Take home messages

• New registrations for Group G herbicides are expanding the ways these herbicides can be used.
• The choice of a spike with knockdowns should be based on activity against the most problematic weeds.
• Care needs to be taken with pre-emergent applications in light soils and where crop tolerance is not high to keep the herbicide out of the crop row.

Group G herbicides – how they work

Group G herbicides inhibit the enzyme protoporphyrinogen IX oxidase (PPO or Protox) in the chlorophyll biosynthesis pathway. The mode of action of these herbicides is quite complex. Inhibition of Protox results in an accumulation of the substrate for the enzyme, protoporphyrinogen IX, which leaks out of the chloroplast into the cytoplasm. Within the cytoplasm, protoporphyrinogen IX undergoes non-enzymatic oxidation to protoporphyrin IX. In the light, protoporphryin IX is converted to a radical, which then reacts with lipids in the cell membrane, destroying the integrity of the cell. This leads to the bleaching symptoms seen with Group G herbicides.

These herbicides are mostly absorbed by the shoots, even when applied to soil. They are typically contact herbicides with typically no translocation out of the treated leaf. This means that good coverage is important for activity. Group G herbicides control broadleaf weeds and usually have little or limited activity against grasses.

Group G herbicide use patterns

The most common use pattern for Group G herbicides in grain production in Australia has been as a spike application with knockdown herbicides to control weeds prior to sowing the crop. The registration of additional Group G herbicides and expansion of registrations in recent years has increased the potential use patterns for these herbicides. Most products are registered for spike applications with glyphosate or paraquat; however, many now have other applications as well. One of the newer use patterns is as a pre-emergent herbicide. Table 1 lists the Group G herbicides registered for use in grain production in Australia and their registered uses.

Table 1. Registered use patterns of Group G herbicides in grain production in Australia

aRegistration of Reflex is expected in 2021

Chemical characteristics and behaviour of Group G herbicides

Much of our thinking about the behaviour of Group G herbicides has been influenced by the products that have been in use for a long time. There is a tendency to think of Group G herbicides having low solubility, resulting in contact herbicide behaviour and limited movement in the soil. Table 2 describes the solubility and binding to organic matter characteristics for various Group G herbicides. There is a wide range in solubility of Group G herbicides, with some more recently registered herbicides having much higher water solubility than has been traditionally associated with Group G herbicides.

Table 2. Water solubility and binding to organic matter characteristics of Group G herbicides.

As can be seen from Table 2, the compounds with low water solubility have high binding to soil organic carbon and those with high water solubility have low binding to soil organic carbon. This means the products with low water solubility will be particularly immobile in soil. For instance, saflufenacil, with high water solubility and low binding to soil organic matter, is highly mobile in soil. Saflufenacil will be particularly mobile in sandy soils with low organic matter.

These properties influence the behaviour of Group G herbicides, both in plants and in soil. For an herbicide to enter a leaf, it needs to cross the waxy cuticle. This is not a problem for a lipophilic herbicide like oxyflurofen. It readily moves into the cuticle following the concentration gradient (Figure 1). However, once it reaches the inside of the cuticle, its low water solubility means it will only slowly permeate the cell wall space. Due to the low water solubility, there is little movement from the site of application. This results in the classic spotting of leaves that is seen from application of these herbicides.

Figure 1. Movement of a lipophilic herbicide across the cuticle driven by the concentration gradient from high in the spray droplet to low in the cell wall space.

As the water solubility of the Group G herbicides increases, there will be more movement within the leaf, resulting in larger areas of damage from each surface droplet. For instance, saflufenacil, being highly water soluble, passes through the cuticle in a different manner and is much more mobile in the leaf. Saflufenacil also has some movement out of the treated leaves, but it is very limited.

The solubility of the herbicides also influences how they will behave on the soil surface. Oxyfluorfen with its low water solubility is used in horticulture to create a surface seal of herbicide to control emerging broadleaf weeds (Figure 2). Due to the low solubility of oxyfluorfen, any breaks in the surface seal, such as what happens with traffic, can allow weeds to emerge without contacting the herbicide. Flumioxazin, with low water solubility, has similar behaviour. However, saflufenacil is much more mobile and creates a wider band of herbicide, making it harder for the weeds to avoid contact.

