Paraquat Fact Sheet

What is Paraquat?

Wheat harvestParaquat is a herbicide (chemical weed killer) used to control a very broad range of weeds (unwanted plants) in more than 100 crops, including cereals, oilseeds, fruit and vegetables, growing in all climates. Weeds shade crops, take their water and nutrients, and make harvesting difficult. The leading manufacturer of paraquat is Syngenta, which (as ICI) developed the active ingredient (AI) in the early 1960’s. Since then, paraquat has made possible many innovations in sustainable farming

Description

Management and conservation of the natural resource base and the use of technological and organizational change in a manner that ensures continued agricultural production from the land for present and future generations. Such practices conserve land, water, and plant and animal genetic resources. They are environmentally non-degrading, technically appropriate, economically viable, and socially acceptable. Sustainability rests on the principle that we must meet the needs of the present without compromising the ability of future generations to meet their own needs. Therefore, stewardship of both natural and human resources is of prime importance.

Authoritative On-line References and Resources

www.nal.usda.gov Information from the USDA's Alternative Agricultural Systems Information Center.
systems, based on its simplification of crop production by effectively controlling weeds and, in doing so, removing the need for ploughing to bury them. This has freed up farmers’ time and also helped care for the soil. Paraquat is one of the most widely used herbicides in the world. It has been approved for use by authorities in nearly 90 countries. When used as recommended, paraquat is effective and safe to users, consumers and the environment.
Key facts about the safe and effective use of paraquat are noted below. This fact sheet also contains a list of referenced scientific papers and other publications and a summary of technical information.

Why Farmers Use Paraquat

Spraying paraquat lets millions of farmers grow better crops more easily, while respecting the environment. Paraquat has a unique set of characteristics:

Control weeds quickly and effectively

  • Paraquat quickly removes weed competition, controlling most annual weeds

    Definition

    Weeds that complete their life cycle within one growing season, or year. From seed to flowering to seed before the year ends.

    Authoritative On-line References and Resources

    The International Weed Science Society represents individual associations around the world.
    and suppressing perennials

    Description

    Weeds that return year after year. Some die back in the winter but their roots remain alive and shoots reappear in spring; some don't die back and grow in size and stature the next season.

    Authoritative On-line References and Resources

    The International Weed Science Society represents individual associations around the world.
    . Its fast action can allow more than one crop to be planted in a single growing season
  • Extensive use of paraquat has resulted in little weed resistance, especially when compared to much more recent introductions, eg, sulfonylurea herbicides. In those cases where resistant biotypes have evolved, they have had no significant agricultural impact.
  • The other main broad spectrum ‘non-selective

    Description

    A chemical product used for eliminating all types of weeds (annual and perennial grasses and broadleaved weeds).

    Authoritative On-line References and Resources

    http://www.weeds.iastate.edu/ An invaluable source of contemporary information about herbicides and weeds from Iowa State University.
    ’ herbicide, glyphosate, has an increasing problem with resistant weeds. Paraquat is seen by many farmers and scientists as pivotal in managing this issue by providing a non-selective alternative. Visit the International Survey of Herbicide Resistant Weeds website (www.weedscience.org) for more information on weed resistance.

Reliable in all weathers

  • Rainfall a few minutes after spraying has little or no detrimental effect on the performance of paraquat. Sufficient paraquat rapidly enters leaves before rain can wash significant amounts away. Even in the tropics, paraquat is consistently rainfast only 15–30 minutes after application.
  • Unlike most herbicides, paraquat is also effective under low temperatures when weeds are making little active growth. This makes paraquat a suitable option for early season seedbed preparation and for many orchard crops in the autumn.

Safe to crops

  • Paraquat has contact action only. Sprayed plant tissues are destroyed so quickly that no significant amounts move from leaves to more distant, unsprayed parts such as roots and hidden, shoot-growing points. Unlike with glyphosate, which is slow acting and does move in plants, minor spraying inaccuracies or drift are not a problem for crops treated with paraquat. Paraquat may be used safely for weed control between plant rows (“inter-row”) in many arable and vegetable crops.
  • Paraquat will not penetrate mature bark and can be sprayed safely around trees and beneath bushes. Using glyphosate risks permanently damaging crops like vines and young trees. The alternative - mechanical cultivations - may damage root systems.
  • In soil, paraquat is biologically deactivated by being rapidly and tightly adsorbed into clay particles. No ‘residual’ activity in soil means that paraquat can be sprayed before crops are planted or seedlings emerge.

Respects the soil

  • Paraquat in soil is unavailable to plants and all forms of animals and micro-organisms, so it cannot affect them.
  • Although paraquat quickly kills weed shoots, roots decompose slowly and provide an anchoring effect for soil on slopes prone to erosion.

Environmental Safety Profile

Paraquat’s unique combination of biological and physico-chemical properties, particularly its fast action and strong adsorption, give it a very robust environmental profile.

