Retaining sufficient ground cover to improve soil moisture retention is the focus of cover cropping trials in Central Queensland.

Growers in this region regularly encounter situations that result in inadequate ground cover, including after tillage and low stubble residue crops. This reduces water infiltration rates and soil moisture profiles, with consequent impacts on productivity and yield potential.

GRDC-supported research led by the Queensland Department of Agriculture and Fisheries (DAF) began in 2023 and is exploring  options for maintaining and increasing ground cover.

Project leader Max Quinlivan says the primary aim is to work with Central Queensland growers to identify and implement new strategies that increase the amount of ground cover in the cropping system. This includes strategies to retain ground cover or regain it when it is lost.

Researchers have been consulting with local growers. The initial meetings took place in March 2023. Mr Quinlivan says the meetings determined growers’ most pressing research questions, with the consensus resulting in cover cropping trials that began in spring 2023.

Grower-driven trials

Guided by growers’ questions, the first round of trials is exploring whether a cover crop planted on a partial soil moisture profile (not enough to produce a yielding crop), and then sprayed out before grain fill, will recover soil moisture in the remaining fallow, and the effect of this on following crops.

A multi-species cover crop strip next to a bare soil control at Emerald Queensland. Photo: Supplied by DAF

The biggest questions asked by growers were: ‘Do you get the water back that you use to grow a cover crop’ and ‘what is the overall cost-benefit?’

Some of the cover crops being trialled include sorghum and millet to grow residual straw, and multi-species cover crop mixes (including legumes, grasses, oilseeds and herbage) to explore potential soil health benefits.

One sorghum trial was sown with a spreader and lightly incorporated to be a low-labour option. Other trials were drilled to replicate a more-expensive but reliable planting operation.

“Harvested sorghum seed planted with a seed spreader could be a cheap and more-reactive option for growers, as it is low-cost and low-effort. Millet and multi-species cover crops would have to be more premeditated and require a little more investment as most growers up here don’t have those seeds in their silos,” he says.

Another scenario the project is exploring is cover crops that can be grown after chickpeas and mungbeans. These crops leave very little ground cover, so would be an ideal situation post-harvest to grow a cover crop. However, Mr Quinlivan warns the challenge is residual herbicide, especially after chickpeas. Residuals can impact on grass species that could be used as a cover crop.

Narrow-spaced sorghum

One of the biggest problems faced by growers, Mr Quinlivan says, is worsening herbicide resistance, forcing growers to plough weeds in. This removes ground cover and causes run-off, especially during heavy rain. The region’s heavy clay soils further exacerbate the run-off issue.

The project is also exploring narrow-spaced sorghum to reduce in-crop opportunities for weeds such as feathertop Rhodes grass. “The current recommended row spacing is one metre to 1.5m, and we’re currently trialling spacings down to 37 to 38 centimetres.”

Mr Quinlivan says the research team expects this will likely result in some yield loss, but along with investigating the extent of this yield impact, researchers are also interested in how the improved crop competition affects weeds and whether it reduces the need for ploughing after a sorghum crop.

“Tillage is an important option in a grower’s toolbox; however, there are so many seasons and situations in Central Queensland where a tillage operation costs a grower the next crop,” he says. “So, we’re trialling proactive options growers could consider to increase ground cover and get land back into production sooner.”

Looking forward

The project is building on DAF and GRDC-supported research in southern Queensland, with a focus on finding solutions for the Central Queensland cropping system. Mr Quinlivan notes one major difference between the two regions is that southern Queensland has long fallow periods; however, Central Queensland is a more opportunistic system, with shorter fallows and sowing opportunities potentially up to 10 months of the year, depending on rainfall.

“I’m looking forward to getting good local information on whether cover cropping can fit an opportunistic cropping system,” he says. “There’s a lot of science that says we will improve infiltration rates in our fallows if we can get more cover, but we don’t have any local information about water use versus water recovery.

“This project will find out whether this is something that can add a fantastic little bump in productivity and profitability for local farmers or, if it doesn’t work, we’ll know for certain so we can park this approach and explore something else.”

With the cover crops having been  trialled over summer, Mr Quinlivan says the plan is to follow with a wheat crop to observe the impacts of cover crops on the following cash crop.

The project is scheduled to run until November 2025. Mr Quinlivan says this will provide an opportunity to see the cover crop impact on the second cash crop and observe different seasonal conditions.

More information: Max Quinlivan, max.quinlivan@daf.qld.gov.au