ORGANIC POLLUTANTS: AN ECOTOXICOLOGICAL PERSPECTIVE - CHAPTER 13 pps

The ecotoxicological effects of herbicides 13.1 Introduction Chapters 5–12 deal with groups of pollutants that have been studied in some depth and detail, largely because they have appre

P A R T 3

Further issues and

future prospects

The ecotoxicological effects of herbicides

13.1 Introduction

Chapters 5–12 deal with groups of pollutants that have been studied in some depth and detail, largely because they have appreciable – sometimes very high – mammalian toxicity and are perceived as human health hazards Some of them are markedly persistent and undergo biomagnification with passage along food chains As has been explained, individual compounds, or mixtures of related compounds, have sometimes been shown to cause adverse ecological effects Although it was necessary to take such pollutants as examples, it is important now to consider the more complex situation

in which organisms are exposed to mixtures of compounds differing in their chemistry and/or mode of action

The present chapter is the first of the final part of the text, in which the emphasis moves on from the detailed descriptions of particular types of pollutants given in the foregoing chapters to address some wider issues Apart from the question of the complexity of pollution in the real world, certain other issues arise directly from the findings of ecotoxicological studies on individual pollutants reported in the earlier text

Herbicides constitute a large and diverse class of pesticides that, with a few exceptions, have very low mammalian toxicity and have received relatively little attention as environmental pollutants Paraquat and other bipyridyl herbicides have appreciable mammalian toxicity and will be discussed in Chapter 14 Dinoseb and

Further issues and future prospects

related dinitrophenols, which act as uncouplers of oxidative phosphorylation in mitochondria, are general biocides that are little used today because of their hazardous nature They will be mentioned, briefly, in section 14.2 Herbicides are, in general, readily biodegradable by vertebrates and are not known to undergo substantial biomagnification in food chains Their principal use has been weed control in agriculture and horticulture, although they have also been used as defoliants in forests (e.g in the Vietnam war), for controlling weeds on roadside verges and in water courses and as management tools on estates and nature reserves This chapter will be mainly concerned with their impact on the agricultural environment A brief mention will be made of their wider dispersal in the aquatic environment

13.2 Some major groups of herbicides

The following brief account identifies only major groups of herbicides not mentioned elsewhere in the text and is far from comprehensive For a more detailed account see

Hassall (1990)

The phenoxyalkane carboxylic acids are among the most successful and widely used herbicides They act as plant growth regulators and produce distorted growth patterns in treated plants Compounds such as 2,4-D, MCPA, and mecoprop (Figure 13.1) are used as selective herbicides to control dicotyledenous weeds in monocotyledenous crops such as cereals and grass They are formulated as water-soluble potassium or sodium salts, or as lipophilic esters, and they are frequently sprayed in combination with other types of herbicides that have different modes of action and patterns of weed control They are applied to foliage and are not soil acting

Ureides (e.g diuron, linuron, isoproturon) and triazines (e.g atrazine, simazine, ametryne) all act as inhibitors of photosynthesis and are applied to soil (see Figure 13.1 for structures) They are toxic to seedling weeds, which can absorb them from soil Some of them (e.g simazine) have very low water solubility and, consequently, are persistent and relatively immobile in soil (section 4.3)

Sulphonylurea herbicides such as chlorsulfuron and sulfometuron are also soil acting, affect cell division and have very high phytotoxicity Indeed, they can be toxic to plants when present in soil at levels low enough to make chemical analysis difficult Carbamate herbicides constitute a relatively diverse group Some, like barban (Figure 13.1), are applied to foliage, whereas others (e.g chlorpropham) are soil acting The latter type affect cell division Other important herbicides, or groups of herbicides, include glyphosate, aminotriazole, chlorinated benzoic acids (e.g dicamba) and phenolic nitriles (e.g ioxynil, bromoxynil)

O

Cl

Cl

CH2COOH

MCPA

O

CH2COOH

Mecoprop

O CHCOOH

(b) Ureides

(d) Triazines

(e) Carbamates

(c) Sulphonyl urea

Cl

Cl

NH

CO

CH3 CH3

N

1

4

2

3

2

1

3

Diuron

Cl

Cl

NH CO

CH3 OCH3

N

Linuron

NH CO

CH3 CH3 N CH(CH3)2

Isoproturon

N 6

N Cl

Chlorsulphuron

O NH 2

3 1

5 6

3

2 4 5 N

1

N

Simazine

N

Et

Cl

5

N

2

N

N

Et

6

4

3

N

Atrazine

N Et

Cl N N N isoPr

N

Ametryne

N Et

SCH3 N N N Et

Chlorpropham

NH

O

CH

Cl

CH3

CH3

1' 2' 3' 4'

CH2Cl

Cl

NH CO

CH2 O

1

4 3 2

Barban

Figure 13.1 Structures of some herbicides.

