzymes have been exudated from living microor­ ganisms or plant roots or are released from dead microorganisms, soil animals, or plant roots. Microorganisms are able to degrade a wide variety of chemicals, ranging from simple polysaccharides, amino acids, proteins, lipids, etc. to more complex materials such as plant residues, waxes, and rub­ bers. Without the degradative capacity of microor­ ganisms, such organic compounds would accumu­ late and pollute the environment. Microorganisms are also capable of degrading synthetic chemical compounds. A variety of mechanisms underlies these activities and a number of factors influences rates and routes of the transformations.

2. Chemical Decomposition

2. Mechanisms of Microbial Decomposition

In soil and water, chemical reactions may change herbicide molecules. Such reactions have been pro­ posed to be the main mechanism for the degradation of only a few herbicides. However, the importance of chemical reactions should not be underestimated, as they may also contribute essential stages to the principal biological routes of degradation. Probably the two most important factors govern­ ing chemical transformation in soil are moisture and pH, although other reagents may also be involved.

Microbial metabolic activities require energy. Most organic materials can serve as a source of energy for at least some microorganisms. Another environ­ mental attribute of microbial metabolism is their ability to adapt through induction or mutation, par­ ticularly toward chemicals that are initially toxic to them. An important criterion used in classifying the en­ zymatic reactions involved in microbial transfor­ mations of herbicides (Table VI) is whether or not

359

Herbicides Table VI

Classification of Microbial Activities in Connection with Decomposition of Herbicides Mechanism

Consequences Enzymatic reactions

Direct degradation of herbicides in central metabolism of microorganisms in which the herbicides serve as energy sources to supply growth (catabolism) and where adaptation phenomena appear.

Repeated application of an herbicide to the same field results in faster decomposition,

Incidental transformation of herbicides by microorganisms via peripheral metabolic processes in the absence of the perfect coordination of the process which is characteristic in central metabolism (cometabolism).

All herbicides may be decomposed by this mechanism,

Incidental transformation of herbicides by extracellular enzymes.

All herbicides may be decomposed by this mechanism.

Nonenzymatic reactions Normal activities of soil-living microorganisms such as change of pH or generation of free radicals or different reactive substances

the microorganisms derive energy from the process. This consideration is significant from a practical viewpoint, since it is of importance in predicting an herbicide's persistence. Thus, catabolic metabolism requires a favorable chemical structure of the herbi­ cide which allows it to be utilized as a carbon and energy source. If the microorganisms do not derive energy from the transformation of an herbicide (Table VI, cometabolism), they depend upon other more ac­ cessible carbon sources, which increase their gen­ eral metabolic activities. Thus, the rate of microbial metabolism can be controlled by changing either the amount of herbicide or other added carbon source, depending upon the type of microbial degradation activity. In general, cometabolism is the prevalent form of microbial metabolism when the amount of herbicide is low in comparison with other carbon sources. In addition to enzymatic reactions, microor­ ganisms may contribute to the transformation of herbicides via nonenzymatic reactions (Table VI). Large pH changes (1 to 2 pH units or more) are often associated with such microbial activities together with changes in nutritional sources. There are ways in which microbial products can promote photo­ chemical reactions. First, microbial products can act as photosensitizer's by absorbing energy from light and transmitting it to the herbicidal molecule. Second, microbial products can facilitate photo­ chemical reactions by serving as the frequently es­

Influence on biological and nonbiological reactions resulting in transformation of herbicides in soil,

sential donors or acceptors (e.g., of hydrogen and O H ) for photochemical reactions. a. Metabolism and Cometabolism The microbial degradation associated with a given herbicide can be enhanced by the rate and frequency of its application (Fig. 3). This phenomenon is ob­ served and has been found to have practical conse­ quences for the persistence of herbicides such as 2,4-D, dalapon, EPTC, MCPA, and TCA. A loss of Decomposition including adaptation phenomena 1st 2nd and subsequent applications

Time

Figure 3 Principal differences between microbial cometabolic decomposition of herbicides and decomposition where adapta­ tion to the herbicide occurs. [From Torstensson, L. (1988). Mi­ crobial decomposition of herbicides in the soil. In "Outlook on Agriculture," Vol. 17, pp. 120-124. Pergamon, Oxford. Re­ produced with permission.]

