A dormant vineyard recently sprayed with glyphosate
A dormant vineyard recently sprayed with glyphosate.

Managing glyphosate resistance

K.C. Harrington1, T.K. James2 and M.D. Parker3

1Massey University, Private Bag 11-222, Palmerston North, New Zealand 
2AgResearch, Ruakura Research Centre, Private Bag 3123, Hamilton, New Zealand

3Foundation for Arable Research, 85 Duncan Rd, RD3, Hamilton, New Zealand

(Updated November 2021)

Background

Glyphosate is the most widely used herbicide in the world.  It effectively kills a very wide range of broad-leaved and grass species, both annuals and perennials.  Because glyphosate translocates down into underground structures, there is usually less regrowth than with most other herbicides.  It is also strongly adsorbed onto soil particles thus losing activity on contact with the soil, making it suitable for use in preparing ground for sowing crops.  Glyphosate also has a lower mammalian toxicity than many other herbicides.  As a result, glyphosate is widely used for weed control throughout New Zealand in areas as diverse as along roadsides, urban areas, most cropping situations and pastoral systems for crop or pasture establishment, and selectively within orchard crops and vineyards.

As glyphosate is generally considered more effective and environmentally more benign than most other herbicide products, it could also be considered to be over-used.  There are many situations in New Zealand where glyphosate might be applied three or four times a year with no other form of weed control used, a recipe for herbicide resistance evolution.  Glyphosate-resistant populations of weeds have developed in many parts of the world, especially USA and Australia (Heap 2021), due to repeated use of the herbicide causing selection pressure for individual plants with mutations that allow them to survive exposure to glyphosate.  Continuing to apply only glyphosate allows these individuals to thrive without competition, multiply up and become the dominant vegetation in these sprayed areas.

In 2013, the first cases of glyphosate resistance were reported in New Zealand, with perennial ryegrass and Italian ryegrass both developing resistant populations following many years of repeated glyphosate applications in Marlborough and Nelson vineyards (Ghanizadeh et al. 2013).  Many vineyards in this area have since been confirmed to have resistant ryegrass present. This strategy outlines options for managing glyphosate-resistant weeds as well as practises to avoid their appearance.

General resistance management and prevention strategy

One of the main strategies to avoid resistance from occurring is to occasionally use a herbicide with a different mode of action or use glyphosate in combination with a herbicide with another mode of action. 

As can be seen in the mode of action table, glyphosate is Group G (or Group 9 in the new system), so alternative herbicides should be from another group.  Glyphosate has a very large number of trade names in New Zealand, including various Roundup formulations, Lion, Polaris Accelerate, Crucial, Turbo, Dryphosate, Deal, Touchdown IQ, Weedmaster, etc.  So alternating between these formulations will not reduce the selection pressure for glyphosate resistance.  Looking for herbicides that are not Group G (Group 9) will help with selection of appropriate alternatives.

Following many sequential applications of glyphosate, some people confuse the development of glyphosate-resistant populations within species that are normally susceptible to glyphosate (such as ryegrass) with the build-up of tolerant species that have never been controlled well by glyphosate, such as white clover, tall willow herb and mallows.  However, similar strategies are required to solve both problems, so if herbicide rotation and mixtures are used correctly, there should be no build-up of these tolerant species or individuals with genes for resistance.

As part of a good herbicide programme, observations should be made of plants not controlled by glyphosate and be aware that this poor control might require changes in the way herbicides are being used rather than simply ignoring the situation. This strategy is called ‘Looking for escapes,’ though it is also possible that these escapes may be due to poor herbicide application, such as inadequate overlap of spray swaths giving a narrow line of uncontrolled weeds in the middle of the herbicide strip. Weeds which ‘escape’ control (not ones which emerge and grow after application, or were not controlled due to poor application) should be destroyed either by hand (roguing) or by the use of another herbicide. Herbicide rotations are only effective if the alternative products selected are able to control the weeds not affected by the glyphosate.

Some suitable herbicide rotations will be suggested below for specific situations.  Another overall strategy though is to not use just herbicides for weed control, but to alternate with other weed control strategies such as cultivation, mowing, growing competitive ground covers or applying mulches.

Also it is best to apply herbicides at the recommended rates wherever possible, as applying lower rates can sometimes help promote development of resistant populations.

