Using paraquat: what all farmers should know

What are the essential things farmers need to know about spraying paraquat? Paraquat is a fast-acting, broad spectrum, post-emergence herbicide. The label of paraquat products must always be read before use. However, farmers have basic questions such as: how big should weeds be; should another herbicide be tank-mixed or an adjuvant used; will rain after spraying affect activity; can paraquat help fight resistant weeds?  This article outlines the main practical aspects of using paraquat for weed control.

Safety before, during and after spraying

Safety is paramount.

Secure Storage

Always store agrochemicals in their original containers and under lock and key, to restrict access to only those people who need to use them and to keep out children, non-users and domestic animals. Never decant pesticides and leave them in unmarked containers. There are five Golden Rules for safe use:
  1. Exercise caution at all times
  2. Read and understand the product label
  3. Wash after spraying
  4. Maintain the sprayer
  5. Use personal protective clothing: wear a long-sleeved shirt, long trousers and waterproof footwear for spraying, plus eye protection and gloves when handling the concentrated product

Appropriate disposal

Used containers should be rinsed three times and the washings sprayed onto weeds or properly disposed of in a rinsate collection system. Empty containers should be collected or disposed of according to procedures approved locally and not be thrown away in the field or in watercourses. These essential points are addressed in more detail here.

Crops and weeds 

Paraquat's technical features

  • Broad spectrum weed control
  • Quick acting and rainfast
  • No soil residual effects
  • Bark protects trees

Paraquat's benefits on farms

  • Clean fields
  • Allows reduced tillage, protecting soil
  • Safe to crops
  • Enables multi-cropping
  • Saves fuel
  • Reduces costs
Paraquat controls a very broad spectrum of weeds in a wide range of field crops, orchard and plantation crops, vineyards and vegetables. It can be used before planting these crops and has no effect on crops following in the rotation because it is deactivated immediately on contact with soil. It can be used between the rows of growing crops as long as green parts of crops are not sprayed. In annual crops, nozzles should be shielded; in perennial crops, paraquat cannot permeate mature bark.  Paraquat quickly burns down the shoots of almost all weeds to leave clean fields immediately ready for planting. Although perennial weeds

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.
will re-grow, their roots will remain to anchor the soil, helping to prevent soil erosion

Description

Displacement of solids (soil, mud, rock and other particles) usually by the agents of currents such as, wind, water, or ice by downward or down-slope movement.

Authoritative On-line References and Resources

http://soilerosion.net/ This site brings together reliable information on soil erosion from a wide range of disciplines and sources. It aims to be the definitive internet source for those wishing to find out more about soil loss and soil conservation.
, and the crop will be allowed time to become more competitive.
Paraquat is best applied when weeds are small (2 – 15 cm). Higher label rates should be used when weeds are large or when the weed cover is dense. Sometimes the use of an adjuvant, typically a non-ionic surfactant (wetter) is recommended. This helps to achieve good contact on leaves that tend to repel water. On tougher weeds, better control can be obtained by adding another herbicide to the sprayer tank. Herbicides known as PSII inhibitors (see the article on herbicide modes of action) enhance paraquat’s action by slowing its effects and allowing it to move further into weeds. Other herbicides can be tank-mixed to add a residual effect to control later emerging weeds. 2,4-D is a good partner for improving activity on broadleaved perennial weeds. A pre-mix formulation with diquat also gives good control of broadleaved weeds

Description

The leaves are "broad" as opposed to the "narrow" leaves of grasses. Also called 'dicots' having two seed leaves, while grasses are 'monocots' having one seed leaf.

Authoritative On-line References and Resources

http://www.iwss.info The International Weed Science Society.
.
When burning down weeds prior to planting, paraquat can be sprayed a week or two after glyphosate to control any surviving weeds. In Australia this is known as the ‘Double Knock’ technique and it successfully insures against actual or potential problems with glyphosate resistant weeds. More information on weeds and weed control and on glyphosate resistant weeds can be found in the Paraquat Information Center Knowledge Bank.

Weather conditions

Paraquat can be applied effectively under a wide range of weather conditions. It is well known for being the most rainfast herbicide. Rain falling only 15-30 minutes after spraying does not result in less activity (Fig.1). a) Paraquat herbicide b) Glyphosate herbicide                       Figure 1.  Paraquat products are rainfast in 15 minutes; glyphosate requires several rain-free hours after spraying. In bright sunlight the effects of paraquat on weeds can be seen after a few hours and weeds may be completely desiccated in a day or two. Under cloudy skies paraquat acts more slowly, but the effect is similar to the addition of a PSII inhibitor herbicide and ultimately weed control may be improved. The same effect of low light-intensity can be seen when paraquat is applied in the evening. A temperature inversionTemperature has little effect and paraquat can be used effectively early in the season in temperate regions. Many herbicides need to wait for warmer temperatures to stimulate weed growth, but paraquat even works when weeds are frosted. However, temperature inversions (when a warm layer of air sits on top of a colder air at ground-level, sometimes seen as mist or smoke apparently trapped close to the ground) need to be avoided because spray droplets may linger as a mist and subsequently drift.

Spraying

Paraquat concentrate should be diluted with clean water. While, unlike many herbicides, paraquat is usually little affected by water pH or content of minerals such as calcium (hard water), dirty water will reduce activity. This is because paraquat will adsorb to suspended particles of minerals and organic matter just as it does in soil. Similarly, if weed leaves are dusty the effect of paraquat may be reduced. The objectives when spraying are to achieve good coverage of target weed leaves while avoiding spray drift onto the crop or other non-target plants. The right water volume and spray nozzles must be used, typically 200 L/ha and flat fan nozzles. Spray pressure should be adjusted to avoid the production of smaller droplets, which are more liable to drift. Spray nozzle height and swathe width should allow overlaps to give even coverage. When spraying between row of, eg maize or vegetables, a nozzle shield can be used to confine the spray to the target weeds.

Re-entry to the sprayed area

Approved intervals for re-entry of workers and animals to fields where paraquat has been sprayed vary according to conditions of local registrations, so the label must be consulted. More answers to questions about using paraquat can be found in the FAQ section of The Paraquat Information Center.

Notes

The brand name for the leading paraquat product is Gramoxone.