With skyrocketing nitrogen (N) prices and a shortage of supply, results from an irrigated maize trial series have delivered good news to growers looking to produce bumper yields while saving on fertiliser costs.

The trial is part of a broader GRDC project to support growers to benchmark and test key learnings from irrigation research. A total of eight action learning groups located from the southern Murray Darling Basin, south-east South Australia, and the South Australian/Victorian Mallee are involved in the project.

GRDC Acting Senior Manager Soils, Nutrition, Agronomy and Farming Systems, Dr Kaara Klepper says the hands-on project was designed to support growers in pushing the boundaries of best practice in irrigated grain production through soil amelioration, agronomic practices and improved decision making.

“We wanted to support growers to validate test results coming from other GRDC irrigation research investments so they could maximise the dollar return on every megalitre of water they applied to crops,” Dr Klepper says.

As part of this project, Field Applied Research (FAR) Australia and the Irrigated Cropping Council conducted trials on irrigated maize across two growing seasons, applying varying rates of N to crops, ranging from 230 to 545 kg/ha, and monitoring what rates produced the highest yields.

FAR Australia Managing Director, Nick Poole, presenting on maize nitrogen trials at a grower field day. Image: GRDC

FAR Australia’s Managing Director, Nick Poole, who leads the agronomy and soil amelioration component of the project, says the purpose of the trial was to look at the importance of the naturally occurring N in the soil, its role in producing high yields and whether optimal yields could be achieved while saving on fertiliser costs. “The results were positive for growers because statistically, across two growing seasons, we weren't able to identify results that showed crops responded to more than 250kg/ha of artificially applied N,” he says.

“Applications of N in excess of 250kg/ha were proven to be largely uneconomic during the season, losing $400/ha depending on the price of N and the exact rates applied.”

Maize grower Daniel Coulthard and his family hosted one of the trial sites on their pivot irrigation system at “Blue Vale”, Peechelba East, northern Victoria, over the 2019/20 and 2020/21 growing seasons.

Mr Coulthard says when they first started looking at fertiliser efficiency to achieve higher yields five years ago, they planted their maize crop using 500kg/ha of urea (230 kg N/ha) and produced their highest yields ever.

After that, Mr Coulthard says to capitalise on the rising price of maize, they continued to increase their urea rates to chase higher yields, applying up to 900kg/ha (414 kg N/ha).

“We didn’t see a significant response from the crop to those higher rates at all, which meant we didn’t get a good return from our inputs,” he says.

During the recent two year trial period, the Coulthards’ maize crops averaged 17 tonnes per hectare with no statistical yield benefit observed from artificial N rates of above 250kg/ha. The site’s highest grain yields were 18 to19 t/ha, which were produced after applications of approximately 250kg N per ha.

Mr Poole says these results were similar across the other trials conducted, showing crops that had N applied at 250kg/ha were producing aspirational yields of above 16t/ha, which was statistically no less than what crops with higher N rates produced.

While the study showed crops were unable to utilise more than 250kg/ha of artificially applied fertiliser, Mr Poole says crops that produced aspirational yields of between 16-19 tonnes/ha commonly removed 400kg/ha of N from the soil.

“This high uptake of N shows the importance of having healthy soils in farming systems, as the presence of organic matter can increase soil’s natural N levels,” he says.

“The trial showed that fertile farming systems, where growers are ensuring they balance their budgets with residues to go back into the soil or including N producing crops like pulses and grain legumes into their rotations, they’re building up the organic matter needed to support high yielding crops.”

While the crops had taken up 400 kg N/ha at harvest, Mr Poole says assessments of crop residue during the trial series showed that approximately one third of N (130kg N/ha) was returned to the soil organic matter pool, with the remainder removed in the grain.

Mr Coulthard says results from GRDC research showed the importance of their mixed approach to soil health, which consisted of artificial fertiliser application as well as applying natural fertilisers such as pig and chicken manure and introducing N rich crops to their rotation like green manure crops.

“Having the trial on our property was beneficial because it gave us confidence in what we do and showed how our inputs and farming system can work across growing seasons to achieve high yields,” he says.

“We’ve been working on implementing methods to get our soil healthy prior to sowing so it can further support our fertiliser applications.”

Dr Klepper says these types of findings confirm using artificial fertiliser alone to achieve high yields is uneconomical and while these systems are running at a net negative N balance, growers need to continue to be vigilant in adopting practices that replenish these N supplies.

Riverine Plains is running an Irrigation Discussion Group alongside these trials to help farmers adopt new practices to optimise irrigated crop yields. To get involved, contact Kate Coffey, kate@riverineplains.org.au. To find out more about the other discussion groups, visit the ICC website.