Further issues and future prospects

13.3 Agricultural impact of herbicides

Since the Second World War herbicides have come to be widely used in agriculture and horticulture in the developed world Frequently, they have been used in ‘cocktails’ containing several ingredients of contrasting modes of action, thus giving control over a wide range of weed species The effectiveness of the application of herbicides together with cultivation of the land is evident in many agricultural areas in Western Europe and North America, where few weeds are seen It is easier to control plants, which are stationary, than to control mobile insect or vertebrate pests Weed species have been very effectively controlled over large areas of agricultural land In Britain concern has been expressed over the near extinction of certain once common farmland species that are of botanical interest, e.g corn cockle and pheasants eye

Ecologically, such a large reduction in weed species represents a major change to farmland ecosystems, and may be expected to have knock-on effects upon other species Certain problems have come to light with the investigation of the status of birds on farmland In one study, the Game Conservancy Council investigated the reasons for

the severe and continuing decline of the grey partridge (Perdix perdix) on farmland in

Britain The study commenced in the late 1960s, and established that the decline was closely related to increased chick mortality (Potts, 1986, 2000; also Chapter 12 in

Walker et al., 2000) The chick mortality was largely explained by a shortage of their

insect food (e.g sawflies) due, in turn, to the absence of the weeds upon which the insects themselves feed An effect at the bottom of the food chain led to a population decline further up It is worth reflecting that such an effect by herbicides could not have been forecasted by normal risk assessment (see Chapters 14 and 15) The herbicides responsible are, in general, of very low avian toxicity, and ordinary risk assessment would have declared them perfectly safe to use, so far as partridges and other birds are concerned! Subsequent work has shown that partridge populations can continue to survive on agricultural land if headlands are left unsprayed, thereby allowing weeds to survive, which will support the insects upon which young partridges feed

This study helped to ring the alarm bells about possible other indirect effects of the wide variety of herbicides used in agriculture More recently, further evidence has been gained of the reduction in populations of insects and other arthropods on farmland that may relate, at least in part, to the removal of weeds by the use of herbicides A survey of farmland birds in Britain has established the marked decline of several species

in addition to the grey partridge, which may be the consequence of indirect effects of

herbicides and other pesticides (Crick et al., 1998; also Chapter 12 of Walker et al., 2000) Species affected include tree sparrow (Passer montanus), turtle dove (Streptopelia purpur), spotted flycatcher (Musciapa striata) and skylark (Alauda arvensis) A study is

currently in progress to attempt to establish the cause of these declines Recently, concern about the side-effects of herbicides used on agricultural land has intensified with the development of genetically manipulated (GM) crops Some GM crops are relatively insensitive to the action of herbicides, thus permitting the application to

them of unusually high levels of certain herbicides The advantage of increasing dose, from the agricultural point of view, is the control of certain difficult weeds From an ecotoxicological point of view, increasing dose rates of herbicides above the currently approved levels may cause undesirable ecological side-effects It is very important that any such change in practice is rigorously tested in field trials, as part of environmental risk assessment, before approval is given by regulatory authorities Such new technology, based on GM crops, should only be introduced if it is shown to

be environmentally safe

One problem that has arisen with the use of herbicides in agriculture is spray or vapour drift When fine-spray droplets are released, especially if applied aerially, they may be deposited outside the target area because of air movements and cause damage there In the first place, this is a question of application technique Herbicides, like other pesticides, should not be applied as sprays under windy conditions In most situations, herbicides are not applied aerially because of the danger of drift Where herbicides have appreciable vapour pressure, there may be problems with vapour drift Under hot conditions, volatile herbicides may go into the vapour state, and the vapour may drift further than the spray droplets Such was the case with early volatile ester formulations of phenoxyalkanecarboxylic acids (Hassall, 1990) Nowadays, formulations are of less volatile esters or of aqueous concentrates of sodium or potassium salts (which are of low volatility) Spray drift of herbicides can cause damage to crops and wild plants outside the spray area The cause of such damage can be hard to establish with highly active herbicides (e.g sulphonylureas), where the phytotoxic concentrations are low enough to make chemical detection difficult

13.4 Movement of herbicides into surface waters

and drinking water

As discussed earlier (section 4.3), pesticides have a very limited tendency to move through soil profiles into drainage water because of the combined effects of adsorption

by soil colloids (important for herbicides such as simazine, which have relatively high

