Deployment of Group 27 herbicides into broadacre weed management systems in the northern cropping belt. How they work, what affects performance and where do they fit?

Take home message

Group 27 herbicides provide a useful tool in northern farming systems particularly in providing an additional mode of action for pre-emergence weed control in certain crops and giving useful selective post-emergence control of a range of broadleaf weeds in winter cereals. Fallow uses have been developed for some products.

Introduction

Group 27 herbicides are inhibitors of 4-hydroxyphenyl-pyruvate dioxygenase (HPPD). The HPPD enzyme is essential for synthesis of plastoquinone. One of the requirements for plastoquinone in plants is a co-factor in formation of carotenoids which protect chlorophyll in plants. Plants sensitive to HPPD herbicides experience oxidative damage and chlorophyll destruction giving rise to the bleaching of plant tissues and eventual death (Mitchell et al., 2001). Plastoquinone is also essential for electron transfer in photosynthesis, so Group 27 herbicides interact with other photosystem I (Group 22) and photosystem II (Group 5/6) herbicides.

HPPD herbicides were first commercialized globally in 1980 with the active pyrazolynate, followed by pyrazoxyfen in 1985 and benzofenap in 1987. However, the first introduction to the Australian broadacre market did not take place until the late ‘90s with isoxaflutole. Since then, the actives pyrasulfotole, bicyclopyrone, mesotrione and topramezone have been introduced as herbicides in the winter cereal market. Development of HPPD chemistry is ongoing. Table 1 provides a brief summary of the main products commercialized across world areas.

Table 1. Herbicides containing HPPD actives

Use patterns

Isoxaflutole

Isoxaflutole stands out from other HPPD herbicides in that it is a proherbicide that is bio-activated to a diketonitrile (biologically active metabolite) by soil and plant enzymes (Jahala et al., 2023). Rainfall facilitates production of diketonitrile which is quite mobile in soil and is readily absorbed by roots and shoots of plants when used as a pre-emergence herbicide. On commercialization in Australia, isoxaflutole quickly became a foundation for weed control in chickpea mainly in combination with simazine and sometimes terbuthylazine. Registration for fallow uses has followed and use in this use pattern has expanded in recent years.

The weed spectrum controlled by isoxaflutole requires addition of herbicides with a complementary spectrum of activity and this applies to both in-crop and fallow use. While isoxaflutole is quite effective in providing pre-emergence activity against Feathertop Rhodes grass (Chloris virgata) and to a lesser extent awnless barnyard grass (Echinochloa colona), it is relatively weak against liverseed grass (Urochloa panicoides). Broadleaf weeds common to the northern region that are not well controlled by isoxaflutole include but is not limited to climbing buckwheat (Polygonum convolvulus), wireweed (Polygonum aviculare), caustic weed (Chamaesyce drummondii), corn gromwell (Bugglosoides arvensis), cowvine (Ipomea lonchophylla).  Moreover, duration of control of sowthistle (Sonchus oleraceus) and flax-leaf fleabane (Conyza bonariensis) with summer applications of isoxaflutole can be relatively short lived where heavy rainfall is received. Suitable tank mix partners for fallow uses include terbuthylazine, flumioxazin or imazapic.

Mesotrione

Callisto is a relatively new introduction into the Australian market as a pre-emergence herbicide in winter cereals. The requirement for incorporation by planting with a tyned implement is a limitation to the deployment of this herbicide in northern cropping regions particularly where zero tillage and disc planters are preferred. Weeds controlled include sowthistle, fleabane, volunteer pulses, wild turnip and wild radish. Pre-emergence herbicides more broadly have not been well accepted in winter cereal production in northern regions with the shift away from conventional tillage practices and for the moment is difficult to see mesotrione being utilized in a widespread manner.

Pyrasulfotole

Pyrasufotole formulated with either bromoxynil (Velocity) or MCPA (Precept) have not been as widely used in northern farming systems relative to the southern and western regions, even several years after commercialization.  In common with HPPD herbicides applied post-emergence, activity is often significantly enhanced with the addition of photosystem II inhibitor herbicides from herbicide groups 5 or 6 (e.g. bromoxynil). The recent release of a standalone pyrasulfotole formulation (Galaxy®) has provided some flexibility in combining either bromoxynil or bromoxynil + MCPA. Recent registration of diflufenican+pyrasulfotole (Infinity Ultra) has facilitated fallow application of pyrasulfotole for the first time with modest re-crop intervals for many crops and in particular, cereals. The diflufenican+pyrasulfotole (Infinity Ultra) label also carries several of the winter cereal uses currently included on the pyrasulfotole+bromoxynil (Velocity) label, but does not mandate the addition of bromoxynil.

