- Ideal growing conditions could lead to a large brome grass outbreak in Western Australia this season.
- Brome grass control is made difficult by a persistent weed seedbank, late germination and unpredictable pre-harvest seed shed.
- A year-round, multi-season integrated weed management strategy is the only successful approach to controlling brome grass.
- Control strategies should be based on WeedSmart’s ‘Big 6’ IWM principles.
With much of Western Australia’s grainbelt enjoying higher than usual soil moisture heading into winter, growers are being urged to consider their brome grass control strategy and plan for a long campaign.
Brome grass (Bromus diandrus var. rigidus or B. diandrus var. diandrus) is widespread throughout the WA wheatbelt. While there is more than one variety of B. diandrus, it is not possible to distinguish between them in the field. The species is highly adaptable, with broad variation in dormancy, germination times and seed production.
Brome populations within a cereal crop can produce up to 4000 seeds per square metre. Uncontrolled plants in a poorly competitive crop may produce over 30,000 seeds per square metre.
The seeds have an inherent dormancy after shedding. So even when conditions strongly favour germination, the seedbank can persist in the soil and germinate in subsequent growing seasons.
This year’s summer rainfall and promising season start means 85 to 90 per cent of the current seedbank may germinate in the coming months.
If left unmanaged, the resulting outbreak could lead to a long-term brome grass problem for many growers.
Brome is not a one-season weed
Brome grass has an average seedbank life of three to four years and is notoriously difficult to control. On non-wetting sandy soils, the dormant seeds can result in patchy and protracted germination of the weed.
These factors help brome grass to avoid pre-sowing weed control tactics and can enable the seedbank to persist through several break crops before germinating in a subsequent cereal crop.
At maturity, brome grass has a very rapid and unpredictable seed shed. Most plants will shed their seeds before harvest, making the seedbank extremely difficult to control. Widespread seed shed can occur within a matter of days. On the other hand, some plants may retain some or all of their seeds until harvest and show no tendency to shed.
Long term strategies are essential
The most important point to be made about brome grass is that it cannot be effectively controlled within one or two seasons like other weed species such as ryegrass.
A multi-year, integrated weed management (IWM) strategy is essential.
The Brome RIM tool enables growers to model the results of different long-term brome grass control tactics. This online tool was developed by CSIRO scientists who modified the original RIM (Ryegrass Integrated Management) model developed by the Australian Herbicide Resistance Initiative (AHRI), with GRDC investment.
A brome grass IWM strategy should include:
Crops can out-compete brome grass to reduce seed numbers at the end of the season. Barley has been found to provide the most effective competition among cereal crops, followed by oat, triticale, wheat and durum.
Plant density can also aid crop competition. At densities above 100 plants per square metre, closer row spacing is generally more effective for out-competing brome grass than simply increasing the seeding rate. Planting double rows or using splitter boots to increase the row width are both effective alternatives to simply reducing the interval between rows.
Crop vigour has a profound effect on a crop’s competitiveness against brome grass and this can be encouraged through variety choice, time of sowing, sowing depth, fertiliser placement and good pest and disease management.
Where seed companies publish data on the competitiveness of their varieties, selections should be based on the variety’s ability to suppress weed seed production but tolerate the presence of weeds and yield well.
Harvest weed seed control (HWSC)
Regardless of when or how much brome grass sets seed, harvest weed seed control (HSWC) should be an integral part of the IWM strategy.
Modelling from the Department of Primary Industries and Regional Development’s Weed Seed Wizard decision making tool has demonstrated that using HWSC to capture even 20 per cent of brome grass seed will help reduce the soil seedbank.
Consideration of post-harvest burning
Brome grass seed can tolerate very high temperatures so burning may be of limited use for reducing the seedbank. Burning may increase the risk of erosion and is not recommended on sandy soils where brome grass tends to dominate.
Crop and paddock rotation
The persistent nature of brome grass seeds means a multi-season control strategy is essential and this includes a diverse rotation and herbicide use program to create a more dynamic environment.
Including at least barley and legume rotations between wheat plantings will help suppress brome grass seed production.
Brome grass remains susceptible to most herbicides, so including a broadleaf legume or canola in the rotation allows a grass-selective herbicide application to target brome grass.
Planting imidazolinone-tolerant Clearfield® cereal crops can also provide an excellent opportunity to reduce the brome grass seedbank in the rotation.
Persistence pays off
There is no ‘silver bullet’ remedy for a brome grass infestation. The only successful approach is a comprehensive IWM strategy based on the WeedSmart ‘Big 6’ weed control principles:
- Use double break crops, fallow and pasture phases
- Where considerable early germination appears, consider a double knock approach
- Rotate and mix herbicide groups
- Use your crop to compete with your weeds
- Never miss the opportunity to stop seed set
- Capture weed seeds at harvest.
This strategy will need to be pursued patiently and persistently for several years to remedy an existing brome grass problem and to keep brome grass under control for the long term.
High brome grass germination rates at the start of this season should be seen as an opportunity to assess the problem and plan a brome grass IWM strategy for the years ahead.
DPIRD – Weed Seed Wizard