Tackling the brome grass seedbank

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Fast fact

Brome grass treatment cut high (35cm) without windrows

Brome grass trial treatment cut high at 35 centimetres without windrows.

Brome grass treatment cut low (10cm) without windrows

Brome grass trial treatment cut low (10cm) without windrows.

Brome grass treatment cut low (10cm) and windrowed

Brome grass trial treatment cut low (10cm) and windrowed.

Narrow windrow burning is significantly more effective at reducing the brome grass weed seedbank when harvested at 10-centimetre height than at 15cm.

Great brome grass (Bromus diandrus) is emerging as a major weed in parts of southern Australia, including Western Australia, South Australia and parts of Victoria, NSW and Tasmania. This is due to reduced pasture, increased cropping intensity, zero-tillage systems, resistance development and the lack of in-crop herbicides for cereals.

A 2016 GRDC study into the impact of weeds found that brome grass was the fourth most important weed after ryegrass, wild oats and wild radish, and was responsible for revenue losses of about $22.5 million.

Reducing the weed seedbank by using weed seed removal techniques at crop harvest is an important management technique for herbicide-resistant weeds. But unlike wild radish and ryegrass, little is known about the effectiveness of seed removal on brome grass.

Burning

Narrow windrow burning is commonly used to reduce ryegrass seedbanks. It involves concentrating harvest residues and weed seed into a narrow windrow where increased fuel load extends the period of high temperatures, improving the kill of weed seeds. Yet little is known about the effectiveness of harvest weed seed management on brome grass.

A Department of Agriculture and Food, WA (DAFWA) trial in wheat at Grass Valley (Northam) found that narrow windrow burning was more effective at removing great brome grass seed when the harvest height was set at 10 centimetres rather than 15cm. This is because brome grass is shorter than mature wheat plants and some brome heads tend to bend down below the typical harvest height of 15cm.

Prior to harvest in 2014 the total number of mature brome grass seeds was estimated to be 9297 seeds per square metre. Of these, 9204 seeds/m2 were determined to be viable. Wheat was harvested with a commercial header to either 10cm or 15cm height and the crop residue was either spread across the plot or windrowed.

Before and after burning, soil and crop residue samples were collected for each treatment and transferred to a tray. Great brome grass seedlings were counted and removed every two to four weeks from April 2015 to August 2016, with the total number of seedlings providing a measure of the number of viable seeds per square metre.

Harvest low

Graphic showing the proportion of viable brome grass seed collected in windrow or destroyed window burning is greater at 10cm harvesting height

Figure 1 The proportion of viable brome grass seed collected in the windrow and destroyed in windrow burning is substantially greater when harvesting at the 10cm height.

When the crop was harvested at 15cm, 3662 viable brome grass seeds/m2 (40 per cent of total viable seed) were collected into narrow windrows. After burning, this was reduced to 313 viable seeds/m2 (3.4 per cent of total viable seed).

When cutting the wheat crop at 10cm high, 6794 seeds/m2 (74 per cent of total viable seed) were collected into windrows and this was reduced to 432 viable seeds/m2 (4.7 per cent of total viable seed) after burning. Harvesting at 10cm also meant a greater proportion of the seed was captured in the windrow, rather than landing on the ground outside the windrow.

The bottom line is that burning the narrow windrows cleaned up 39 per cent of viable seed in the windrow at the 15cm harvest height and 69 per cent of viable seed at the 10cm height (see Brome grass trial treatment photos, right).

More information:

Dr Abul Hashem,
08 9690 2136,
abul.hashem@agric.wa.gov.au

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Low rainfall grass weeds need whole-of-system strategy

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Beat down annual ryegrass – not your profit

GRDC Project Code UA00149

Region West