The combination of weeds and water repellent soils is fast becoming one of the biggest crop yield constraints in WA's central and upper great southern region.

For Wagin growers Wade and Holly Brockway, water repellence is a major inhibitor to crop establishment, particularly on their sandy duplex soils.

And without good early crop establishment, Wade says, there is limited crop competition against weeds - particularly after a late break to the season.

"We need to overcome the water repellence challenge to manage weeds early in the growing season," he says.

Crop competition with weeds

In these water-repellent soils, late-germinating weeds can also be a problem in short crops that have not established well - meaning the crop cannot out-compete the weeds at any stage throughout the season.

"It's a perfect storm unfortunately, and so we are trying a few different techniques to overcome this major constraint," Wade says.

The Brockways have only just started to test the value of soil amelioration strategies and invested in a modified one-way disc plough, often known as a Plozza plough. Early visual results, compared to untreated control strips, are looking very promising.

After a very dry summer (2018-19) - and a late break to the 2019 season - water repellence was particularly obvious on many duplex soils in the region. Wade says the oats planted into 160 hectares of his ploughed soils looked robust from germination and weeds were never able to take hold.

Plozza plough an easy call

While he considered using a mouldboard plough, the smaller investment plus the simplicity of the Plozza plough made it an easy decision.

"For a total investment of $8000, we are seeing some really impressive visual results after just one season," Wade says.

Early in the 2019 season, the Brockways ploughed one paddock after 15 millimetres of rain. But, with no significant rain events for several months after, there were erosion issues in that paddock.

They also ploughed a paddock at seeding time, planting directly afterwards, seeing significantly more positive results.

"That was an early lesson for us," Wade says.

"In 2020, we will be ploughing and then immediately seeding the paddock. Or we will even wait until the bulk of our seeding program is over and we will then plough so we don't see our paddock blow away."

An older Case Steiger 320-horsepower four-wheel-drive tractor has been given a new lease of life and is now used exclusively to pull the Plozza plough, often through some soft sandy paddocks.

After ploughing, the seedbed is flattened with a steel roller in a separate pass to allow for more accurate seed placement.

Lime, phosphorus, potassium and other trace elements have been applied before the ploughing process and incorporated to improve the nutrient content and pH of the soil profile.

Wade says the crop looked better than the control strips all season in 2019 and, while the ploughing did not bury all the weed seeds, there was a distinct difference in weed germination in the ploughed areas.

"Looking at the soil profile after the plough has been through, we can see the topsoil in a 45-degree line, down to 350mm," he says.

"This means that while some of the weed seeds are buried, it is not a complete inversion - as you might see with a mouldboard plough."

The Brockways are also using banded soil wetters on some paddocks to improve early germination.

But while this appears to help in some years, Wade says, there has not been an obvious biomass response later in the growing season.

Advice on tactics

The Brockways' agronomist, Wade Longmuir, from Elders Wagin, says many growers throughout the central and upper great southern region have expressed concern about water repellence in their duplex soils.

"Growers are telling me that it's a challenge to attack the weeds in these soil types because it's hard to know when throughout the season to apply the herbicide," he says.

"Any early knockdown will miss the later germinating weeds and a later application may mean the earlier established weeds have already out-performed the crop."

Mr Longmuir says crops on the ploughed areas on the Brockways' property will almost certainly out-yield any other crops in the rotation.

"After what I have seen all season, not just in terms of the health of the crops and the increase in biomass, but also in regard to a reduced weed load, I'm confident these oats will yield incredibly well," he says.

Growers are telling me that it's a challenge to attack the weeds in these soil types because it's hard to know when throughout the season to apply the herbicide.
Elders agronomist Wade Longmuir
Mr Longmuir says many growers in the southern regions of the WA grainbelt, such as the Brockways, are now in a maintenance phase in regard to liming and so incorporating this lime could prove particularly beneficial for soil health and crop establishment.

"One of the big challenges of this region is the huge amount of different soil types every grower has to deal with, even within the one paddock," he says.

"So there is no one-size-fits-all approach unfortunately."

Almost one-third of the soils on the Brockways' property are water repellent, so the ploughing program is expected to continue for many years.

"It's a slow process, but we hope to do around 200ha every year to overcome this constraint," Wade says.

Water repellence a dynamic constraint

Western Australian Department of Primary Industries and Regional Development (DPIRD) soil scientist, Dr Stephen Davies, says water repellence is a dynamic constraint and its severity and expression can change throughout the season - and from season to season - in response to changing climatic and soil conditions.

"Soil water repellence will become more pronounced at different times throughout the year in response to changing temperature and soil moisture," Dr Davies says.

"While we don't have firm evidence to say that water repellence in the soil is increasing, we do believe seasonal variations and farming system impacts can influence the expression of the constraint."

Dr Davies says no-till farming systems, later and less reliable breaks to the season, and the increase of dry seeding practices could all exacerbate the expression of water-repellent soils.

"Since we have been using no-till strategies for a long time now, there is a build-up of organic material in the surface layer of the soil, which includes a layer of hydrophobic organic compounds that cause water repellence," Dr Davies says.

"Our seeding systems, particularly the use of typical knife-points which allow repellent topsoil to fall into the furrow with the seed, combined with later, drier breaks to the season can also aggravate the problem."

Panel members on tour

In September 2019, GRDC panel members visited the southern grainbelt region as part of their annual spring tour - with water repellence identified as a major constraint for many growers in the region.

GRDC Western Panel chair Darrin Lee says more research will be undertaken in coming years to help growers understand the best strategy to overcome water-repellent duplex soils.

"The spring tour was an opportunity for all panel members to better understand the yield constraints facing growers in this region, plus their research priorities going forward," Mr Lee says.

He says growers interested in the future direction of GRDC investment can visit the GRDC website to look at details of Key Investment Targets developed as part of the organisation's five-year RD&E Plan launched last year.

The 2019 spring tour also visited the Geraldton port zone.

More information: Dr Stephen Davies, 08 9956 8515.