Glyphosate resistance 2008 update

In 2008 the problem of weeds that have become resistant to glyphosate has continued to increase.

New outbreaks have been reported in Australia, Brazil, Paraquay and the USA, including one species previously unrecorded as being resistant¹. Urochloa panicoides (liverseedgrass) was found in New South Wales, Australia infesting grain sorghum and wheat crops².

In addition, populations of Chenopodium album (lambsquarters, fat hen) with less than usual sensitivity to glyphosate have been noted in US soybean fields³.

Glyphosate has been called a ‘once-in-a-century’ herbicide because of its unique combination of high efficacy and low impact⁴. Its popularity ironically now threatens its key role in farming because of weed resistance, and a more diverse approach to managing weeds has been called for by leading weed scientists. This would include using cultural techniques and ensuring different herbicide modes of action are employed in an integrated approach to weed management.

Problems continue in 2008

Trends over recent years suggest that there will be more outbreaks of resistant weeds recorded before the year is out¹. In 2006, 15 outbreaks were logged and in 2007 there were nine. These past couple of seasons have seen cases reported which well illustrate the general scope and scale of the problem.

Most outbreaks occur in glyphosate resistant GM crops where weed control relies very heavily on glyphosate. Others occur in situations where there has been intensive use of glyphosate for many years.

In 2008, cases in GM crops, especially soybeans, have involved Palmer amaranth (Amaranthus palmeri) in many crops in Mississippi, USA and sourgrass (Digitara insularis) in Brazil and Paraquay. In 2007, cases were reported from non-crop situations where glyphosate has become the standard means of weed control, on roadsides in California and along railways in the Czech Republic where Conyza species (horseweed, marestail, fleabane) have become resistant.

Weeds which have evolved resistance to glyphosate are typically those with complex and diverse genetics which offer the greatest opportunity for resistance genes to be present. The first weed observed to be glyphosate resistant, Lolium rigidum, is a good example, as are others like Conyza canadensis (horseweed, marestail) and Amaranthus species (waterhemps) (Table 1). Wide germination windows are also a feature. Waterhemps have become a serious problem in Missouri, USA, with some populations having multiple resistance to three herbicide modes of action. As waterhemp pollen is tiny, 10 times smaller than corn pollen, it can travel maybe one kilometre, and as Missouri State agronomist Julie Abendroth says “Pollen spread is a big factor in resistance – if you are managing it well, but your neighbour isn’t, this is a problem”⁵.

Table 1. Weeds officially confirmed as having populations resistant to glyphosate¹

Recent research has confirmed that weeds become resistant in two ways. In some cases the movement of glyphosate out of sprayed leaves to growing points is very poor⁶. This is often the case in Lolium rigidum and Conyza canadensis. In other resistant populations the enzyme EPSP synthase which is normally inhibited by glyphosate is subtly altered by a single amino acid mutation making it much less sensitive, eg Lolium multiflorum and Lolium rigidum⁷,⁸.

Why has glyphosate become less effective and what lessons are farmers and weed scientists learning about how to avoid weed resistance?

Lessons learnt

Leading weed resistance expert Prof. Steve Powles, director of the Western Australia Herbicide Resistance Initiative has recently looked at what is now known about glyphosate resistant weeds – and how to overcome the problem⁹.

Powles believes that glyphosate had been widely used for more than 20 years before resistant weeds were noticed for two main reasons. First, because glyphosate has no residual effect in the soil flushes of weeds emerging after application do not come under any selection pressure. Second, it was used as only part of a diverse weed management system involving other control methods: mechanical, cultural and a wide variety of selective herbicidesDescription 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 http://www.weeds.iastate.edu/ An invaluable source of contemporary information about herbicides and weeds from Iowa State University.. Using different methods tends to ensure any weeds surviving glyphosate are killed by other means.

With intensive use of a single herbicide mode of action, resistant weeds are bound to arise in time. However, opportunities for resistant weeds to survive and thrive have been multiplied by the success of glyphosate tolerant GM crops in which weed control often relies only on glyphosate. This simple weed control system is very cost-effective for farmers, but time has shown that it can clearly be abused. Similarly. no-tillDescription 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 http://www.no-till.com/index.htm A portal for on-line information about no-till farming. systems, which have many benefits, including preventing soil erosionDescription 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., sequestering carbon and saving fuel, are becoming more popular, but mean that weeds are not buried by ploughing¹⁰.
Another Australian weed resistance ‘guru’, Prof. Chris Preston (University of Adelaide) toured Arkansas, USA, this summer and commented on the glyphosate resistant Palmer amaranth he saw: “Farmers here have used Roundup Ready cropping with glyphosate as the only herbicide while pulling back on tillage. In that system, the only thing that’s really controlling the weed is glyphosate. Do that for a decade and you’ll have problems.”¹¹

Solutions

In New South Wales, Australia, this year’s discovery of resistant liverseedgrass has followed last year’s confirmation of resistance in Echinochloa colona (barnyardgrass, jungle rice). Local weed scientists have emphasised the need for integrated weed managementDescription A decision support system for crop protection which focuses on long-term prevention or suppression of pest problems with minimum impact on human health, the environment, and non-target organisms. IPM takes into consideration all available pest control techniques and tactics (cultural, mechanical, biological, chemical). IPM emphasizes the growth of healthy crops for better productivity with the least possible disruption to agroecosystems. Authoritative On-line References and Resources http://www.ipmcenters.org "The USDA's National Site for the Regional IPM Centers' Information System provides information about US commodities, pests and pest management practices, people and issues." (IWM), including different modes of action herbicides, to give the diversity needed. “Integrated weed management is like an insurance policy,” Andrew Storrie, Dept. of Primary Industries weed specialist said: “Pay a small premium now in adopting IWM, or risk paying a bigger premium later when herbicide resistance occurs.”²

