More modes of action needed to support glyphosate tolerant crops

Resistant weeds (C. canadensis pictured): The Achilles heal of glyphosate resistant cropsThe introduction of glyphosate tolerant (GT) crops has been the one of the most significant technological changes in agriculture, along with mechanization and hybrid seed breeding. Farmers around the world are reaping the benefits of a simplified weed control system, saving costs, time and labor and increasing operational flexibility. Adoption has been rapid, particularly in the Americas. In the most important soybean growing US states, Brazil and Argentina around 90% of crops are GT. Globally 148 million ha were sown to GM crops across 29 countries in 2010 of which close to 90 million ha had a herbicide tolerance trait1. Three crops dominate this technology adoption, soybeans, cotton and corn. The technology has also spurred-on growers to further simplify their farming by reducing plowing and taking-up 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.
practices such as 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.
. Abandoning the plow enhances soil quality and reduces erosion, while increasing 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.
However, relying heavily on a single herbicide mode of action to control weeds, especially in no-till, encourages the evolution of resistant populations. Although glyphosate had been widely used for more than 20 years before the first isolated cases of resistance from areas of heavy use appeared, the further expansion of its use in GT crops soon led to the appearance of many more outbreaks. Previous articles on glyphosate resistant weeds have been published on the Paraquat Information Center (eg Paraquat protects glyphosate in Aussie Double Knock ; Paraquat is glyphosate's bodyguard) and up to date information is posted on the International Survey of Herbicide Resistant Weeds website2.

US benchmark study

University researchers in the US have recently reported on a benchmark study designed to show how best management practices (BMP) to avoid resistance, involving the use of more diversity in herbicide modes of action, can maintain the benefits of GT cropping systems3. Following a survey of 1200 farmers across six states, 156 fields of at least 16 ha were selected as sites in a large-scale field trial program. Covering the most important rotations, each field was split in two. One half received the farmer’s own weed control program and on the other half weeds were controlled according to BMP recommendations made by the state university extension services.

Main rotations included in benchmark study

Cropping System States
Continuous GT soybeans Illinois, Indiana, Mississippi, N. Carolina
Continuous GT cotton Mississippi, N. Carolina
Continuous GT maize Iowa, Nebraska
GT maize/GT soybeans Illinois, Indiana, Iowa, Mississippi, Nebraska, N. Carolina
GT soybeans/Non-GT maize Illinois, Indiana, Iowa, N. Carolina, Nebraska
GT soybeans/Non-GT maize Mississippi
The initial grower survey indicated that under the most GT-intensive rotations around 60% of soybeans fields received only glyphosate for weed control3. This figure was lower in maize (49% in continuous maize; 30% in maize-soybean rotations) and lowest in cotton (13%), because of the wider use of residual herbicides. Comparing BMP recommendations for each field to those of the respective farmers indicated that, worryingly, most typical weed control regimes fell a good way short of best practice, with few modes of action being employed (Figure 1). Figure 1.  Farmers use fewer modes of action than recommended.Glyphosate was clearly being over-relied upon. Weeds harder to control with glyphosate were likely to become dominant, as well as possibly resistant. Crop rotation and tillage are useful components of an integrated approach to weed management. However, the survey had shown that more than 30% of farmers did not rotate their crops and a similar proportion rotated to other GT crops. With the increasing popularity of no-till, opportunities for weed control by tillage were also becoming seriously limited, despite its environmental benefits. The BMP programs used tank-mixes of glyphosate with other herbicides, many having soil residual action, either pre- or post-emergence4. So far, only results from the first two seasons of trials, 2006 and 2007, have been published4. These showed that both rotation and herbicide program were having a significant effect on the success of weed control. Continuous GT maize represents a worst-case with no rotation either of crop or away from GT varieties. In this cropping system weed population density at harvest was reduced by 50% under BMP in the first year4. Unfortunately, farmers tend to only change weed control strategies once resistance has been recognized in their fields. This generally results in substantial cost increases to attack established resistant populations. In this study, BMP programs were usually about 20% more expensive, but yields tended to be slightly greater such that ultimately net returns were the same in all cropping systems for farmers’ and BMP programs5. In the long-term, preventing the development of glyphosate resistant weeds and avoiding the need for expensive control strategies will generate additional substantial savings.

Role for paraquat

This US study focused on introducing additional modes of action by including tank-mix partners for glyphosate. Paraquat cannot be mixed with glyphosate, but can be successfully used in GT cropping systems to provide broad-spectrum weed control before crops are planted, allowing glyphosate to be used for post-emergence weed control. Paraquat has a very distinctive mode of action. Weed control systems integrating paraquat retain the benefits of glyphosate while markedly reducing the risk of resistant weeds appearing. Examples of such regimes include the use of paraquat + diuron mixtures for burndown in no-till cropping in Brazil; and the ‘double knock’ system of an application of glyphosate followed-up by paraquat which is popular in Australia. Where no-till has been successfully established it is important not to revert to plowing for weed control. Crop rotation can help, but to preserve the benefits of glyphosate it is vital to use more diversity in herbicide modes of action and in this paraquat has a key role to play.


  1. ISAAA (2010). Brief 42: Highlights of "Global Status of Commercialized Biotech/GM Crops: 2010
  2. International Survey of Herbicide Resistant Weeds
  3. Shaw, D R et al (2011). Benchmark study on glyphosate-resistant cropping systems in the United States. Part 1: Introduction to 2006–2008. Pest Management Science, 67, 741-746
  4. Wilson, R G et al (2011). Benchmark study on glyphosate-resistant cropping systems in the United States. Part 4: Weed management programs and effects over time and geography on weed populations and soil seedbanks. Pest Management Science, 67, 771-780
  5. Weirich, J W et al (2011). Benchmark study on glyphosate-resistant cropping systems in the United States. Part 5: Effects of glyphosate-based weed management programs on farm-level profitability. Pest Management Science, 67, 781-784


The brand name of the leading paraquat product is Gramoxone.