Research tackles brome's changing nature

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The University of Adelaide’s Dr Sam Kleemann says crop rotations that include break crops of lupins and triazine-tolerant canola can achieve a reduction of up to 96 per cent in the brome grass seedbank.

PHOTO: Alistair Lawson

Brome grass has long been a problem for many growers across the southern region, particularly those in low-rainfall areas. With the advent of no-till farming and continuous cropping, particularly wheat/wheat rotations, brome grass has become a bigger control headache.

According to the GRDC’s Impact of weeds on Australian grain production report, brome grass is said to cause $21 million in lost revenue to southern region growers and is the third-ranked residual (in-crop) weed behind annual ryegrass and wild oats.

Figure 1 Differences in germination and seedling emergence between in-crop and fenceline populations of brome grass collected from the same farm at Warnertwon, SA.

SOURCE: Sam Kleemann

As part of a GRDC investment, the University of Adelaide has been researching brome grass behaviour and management. This research has shown that there are higher levels of seed dormancy in brome grass populations collected from cropping paddocks than fencelines or roadsides (see Figure 1).

According to University of Adelaide research scientist Dr Sam Kleemann, these results clearly indicate that management practices used by growers to control brome grass have caused a shift in weed population behaviour.

Seed dormancy

“This increase in seed dormancy has been caused by selection for individuals in these populations that possess greater seed dormancy to escape pre-sowing weed-control tactics such as tillage or knockdown herbicides,” he says.

“The process of selection for increased seed dormancy would be similar but slower than selection for herbicide resistance.

“Over time, weed management in cropping paddocks would select for biotypes that possess higher dormancy and select against or remove those with low dormancy.”

Dr Kleemann says the reasons for this dormancy are two-fold.

First, research has shown that the addition of gibberellic acid overcomes the dormancy and makes the seed germinate. Dormant populations of brome grass seed have been shown to germinate after exposure to low temperatures of about 5ºC. This chilling process has been shown to increase gibberellic acid production within the seed.

“In the field, this means that the dormant brome grass requires not only moisture, but a period of colder temperatures to germinate,” Dr Kleemann says. “Therefore, larger germinations of brome would not be expected until cooler, moist conditions in late autumn/early winter, allowing it to evade early control tactics and emerge in crops.”

Dr Kleemann says another biological mechanism that appears to be contributing to delayed emergence is the strong inhibitory effect of light on seed germination.

“Strong photo-inhibition is likely to aid brome survival in the field by enabling seeds to remain ungerminated on the soil surface until sowing of the crop, thus preventing seedlings from being killed by seedbed preparation,” he says.

“This feature of brome grass ecology also goes some way to explaining why it has proliferated under no-till, where seeds remain on the soil surface until being buried by the sowing pass, removing the inhibitory effect of light.”

Break crops

In order to tackle late-germinating brome grass, University of Adelaide research conducted at Balaklava, South Australia, has shown that crop rotations which included break crops of lupins and triazine-tolerant canola achieved a reduction of up to 96 per cent in the brome grass seedbank.

Clearfield® cereal varieties have also been effective in helping to control brome grass, with rotations of Clearfield® wheat followed by TT canola achieving a 93 per cent reduction in the brome grass seedbank.

“The effectiveness of combinations of pre and post-sowing herbicides, plus seed-set control tactics, used in these rotations were able to deplete the seedbank from about 600 seeds per square metre to manageable levels of less than 30 seeds/m2 in three years,” Dr Kleemann says.

“The use of Weedmaster® DST® (glyphosate) at windrowing or desiccation also provides an opportunity for seed-set control in canola.

“However, spray-topping with paraquat or glyphosate will only have limited benefit on late germinations and growers should ensure they also use effective pre-emergent and post-emergent grass-selective herbicides in these phases.”

GRDC Research Codes UA00060, UA00149, UCS00020, UQ00080

More information:

Dr Sam Kleemann
samuel.kleemann@adelaide.edu.au
0418 256 475