module 15
Herbicide application and fate in the environment
Objectives
At the end of this module, you will:
- understand the importance of correct herbicide application
- be aware of the importance of following the directions on the herbicide label
- be able to describe how different types of herbicide applicators work
- understand the causes of herbicide drift and how to minimise drift
- understand the processes that dissipate herbicides in the environment
- understand routes of entry of herbicides into plants
- be aware that incorrect choice of herbicide or application strategy can result in unintended damage to non-target organisms
Module outline
- Introduction
- Application methods and equipment
- Fate of herbicides in the environment
- Plant processes
- Summary
- References and further reading
Introduction
Herbicides are a major means of weed control in many situations. Herbicides are usually applied as sprays, but it may be more appropriate to apply herbicides in some other manner (eg. granules, powders, pellets etc.). Even where a spray option is chosen, a variety of equipment with different characteristics is available. It is important to understand the principles of herbicide application so that the correct equipment is chosen and used in the correct way. This section will cover the major issues related to application of herbicides and their fate in the environment.
Application methods and equipment
As stated above choice of equipment is vital for the correct application of a herbicide to efficiently control weeds without harm to desired plants, operators or the environment. The following reading considers in detail the characteristics of the various equipment used for applying herbicides. The reading also covers how spray equipment works.
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Fate of herbicides in the environment
Once herbicides are applied they are subject to processes that work to degrade or move a herbicide in the environment. Herbicide leaving the applicator may drift from the intended site of application, land on soil, water or plant tissue. Herbicide drift is a major environmental issue and should be avoided at all costs. Several Australian states are enacting legislation applying penalties for “chemical trespass”. The major causes of herbicide drift are spraying when conditions are not appropriate and applying small droplets. The following reading discusses these issues in detail and provides methods to limit herbicide drift.
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Understanding how herbicides behave in the environment is important to avoid off-target damage or damage to susceptible crops. Most herbicides are readily degraded in the environment, principally by microorganisms. However, others may persist sufficiently to damage susceptible crops that are later planted. Other herbicides may leach in certain soils, or enter waterways where they pose a risk to aquatic species. Figure 12.1 illustrates the various processes that may act to dissipate a herbicide.
Figure 15.1. Herbicide reaching the
soil can be lost through a variety of processes
including absorption by plant roots, volatilisation, adsorption to soil components,
leaching, or microbial, chemical or photochemical decomposition.
Recently, there have been a number of examples where crop damage has occurred as a result of poor understanding of herbicide characteristics and failure to follow directions on product labels. Two such examples has been the use of simazine for weed control around young grapevines, and 2,4-D around tomato plants, both with a substantial damage bill. The following reading briefly covers the theoretical aspects of herbicide dissipation in the environment.
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You should note the example of the persistence of sulfonylureas given in the reading above. This soil persistence is an important issue in most Australian cropping systems and severely limits rotational alternatives in highly alkaline soils such as those in parts of South Australia and Victoria. This module is discussed at length in Sarmah et al. (1998).
The same issues occur for the imidazolinone herbicides, except these herbicides tend to persist in acid soils. You should also note from the reading the ways in which herbicide behaviour in the soil is used to provide herbicide selectivity for crop species. Particular examples are paraquat and glyphosate that are strongly adsorbed to soil. These herbicides are frequently used to control weeds prior to sowing a crop. The strong soil adsorption means the herbicide in the soil will not damage the crop.
Plant processes
Herbicides which land on plant surfaces can be washed from the leaf surface by rainfall, absorbed, remain bound in the cuticular waxes, or crystallise on the leaf surface. Some herbicides are rapidly absorbed by leaves and are not affected by rainfall. Paraquat is an example and is described as having a short rainfast period. In contrast, some formulations of glyphosate have a rainfast period of 6 h and loss of efficacy can result if rain falls within this period.
However, some newer formulations of glyphosate have improved rainfastness. The following reading discusses the major pathways for herbicide entry into plants and the possible fates of herbicides once they enter plants. The reading only briefly considers the complexity of processes resulting in metabolism of herbicides within plants. These processes are very important, as they are the main reason for herbicide selectivity in crops. Detailed discussion of herbicide metabolism in plants can be found in Cole (1994).
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Summary
Correct application of herbicide involves:
- identification of the weeds present
- correct choice of herbicide
- correctly adjusted and calibrated equipment
- appropriate safety precautions
- proper clean up
- recording and monitoring of results
- following the label directions
If all of these are followed, then an excellent result should be achieved with minimum off-target damage. The fate of herbicides in the environment is complex and involves one or more of the following processes:
- absorption to soil components
- volatilisation
- leaching and surface run-off
- decomposition by microbes
- chemical or photochemical degradation
Once inside the plant, further decomposition may occur. It is important that these processes are understood to obtain maximum efficacy of herbicides with as little off target damage as possible.
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References and further reading
Ahrens, RH, (ed.) 1994, Herbicide handbook, Weed Science Society of America, Champaign, Il.
Bowmer, KH 1987, ‘Herbicides in surface water’, in DH Hutson & TR Roberts, (eds) Herbicides, John Wiley and Sons, Chichester, UK, pp. 271- 355.
Cole, DJ 1994, ‘Detoxification and activation of agrochemicals in plants’, Pesticide Science, vol. 42, pp. 209-222.
Cunningham, G, Hughes, P & Harden, J 1996, Workshop manual on efficient pesticide use in citrus.
Grover, R (ed.) 1988, Environmental Chemistry of Herbicides, Vol I, CRC Press, Boca Raton, Fl.
Grover, R & Cessna, AJ (eds) 1991, Environmental Chemistry of Herbicides, Vol II. CRC Press, Boca Raton, Fl.
Hance, RJ 1987, ‘Herbicide behavior in the soil with particular reference to the potential for ground water contamination’, in DH Hutson & TR Roberts, (eds) Herbicides, John Wiley and Sons, Chichester, UK, pp. 223- 247.
Hassall, KA 1990, The biochemistry and uses of pesticides, 2nd edn. Macmillan Press.
Kent, J 1994, Pesticides in Australia - resource manual, Charles Sturt University.
Matthews, G 1979, Pesticide application methods, Longman.
Parsons, JM, (ed.) 1995, Australian Weed Control Handbook, 10th edn, Inkata Press, Melbourne.
Riley, D & Eagle, D 1990, ‘Herbicides in soil and water’, in RJ Hance & K Holly (eds), Weed control handbook: Principles, Blackwell Scientific Publications, Oxford, pp. 243-259.
Rural Training Council of NSW Ltd 1994, National farm chemical user training program (New South Wales) reference manual, NSW Farmers Association.
Sarmah, AK, Kookana, RS & Alston, AM 1998, ‘Fate and behaviour of triasulfuron, metsulfuron-methyl, and chlorsulfuron in the Australian soil environment - a review’, Australian Journal of Agricultural Research, vol. 49, pp. 775-790.
Somervaille, A 1989, ‘Avoiding spray drift conditions’, Herbicide application guidelines, Queensland Department of Primary Industries.
Southcombe, E (ed.) 1985, ‘Application and Biology’, BCPC Monograph No. 28.
Taylor, AW & Glotfelty, DE 1988, ‘Evaporation from soils and crops’, in R Gover (ed), Environmental Chemistry of Herbicides, Volume 1, CRC Press, Boca Raton, Fl. pp. 89-130.
Torstensson, NTL 1987, ‘Microbial decomposition of herbicides in soil’ in DH Hutson and TR Roberts (eds), Herbicides, John Wiley and Sons, Chichester, UK. pp. 249-270.
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Self assessmentDo you know:
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