Paraquat and new spray hood fight superweeds

Cotton grown using conservation tillage is threatenedParaquat is in the front-line of US cotton growers’ defenses against glyphosate resistant Palmer amaranth, often described as a ‘superweed’. This season an advanced design spray hood will enable weeds up to four feet tall (120 cm) to be controlled by paraquat, while shielding late growth-stage cotton from any leaf scorching. The hood has been designed for use at all growth-stages up until ‘layby’ (when the rows close). Externally the hood is streamlined to glide easily between cotton rows without knocking fruit off more advanced plants.  Inside, the hood has been designed to maximise spray coverage. As the sprayer moves over the crop, weeds are guided into the hood and knocked down by a horizontal bar before being sprayed from three nozzles, positioned above and to each side1.

Conservation tillage

Glyphosate resistant weeds, like Palmer amaranth (Amaranthus palmeri, also known as Palmer pigweed), threaten not only the productivity and profitability of infested crops, but also the future of sustainable farming


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 Information from the USDA's Alternative Agricultural Systems Information Center.
systems. Glyphosate, described as a “once in a century” innovation2, has been a major factor in the increased adoption of conservation tillage systems, especially 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.
, because it enabled virtually all weeds to be controlled without plowing.

Conservation tillage benefits under threat

  • Better soil
  • More biodiversity


    The variety of life in all its forms, levels and combinations. Includes ecosystem diversity, species diversity, and genetic diversity (IUCN, UNEP and WWF, 1991).

    Authoritative On-line References and Resources EarthTrends is a comprehensive online database, maintained by the World Resources Institute, that focuses on environmental, social, and economic trends. Statistics on biodiversity indicators are available.
  • Reduced costs
  • Lower greenhouse gas emissions
According to the latest assessments by the US Department of Agriculture, 35% of US cropland is no-tilled. Around 50% of soybeans, 30% of corn and wheat, and 24% of cotton are grown in no-till fields3. However, although the overall rate of increase in the area of no-till cotton grown has been around 1.4 percentage points each year, some states have recorded reductions recently. This is worrying because no-till has very significant economic and environmental benefits. An earlier article reported how glyphosate resistant weeds in the US Cotton Belt are turning some farmers back to deep plowing in a bid to contain them. How has this happened?

Glyphosate tolerant cotton

Glyphosate tolerant cotton has been grown in the US since 1997 and was rapidly adopted. Before glyphosate, selective herbicides


A chemical product used for eliminating certain types of weeds only (ie either grasses or broadleaved weeds) and not affecting specified crops.

Authoritative On-line References and Resources An invaluable source of contemporary information about herbicides and weeds from Iowa State University.
in cotton relied on rain for activation, had narrow spectra of weed control or could damage the crop, such that within 5 years 90% of fields were being planted with glyphosate tolerant varieties4. Surveys have indicated cost advantages running to hundreds of millions of dollars per annum. Lower overall costs of weed control, eg labor, herbicides, machinery and fuel, have been the major contributors to the economic benefits, although less tangible benefits of simplicity, flexibility and freeing-up of labor and management time for other purposes are also believed to have been very important in the popularity of the technology5. Some evaluations of the benefits of herbicide tolerant crops have pointed out the simplified weed control regimes, eg “ … fewer herbicide modes of action used during the growing season …”5. Unfortunately, over-simplification of weed control is what has resulted in the evolution of glyphosate resistant weeds. In cotton, resistant populations of horseweed (Conyza canadensis) were first recorded in Tennessee in 2001, followed by giant ragweed (Artemisia trifida) in 2007, while resistant Palmer amaranth first appeared in Georgia cotton fields in 2005 and now infests between one and two million acres (400,000 – 800,000 ha) of cotton and soybean crops in that state alone6.

Paraquat: a better solution

Ideally, Palmer amaranth needs to be removed early because even quite small plants will compete with the crop and reduce yield. If rogue plants are allowed to mature and set seed, every one of them can release around half a million seeds, each potentially another glyphosate resistant weed. Over the past couple of seasons, farmers in many southern US states have had to resort to using hoes and machetes to attack infestations of palmer pigweed no longer controlled by glyphosate. Using paraquat and the new spray hood, with the option of adding diuron for large weeds, will provide a much better solution. Reintroducing some diversity in approaches to weed control back into cotton farming is key to fighting the so-called ‘superweeds’. Using soil tillage is undesirable as already discussed. Cover crops


Cover crops are primarily planted not to be harvested for food but to reduce soil erosion, control weeds and improve soil quality. They are usually plowed or tilled under before the next food crop is planted, in which cases the "cover crop" is used as a soil amendment and is synonymous with "green manure crop."

Authoritative On-line References and Resources ATTRA is the US National Centre for Appropriate Technology's Sustainable Agriculture Information Centre.
can help and are a recognised part of conservation agriculture. However, when it comes to herbicides, paraquat’s distinctive mode of action is a big advantage. The leading breeder of glyphosate tolerant cotton varieties is offering cotton farmers a rebate on their weed control costs if certain alternative mode of action herbicides, including paraquat, are used in their 2011 season program.
Read more about the fight against Palmer amaranth and other herbicide resistant weeds at


  1. Delta Farm Press, 10 February 2011.
  2. Duke, S O & Powles, S B (2008). Glyphosate: a once-in-a-century herbicide. Pest Management Science, 64, 319-325.
  3. Horowitz, J, Ebel, R & Ueda, K (2010). "No-till" farming is a growing practice. USDA Economic Information Bulletin No. 70.
  4. Dill, G M, CaJacob, C A & Padgette, S R (2008). Glyphosate-resistant crops: adoption, use and future considerations. Pest Management Science, 64, 326-331.
  5. Gianessi, L P (2008). Economic impacts of glyphosate-resistant crops. Pest Management Science, 64, 346–352.
  6. International Survey of Herbicide Resistant Weeds


The brand name for the leading paraquat product is Gramoxone.