Specific situations where glyphosate is currently being over-used at times within New Zealand are listed below with suggestions on how to avoid herbicide resistance from developing:

o   Vineyards and orchards

o   Roadsides, railways, amenity areas and waste areas

o   Fence-lines and headlands in arable crops and pastures

Vineyards and orchards

Although our first cases of glyphosate resistance have developed in vineyards, similar selection pressures are also being applied within pipfruit orchards, kiwifruit and other perennial fruit crops.  Because glyphosate is more effective and less expensive than alternative herbicides, and also less likely to result in herbicide residues within fruit which could cause market access problems like alternative herbicides, or contaminate ground-water, many growers have moved to using only glyphosate to control weeds.  This has often led to a build-up of tolerant weed species such as mallows and tall willow herb.

Where possible, growers should try to rotate and alternate herbicides during and between seasons.  Amitrole (Group F3) is a broad-spectrum translocated herbicide generally not permitted while fruit are on vines or trees, but can be used in late winter to clean up weeds prior to the growing season, or after fruit harvest to deal with weeds that have built-up during the season, especially perennial weeds. Unfortunately a few of the glyphosate-resistant ryegrass populations found in Marlborough appear to also be resistant to amitrole (Ghanizadeh et al. 2014).

Glufosinate (Group H or 10) (examples include Buster, Bash, Biffo, Fiestar, Nirvana, Agpro Glufosinate and in Vixen and Bammer) is permitted in most fruit crops and, although it doesn’t translocate into root systems, can be a useful product to rotate with glyphosate in summer, especially for annual weeds. In some cases overseas weeds have developed simultaneous resistance to glyphosate and glufosinate, even though glufosinate has never been used, and this cross-resistance to glufosinate has also been found with some of the glyphosate-resistant ryegrass populations studied in New Zealand (Ghanizadeh et al. 2015a). The level of resistance to glufosinate is not large though, so 7.5 L/ha of products containing glufosinate should kill young ryegrass plants successfully whether they have glyphosate resistance or not.

Paraquat (Group D or 22) (examples include Gramoxone, Flash, PQ 200 and Agpro Paraquat, and also sold mixed with another Group D herbicide, diquat, as Preeglone, PDQ, Kuatout and Speedy), is now not permitted by some industry bodies to be used in some fruit crops, especially SWNZ vineyards.  However this is a herbicide that has been shown to be useful for rotating with glyphosate in fruit crops in Australia when glyphosate resistance has caused problems, although like glufosinate it too does not move into root systems.  Experience in Australian vineyards unfortunately has shown that rotating with just glyphosate and paraquat can lead to resistance to both herbicides in rigid ryegrass (Lolium rigidum) so alternatives like paraquat should not be used in isolation and repeatedly.

These are the main broad-spectrum post-emergence herbicides used in fruit crops.  But there are some herbicides used for specific weeds, especially Group A (Group 1) grass-killers such as fluazifop (Fusilade Forte) and clethodim (eg Centurion, Arrow, Cleo, Cletho, Sequence, Vega) which can be used for grass weeds that have not died following application of these other herbicides.  Fluazifop can give good control of glyphosate-resistant Italian ryegrass but is ineffective on perennial ryegrass which would need to be treated with clethodim. Haloxyfop (eg Gallant Ultra, Ignite, Valiant, Crest, Haloxyken) also gives good control of ryegrasses but is only permitted in pipfruit and citrus orchards. Avoid overuse of Group A herbicides as resistance can develop quickly to these, which is why vineyard managers must seek permission from SWNZ before using them on resistant ryegrasses.

Although still frowned upon by a few industry quality management standards, residual herbicides are generally now considered as acceptable for helping prevent development of glyphosate resistance and thus allow more sustainable weed control.  Having a herbicide such as indaziflam (Group L or 29; Alion), flumioxazin (Group E or 14; Chateau) or terbuthylazine (Group C1 or 5, eg Asset, Terb 500, Terbaflo, Magneto, Timberwolf) controlling weeds as they germinate for several months each year will reduce the number of weeds that need to be controlled by glyphosate.  The full range of orchard residual herbicides and information on how to rotate these correctly can be found in our orchard triazine resistance strategy (Harrington 2020). Given the issues with amitrole, glufosinate and paraquat mentioned above, residual herbicides when properly used are particularly helpful for preventing a buildup of glyphosate resistance in orchards and vineyards.

