Cutting low lifts harvest weed-seed control

Research across 25 sites in Western Australia supports the theory that harvesting stubble at the lowest practical level increases weed-seed collection

Photo of Dr Michael Walsh

University of Sydney's Dr Michael Walsh says there needs to be a balanced, risk-aware approach to slowing down the harvester to cut at a low level.

PHOTO: Evan Collis

New research has backed up the theory that harvesting crops as low as possible gives the greatest harvest weed-seed control, particularly when it comes to ryegrass.

Sampling in 25 wheat crops across the Western Australian grainbelt demonstrated a clear correlation between harvest height and ryegrass weed-seed collection. The results from this sampling determined that a theoretical zero-centimetre harvest height would capture the maximum number of ryegrass seeds, although University of Sydney director of weed research Dr Michael Walsh acknowledges that it is not realistic for harvesters to be set at that level.

“But this research clearly demonstrates the best result achieved in terms of harvest weed-seed control is at the lowest practical cutting height,” Dr Walsh says.

In each sampled quadrat area, wheat and ryegrass plants were harvested at 40cm, 30cm, 20cm, 10cm and 0cm. These samples were then processed to determine the distribution of plant biomass and ryegrass seed through the crop canopy.

Dr Walsh says the results showed that 70 per cent of ryegrass seed production would have been collected when the crop was harvested at 10cm, compared to 24 per cent at 40cm.

Contrary to some lines of thought, he says, reducing the harvest height from 30cm to 10cm only increases the crop biomass (straw) collection by 14 per cent.

“So when you do cut lower, you are not dramatically increasing biomass collection, as previously thought, yet the increase in weed-seed collection is significant.”

However, he says that the dilemma for many growers considering harvest weed-seed control is that the need to harvest lower to maximise weed-seed collection may reduce the speed of harvest. A longer harvest period increases the risk of harvest rainfall events and lost grain quality. This risk is particularly high in summer-dominant rainfall areas. 

But Dr Walsh believes the study provides a clearer picture of the potential impact on harvest operations, allowing weed-seed control to be planned accordingly.

“Clearly there needs to be a balanced approach towards slowing down the harvester to cut at this low level, and harvesting at a speed to reduce the risk of grain quality problems if harvest is extended,” he says.

“It may be that growers apply this research to some paddocks or parts of their paddocks, but not to others,” he says.

Dr Walsh says harvest weed-seed control is now an established practice. “We now know that implementing harvest weed-seed control practices are critical when it comes to managing the weed seedbank. If you control the seedbank you will control the weed.”

More information:

Dr Michael Walsh,
02 6799 2201,


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