Glyphosate Resistant Kochia

Since kochia cross-pollenates easily, is a tumbleweed, and has wind-blown pollen, it is difficult to know if the glyphosate resistance found in western Canada was introduced from elsewhere, or if it evolved independently at the various sites already confirmed.

Causes of glyphosate resistance in kochia

Many herbicides work by linking to a target enzyme site to disrupt whatever critical process it controls. Most herbicide resistant weeds in western Canada use a change in the "shape" of that target enzyme to prevent the herbicide from linking to the site. Because the herbicide, no matter how much of it there is in the plant, can no longer connect to the target enzyme, the resistance is absolute.

The mechanism used by glyphosate resistant kochia is very different. Instead of a mutation that changes the "shape" of the target site, the EPSPS enzyme (the target enzyme for glyphosate) in kochia remains susceptible and glyphosate applied to the plant can still link with the enzyme. Glyphosate resistant kochia simply produces much more than it needs by making many copies of the gene responsible for the production of EPSPS enzyme. As a result, all of the glyphosate entering the plant is tied up by the extra EPSPS enzyme and there is still enough EPSPS enzyme remaining to allow the resistant plant to function normally.

Currently, glyphosate resistant kochia populations found in Canada are resistant to roughly five times the normal rate for kochia control. Increasing the rate of glyphosate in an attempt to control these populations, aside from being impractical in most cases, is a poor choice for managing glyphosate resistant populations. Increasing glyphosate rates is more likely to result in the selection of individual plants with even higher numbers of copies of the EPSPS gene, and push the level of resistance in the populations to greater than the current five times normal rate. Kochia management strategies other than higher glyphosate rates will be more successful over the long term.

Spreading

Kochia is a tumbleweed and as it tumbles across the landscape it drops seed to the ground along its path. One of the initial tell-tale signs that kochia is resistant to a particular herbicide group is that the resistant kochia will grow in relatively straight lines across fields treated with that herbicide, while susceptible kochia all around it is controlled. This also occurred initially with Group 2 herbicide resistance. A tumbling kochia plant has the potential to travel great distances over the landscape if it does not encounter anything to block its progress.

Kochia also has wind-borne pollen that may allow plants to cross-pollenate with other kochia plants downwind. In this case cross pollination is likely between plants in neighbouring fields and pollen may in very rare cases travel further than this.

As with other weeds, kochia can also be transported on vehicles and equipment that have been used in infested fields and have not been cleaned prior to leaving. In addition, resistant seed may spread along the travel route of the trucks shipping contaminated grain without cover. This could be a great distance from the original field. Research has documented that larger farmers generally have higher levels of resistance, with the cause suspected to be the failure to clean equipment and vehicles as they move from field to field.

While glyphosate resistant kochia will inevitably spread from established populations, it is also as likely that additional new populations will have their origins in home-grown evolution events. The further away documented sites are from other known sites, the more likely they were the result of unique evolution events at that distant new site.

Resistant plants form a pattern in the field

Because of its tumbleweed habit, resistant kochia plants will, in the initial stages, form a tell-tale line across the field, where the mature resistant mother-plant previously dislodged and tumbled across the field in the wind. As resistance development progresses, a cross hatch pattern may form in the field as plants blow across the field in various directions. Eventually there will be fewer and fewer gaps in the stand until all of the plants in the field are resistant.

Kochia seed soil viability

The majority of kochia seed has very low dormancy and emergence declines very rapidly following the first growing season for new seed. Very little seed remains in the soil after a period of three years, when seed production, or re-introduction, can be completely eliminated. There may be a very small number of seeds that remain viable in the soil from five to 10 years. Burial may extend soil persistence slightly but the overall trend remains the same.

There is no evidence that shows that seed viability changes when a weed develops resistance.

Glyphosate resistant kochia and chemfallow

Glyphosate resistant kochia has the potential to develop in any system where glyphosate is used as a stand-alone herbicide on a regular basis. This can include fallow, glyphosate tolerant crops, or bare ground management in situations like orchards or industrial sites such as oil or gas wells.

All of the resistant populations confirmed to date have been collected from fields where chemical fallow was practiced as a part of the rotation. This may be due to the herbicide use patterns or simply because the tell-tale lines of resistant kochia are more obvious in fallow than in fields with crop cover.

Is glyphosate resistant kochia developing from non-cropland uses and then spreading into fields?