Several products exist on the market for mixing with glyphosate to improve control of tolerant species such as mallows, and these include carfentrazone (eg Shark, Hammer Force, Nail, Torus, Wallop), oxyfluorfen (eg Ox, Browndown Zap'd, Brownout, Burnout), fluroxypyr (eg Solstar, Tomahawk, Starane Xtra, Tandus, Tyrant) and saflufenacil (Sharpen).  These are all herbicides with modes of action different to that for glyphosate so may help prevent glyphosate-resistant biotypes from developing.  With all of these alternatives though, check first that they are registered for use in the fruit crop being considered using the Novachem Manual (Agrimedia 2021) or on-line sample labels for New Zealand from the chemical’s proprietor.

Once trees and vines are well-established, it should be possible to have the mown sward between the crop rows growing almost up to the base of the tree or vine without affecting crop yields.  With off-set mowers that can mow under the branches, herbicide really only needs to be used for keeping vegetation away from the base of the tree or vine to facilitate the mowing process.  By keeping the sprayed strip as narrow as possible, less weeds need to be sprayed, possibly reducing the chances of resistant individuals appearing, but also making it less costly per hectare of crop if more expensive alternative herbicides are used.  If it is possible to direct mown clippings on to the narrow sprayed strip, this may form a thick enough mulch to reduce the germination of new weeds.  Grazing of vineyards and orchards through winter may also reduce seed production for some weed species though won’t have much effect on other species such as established ryegrasses.  In some systems, use of flame-weeders or shallow cultivators might be an option for controlling weeds around the base of trees and vines.

Research showed that the resistance disappeared from a glyphosate-resistant ryegrass population from Marlborough when treated with glyphosate under cold conditions similar to those encountered in late-winter (Ghanizadeh et al. 2015b).  This has also been shown in a number of research projects overseas, that the non-target-site mechanism of resistance found in some resistant ryegrass in New Zealand will not protect plants from the effects of the herbicide under cold conditions. This allows growers to remove some resistant plants using late-winter spraying of glyphosate, which can then be followed by an application of a residual herbicide such as indaziflam, flumioxazin or terbuthylazine to prevent further plants from establishing. Those ryegrass plants with target-site resistance will not be controlled by this winter spraying though so will then need to be treated with clethodim.

Roadsides, railways, amenity areas and waste areas

Constant use of just glyphosate in situations such as roadsides, railways, urban areas and general waste areas provides a very strong selection pressure for development of glyphosate-resistant biotypes of weeds.  A common strategy that has been used to avoid build-up of tolerant species is to add metsulfuron (eg Answer, Agpro Meturon, MSF 600, Eradicate, Reply) to the glyphosate from time to time, or every time it is used.  Using metsulfuron too frequently increases the risk of resistance developing to this herbicide too, so it probably only needs adding once every three or four applications.  Another option is to add amitrole to the glyphosate occasionally, or to use amitrole by itself.  Adding either metsulfuron or amitrole to the programme will also reduce the chance of glyphosate resistance from developing.

Ideally, a residual herbicide should also be added to the mixture.  This reduces the chance of glyphosate resistance developing both by reducing the regularity with which glyphosate needs to be applied plus by adding a herbicide with another mode of action to the control programme.  Although residual herbicides are used less often now due to risks of contamination of water-ways, in some situations this risk may not exist, and also a few residual herbicides such as oxyfluorfen are less likely to leach into waterways than others.  In gravelly areas such as roadsides and railways, however, residual herbicides are often less effective, providing little added benefit to glyphosate alone, so are mainly useful for their knockdown value.

Many residual herbicides are available on the market that might be used in waste areas, including simazine, terbuthylazine, indaziflam, flumioxazin, oxyfluorfen, diuron, bromacil and imazapyr (Agrimedia 2021).  Some of these are available as mixtures, such as TAG G2 and Terminator G4 (glyphosate + terbuthylazine + amitrole + oxyfluorfen).

In some waste areas, it may be possible to make a few small changes to the site to make it more suited to being mowed, removing the need for herbicides and providing a better surface for reducing nutrient and hydrocarbon flow into drains than bare gravel.  Or perhaps areas can be covered in concrete or bitumen to make weed growth less likely.  Some areas might be suitable for converting into gardens with bark mulch and perennial shrubs to remove the need for herbicides.