Herbicides

360 application efficiency of certain soil-applied herbi­ cides may result through the development of micro­ bial populations capable of rapidly degrading the substances. The reduced pesticide efficiency dif­ fers from the separate problem that arises when the pests themselves develop resistance to chemical treatment. The term cometabolism has no succint definition and has been used in several different ways. In stud­ ies of herbicide decomposition, it is often necessary to distinguish between degradation by catabolism in connection with adaptation phenomena and that caused by other metabolic transformations. The use of the term cometabolism for these transformations has resulted from the focus of many investigators on the microbial interactions with several substances and on the interrelation between their metabolism. Therefore, the use of the term might be justified. From a practical viewpoint it is of interest that adaptation phenomena do not appear during cometabolic decomposition of herbicides. This means that repeated applications of the substance have no influence on the rate of its disappearance from the soil (Fig. 3).

of turnover and mineralization of organic substrate are largely governed by the activity of the soil biomass. Inhibition of microbial activity by a low or high temperature, drought, waterlogging, ex­ tremes of pH, or xenobiotic substances may re­ sult in the persistence in soil of potentially decom­ posable and mineralizable compounds such as herbicides. From a number of studies, we know that herbi­ cides may be transformed by a single organism, but we also know that the normal degradation of native organic matter proceeds in a sequence of metabolic steps carried out be a wide variety of microor­ ganisms. With an increasing biomass in soil, the chances also increase that the microorganisms synthesizing the ' 'right" enzymes are present in a number that forms the basis for a high rate of decom­ position of herbicides and other chemicals in the soil (Fig. 4).

b. Rate of Degradation

The toxicity to mammals of any particular herbicide can readily be determined and compared with that of other herbicides in appropriate tests on laboratory animals. However, a proper evaluation of any toxic hazards that a compound might present to humans requires that additional factors be taken into ac­ count. There is a need to investigate all potentially adverse effects found during routine toxicological investigations and to undertake such additional studies as are appropriate to assess the significance of these effects. Since every herbicide is on some occasion likely to come into contact with the skin or be ingested, toxicity data are required on oral ingestion and skin application. Similarly, inhalation data must be fur­ nished when there is danger of uptake by the respira­ tory tract. Repeated use may also be adequately covered in the toxicological study. To define acute and short-term hazards resulting from use, to classify, and to assist in the design of longer-term tests, all herbicides must be assessed by certain primary toxicological studies. The singledose toxicity by appropriate routes of administra­ tion is established with an L D 5 0 value for at least two common laboratory species, preferably including a rodent and a non rodent. Examples of typical L D 5 0 values are given in Table VII.

The rate of microbial decomposition of herbicides in soil is a function of three variables: (i) the availabil­ ity of the chemical to the microorganisms or enzyme systems which can degrade it; (ii) the quantity of these microorganisms or enzyme systems; and (iii) the relative activities of these organisms or en­ zyme systems. Edaphic factors, such as contents of organic matter and clay, moisture level, tempera­ ture, pH, aeration, and nutrient status, are of impor­ tance as moderators and driving factors. The degree to which an herbicide is decomposed in soil is determined by the adsorption/desorption characteristics of the substance and the simulta­ neous occurrence of the substance and microor­ ganisms at the same site. The distribution of soil microorganisms is not uniform throughout the soil profile. The organism density is highest in the upper­ most part of the soil and declines with depth. This means that if an herbicide is mobile and passes through the topsoil layer into the subsoil, the chance of its becoming microbially degraded is considerably reduced. Additionally, within the topsoil there are differences in microorganism distribution with higher densities occurring in the rhizosphere. The role of microbial biomass in the transfor­ mation of organic matter in soil is crucial and the rate

Ι1Λ Toxicology Λ. Toxicity of Herbicides

361

Herbicides

Biological activity oi the soil Figure 4 The importance of soil biological activity for rate of decomposition of herbicides in the same soil. [From Torstensson, L. (1988). Microbial decomposition of herbicides in the soil. In "Outlook on Agriculture," Vol. 17. pp. 120-124. Pergamon, Oxford. Reproduced with permission.]

