Australia
June 29, 2015
A large and increasing number of growers now have high levels of resistance to all Group A herbicides. This is leading to a major switch to the use of pre-emergent herbicides as the mainstay of many herbicide programs targeted at ryegrass. However, in spite of resistance, many populations of ryegrass can still be controlled by one or more Group A herbicides and many users still rely on post-emergent applications of Group A herbicides to manage escapes in wheat and broadleaf crops.
To maximise performance of Group A herbicides, there are a number of simple ground rules that need to be followed:
Group A herbicides do not translocate easily within the grass weed. This means attention to application is critical to ensure good uptake and translocation.
- Always ensure excellent spray coverage. This typically involves water rates of 70 to 100L/ha to optimise results.
- Good coverage is more difficult to achieve on small grass weeds with narrow upright catching surfaces. Medium or medium coarse spray quality with high carrier volumes will normally provide the best results. Do not use extra coarse droplets.
- Group A herbicides need to penetrate the leaf to enter the plant and commence translocation to the site of activity. The choice of surfactant can greatly influence the amount of herbicide entering the leaf, so always follow label directions for individual herbicides.
- Group A herbicides need to translocate to the meristematic region (growing points) of the plant, where cell division is occurring. Group A herbicides disrupt an enzyme pathway that occurs at the growing point which stops the production of new leaves, and causes abscission at the base of leaves present at application. This allows agronomists to easily pull out the main leaf 1 to 2 weeks after application, a typical test to ensure that the herbicide has translocated and is working effectively.
- Due to weak translocation, the less distance between herbicide uptake into the leaf and movement to the growing point, the quicker and more effective the Group A herbicide will be. Therefore Group A herbicides work much better on small weeds, with performance rapidly declining once plants start to tiller. Always treat small weeds and don’t delay application waiting for further germinations.
- Any stress placed on the plant, in particular waterlogging, frost or moisture stress, will significantly reduce translocation to the growing point, which will result in reduced weed control. In particular, kill rates when application occurs soon after a frost event are often very poor.
Understand the resistance profile of the herbicide. Multiple different target site mutations have been identified that lead to herbicide resistance. Metabolic resistance can also occur. Due to the number of resistance mechanisms, and potential for combinations of different mechanisms within individual plants, it is highly likely that different resistance profiles will occur between paddocks and even within paddocks.
Irrespective of why a weed is resistant, it is critical to know which herbicides still work in a particular paddock.
Use resistance tests regularly. Seed can be collected from survivors at the end of the growing system or whole plant tests can be conducted on live plants within the growing season. The quick turnaround of in-season tests may allow more informed herbicide selection.
Increasing application rate of Group A herbicides can, in some instances, lead to increased control of populations that may be developing resistance. However where high level target site resistance occurs there may be no dose response when use rates are increased within label limitations.
The maximum registered rate for control of ryegrass with selected Group A herbicides is as follows. Always check the product label for registered crops and use situations:
In recent years, with the development of resistance, some users have increased application rates of clethodim, or moved to tank mixtures. Some clethodim brands have increased the label rate to 120gai/ha (e.g. 500mL/ha of 240g/L formulation). At this rate it is not surprising that increased control of some populations of resistant ryegrass has been observed.
For those that are interested, a number of the annual ryegrass populations that are rate responsive to clethodim have a target site resistance that partly occludes, but does not block the binding site for clethodim.
Another common strategy that has been employed in the early stages of the onset of resistance is to tank mix different Group A herbicides.
A common mixture in early days of resistance was to apply 38mL/ha of 520g/L haloxyfop (20gai/ha) + 250mL/ha of 240g/L clethodim (60gai/ha), although currently it is more common to see users apply full rates of both products i.e. 52gai haloxyfop + 60gai clethodim per hectare.
Another mixture which is gaining popularity is 80g/ha of 250g/kg butroxydim (20gai/ha) + 350mL/ha of 240g/L clethodim (84gai/ha).
It is not surprising that control improves in some situations when the additive effects of increased rates of various Group A herbicides are combined. If however the mechanism of resistance present confers resistance to both the herbicides used (as is often likely to be the case), mixing two products may still not achieve control and hence lead to a costly spray failure.
However, continuing to increase rate is not practical in many circumstances.
- Care must be taken not to exceed maximum use rates for each registered herbicide, or there is potential for exceeding MRL’s at harvest, resulting in grain rejection. Further increase in application rates (i.e. the 1L rate of clethodim used in experimental trials) cannot be supported for commercial use without changes to labels to address residues, environment and operator and crop safety.
- Increasing application rate will not provide control of all resistant populations.
- Potential crop damage can be observed. For example:
- Canola was generally considered to be ‘safe’ to Group A herbicides when these herbicides were initially developed, at use rates and application timings covered on registered labels. However, with the practice of now applying higher rates of clethodim and/or clethodim mixtures, and/or application during flowering which is outside the recommended window for use of clethodim, injury to the crop may occur. Further increasing application rate is likely to further increase the risk of crop injury.
- In development trials, increasing application rates have also been shown to cause increased flower abortion in some crops. However withholding periods on most herbicide labels prevent application to flowering crops, so this effect is not observed in commercial practice where label directions for use are followed.
Finally, when dealing with resistant ryegrass populations, harvest weed seed management should be targeted at any survivors to prevent weed seed bank replenishment.
GRDC Factsheet on group A herbicides in fallow
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