Soil adsorption and degradation

  • Soils of various types are capable of adsorbing vast quantities of paraquat. Clay content is important, but even for lighter soils the adsorption capacity in the top few centimetres is capable of deactivating many kilograms of paraquat per hectare.
  • Tiny amounts of paraquat are continuously released from clay into soil water. There, soil microbes degrade it to carbon dioxide, ammonia and water, preventing soil accumulation.
  • No leaching

    Description

    The natural process by which water soluble substances are carried downward through the soil into groundwater.
    , run-off

    Description

    The occurrence of surplus liquid (like rain) which originates up-slope and is collected beyond the ability of the soil to absorb it. The surplus liquid then flows away over the surface to reach the nearest surface water (pond, lake, river).

    Authoritative On-line References and Resources

    US Geological Survey's Water Science School
    or drainage to ground water
  • Although highly soluble in water, paraquat does not leach into ground water because of its very strong soil adsorption. Paraquat is virtually immobile in soil.

Crop residues

  • There is no issue with paraquat residues on harvested crops. Paraquat is sprayed to control weeds around plants grown for food. Any paraquat landing on leaves finds it very difficult to enter and even more difficult to move to grains or tubers. Furthermore, UV radiation in sunlight degrades paraquat on leaf surfaces and soil absorption prevents entry via roots. Paraquat can be used on crops to aid harvest, but grain and tubers are not exposed.

Safety to livestock, domestic animals, wildlife and soil fauna

  • Provided spray deposits have dried, there is no health risk to domestic animals entering fields sprayed with paraquat, or to livestock grazing on vegetation sprayed at normal recommended dilutions. Paraquat binds tightly to plant tissues, so is not easily absorbed in animals. Any trace amounts are rapidly excreted.
  • Wild bird populations monitored over a five year period on a farm where paraquat was sprayed much more intensively than usual, including beneath hedgerows and along fences, were not adversely affected in terms of population density or variety of species. Long-term wildlife surveillance programs in France and the UK have shown that paraquat is not a significant cause of hare deaths or population decline.
  • As paraquat is not biologically active in the soil it has no adverse effects on soil fauna.

Safety in water

  • Used as recommended, paraquat is not hazardous to fish or invertebrates because it cannot leach into water. Even if paraquat spray should drift onto ponds, rivers or ditchwater, it is rapidly removed by adsorption to plants and sediments, and by microbial degradation. Extensive ecological studies have shown that paraquat is not a risk to aquatic environments.

User Safety

Manufacturing and formulation

  • Paraquat is manufactured by Syngenta according to FAO guidelines which specify purity and accuracy of product concentrations. Syngenta has added several safety features. A dye gives Syngenta paraquat products a distinctive color; a stenching agent gives an alerting smell; and an effective emetic induces vomiting. These precautions are additional Syngenta stewardship

    Description

    The responsible and ethical management of a crop protection product in a way that takes full and balanced account of the interests of users, future generations, and other species.

    Authoritative On-line References and Resources

    http://www.croplife.org/ CropLife International is a global federation representing the plant science industry and a network of regional and national associations in 91 countries. These organisations are committed to sustainable agriculture through innovative research and technology in the areas of crop protection, non-agricultural pest control, seeds and plant biotechnology.
    features to minimize the risk of accidental ingestion. Other manufacturers do not necessarily use such manufacturing and safety features. Paraquat is diluted typically 50- to 100-fold in water for use.

Operator exposure and safety

  • Paraquat is not volatile and spray droplets are too large to enter the lungs, so the skin is the most likely potential route of exposure. Because paraquat is poorly absorbed (<0.3%) through human skin, in normal use exposure will be very low - well below levels where adverse effects might be expected. Many studies have confirmed this over the last 50 years.

Oral toxicity

  • Like other pesticides, paraquat concentrate can be fatal if swallowed in sufficient quantities. Utmost care should always be taken to avoid ingesting pesticides. Minor exposure is possible from contaminated fingers or gloves, or spray drift, but in negligible amounts and this exposure would not be expected to cause harm in any normal practice.

Skin and eye irritation

  • Although it is classed as an irritant, simply washing with water will easily remove accidental splashes of paraquat from skin and eyes.

Inhalation

  • Paraquat has no vapor and operators cannot inhale spray droplets produced by normal knapsack or tractor sprayers. To enter air spaces in the lungs, a particle must be less than 10 microns in diameter, but most spray droplets are 10- to 20-times larger. The relative size difference of a normal spray droplet to a droplet able to be inhaled into the lungs is comparable to that of a football to a grape. Spray droplets cannot be inhaled into lungs.

Long-term effects

  • Paraquat does not cause cancer. It is not a reproductive or developmental toxin, and it has no toxic effects on nervous or endocrine systems.

Consumer safety

  • Under normal use conditions, paraquat does not accumulate in the food chain because it is not stored or accumulated in the body. Residues present no risk to consumers.