Table VII L D 5 0 Values for Some Herbicides Established in Rat or Rabbit L D 5 0 Value (mg/kg body weight) Herbicide Chlorsulfuron Dicamba Dichlobenil Dinoseb

0

Oral

Dermal

5550--6300

>3400 (R) >2000 (R)

1700 >3160 58

1350 (R) 80-200

4050

>5000 (R) >5000 (R)

Hexazinone

1690

>5280 (R)

Linuron

4000

>5000

MCPA

700

>1000

EPTC

2525

Glyphosate

Metribuzin

2200--2345

>20000

Phenmedipham

>8000

>4000

Simazine

>5000

>3100

Sulfometuron methyl

>5000

>2000 (R) >3000

Terbuthylazine Triallate Trifluralin a

2000 1675- 2165 >10000

(R), rabbit; all others are for rat.

8200 >2000 (R)

When primary toxicological studies are com­ pleted, the future studies needed for the toxicologi­ cal assessment of the particular compound is planned. They include metabolic studies in animals, neurotoxicology, risk of potentiation, long-term (chronic) toxicity and carcinogenecity studies, mu­ tagenicity, tests for reproductive toxicity and embryotoxicity (including teratogenesis) as well as tests on immunological response and the enzyme system. Finally, the results of the animal experiments must be extrapolated to humans and any information which gives an indication of human sensitivity to the herbicide under consideration is of value. This addi­ tional information is derived from epidemiological studies and health controls of workers in the pesti­ cide chemical factories and in the field. It may be obtained from the investigations of incidents of acci­ dental or deliberate overdosage. Volunteers have sometimes submitted themselves to trials in which accurately measured doses have been administered under conditions where strict biochemical monitor­ ing of the effects is possible.

362

0. Exposure to

Herbicides

Herbicides

1 . Application The potential hazard for occupational exposure to pesticides has been a concern for many years. In particular, acute toxicity has been the focus of occu­ pational health programs. However, since most her­ bicides have low acute toxicity in comparison with many other pesticides, only a few occupational health studies have been made on them. An increased awareness of long-term effects has reinforced the aim of reducing the uptake of any chemical. This does not mean that full protection is sought in every case. The degree of protection should be balanced against known suspected effects of the chemical used. Exposure studies provide a basis for the use of relevant protective equipment and alterations to working procedures. Observations of actual work procedures must in­ clude all the various steps involved (e.g., handling the herbicide container, mixing, loading the tank, spraying, adjusting booms and cleaning nozzles, cleaning the spraying equipment, changing clothes, personal hygiene, eating and smoking). The subject must be followed continuously for the entire session to cover unexpected events. For instance, in an in­ terview study of 240 farmers and spray men, onethird said that they would clean the nozzles in the field by blowing through them with their mouth. Since many herbicides are applied as sprays, it is popularly believed that the greatest risk of exposure is by inhalation. Herbicides can be airborne as va­ pors, aerosols, or dusts. Measurements of exposure in the breathing zone give the sum of what is inhaled and what is swallowed. In general, airborne expo­ sure is small in comparison with dermal exposure. On the other hand, respiratory and oral exposure lead in most cases to a more efficient uptake than dermal exposure. Determination of skin contamination is not in it­ self a way of assessing dermal absorption. However, controlled experimental volunteer studies in hu­ mans with phenoxy herbicides have shown that the dermal absorption is the main route of uptake. It is obvious that people working with pesticides in the field will be exposed. In many cases the same worker mixes, loads, and sprays the herbicide. In other cases there are special mixers, loaders, spray men, and flagmen, who are exposed to differ­ ent extents. Another group of potentially exposed people are those entering the field shortly after spraying for thinning, picking, etc. Different