For in-crop use in winter cereals, pyrasulfotole has provided excellent control of brassica weeds, sowthistle, climbing buckwheat (bindweed) with good control of some hard-to-kill species such as corn gromwell.  The capability of being safe to cereal crops at early growth stages is an advantage shared by other selective HPPD herbicides. On the other hand, the requirement for good weed coverage, limits effectiveness in providing weed control in crops approaching the end of tillering.

Bicyclopyrone

Bicyclopyrone, as a co-mix with bromoxynil (Talinor), has been registered for use in winter cereals but has not been widely adopted in the northern cropping region. The incidence of crop effect in wheat on occasions most likely associated with large diurnal temperature fluctuations has led to caution in this region while in southern regions, this has not been a significant concern.

Topramezone

Topramezone, as Frequency herbicide, is still in the very early stages of commercialization in Australian cereal markets. Topramezone (Frequency) shows similar activity to pyrasulfotole with topramezone (Frequency) having the flexibility of mixing with either bromoxynil or MCPA. Activity on wild oats (suppression) is also covered by a label claim for topramezone (Frequency) when used in combination with bromoxynil.

Crop safeners

Most post-emergent Group 27 herbicides contain a crop safener in the formulation to improve cereal selectivity (mefenpyr-diethyl for Velocity, Precept and Infinity Ultra; cloquintocet-mexyl for Talinor and Frequency). However some solo pyrasulfotole products do not contain a safener in the formulation.

Spray adjuvants

Post-emergent HPPD herbicides in use as post-emergence herbicides all require the addition of a methylated seed oil adjuvant for optimum activity.

Resistance

Extensive use of HPPD herbicides in the US has led to the evolution of resistant populations of several important weeds. Interestingly resistance has also appeared in wild radish in northern Western Australia following several years of application of pyrasulfotole. While this may be a concern for the northern cropping region, the existing diverse nature of rotations and weed management strategies will be somewhat protective for the integration of HPPD herbicides in this region.

Future developments

A number of HPPD herbicides are widely used in maize in other countries (including the Americas and New Zealand).

HPPD tolerant soybean is commercialised in the US using ixosaflutole as a pre-emergence herbicide. Lines of sorghum with tolerance to tembotrione have been identified (Pandian et al., 2020) and the likelihood this could be expanded to other HPPD herbicides is real. The usefulness of an additional mode of action in sorghum growing regions in Australia cannot be overestimate given the dependence on herbicides from Groups 4 (auxins), 5 (triazines) and 15 (s-metolachlor).

A limitation for this prospect will be the very significant development cost of bringing a tolerance trait and suitable herbicide to market and necessarily this might exclude the attractiveness of older actives.

Development of HPPD herbicides as a group is ongoing and it is likely that some actives will be evaluated in Australian markets and in particular for fallow uses where the spectre of resistance to commonly used herbicides is a significant concern.

References

Mitchell G, Bartlett DW, Fraser TE, Hawkes TR, Holt DC, Townson JK, Wichert RA (2001) Mesotrione: a new selective herbicide or use in maize. Pest Manag Sci 57:120 – 128

Jhala AJ, Kumar V, Yadav R, Jha P, Jugulam M, Williams MM II, Hausman NE, Dayan FE, Burton PM, Dale RP, Norsworthy JK (2023) 4-Hydroxyphenylpyruvate dioxygenase (HPPD)-inhibiting herbicides: past, present and future. Weed Technol. 37:1-14 doi:10.1017/wet.2022.79

Pandian BA, Varanasi A, Vannapusa AR, Sathishraj R, Lin G, Zhao M, Tunnell M, Tesso Tm, Liu S, Prasad PV, Jugulam M (2020) Characterization, genetic analyses, and identification of QTLs conferring metabolic resistance to a 4-hydroxyphenylpyruvate dioxygenase inhibitor in sorghum (Sorghum bicolor). Front Plant Sci 1:596581

Contact details

Andrew Somervaille
Email: somervailleandrew@gmail.com

Date published
March 2024

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