What makes harvest weed seed control more efficient?

with Michael Walsh, UWA Senior Research Fellow, Australian Herbicide Resistance Initiative

Keeping paddocks weed-free is far easier and cheaper than doing battle with high density weed populations. Harvest weed seed control (HWSC) measures have proven their worth in reducing the amount of seed that returns to the seed bank each year and can quickly reverse a weed-density trend.

AHRI researcher Dr Michael Walsh specialises in studying the effectiveness of HWSC tactics such as chaff carts, narrow windrow burning, Harrington Seed Destructor and the like. He says all these tactics can achieve very similar results and the choice of tactic used will depend on factors such as crop type, location and markets.

Dr Michael Walsh, UWA Senior Research Fellow, Australian Herbicide Resistance Initiative says growing the most competitive crop possible will increase the efficiency of your chosen harvest weed seed control method.

“Something that we have looked at recently is the effect of crop competition on practical aspects of implementing HWSC,” says Michael. “In a nutshell, increased crop competition via crop density forces weeds to grow taller as they compete for light.”

“The result is that shade intolerant annual weeds produce less seed and the seed is located higher in the crop canopy where they are more easily captured using HWSC tactics.”

Michael says this GRDC supported research adds more weight to the benefits of implementing crop competition as a weed management strategy to achieve higher yields, support herbicide applications and ensure HWSC measures are as effective as possible.

This may mean that growers who effectively use crop competition to support their chosen HWSC tactic can potentially lift the harvest height a little and still be confident that they are capturing more than 90 per cent of the weed seed present at harvest.

In highly competitive crops annual weeds are forced to grow taller and tend to produce less seed.

What effect does crop competition have on weed plant structure?

Short answer: Weeds tend to grow taller.

Longer answer: Where crop biomass is low and there is a sparse, open crop canopy annual ryegrass populations tend to adopt a prostrate growth habit where seed is retained at low heights and more difficult to capture at harvest. In high biomass crops the combination of reduced seed production and elevated seed retention greatly reduces the amount of annual ryegrass, wild radish, brome grass and wild oats seed that potentially avoids HWSC.

How do seasonal conditions affect HWSC efficiency?

Short answer: In low rainfall years, it is important to harvest as low as possible.

Longer answer: In years that do not support robust crop growth the majority of annual ryegrass seedheads are likely to be located below a 20 cm harvest height. To capture more of this seed at harvest will require a lower harvest height. In more favourable years, over 75 per cent of the ryegrass seedheads are likely to be above 30 cm.

Can crop density influence weed seed production?

Short answer: Yes, weeds growing in high density crops produce less seed.

Longer answer: In a pot trial, wheat sown at the highest density (400 plants/m2) reduced seed production in annual ryegrass, wild radish, brome grass and wild oats seed production by 80, 93, 97and 96% respectively. Even at a crop density of 60 plants/m2 weed seed production was reduced by more than 50 per cent.

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