Emergency first aid

If paraquat is swallowed:
  • Try to vomit, if you have not already done so.
  • Mix 150g Fuller’s Earth or activated charcoal in 1L water, shake well, drink the Fuller’s solution and vomit into a bag.
  • Mix Epsom salts or other purgative into water, shake well and drink.
  • Call the hospital emergency room and go immediately, taking the vomit bag with you.

Registration of Paraquat

All crop protection products must be approved by governments and regulatory authorities before being sold. Paraquat has been researched more thoroughly, subjected to more scientific tests and field studies, over a longer period than probably any other crop protection product. It has been studied by the US Environmental Protection Agency (EPA), the UN Joint Meeting on Pesticide Residues (JMPR), which comprises experts from the World Health Organization (WHO) and the Food & Agriculture Organization (FAO), and many universities and government authorities. Countries register paraquat only after detailed and costly evaluation of risks and benefits. The product’s effectiveness, its short- and long-term toxicity, operator safety, and environmental behavior, including residue levels in food crops and effects on wildlife, are all thoroughly studied and reviewed. Some countries, particularly in the developing world, do not have the capacity to evaluate and regulate crop protection products. Syngenta insists that paraquat products sold in developing countries have current regulatory approval in an OECD (Organization for Economic Co-operation & Development) country. By giving approval for its use, based on expert scrutiny of mandatory information, government authorities around the world agree that paraquat can be safely handled and sprayed by users taking normal precautions to protect themselves, the public and the environment.

References

Paraquat’s practical benefits for crop production

Chee et al (1993). Planters' Bulletin, 216, 80-86. Chia et al (2003). 19th Asian-Pacific Weed Science Society Conference. Derpsch et al (1991). Controle da erosao no Paraná, Brasil: sistemas de cobertura do solo, plantio direto e preparo conservacionista do Solo. Eschborn, Germany, GTZ. ISBN 3-88085-433-5, pp. 76, 101, 112, 113, 268. Grime (1979). Plant Strategies and Vegetation Processes, p222. Wiley: New York. International Survey of Herbicide Resistant Weeds website. www.weedscience.org. Schmidt et al (1994). Australian Journal of Agriculture, 45, 547-564.

Paraquat’s benefits for soil care

Hayward et al (1980). Outlook on Agriculture 10, 255-261. Roldan et al (2003). Soil and Tillage Research, 72, (1), 65–73. Shui Jian-guo et al (2004). 13th International Soil Conservation Organization Conference, Brisbane, July 2004.

Paraquat’s social benefits

Barnett (2002). The XIV International Conference on HIV/AIDS, Barcelona. Sorrenson (1998). Economics of no-till

Description

Also known as conservation tillage or zero tillage is a way of growing crops from year to year without disturbing the soil through tillage ie cultivating the soil usually with tractor-drawn implements.

Authoritative On-line References and Resources

No-till.com:  A portal for on-line information about no-till farming.
compared to traditional cultivation on small farms in Paraguay
.
Asunción: MAG/GTZ Soil Conservation Project.

Paraquat’s safety to users

Schenker (2004). American Journal of Respiratory and Critical Care Medicine, 170, 773-779. Wester et al (1984). Journal of Toxicology and Environmental Health, 14, (5-6), 759-62.

Paraquat in soil and water

Helling (1971). Proceedings of the Soil Science Society of America, 35, 737-743. Roberts et al (2002). Journal of Agricultural and Food Chemistry, 50, 3623–3631.

Paraquat’s safety to wildlife

Edwards et al (2000). Agriculture, Ecosystems and Environment, 79, 95-103.

Safety to soil fauna

Satchell (1983). Earthworm Ecology (Chapter 10). Institute of Terrestrial Ecology, UK.

Technical Data

 Chemical and physical properties of paraquat
The active ingredient paraquat is a non-volatile white crystalline solid, melting and decomposing at 300°C. Extremely soluble in water, it is practically insoluble in most organic solvents. Paraquat is formulated as the dichloride salt.
Common Name: paraquat  Chemical Name: 1,1’-dimethyl-4,4’-bipyridylium ion
Empirical Formula: C12H14N2Cl2 Structural Formula:
Molecular Weight: 257
Water solubility:
620 g/l (20 ºC)
pH Stability:
Acidic: stable
Neutral: stable
Alkaline: hydrolysed  
Vapor Pressure: 1 x 10-9 mm Hg
Photostability: Decomposed by UV radiation in aqueous solution
Mode of Action
Paraquat acts in the chloroplasts of green plants. Here, photosynthetic systems absorb light energy to produce sugars for plant nutrition. Paraquat precisely targets the biochemical system known as Photosystem I. This produces free electrons, which drive photosynthesis. The paraquat ion reacts with these electrons to form “free radicals.” Oxygen rapidly converts free radicals to superoxides. These readily react with the unsaturated fatty acid components of cell membranes. As a result of these dramatic chemical changes, membranes are destroyed, and cell contents leak and mix causing further destruction. This entire process occurs so quickly that there is no measurable translocation of paraquat.