methods of application give rise to different degrees of exposure. Thus tractor spraying usually exposes the driver more than does airplane spraying. Knap­ sack application frequently gives rise to leakage and an increased risk of dermal contamination. Other workers who are at risk of exposure are those in­ volved in the manufacture of herbicides (i.e., pro­ duction, formulating, and packaging staff). 2. Food and Water Many publications and recommendations have been prepared by both national and international commit­ tees relating to the levels at which residues of herbi­ cides may persist on crops to which they have been applied. Since herbicides are used to kill or restrict the growth of plants, problems of residues in the harvested crop will not often arise. Residues may sometimes persist in straw or fodder crops, and stock may have access to treated crops. The maximum residue limits should not exceed those resulting from good agricultural practice and should not give rise to intakes exceeding the accept­ able daily intake (ADI). On these criteria, good agri­ cultural practice is defined as the officially recom­ mended or authorized usage of herbicides under practical conditions at any stage of the production, storage, transport, distribution, and processing of food or other commodities. Also bearing in mind that variation in requirements within and between regions, and taking into account the minimum quan­ tities necessary to achieve adequate control, herbi­ cides must be applied in a manner that leaves a residue which is the smallest amount practicable and which is toxicologically acceptable. Groundwater was once thought protected from chemicals applied on the soil's surface, but current evidence shows the presence of a great number of different chemicals, including herbicides, in the groundwater of many countries. The ability to detect increasingly lower concentrations of pesticides, in­ creased knowledge of and concern about potential chronic effects, and the finding of low levels of pesti­ cides in drinking water has made the public gener­ ally more suspicious of the beneficial aspects of pespesticide usage. However, from a toxicological point-of-view, the chemical residues can be handled in the same way as those in food. 3. Third Parties The use of poisons for suicidal intent has led to use of the herbicide paraquat for this purpose. As a group, however, herbicides are not sufficiently toxic

363

Herbicides

to be lethal, although the solvent may have lethal effects in some instances.

I/. Side Effects and Fauna

on Flora

To use herbicides effectively and economically, and to avoid unacceptable side effects in the environ­ ment, we have to know their appearance and persis­ tence in the environment as discussed in Sections II and III. Most herbicides today have a good selectiv­ ity in their action against weed species. However, despite this selectivity, effects on nontarget organ­ isms have been observed. This may involve direct toxic effects on sensitive species (e.g., certain plants and soil microorganisms) or indirect effects caused as a consequence of the weed treatment, (e.g., on insects and wildlife).

A. Soil Organisms 1 . Microorganisms Microorganisms usually occupy a volume of less than 0. l% of the soil, but are responsible for numer­ ous transformations that cycle elements and energy in nature. However, population densities of bacteria 9 may be quite high (l0 /g of soil is not uncommon) and the length of fungal hyphae may be some thou­ sands of meters per gram of soil. Therefore, the biomass of microorganisms per hectare may reach several tons. The diversity of species of microorganisms in soil is great. The microbial population exists in a dy­ namic equilibrium formed by interactions of abiotic and biotic factors that can be altered by modifying environmental conditions. If this equilibrium is dis­ turbed by an herbicide, it may lead to consequences for the turnover of carbon and nutrients in the soil resulting in changes to primary production. It can generally be stated that herbicides affects growth and other activities of soil microorganisms. This leads to changes in the composition of species of microflora and in interactions between different species. However, in the majority of cases where herbicides have been tested, the influence has been concluded to not result in any effect of appreciable importance for the function of the soil ecosystem. In some cases, herbicides have contributed a more or less potent effect on several microor­ ganisms or microbial processes in soil (e.g., organ­

isms involved in nitrogen fixation, or nitrification, as well as mycorrhiza, and plant pathogens). Examples of such substances are amitrol, atrazine, diallate, dicamba, dinoseb, paraquat, simazine, and sodium chlorate. 2. Animals About 10% of energy flow in soil is caused by soil animals. The microbial decomposition of organic matter is, in most cases, dependent on interactions with soil animals. It is difficult to identify herbicide effects on the soil fauna in practical agriculture because herbicide application is only one component of a system of mangement. Some herbicides like atrazine, dalapon, dinoseb, linuron, and simazine may have a signifi­ cant effect on the soil fauna, although several stud­ ies suggest that the effects are largely indirect. A decrease in the overall number of soil fauna species could primarily be caused by the destruction of the plant cover so that the soil fauna would be second­ arily exposed to greater physical stress from ex­ treme temperature and drought.