Figure 2. Some Group G herbicides with low water solubility create a surface seal on the soil surface. The herbicide is absorbed by the weed shoot as it emerges through the herbicide zone. Any break in the herbicide zone could allow the weed to avoid picking up the herbicide (left) and survive. More soluble herbicides have a wider herbicide band (right) making any breaks in the surface seal less important.

The other factor in the activity of Group G herbicides is the ability of plants to detoxify them. There are variations between the herbicides and between species in the rate at which the herbicides can be detoxified. The combination of exposure, movement and detoxification capability of the herbicides influences the weed spectrum. Because of their limited water solubility and the growth pattern of grasses, Group G herbicide are more effective against broadleaf weeds than grasses. The more water-soluble compounds will tend to have higher grass activity; however, this can be reduced by high rates of detoxification.

Fitting Group G herbicides into Australian grain production systems

With the introduction of new herbicides and new uses of existing Group G herbicides, there will be increased choice in their use. There are now many herbicides that can be used as spikes with knockdown herbicides. The choice of product for this use should be dictated by the main weeds of concern. Where mallows are the main concern, carfentrazone-ethyl remains a good choice. For fleabane, saflufenacil would be a better choice. In situations where glyphosate-resistant ryegrass is an issue, tiafenacil will provide the highest level of efficacy.

Plant back restrictions need to be considered with some of the newer Group G herbicides. Most have no plant back restrictions when spike rates are used. However, plant backs to canola for tiafenacil and saflufenacil range from 1 to 6 weeks, depending on the rate used.

Starting with flumioxazin, registrations for Group G herbicides for pre-emergent weed control have been developed. Terrain (flumioxazin) is registered for use prior to wheat and some pulses, Reflex (fomesafen) is registered for use prior to pulse crops and Voraxor (saflufenacil + trifludimoxazin) is registered for use prior to wheat, barley and durum, primarily for the control of broadleaf weeds. It is important to keep the herbicide away from the crop row where there is insufficient crop safety. This means knife-points and press-wheel seeding equipment should be used with pre-emergent uses of these herbicides. Pulse crops have some tolerance to fomesafen, and this herbicide can also be used post-sow, pre-emergent (PSPE) on all winter pulse crops, except lentils.

The more water-soluble products (Reflex and Voraxor) will tend to provide better weed control due to their greater movement within the soil. However, that also increases their risk of producing crop damage, particularly in lighter soils. Factors that allow movement of the herbicide into the crop row will exacerbate crop damage.

Carfentrazone and pyraflufen-ethyl can be used mixed with MCPA (2-methyl-4-chlorophenoxyacetic acid) to control broadleaf weeds post-emergent in cereal crops. Saflufenacil is registered for control of green material late in pulse crops and flumioxazin and Voraxor are registered for fence line weed control.

Should we worry about resistance to Group G herbicides?

Currently there is no known resistance to Group G herbicides in Australia. However, there are 13 weed species across the world with resistance to Group G herbicides. Most of these are broadleaf weeds; however, resistance has occurred in three grass weeds: annual ryegrass, wild oats and crowsfoot grass.

Where resistance to Group G herbicides has occurred, it has been typically in situations where Group G herbicides have been used intensively, including horticulture, turf and soybeans. As yet in Australia, Group G herbicides have not been used intensively in grain production. Mostly their use has been as spikes with knockdown herbicides, which targets a smaller percentage of the weed population. The expanded use patterns for Group G herbicides in grain production is likely to increase the selection pressure for resistance. Choices will need to be made about where Group G herbicides fit best in rotations.

Resistance to Group G herbicides is often the result of target site mutations, but non-target site resistance mechanisms are also known. There are a number of known mutations in PPX2, the gene for the target enzyme, which result in variations in the amount of resistance to different Group G herbicides. Resistance when it does occur in Australia is likely to be unpredictable and herbicide testing will be a useful tool in managing resistance.

Useful resources

GRDC Fact Sheet – Mixing knockdown partners with Group G herbicides

https://grdc.com.au/__data/assets/pdf_file/0028/381736/10408-GRDC-Fact-sheet-Group-G-herbicides.pdf

Specific guidelines for Group G herbicides

Contact details

Dr Chris Preston
School of Agriculture, Food & Wine, University of Adelaide
0488 404 120
christopher.preston@adelaide.edu.au