When Preston was in the US this summer, he remarked on a new technique called seed-set control. This involves switching to crops with waxy leaves like peas or lentils and spraying a paraquat-based product late in the season. The resistant weeds in the crop are then killed before they can set seed and replenish the soil seedbank. The spray tends to run offDescription The occurrence of surplus liquid (like rain) which originates up-slope and is collected beyond the ability of the soil to absorb it. The surplus liquid then flows away over the surface to reach the nearest surface water (pond, lake, river). Authoritative On-line References and Resources http://www.sowap.org/index.htm SOWAP (Soil and Water Protection) is a collaboration between industry, NGOs, universities and farmers to test a range of site-specific soil management methods, based on the concept of conservation tillage. It has looked at economic and environment aspects including effects on soil erosion and run-off. the crop, but farmers will accept a little damage to get on top of the glyphodate resistant weeds problem¹¹.

Other Australian researchers have developed computer models of IWM in action. Simulations run over a 30 year period showed that using IWM could prevent glyphosate resistant weeds arising in GM cotton¹². Work on understanding how to reduce the chances of resistance arising in Palmer amaranth in US GM cotton using simulation models is due to be published soon. This showed that introducing different modes of action was key. Just using single annual applications of glyphosate would lead to resistant Palmer amaranth in as little as four years in the worst case scenario.

In the US, Ohio State University weed specialist Prof. Mark Loux, says that most of the marestail (Conyza canadensis) seen in Roundup Ready soybean fields in Ohio in summer 2008 will have been glyphosate resistant¹³. Loux stresses the importance of planning to take out glyphosate resistant marestail before planting the crop and flushes of new emergence must be prevented before the new crop canopy closes to shade out weeds. He has found that the best option is to add a residual herbicide and 2,4-D ester to paraquat and use this mixture to burn down marestail while it is still fairly small, in the rosette stage.

David Hallauer, a Kansas State University extension agent also advocates early strikes against problem weeds like marestail, especially if resistance is suspected, by using autumn applications of atrazine rather than waiting until spring¹⁴. Adding an adjuvant and paraquat or co-applying with liquid nitrogen fertilizer gives enhanced burndown. Of course, care must be taken with residual herbicides to ensure no problems with leaching or damage to the crop when planted. Using a post-emergence programme based on paraquat avoids worries about leaching and has the advantage that any new crop can be planted immediately.

Volunteers from previous crops are also weed issues and if they are from glyphosate tolerant crops, then alternative control methods are needed. Loux at Ohio State and colleagues at the University of Illinois, Penn State University and Purdue University have found that paraquat as Gramoxone Inteon, with or without metribuzin was the best option for controlling volunteer corn up to more than 25 cm tall, although best results were with seedlings less than 15 cm tall¹⁵. The alternative non-selectiveDescription A chemical product used for eliminating all types of weeds (annual and perennial grasses and broadleaved weeds). Authoritative On-line References and Resources http://www.weeds.iastate.edu/ An invaluable source of contemporary information about herbicides and weeds from Iowa State University. herbicide glufosinate (Liberty) was only effective on very small seedlings.

References

1. International survey of herbicide resistant weeds http://www.weedscience.org/in.asp
2. http://bombala.yourguide.com.au
3. Westhoven AM, Davis VM, Gibson KD, Weller SC & Johnson WG (2008). Field presence of glyphosate-resistant horseweed (Conyza canadensis), common lambsquarters (Chenopodium album), and giant ragweed (Ambrosia trifida) biotypes with elevated tolerance to glyphosate. Weed Technology, 22, (3), 544-548
4. Duke SO & Powles SB (2008). Glyphosate: a once–in-a-century herbicide. Pest Management Science, 64, 319-325
5. http://agebb.missouri.edu/news/ext/showall.asp?story_num=4597&iln=52
6. Preston C & Wakelin AM (2008). Resistance to glyphosate from altered herbicide translocation patterns. Pest Management Science, 64, 372-376
7. Jasieniuk M, Ahmad R, Sherwood AM, Firestone JL, Perez-Jones A, Lanini WT, Mallory-Smith C & Stednick Z (2008). Glyphosate-resistant italian ryegrass (Lolium multiflorum) in California: distribution, response to glyphosate, and molecular evidence for an altered target enzyme. Weed Science, 56, (4), 496-502
8. Simarmata M & Penner D (2008). The basis for glyphosate resistance in rigid ryegrass (Lolium rigidum) from California. Weed Science, 56, (2), 181 – 188
9. Powles SB (2008). Evolved glyphosate–resistant weeds around the world: lessons to be learnt. Pest Management Science, 64, 360-365
10. Vila-Aiub MM, Vidal RA, Balbi MC, Gundel PE, Trucco, F & Ghersa, CM (2008). Glyphosate-resistant weeds of South American cropping systems: an overview. Pest Management Science, 64, 366-371
11. http://deltafarmpress.com/cotton/resistant-pigweed-0925/
12. Werth JA, Preston C, Taylor IN, Charles GW, Roberts GN & Baker J (2008). Managing the risk of glyphosate resistance in Australian glyphosate-resistant cotton production systems. Pest Management Science, 64, 417-421
13. http://www.wnewsj.com/
14. http://www.hpj.com/archives/2008/oct08/oct13/
15. http://corn.osu.edu/story.php?setissueID=230&storyID=1435