Take home messages

• The only soybean varieties with tolerance to halosulfuron-methyl (ie. Sempra®) are New Bunya HB1, Mossman HB1 and Kuranda HB1
• APVMA Permit number PER88483 for Sempra® (or other registered products containing 750g/kg halosulfuron-methyl as the only active constituent) is for the control of nutgrass (Cyperus rotundus) in soybean, effective from 31 March 2020 to 31 March 2023. Note the restraints, critical use comments and withholding periods on the permit
• Temporary maximum residue limits (tMRLs) have been established to allow treated produce to be used domestically for human and animal consumption. Growers are advised to discuss use of halosulfuron-methyl with marketers of final produce to ensure that the residue limits of importing countries are not exceeded
• Control established weeds with knockdown herbicides/ land preparation prior to planting the crop
• Ensure correct crop stage. For optimal results, apply 65–130 g/ha of a 750g/kg formulation early post-emergence to the crop (up to 1st trifoliate leaf stage), when the majority of the nutgrass is at the 3–4 leaf stage
• Coverage is important. Apply in a minimum 80 L water/ ha utilising a COARSE spray quality
• Always follow adjuvant recommendations on the label of the halosulfuron-methyl product being used. For example, Sempra® recommends application with either Supercharge® Elite (Paraffin oil 471g/L) or Banjo® (Methyl esters of canola oil 725g/L) at 1L/100 L.  Some halosulfuron-methyl formulations recommend a non-ionic surfactant/wetter.
• Only ONE (1) application is permitted per season
• Read and understand the conditions of the PERMIT and the herbicide label prior to use
• It is advisable to only use ONE (1) application of a Group B herbicide per season.

The Australian Soybean Breeding Program has recently released three new soybean varieties: New Bunya HB1, Kuranda HB1 and Mossman HB1.  All three varieties possess the ALS 1 gene conferring tolerance to halosulfuron-methyl, the active ingredient in Sempra®, a sulfonylurea herbicide commonly used to control nutgrass.

Sulfonylurea (SU) herbicides inhibit the acetolactate synthase (ALS) enzyme in plants. The ALS enzyme, which is responsible for manufacture of certain branch-chain amino acids, subsequent protein manufacture and plant growth, is blocked at the growing point, resulting in plant death. Sulfonylurea herbicides are taken up by the roots, shoots and foliage. Herbicides in this group differ in whether their activity is predominantly through root and shoot uptake, or foliar uptake, or both.

Halosulfuron-methyl is a Group B herbicide that is absorbed through roots, shoots and foliage with the active ingredient being translocated through the plant to the nutgrass ‘nut’ (tuber) below the soil surface. Nutgrass exhibits a complicated interconnected network of tubers through underground rhizomes (stems) howeverhalosulfuron-methyl does not translocate through these rhizomes to attached tubers, so subsequent emergence from adjacent tubers may require alternative control measures.

Under APVMA permit PER88483 (which expires 31 March 2023) halosulfuron-methyl may be applied as a foliar spray to soybean early postemergence (up to the 1st trifoliate leaf stage of the crop) to target nutgrass at the 3–4 leaf stage.

Symptoms of successful weed control are a gradual yellowing of foliage and seed heads followed by desiccation. Initial symptoms may take 7 to 10 days to be noticeable, with full effects occurring 4 to 6 weeks after treatment.

Characteristics of the varieties

New Bunya HB1

Regional adaptation – New Bunya HB1 is expected to have similar adaption as its parent variety Bunya. It is well-adapted to irrigated cropping across inland southern Qld and to northern NSW with planting window from late November though to mid-January. New Bunya HB1 may also be grown as a winter season crop in the Burdekin Irrigation Area with a planting window from May through to the end of June.

Market suitability – New Bunya HB1, is suitable for crushing, full fat, and a wide range of human consumption markets. It is anticipated that it will be strongly sought after by tofu makers.

Breeding – New Bunya HB1, is largely derived from its parent variety Bunya but with tolerance to halosulfuron-methyl herbicides, resistance to powdery mildew and resistance to seed coat splitting transferred in to via by backcrossing.

Kuranda HB1

Regional adaptation – Kuranda HB1 appears to be well adapted to cropping in coastal environments in Qld from around Nambour to the wet tropics and may also be grown as a winter season crop in the Burdekin Irrigation Area with a planting window from May through to the end of June.

In the southern end of the cropping range Kuranda HB1 is best adapted to planting dates from late December though to early February as at earlier planting dates it may become excessively vegetatively vigorous.

Market suitability – Kuranda HB1, is suitable for crushing, full fat, and a wide range of human consumption markets.

Breeding – Kuranda HB1, is largely derived from its parent variety M103-22 which was a selection from Moonbi x Fraser with some parentage from a breeding line selected from Stuart, x sib of Cowrie.  Kuranda HB1 combines broad adaptation and high yield potential with tolerance to halosulfuron-methyl and high resistance to many diseases such as soybean rust, purple seed stain and powdery mildew.