B.

Insects

Insects in agricultural areas may carry out many activities such as causing crop damage (or con­ versely being "natural enemies" to the pest insects), pollinating a wide range of plants, or being food for many bird species. Herbicides generally have minor toxic effects on insects. The indirect effects are probably more important. Removal of weeds greatly decreases insect densities in small grains and other crops, hence some weed growth is essential if high insect densities are to be maintained.

C.

Plants

The regular use of herbicides in agriculture since the end of 1940s has caused a decrease in the occurrence of weeds in fields, both in the number and diversity of species. Weed species that are sensitive to herbi­ cides now represent a smaller portion of the weed population, while the numbers of insensitive species have increased, particularly grass weeds. Within certain weed species there has been observed a change toward more tolerant and/or resistant specimens/populations (Table VIII). In the field corridors (e.g., the unplanted areas of the field) and in the immediate surroundings of the field, the use of herbicides and fertilizers has caused

364

Herbicides Table VIII Increased Tolerance/Resistance in Weeds after Long Continuous Use of Herbicides Herbicide

Weed

Haloalkanoic acids Dalapon

Quackgrass (Elymus repens)

Phenoxyalkanoic acids 2,4-D

Dandelion {Taraxacum vulgare)

tolerant

False camomile {Matricaria maritima ssp. inodora) Canada thistle {Cirsium arvense)

tolerant tolerant

Phenyl ureas Metoxuron

Annual bluegrass {Poa annua)

tolerant

Pyridines Paraquat

Annual bluegrass {Poa annua)

resistant

Allseed {Chenopodium polyspermum) Galinsoga {Galinsoga ciliata) Ragwort {Senecio vulgaris) Common lambsquarters {Chenopodium album) Speedwell {Veronica persica) Common chickweed {Stellaria media) Spreading orache {Atriplex patula) Ragwort {Senecio vulgaris) Shepherd's purse {Capsella bursa-pastoris) Common lambsquarters {Chenopodium album) Annual bluegrass {Poa annua)

resistant resistant resistant resistant tolerant resistant resistant resistant tolerant tolerant resistant

MCPA

Triazines Atrazine

Simazine

a trivialization of wild flora. These areas often have been islands with the few plant remnants from an older type of landscape. Quite another problem involves what may happen if certain weeds are not removed from a grain crop. Different species of the fungus Fusarium may grow in the crop (including its weeds) on the field. Under certain conditions, the Fusarium spp. produce my­ cotoxins such as trichothecenes of different kinds. The trichothecenes are highly toxic and are a threat to major grain consumers like poultry and swine. A positive correlation has been found between the oc­ currence of quackgrass (Elymus repens) and con­ centrations of deoxynivalenol, one of the tricho­ thecenes, in grain.

D.

Tolerant/resistant

Wildlife

Herbicides have seldom been reported as directly toxic to wildlife. However, substances used for de­ foliation or other drastic changes of the vegetation cover may alter the possibilities for both mammals and birds to find food and cover. The structure of forest bird communities is strongly influenced by the successional stages of the vegetation. Herbicides