It is impossible to determine this. No particular industry/sector that uses herbicides to manage weeds is immune to the evolution of resistance. But considering the relative size of the acreages being treated in cropland versus industrial sites, the odds of finding a resistant biotype are much greater on the larger tracks of land being sprayed in agriculture. Industrial (non-cropland) sites also frequently use soil active, non-selective herbicides in addition to glyphosate applications to keep bare ground sites free from vegetation and minimize the number of times an area requires treatment in a season. The use of a combination of herbicide Groups greatly reduces the risk of developing resistance to glyphosate on non-cropland sites.

Continuous cropping

Continuous cropping may actually reduce the risk of glyphosate resistant kochia by maintaining crop cover on the field and limiting another application or two of glyphosate in the rotation. Kochia is relatively uncompetitive compared to other plants and does best in areas that have no other cover, such as fallow or areas of marginal crop production such as saline patches or seeding misses. Tall stature crops can also act as a physical barrier to kochia tumbling onto the land while they are standing. Avoiding fallow and maintaining a diverse crop rotation with a mixture of cereal and broadleaf crops, will minimize the opportunity for glyphosate resistant kochia to evolve and make it more difficult for known populations to spread.

Tillage

Tillage will control kochia spread when used early in the life cycle of the kochia plant by preventing plants from producing seed. The use of tillage as a management option would have to be balanced against the many soil health benefits that have been gained from eliminating tillage.

The use of targeted tillage could be one of the immediate measures taken when kochia appears as tell-tale lines across a fallow field. By tilling green kochia lines before the kochia begins to produce seed, the spread of the resistant biotype can be effectively controlled while retaining most of the benefits of minimal disturbance over the majority of the field. Even a small garden/utility tractor and 3 point hitch rototiller or disc may be sufficient to manage the narrow strips.

Tillage has also been found to stimulate additional weed germination in some cases which helps to reduce the number of seeds carrying into future years in the "seed bank". On the other hand, burying weed seeds can also preserve seeds and extend their dormancy. Weed seeds left on the soil surface are susceptible to more rapid degradation from extreme swings of hot-cold and wet-dry, quicker germination, consumption by small animals, insects and attack from disease organisms, that reduce their numbers more quickly.

Close mowing will also provide nearly the same effect as tillage while retaining the benefits of zero-till. Unfortunately no matter how close the mow, it is possible for kochia to regrow from buds below the cut line.

An alternative to tillage in fallow is to under-seed sweet clover with the crop previous to the fallow year and use as a cover crop to compete with kochia. Sweet clover residues have been shown to have a highly suppressive effect on the growth of kochia when seeded as a forage, or green manure crop on fallow land. Sweet clover was found to reduce the emergence of kochia on those sites by 80 per cent or more the following year, compared to conventional fallow systems.

Difference in speed of resistance development between using one or two applications at a low rate or 1 application at a higher rate when performing glyphosate applications to glyphosate tolerant crops

Because it is not clear how kochia evolves the multiple copies of the EPSPS gene that increase an individual plant's tolerance to glyphosate, it is not understood how rate influences the development of this type of herbicide resistance. What we do understand however, as with any other type of resistance, is that as long as the herbicide rate is sufficient to kill susceptible kochia plants, resistance plants will increase relative to susceptible plants in the sprayed population.

As is the case with all forms of resistance, each repeated application, provides one more opportunity for susceptible plants to be eliminated from the population taken as a whole. Each time a herbicide from the same resistance group is used; there is an opportunity for more susceptible members of the population to be eliminated, moving the selection process closer to a completely resistant population.

The best strategy to prevent this problem is to mix glyphosate with another herbicide that also controls kochia and never apply glyphosate alone if mixing is an option.

Minimize the impact of glyphosate resistant kochia

Because of its tumbleweed mobility, and tendency to cross-pollenate, it is very difficult to prevent glyphosate resistant kochia from entering any particular field. That being said, it is possible to minimize the impact of that spread and prevent widespread establishment of the resistant biotype on a field by treating each field as though it has glyphosate resistant kochia from now onward. By adopting practices to manage glyphosate resistant kochia before it is visible, individual plants that happen to appear will be addressed before they have a chance to increase and cause management challenges. These management practices include a combination of chemical, mechanical, cultural and rotational options.

If you do see plants that you suspect may be glyphosate resistant, physically remove and destroy those plants immediately, before they have a chance to multiply. Hand picking and burning are good management options for individual plants or smaller tell-tale lines. Larger lines or patches may need to be managed with mowing and spot-tillage before they mature and go to seed. By minimizing the opportunity for glyphosate resistant kochia to gain a foothold in a field, the long-term impact of resistance can be minimized.

