Paraquat protects glyphosate in Aussie Double Knock

Australian farmers are using a Double Knock including paraquatAustralian researchers have found that spraying paraquat in a ‘Double Knock’ system is a very effective way to restrict the evolution of glyphosate resistant weeds. Survivors of glyphosate burndown are sprayed with paraquat up to two weeks later. The Double Knock ensures that weeds are hit with two different modes of action. The Double Knock system has been modified over the years, with different options being developed, but each retains the principle of diversity in weed control.  It was pioneered when paraquat was first introduced as a burndown herbicide in Australia. A spray was followed by soil cultivation. Nowadays, with the extensive use of glyphosate and the popularity of no-till


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  A portal for on-line information about no-till farming.
, an alternative option is for the Double Knock to be given as glyphosate followed by paraquat. The system works best when glyphosate is given a little time to translocate through weeds to the roots; when paraquat is applied while weeds are still green; and when early rains encourage a new flush of germination1.
The world’s first case of weed resistance to glyphosate was recorded in Australia in 1996 when a population of annual ryegrass (Lolium rigidum) in Victoria failed to be controlled2. By 2010, confirmed cases of glyphosate resistance in annual ryegrass had reached 981. Most are in no-till cereal fields or in orchards or vineyards – all examples of long-term intensive glyphosate use. Researchers at WAHRI (Western Australian Herbicide Research Initiative) calculated that after 20 years’ annual use of glyphosate in an arable rotation, more than one field in three with an annual ryegrass problem would be expected to have a weed population resistant to glyphosate1. Calculated probability of glyphosate resistant ryegrass evolvingThe chart opposite also shows that the simulation calculated that if the herbicide used in land preparation was alternated between glyphosate and paraquat, only one field in five would be expected to have resistance after 30 years, compared to nearly 90% of fields sprayed only with glyphosate. A ‘Double Knock’ regime with paraquat cleaning-up after glyphosate was predicted to keep all fields free of glyphosate resistant ryegrass for at least 30 years3. Other important Australian weeds that are hard to control with glyphosate are also being tackled by a paraquat Double Knock approach. Conyza bonariensis (flaxleaf fleabane) is a major weed problem in S. Queensland and N. New South Wales, in particular4,5,6. It is a prolific seeder, emerging in different seasons. One plant can produce 100,000 airborne seeds and it is imperative to prevent any survivors from weed control from setting seed. Seeds germinate at or near the soil surface, so, unfortunately, it is encouraged by no-till or other conservation tillage


Any tillage and planting system that covers 30 percent or more of the soil surface with crop residue after planting to reduce soil erosion by water.

Authoritative On-line References and Resources

Purdue University-based Conservation Technology Information Centre.
methods. Another issue is that the weed is quite tolerant of glyphosate. Although no official cases of actual glyphosate resistance have been recognised yet in Australia, several other countries have resistance2, so it is important to fight this threat. The standard treatment is to attempt to control young weeds with a mix of glyphosate and 2,4-D. Recent field trials have shown that this resulted in only 67% control. However, when paraquat or the mixture of paraquat and diquat popular with Australian farmers was sprayed 5 – 7 days afterwards to give the ‘Double Knock’, control levels were raised to over 95%.
Another problem weed in Queensland is Chloris virgata (feathertop Rhodes grass). Farm-scale trials using large plots have shown up to complete control of mature stands by using a ‘Double Knock’ including paraquat 11 days after an earlier glyphosate spray7,8.
Other herbicides have been tested as partners for glyphosate in the ‘Double Knock’ system, but paraquat cannot be beaten for its unsurpassed speed of action, the spectrum of weeds controlled and for being completely inactivated on contact with soil.
In summary, controlling weeds before planting with a Double Knock of glyphosate and paraquat:
  • Gives faster and broader spectrum annual weed


    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.
    control than glyphosate alone
  • Gives perennial weed


    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.
    control from glyphosate
  • Quickly leaves a clean seedbed only covered by desiccated weeds – important for soil residual pre-emergence herbicides
  • Protects glyphosate from weeds evolving resistance to this essential tool for food production.


  1. WAHRI newsletter, Autumn 2010.
  2. International Survey of Herbicide Resistant Weeds
  3. Neve, P, Diggle, A J, Smith, F P and Powles, S B (2003). Simulating evolution of glyphosate resistance. II. Past, present and future use glyphosate use in Australian cropping. Weed Research, 43, 418 - 427.
  4. Werth, J, Walker S, Boucher, L & Robinson, G (2010). Applying the double knock technique to control Conyza bonariensis. Weed Biology and Management, 10, (1), 1-8.
  5. Walker, S, Widderick, M & Cook, A (2010). Spraying young flaxleaf fleabane is the key for best control. Australian Grain, March – April 2010, 4–6.
  6. Queensland Department of Employment Economic Development and Innovation (Primary Industries).  Flaxleaf fleabane
  7. Get Farming Australia (2010). Mature feathertop Rhodes rolled with robust double-knock spray
  8. Queensland Department of Employment Economic Development and Innovation (Primary Industries) (2009). Rain prompts urgent feathertop Rhodes offensive


The brand name of the leading paraquat product is Gramoxone and the leading paraquat + diquat mixture in Australia is Spray.Seed.