Fence-lines and headlands with crops and pastures

Some pastoral farmers apply glyphosate frequently along the bottom of electric fence-lines to stop vegetation growing up into the fence and shorting it out.  The comments above about adding in products such as metsulfuron, amitrole and a residual herbicide apply here as well.  However, the best strategy may be simply to not have the bottom wire electrified or raise the bottom wire as livestock frequently graze pasture very hard under fence-lines due to less dung and urine being deposited here.  Removing electrified wires at the base so that grazing is not discouraged may provide perfect vegetation control, without needing to rely on non-selective herbicides which create bare ground and thus encourage weeds to establish.

Around the edges of arable crops there is usually an area of bare ground known as a headland where no crop is grown so that harvesters, other machinery and crop plants do not get too close to fence-lines, especially where machinery is turning at the end of rows.  As this soil is left bare, weeds generally grow, and some farmers control these weeds year after year in frequently cropped paddocks using glyphosate to prevent weed seed production, contamination of the crop (eg by weeds such as yellow bristle grass) and to keep paddocks tidy.  If done too often, this could lead to glyphosate resistance developing.  Although some of the herbicides mentioned above could be added to the glyphosate to reduce this risk, residues from these herbicides may affect future use of this land. 

One possibility is to cultivate or mow the headland on occasions to keep weeds controlled.  Or if the paddock was previously in pasture, the pasture could be left unsprayed and uncultivated in this zone.  Recent trials by the Foundation for Arable Research (FAR) have found establishment of weed suppressing swards of species such as clovers (red or white), lucerne, prairie grass or other pasture grasses in this zone can decrease weed growth without relying on glyphosate.  In addition they provide a buffer zone for nutrient runoff, and in the case of the legumes provide additional nitrogen to the outside crop margin.  These suppressive species can also be sprayed with selective herbicides to control troublesome species such as yellow bristle grass.  On dairy farms in particular, these headland species can also provide extra feed when the crop stubble is grazed.

Applying glyphosate across a paddock prior to planting shouldn’t result in resistance developing, even if done every year, because any inter-row cultivation or selective herbicides used within the crop should deal with any resistant individuals that survive.  However, if patches of a weed survive the glyphosate spraying which would have been expected to die, it might pay to cultivate the paddock prior to sowing just to make sure they are controlled. Large patches of surviving weeds may not be controlled by the normal herbicides used in the crop, especially many of the selective grass-killing herbicides which will not control well-established patches of ryegrass. As the glyphosate resistance trait in ryegrasses is known to be carried in pollen, it is possible that it might spread into pastures from nearby areas where glyphosate has been overused.

Acknowledgements

The assistance of colleagues within the Herbicide Resistance Task Force and also the SFF/FAR Glyphosate Resistance project in providing constructive comments on this strategy was appreciated.  Funding from the Sustainable Farming Fund and collaborating funding bodies helped bring some of the information used in this strategy together, and Hossein Ghanizadeh shed new light on some aspects of the glyphosate-resistant ryegrass problem with his PhD work at Massey University.

References

AgriMedia 2021. New Zealand Novachem Agrichemical Manual.  www.novachem.co.nz 

Ghanizadeh H, Harrington KC, James TK, Woolley, DJ 2013.  Confirmation of glyphosate resistance in two species of ryegrass from New Zealand vineyards.  New Zealand Plant Protection 66: 89-93.

Ghanizadeh H, Harrington KC, James TK, Woolley, DJ 2014.  A glyphosate-resistant perennial ryegrass population is also resistant to amitrole. New Zealand Plant Protection 67: 331.

Ghanizadeh H, Harrington KC, James TK, 2015a. Glyphosate-resistant Lolium multiflorum and Lolium perenne populations from New Zealand are also resistant to glufosinate and amitrole. Crop Protection 78: 1-4.

Ghanizadeh H, Harrington KC and James TK, 2015b.  Glyphosate-resistant population of Lolium perenne loses resistance at winter temperatures. New Zealand Journal of Agricultural Research 58: 423-431.

Harrington KC 2020. Managing triazine resistance in orchards. http://resistance.nzpps.org/index.php?p=herbicides/triazine_orchard

Heap I 2021.  International survey of herbicide resistant weeds. http://www.weedscience.com