tolerant

can alter the successional stage of a given habitat and thus affect nesting birds. Several studies, on the effect of phenoxy acids (2,4-D, and 2,4,5-T) and glyphosate on bird communities have reported den­ sities altered by spray treatment. Field margins, defined as the unplanted areas of the field and its boundary, and the outermost edges of the crops themselves, are important habitats for many bird species, especially species of gamebirds. Partridges (Perdix perdix and Alectoris rufa) nest almost entirely in field boundaries. After hatching, the chicks of these species move into crops to feed, as do chicks of the pheasant (Phasianus colchicus). Chicks of grey partridges (P. perdix) in particular, feed in cereal fields and during the first 2-3 weeks of life their digestive systems are not well adapted to plant food. Therefore the presence of large numbers of insects, which at this crucial age form their staple diet, are vital to their survival. Increased herbicide use over the last four decades has removed the host plants of these insects and, more recently, the use of insecticides to control aphids has caused direct mor­ tality of other insect species. The consequent reduc­ tion in the numbers of chick-food insects caused by the disruption of food chains has been a major factor in the decline of the wild grey partridge.

Herbicides

Ε. Aquatic

365

Organisms

Certain herbicides are used against aquatic weeds. However, as was discussed in Section II, many other herbicides can also appear in water systems where they may cause effects on the aquatic flora and fauna. Herbicides with effects on photosynthetic systems have been toxic at the parts per billion level. Other types of herbicides may be toxic at the parts per million level.

VI.

Regulations

As in so many factors which may be on the surface beneficial to humans (such as the use of herbicides to protect crops) hazards to human and animal health and the environment may also occur if their use is not properly controlled. This type of problem is not restricted to herbicides, but arises with all other chemicals encountered or used by humans in daily life. Consequently, before an herbicide can be allowed to pass into general use, sufficient information must be available for a reasonable estimate to be made of its potential for good and harm. The correct mea­ sures can then be devised for its control and recom­ mendations made on how surplus herbicides and containers can be safely disposed of with the aim of reducing the risks to the community and the envi­ ronment. Most countries regulate the sale and supply of herbicides and other pesticides and, for that pur­ pose, call upon pesticide manufacturers to submit information on the effect and safe use of their prod­ ucts. Differences exist between countries on the ex­ tent and scope of these requirements. However, the type of information needed includes: 1. the formulation of the proprietary product; 2. identity of the active ingredient; 3. chemical properties of the active, pure, and technical ingredients;

4. physical properties of the active, pure, and technical ingredients; 5. intended uses and methods of application; and 6. experimental data on efficiency. Data on herbicide residue including statements on principal residues (parent compound, breakdown products, and metabolites), and methods of residue analysis are also important particularly in edible crops, food, or feedstuffs. These must be supple­ mented by experimental data on toxicity in animals, including acute, short-term and long-term toxicity, carcinogenecity, neurotoxicity and reproductive studies, including teratogenicity and mutagenicity. Observations in humans and information on diagno­ sis and treatment as well as environmental and wild­ life data must analyze degradation in soil and water, adsorption and mobility in soil, effects on soil flora and fauna, and toxicity to birds, fish and bees. Fi­ nally, conditions of safe disposal of surplus herbi­ cides and herbicide containers are essential. Related Articles:

DIOXIN, HEALTH EFFECTS; OR­

GANIC MICROPOLLUTANTS IN LAKE

SEDIMENTS;

PEROXISOMES; PESTICIDES AND FOOD SAFETY.

Bibliography Garner, W. Y., Honeycutt, R. C , and Nigg, Η. Ν., eds. (1986). "Evaluation of Pesticides in Groundwater." ACS Symp. Ser. 315. Washington, D.C. Grover, R., ed. (1988). "Environmental Chemistry of Herbi­ cides." Vol. I. CRC Press, Boca Raton, Florida. Hutson, D. H., and Roberts, T. R., eds. (1987). "Progress in Pesticide Biochemistry and Toxicology." Vol. 6. "Herbi­ cides." Wiley, New York. Racke, K. D., and Coats, J. R., eds. (1990). "Enhanced Biode­ gradation of Pesticides in the Environment." ACS Symp. Ser. 426. Washington, D.C. Richardson, M., ed. (1986). "Toxic Hazard Assessment of Chemicals." The Royal Society of Chemistry, Burlington House, London. Worthing, C. R., ed. (1987). "The Pesticide Manual: A World Compendium." 8th ed. The British Crop Protection Council. Lavenham, Suffolk.