WithTheGrain: Multi-year strategies key in brome and barley grass control
Author: Alistair Lawson | Date: 26 Mar 2018
Carefully developed, multi-year management strategies of at least three seasons are a critical factor in growers gaining the upper hand against barley grass and brome grass.
Both barley grass and brome grass have become increasingly difficult to manage in-crop, particularly for growers in the low- to medium-rainfall areas of the southern region.
According to University of Adelaide weeds researcher Dr Gurjeet Gill, there are a number of potential reasons for increasing infestations of brome and barley grass.
These reasons include:
- Early or dry seeding in the low-rainfall zone (LRZ), reducing the effectiveness of pre-sowing weed control
- The dominance of cereals in LRZ with limited control options available
- Potential changing behaviour of brome and barley grass.
The alleged changing behaviour of brome and barley grass has been researched by Dr Gill and his colleagues.
Dr Gill says commonly used crop management practices have selected for greater seed dormancy, which is reflected in delayed weed emergence associated with a response to cold temperatures (Figure 1).
“This has major implications for the effectiveness of pre-seeding knockdown herbicides because only a small proportion of the brome and barley grass population have emerged before seeding and places greater pressure on in-crop control options,” he says.
Laboratory tests have shown exposure of some brome grass populations to prolonged chilling (four days at five degrees Celsius) significantly increased germination (Figure 2).
Dr Gill says this feature restricts germination of such populations to late autumn and winter (May-June) when temperatures drop below 5°C, which is generally after crop emergence.
“Further to this, exposure to light strongly inhibited seed germination in populations from cropping paddocks,” he says.
“This light response is likely to have adaptive value under no-till farming systems – that is future populations can adapt to this environment – because most of the weed seeds remain on the soil surface until burial following the sowing pass and are able to germinate in the dark.”
A further complicating factor in brome grass management and control is its long seedbank persistence which further highlights the need for multi-year management strategies.
In wheat crops, brome grass is capable of producing more than 12,000 seeds per square metre and therefore even small escapes can help to rapidly replenish the seedbank.
“One year of poor control can lead to a blow out in brome grass populations,” Dr Gill says.
“Brome grass seeds can remain viable in the soil for up to three years. More than 20 per cent of brome grass seed remains viable in the field after 12 months, with populations taken from Balaklava, SA, recording 25 per cent viability 12 months after seed set.
“This is likely to be even higher on non-wetting sands. Logic dictates that because there is less germination on non-wetting sands there is therefore a chance of greater seedbank persistence because there is less plant establishment.”
Dr Gill says three consecutive years of good management are required to deplete the brome grass seed bank.
This could include a legume pasture or break crop in year one with the option to crop-top, followed by triazine tolerant canola in year two, again with the option to crop top, Clearfield® wheat in year three and metribuzin and trifluralin in a barley crop in the fourth year.
For Victorian growers, the inclusion of Roundup Ready or RT canola could also be an effective measure for brome grass population management in the canola phase of the rotation.
A similar strategy also applies to barley grass, where Sakura® (pyroxasulfone) is a highly effective option for the wheat phase, Dr Gill says.
“The mechanisms responsible for the change in barley grass behaviour are very similar to brome grass,” Dr Gill says.
“Populations of barley grass from cropping systems need prolonged chilling – just like brome grass – to break seed dormancy.
“This four to five-week chilling period requirement is likely to delay barley grass emergence until well after seeding.”
The seed retention characteristics of both brome and barley grass (Figure 3) make incorporating harvest weed seed management into the strategy difficult, given the first step in harvest weed seed management is getting the seed into the harvester.
According to Dr Gill, barley grass reaches maturity much earlier than the crop, retaining just six per cent of seeds at crop maturity in southern region trials and making it difficult to capture the weed seeds in the harvester.
In comparison, brome grass is slower to reach physiological maturity, retaining 75 per cent of its seeds at crop maturity.
However, trials have shown that between 30 per cent to 80 per cent of brome panicles lodged below 15 centimetres harvest height, indicating the success of getting brome grass into the harvester is extremely variable.
Seedbank management by burning can also be challenging, with 350°C for 60 seconds required to kill barley grass seeds and 400°C for 60 seconds required to kill brome grass seeds, both of which are difficult to achieve on a paddock scale.
“This is why we advise growers to use carefully developed multi-year management strategies to control both brome and barley grass,” Dr Gill says.
“Good crop rotations with multi-year breaks, competitive crops and rotating chemistries are vital aspects of a good management strategy to help reduce the weed seedbank.”
With GRDC investment, the CSIRO has recently developed the Brome RIM tool, based on the Ryegrass Integrated Management (RIM) tool, to help growers evaluate the profitability of long-term management strategies on brome grass.
GRDC research codes: UA00156, UWA00171, UCS00020, UA00158, CSP00186
Dr Gurjeet Gill, 08 8313 7744, firstname.lastname@example.org
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