Kow values), metabolism (important for water-soluble and readily biodegradable herbicides such as 2,4-D and MCPA) and in some cases volatilisation In reality, however, there are complications In the first place there may be run-off from agricultural land into neighbouring water courses after heavy rainfall Soil colloids, with adsorbed herbicides, can be washed into drainage ditches and streams There is

an additional problem with certain soils high in clay minerals (Williams et al., 1996;

and section 4.3) During dry periods these soils shrink and develop deep cracks If heavy rains follow, free herbicides located near the soil surface, and colloids with adsorbed herbicides, can be quickly washed down into the drainage system without passing through the soil profile In the Rosemaund experiment, the herbicides atrazine, simazine, isoproturon, trifluralin, and MCPA were all detected in drainage water after

Further issues and future prospects

heavy rain The respective maximum concentrations in micrograms per litre (ppb)

were – 81, 68, 16, 14 and 47 (Williams et al., 1996) These levels were reached after

normal approved use of the herbicides and raise questions about possible effects on aquatic plants growing in receiving waters As mentioned elsewhere (section 10.3.4) the level of carbofuran found during the same study was sometimes high enough to

kill freshwater shrimps (G pulex) used as a bioindicator (Matthiessen et al., 1995).

Recent surveys have been providing more information on the levels of herbicides in rivers In one study a number of different herbicides were detected in the River Humber,

UK (House et al., 1997) A number of triazines were detected in the Rivers Aire,

Calder, Trent, Don and Ouse, the most abundant of them being atrazine and simazine The results for simazine showed peaks in the spring and again in the early autumn of

1994 for some rivers, the latter coinciding with the first major storm of the year (Figure 13.2) The maximum level of simazine recorded was 8µg/L This was high enough to be toxic to phytoplankton and algae, but was not sustained Phenyl ureas and phenoxyalkanoic acids were also detected Concentrations were generally low, but levels of the following herbicides were detected up to the maximum value (µg/L) given in brackets: diuron (< 8.7), chlortoluron (< 0.67), mecoprop (< 8.2) These high levels were sporadic and transitory However, they were sometimes high enough to cause phytotoxicity, and more work needs to be carried out to establish whether herbicides in contaminated streams and rivers are having adverse effects upon populations of aquatic plants

With the acceptable concentrations of herbicides in drinking water being taken to very low levels by some regulatory authorities (e.g the EC), there has been interest in low levels of atrazine present in groundwater and in drinking water This finding illustrates the point that mobility of pesticides becomes increasingly evident as the sensitivity of analysis improves

13.5 Summary

As the first chapter in the final part of the book, contamination by herbicides is taken

as an example of the complexity of pollution in the real world A wide variety of compounds of diverse structure, chemical properties and mechanism of action are used as herbicides Very few of them have appreciable toxicity to animals, and they do not usually undergo significant biomagnification with movement along food chains Important groups of herbicides are phenoxyalkane carboxylic acids, ureides, triazines and carbamates Herbicides are often applied as mixtures of compounds with contrasting properties

The successful use of herbicides and associated cultivation procedures has greatly reduced the populations of weed species in many agricultural areas, sometimes bringing species of botanical interest to near extinction Intensive weed control in cereal farming has been shown to cause the reduction of certain insect populations and the reduction

of the grey partridge The decline of some other insectivorous birds on agricultural land may have a similar cause The introduction of GM crops with high tolerance to herbicides may lead to increases in dose rates of herbicides on agricultural land with attendant ecotoxicological risks

Significant levels of herbicides have also been detected in rivers, although these are usually transitory Heavy rainfall can move herbicides from agricultural land to nearby ditches and streams because of run off and percolation of water through deep fissures

in certain soils that are high in clay

Figure 13.2 Atrazine levels in the Humber River area Comparison of the concentration of simazine and river discharge over one annual cycle for (a) River Trent at Cromwell Lock and (b) River Aire at Beale.

, Simazine concentration;
, river discharge From House et al (1997) with permission.

(a)

(b)

g/L) Discharge (m

Date

0.6

0.5

0.4

0.3

0.2

0.1

0

07/03/94 26/04/94 15/06/94 04/08/94 23/09/94 12/11/94 01/01/95 20/02/95

0

0.50

0.45

0.40

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0

350 300 250 200 150 100 50

Date

07/03/94 26/04/94 15/06/94 04/08/94 23/09/94 12/11/94 01/01/95 20/02/95 11/04/95

250 200 150 100 50 0

Further issues and future prospects

13.6 Further reading

Ashton, F M and Crafts, A S (1973) Mode of Action of Herbicides This book describes the mode

of action of major types of herbicides.

Hassall, K A (1990) Includes a readable account of the biochemistry of herbicides.

Potts (1986) The Biochemistry and Use of Pesticides An authoritative account of the factors responsible

for the decline of the grey partridge on agricultural land.