Mossman HB1

Regional adaptation – Mossman HB1 is a grain option in the Burdekin and Atherton Tablelands or  green manure option in many cropping environments due to its long duration and high biomass production.

Market suitability – It is anticipated that grain of Mossman HB1 will be accepted into most crushing, stockfeed and human consumption markets.

Breeding – Mossman HB1 is closely related to its parent variety Leichhardt but with the additional inclusion of a light hilum and herbicide tolerance derived by backcrossing.

Tolerance of HB1 soybean varieties to Halosulfuron-methyl

Figure 1. Yield of soybean varieties grown at Gatton in 2018 with and without a treatment of halosulfuron-methyl at 130 grams per hectare. Within a cultivar, all treatment effects were statistically significant (P< 0.05).

When conventional herbicides are used and direct comparisons are possible, the HB1 varieties yield similarly or higher than their recurrent parents. In instances where a treatment of halosulfuron-methyl at 130 grams per hectare was used in weed-free field trials, yields of HB1 varieties were observed to be higher in the treated plots than in plots treated with nil herbicide application (Figure 1). The cause of this apparent yield-gain requires further investigation although similar effects are reported in the USA and Brazil.

What weeds does halosulfuron-methyl control?

In Australia, there is limited data available on the efficacy of halosulfuron-methyl on weeds other than nutgrass, Navua sedge and Mullumbimby couch. It is, therefore, recommended to discuss with your agronomist complementing halosulfuron-methyl targeting nutgrass with other herbicides registered for soybean production.

Successful application

Attention to detail with application set-up is crucial to ensure desirable coverage and uptake of halosulfuron-methyl applied as a selective, post-emergence foliar spray.

For successful results it is important to ensure the following:

• Spray rig is accurately calibrated
• Apply halosulfuron-methyl in a minimum of 80L water/ha utilising a COARSE spray quality
• Add an adjuvant as per the herbicide label
• Do not spray when crop and/or weeds are stressed due to low relative humidity, temperature extremes, waterlogging, or when severe root or foliar diseases are present
• Do not spray if storms or heavy rain is anticipated within at least 48 hrs
• Crop nutrition is managed to avoid nutritional deficiencies and plant stress

Plant back periods

Pesticide residues need to be taken into consideration in determining plant back intervals to subsequent crops and resulting future cropping opportunities.

In the circumstance of a failed HB1 soybean crop where halosulfuron-methyl has already been applied to the failed crop, another HB1 soy variety (tolerant of halosulfuron-methyl) can be re-planted, however, halosulfuron-methyl must not be re-applied to the re-planted crop. If subsequent weeds germinate, manage with conventional herbicide options registered for use in soybean production, excluding other Group B herbicides (eg. imazethapyr, imazamox, flumetsulam).

Break down in soil

Sulfonylurea herbicides are extremely active at low use rates in the soil and can be very damaging to many broadleaf rotational crops. It is essential that plant back recommendations are followed.

The primary factor for degradation of all SU herbicides in low pH to neutral pH soils is a hydrolysis chemical reaction.  In alkaline soils at higher pH levels, this reaction slows or stops and breakdown reverts to a far slower microbially driven breakdown process. Soil pH is the key factor driving breakdown while maintaining soil moisture over the warmer spring/summer months is the most important factor assisting microbial degradation.

Factors important for breakdown include:

• soil moisture – increased breakdown with higher soil moisture
• soil temperature – increased breakdown with warmer soil temperature
• organic matter in cropping soil is crucial as it provides a food source for soil microbes
• soil pH –slower breakdown in alkaline (high pH) soils
• time.

No one path works in isolation. A combination of the factors above results in effective breakdown of SU herbicide residues. Always follow the product label with respect to plant back timeframes to subsequent crops and discuss your particular situation with your local agronomist.

Herbicide resistance

Sulfonylurea herbicides are historically the fastest herbicides to select for herbicide resistance, with resistance occurring in as little as 4-6 applications in some weed species. To delay herbicide resistance, only use 1 application per crop; ensure the herbicide is applied for maximum efficacy and ensure there are no survivors that are allowed to set seed.

Further information

• APVMA PERMIT PER88483
• Nufarm product information and contact details
• Soy Australia factsheets for new HB1 varieties
• NSW DPI Summer Crop Production Guide – Soybean
• GRDC Soybean Northern Region Grow Notes™

Acknowledgements

Thanks to Rob Walker and Matthew Moyle from Nufarm for valuable technical advice in the preparation of this paper.

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the authors would like to thank them for their continued support. The Australian Soybean Breeding Program is a co-investment by CSIRO, NSW DPI and GRDC.

Contact details

Dr Andrew James
CSIRO Agriculture and Food
306 Carmody Rd
St Lucia QLD 4067
Email: Andrew.James@csiro.au

Dr Natalie Moore
NSW Department of Primary Industries
Experiment Farm Rd
Grafton NSW 2460
Email: Natalie.Moore@dpi.nsw.gov.au

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