WeedSmart's Chris Davey looks at crop competition

The WeedSmart Big 6 integrated weed management program starts with maximising crop competition to suppress weed seed production and then adds the other tactics, such as mixing and rotating herbicide modes of action, preventing seed set and harvest weed seed control, to reduce the impact of herbicide resistance.

WeedSmart southern extension agronomist Chris Davey said crop competition was a quiet achiever in weed control that also had a synergistic effect when applied with other tactics.

"When there is a competitive crop growing, the herbicides work better and harvest weed seed control works better," Mr Davey said.

"By suppressing weed numbers, growers can avoid the difficulties that come with herbicide resistance and weeds have less control over crop choices."

There are at least six ways to boost crop competition - increased seeding rate, narrower row spacing, planting east to west, addressing soil constraints, choosing vigorous or competitive varieties and correct sowing time.

"Everyone has seen weeds flourish in a bare patch in-crop, so the quicker the crop can cover the inter-row space the better," Mr Davey said.

"This is where pre-emergent herbicides and varieties with rapid early growth work well together.

"More competitive crops also assist with holding weed seed heads in the canopy ready for destruction with harvest weed seed control tactics."

When it comes to choosing crops and varieties, growers should look for options with vigorous early growth and plant architecture which provide maximum shade of the soil surface and weeds.

Including competitive crops and varieties in a diverse rotation maximises the benefits.

In the cereal phase, barley is widely considered more competitive than wheat, but within the suite of both wheat and barley cultivars, some offer more competitive traits, such as rapid early growth and denser canopy.

Adding a hay crop can also have significant weed control benefits in some farming systems.

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Whereas in the break crop phase, some canola cultivars are more competitive than others, and there is a known benefit of sowing larger seeds.

Pulses have shown they can be quite competitive against weeds, particularly when sown on narrow row spacing.

Mr Davey said other tactics, such as crop-topping, swathing and manuring, could also be implemented through the rotation to drive down the weed seed bank.

"Field pea is a particularly effective brown manure crop to boost soil health and nutrition while stopping weed seed set," he said.

"There are distinct benefits in using consecutive break-crops and cereals to target grass and broadleaf weeds, respectively.

"Two or more years are often needed to fully disrupt the weed ecology."

On its own, variety choice is a very useful tactic, but it can also be implemented alongside other crop competition tactics.

That includes planting on narrow rows, increasing the seeding rate, establishing an even plant stand, sowing at the optimal time and addressing soil constraints.

Growing a competitive crop supports harvest weed seed control (HWSC) as fewer weeds are present and they tend to have weed seed heads held high in the crop canopy.

Mr Davey said all HWSC tactics relied on the weed seed entering the harvester front.

"A well-grown crop can hold the weed seed heads at cutterbar height and the extra bulk can maximise weed seed collection," he said.

"Once in the harvester, all HWSC tactics provide similar benefits in destroying weed seed.

"Some growers implement HWSC intermittently, such as narrow windrow burning canola but not cereals, while others use tactics such as chaff-lining or seed mills in every crop every year."