Is Palmer amaranth the worst glyphosate resistant weed?

Glyphosate resistant Palmer amaranth choking cotton, Georgia, USA.Officially, there are now 31 weed species around the world confirmed as having biotypes resistant to glyphosate. Over the past four years, new cases of glyphosate resistant populations have been confirmed in Argentina, Australia, Brazil, Canada, Greece, Italy, Japan, Mexico, New Zealand, Portugal and the USA. This is, no doubt, just the tip of iceberg, because once a species has several resistant populations in a region, a new one is no longer a surprise and the emphasis shifts from confirming new outbreaks to fighting the problem.  First confirmations of glyphosate resistant populations of six new species have been added to the list of resistant species maintained by the International Survey of Herbicide Resistant Weeds1 since the last update by The Paraquat Information Center (Table 1). Table 1. New species of glyphosate resistant weeds confirmed since 2011.
Weed species  Country Situation Year
Amaranthus quitensis 
Mucronate pigweed 
Argentina Soybeans 2013
Amaranthus spinosus 
Spiny amaranth
USA Cotton 2012
Bidens pilosa 
Hairy beggarticks
Mexico Citrus 2014
Brachiaria eruciformis
Sweet summer grass
Australia Fallow 2014
Chloris elata
Tall windmill grass
Brazil Soybeans 2014
Sonchus oleraceus
Annual sowthistle
Australia Cotton, fallow 2014

Palmer amaranth problem growing in the US

In 2014, the US farming media have been reporting the increasing problems caused by Amaranthus palmeri. A member of an important family of weeds commonly known as pigweeds or waterhemps, Palmer amaranth is especially aggressive and invasive, growing to over two metres (six feet) tall. Even in competitive situations individual plants are capable of producing more than 100,000 seeds.2 While glyphosate resistant Palmer amaranth has been known since 2005, it is no longer a problem confined to southern states. The northward spread has been particularly remarked upon recently. The tiny seed is easily spread by farm equipment and in cotton by-products that are fed to dairy cows, and even potentially by birds. The first case of glyphosate resistant Palmer amaranth in Illinois was confirmed in 2013 and there are fears that in the near future some populations, which have now reached as far as South Dakota, could also be resistant.3 Two recent papers published in the journal Weed Science illustrate the scale of the problem posed by glyphosate resistant Palmer amaranth and the efforts of US cotton farmers to combat it. Scientists at the University of Arkansas investigated how far and how fast glyphosate-resistant Palmer amaranth will spread in cotton.4 What would happen if a single plant were allowed to survive? Experiments involved simulating the seed-shed from a single plant into a 1m2 circle. Conducted at locations with no history of Palmer amaranth infestation, in the following season glyphosate-resistant Palmer amaranth were found over 100m away and in the third season, the weed infested at least 95% of all fields in the study, resulting in complete crop loss. The original dispersal was believed to be due to wash-off by rain. Surveys were conducted by the University of Georgia to assess the impact of glyphosate-resistant Palmer amaranth on weed management in cotton in the state.5 They found that growers’ input costs had more than doubled since 2005 when the first resistant population had been confirmed. Apart from increased herbicide costs, using a variety of modes of action, more than 50% of the crop now has to be hand-weeded, costing on average $57/ha. Mechanical cultivations before, during and after cropping are increasing employed. Nevertheless, this greatly diversified approach to weed management is proving to be successful.

Recommended control methods

Purdue University2 recommends the following approaches to managing glyphosate-resistant Palmer amaranth and other weeds in corn and soybeans:
  • Plough deep to bury seeds
  • Use 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.
  • Hand weed
  • Monitor ditches and field borders
  • Harvest heavily infested fields last
  • Use burndown herbicide before planting: paraquat + metribuzin gives good control of even large Palmer amaranth plants according to research at the University of Tennessee.
  • Use a pre-emergence residual herbicide
  • Rotate crops to allow use of herbicides with different modes of action

Paraquat can help

Paraquat has an important role to play in preventing the development and spread of glyphosate resistant weeds. Having a different mode of action, it can replace or complement the use of glyphosate to control weeds which might otherwise form the basis of a new resistant population. Several articles on The Paraquat Information Center illustrate the benefits of using paraquat to preserve the effective use of glyphosate. For example, in Brazil a popular use of paraquat is to spray just before or just after planting soybeans following a burndown application of glyphosate. This cleans up weeds that may have survived. A similar technique is widely used in Australia. The so-called ‘Double-Knock’ of glyphosate followed by paraquat works best when rain after the glyphosate spray has encouraged a new flush of weeds to emerge. Paraquat takes care of these as well as any survivors of the glyphosate spray. Paraquat really can act as glyphosate’s 'bodyguard' and preserve what has been called a ‘once-in-a-century herbicide’.6 The in-depth article in the Paraquat Information Center Knowledge Bank on Glyphosate Resistant Weeds has been updated.


  1. International Survey of Herbicide Resistant Weeds 
  2. Legleiter, T and Johnson, W (2013). Palmer amaranth biology, identification and management. Purdue University Extension. WS-51 
  3. North Dakota State University Crop and Pest Report, 26 June 2014 
  4. Norsworthy, J K et al (2014). In-field movement of glyphosate-resistant Palmer amaranth (Amaranthus palmeri) and its impact on cotton lint yield: evidence supporting a zero-threshold strategy. Weed Science, 62, (2), 237-249.
  5. Sosnoskie, L M and Culpepper, A S (2014). Glyphosate-resistant Palmer amaranth (Amaranthus palmeri) increases herbicide use, tillage, and hand-weeding in Georgia cotton. Weed Science, 62, (2), 393-402.
  6. Duke, S O and Powles, S B (2008). Glyphosate: a once-in-a-century herbicide. Pest Management Science, 64, (4), 319-325


The brand name of the leading paraquat product is Gramoxone.