Saline areas, that are in or border fields, are a haven for kochia that can re-infest production areas. These often produce little or no crop so they are better managed to prevent kochia growth by seeding salt tolerant perennial forages to outcompete kochia as well as other salt tolerant weeds, like foxtail barley.

Tank mix partners for use in:

• Chemfallow
• Pre-seed burnoffs - prior to oilseeds, pulses and cereals
• Glyphosate tolerant crops
• Chemfallow and preseed burnoff herbicides:

* Note that the high rate of CleanStart (30 mL of the Aim component per acre) must be used to control glyphosate resistant kochia.

In-Crop herbicides

For crops without the glyphosate tolerance trait, herbicide options for controlling kochia in-crop are the same, regardless of whether the kochia is resistant to glyphosate or not. See the Guide to Crop Protection for information for herbicide options for weed control.

However, there are no herbicide tank mix options for the control of glyphosate resistant kochia in canola or soybean with the glyphosate resistant trait. Edge may be applied and incorporated prior to seeding any variety of canola or soybean, including glyphosate tolerant varieties. Canola producers may also plant Liberty Link (glufosinate tolerant) varieties and apply Liberty to control glyphosate resistant kochia.

Glyphosate tolerant corn growers may mix glyphosate with DyVel DSp to control glyphosate resistant kochia. Corn producers may also consider sequential applications of several herbicides with activity on kochia. See the Guide to Crop Protection for herbicides registered for use in corn.

When selecting herbicides, producers should keep in mind that all kochia populations should be considered to be resistant to Group 2 herbicides. This means that Group 2 herbicides are no longer a management option for kochia.

Do late season applications - pre-harvest - have any impact on seed viability and will the impact be similar on resistant populations?

Susceptible green weeds (including kochia) killed using a pre-harvest glyphosate application will prevent any further seed from being produced by those plants. Any weed seeds on those plants that have reached physiological maturity (<30% moisture) will not take in glyphosate from the sprayed plant, and will survive a pre-harvest treatment. There may be a small number of immature seeds that will take in glyphosate when plants are sprayed with pre-harvest glyphosate and be rendered non-viable. Because this number is so low, attempting to manage weed seed viability with pre-harvest glyphosate is not a good strategy.

In glyphosate resistant kochia populations, those seeds rendered non-viable by glyphosate in susceptible populations will survive. The number of these seeds is so low however that they are unlikely to contribute a significant amount of additional resistant seed to the seed bank.

Control

Glyphosate resistant kochia may be present in a field for a few years before it exhibits the tell-tale lines across a field that are a sign that herbicide resistance is present in tumbleweeds such as kochia. Taking proactive steps, such as mixing glyphosate with other herbicides (that will also control kochia on their own), will reduce the risk of glyphosate resistance evolving or becoming established should tumbling plants introduce it to your land.

Including forages in a crop rotation

Including perennial hay stands, or even annual forage stands, in the rotation is an excellent management option for kochia, including glyphosate resistant types! Kochia does not grow well in locations with dense ground cover. Cutting of the forage stand for hay has the added benefit of removing the top-growth of the kochia plant, minimizing the amount of new seed returning to the soil.

Kochia also has palatability and nutritive qualities in hay similar to alfalfa and may be fed to livestock without harm as long as the proportion of kochia in their diet remains lower than 40 per cent of the total diet. Kochia may contain compounds such as saponins, alkaloids, oxalates and nitrates that can cause toxicity problems in livestock fed more than 40 per cent, but are typically not a concern if present in lower proportions. One of the suggested pathways of kochia introduction to North America was as a salt and drought tolerant annual forage.

Can the government pass a law to force people to take steps to control glyphosate resistant kochia?

Kochia is designated a Noxious Weed under The Weed Control Act. The Weed Control Act is a provincial law that gives municipalities the power to force private landowners to control regulated weed species (those designated Prohibited, Noxious or Nuisance). By appointing a weed inspector, rural and urban municipalities would be able to exercise their authority to order any kochia populations controlled. Taking this one step further, fields exhibiting the tell-tale lines of resistant kochia could be found by municipal weed inspectors on the lookout for the problem and the landowner ordered to address the issue in an appropriate manner. This type of early detection system and central co-ordination of control efforts would be very helpful in efforts to limit the spread of glyphosate resistant kochia.

What should I do if I think I have glyphosate resistant kochia?

If you think that you have glyphosate resistant kochia, contact your Saskatchewan Agriculture Regional Crops Specialist or the Saskatchewan Agriculture Knowledge Centre for more information. The Agriculture Knowledge Centre may be reached through toll free telephone at 866-457-2377, or email at aginfo@gov.sk.ca.