Two years of disastrous rainfall in some Australian grain growing regions have focused researchers’ minds on how best to extract maximum yields during wet seasons.

Flooding and waterlogging in eastern Australia had a catastrophic effect on some areas, with many crops destroyed or yields greatly reduced.

Southern Farming Systems (SFS) researcher Greta Duff told a recent GRDC Grains Research Update that it was critical to understand the crop growth stage during waterlogging to determine potential yield impacts.

“Under waterlogged conditions, nitrogen is lost from soils through denitrification and leaching. During this period, plants also have a limited ability to uptake nutrients,” Ms Duff said.

Waterlogging issues and drainage solutions were a high priority for farming systems in the high-rainfall zones (HRZ) of Victoria and Tasmania, she said. “The HRZ of Victoria and Tasmania is particularly prone to waterlogging conditions, with high rainfall and sodic (dispersive) soils.”

Southern Farming Systems researcher Greta Duff speaks at the Bendigo GRDC Update. Photo: Andrew Cooke

Following higher-than-average rainfall in these areas in 2021, particularly through June, July  and August, a series of trials was established in Victoria – at Hamilton, Streatham and Vite Vite North – and Tasmania to look at the effects of different crop nutrition regimes post-waterlogging on crop yields.

“In 2021, we saw the classic symptoms of yellowing and reduced growth and vigour, which is indicative of waterlogging. But it is very hard to set up a trial for waterlogging, as you don’t know what is going to happen (climatically) year-on-year,” she said.

For the trials, different treatments of urea, sulfate of ammonia, urea ammonia nitrate and trace elements were applied across the sites.

In the waterlogged canola trial site at Streatham, soil moisture probes showed 90 per cent plant-available water in July and 97 per cent plant-available water in August, “which is a good indication that those plants were struggling”.

Different rates of urea fertiliser were based on soil tests and expected crop yield calculations.

At Vite Vite North, the trial comprised waterlogged faba beans. Moisture probes showed 95 per cent plant-available water from mid-June to mid-July and up to 100 per cent in August.

Urea was applied at 10 per cent flowering, and researchers noted standard waterlogging symptoms in pulses of reduced vigour and yellowing.  “Applying urea to faba beans is not standard practice, so we used 60-kilogram-per-hectare increments just to see if we could detect any differences in outcomes,” Ms Duff said.

Waterlogged wheat at Hamilton showed obvious symptoms of yellowing, along with 97 per cent plant-available water in July and 100 per cent in August. Treatments included full (145kg/ha) and reduced rates of fertiliser nitrogen at different timings and rates.

At Hagley, Tasmania, waterlogged wheat was in a low-lying area of a grower’s paddock. Nitrogen was applied at growth stage 32, GS39 and GS45; sulfate of ammonia was applied at GS33 and trace elements were applied at GS39. Treatments were at full (180kg of nitrogen/ha) and reduced (100kg/ha) rates.

Hagley grain yields

Ms Duff said the trial results showed that crop nutrition post-waterlogging did not influence grain yield, except in the trial at Hagley, Tasmania, where yields increased with higher rates of applied nitrogen.

“At Streatham, we think that the timing of waterlogging might have reduced negative impacts on grain yield despite the visual differences in plants after the waterlogging occurred. Although the crops looked patchy, perhaps what was happening underneath the soil was a different story in terms of how stressed the plants were, and nitrogen and oxygen availability.

“At Vite Vite North, there were no significant differences between treatments, and in fact the average yield was eight tonnes/ha, which again shows that the timing and severity of waterlogging will influence the yield response.

“In Hamilton, average grain yields were 5.5t/ha across all treatments, and grain protein was 13 per cent on average.

“The results at Hagley were statistically significant, which was exciting. The average yield was 9.4t/ha, with the nil treatment being 2t/ha lower than the nitrogen-based treatments.

“The urea plus sulfate of ammonia and trace elements treatment was the highest-yielding, at 10.2t/ha. But we also found that yield did not increase consistently with higher rates of nitrogen, but grain protein did.

“However, the total amount of nitrogen applied was a bigger influence on grain yield than the treatment (rates and times) itself.”

Ms Duff said the aim of the trials was to provide growers with knowledge and resources to help salvage their paddocks after waterlogging occurred. “Growers want a solution to waterlogging and a starting point from which to salvage their crops after it occurs.”

Severe effects on young seedlings

She said waterlogging close to sowing affected germinating seeds and young seedlings and, because these plants did not have well-established root systems, the effects could be severe.

“If a soil is waterlogged during June–July in south-western Victoria or Tasmania and the crop is well-established, final yields may not be severely impacted, as soils are cold, the demand for oxygen is low and plant growth is slow.

“Established plants will be most affected when they are rapidly growing. As such, prolonged waterlogging during the warmer spring period is when yield penalties may be most severe.

“It is important to note that waterlogging can occur even when water is not pooled on the surface. As long as it’s happening in the plant root zone, then that plant is under anaerobic stress and is struggling to diffuse its oxygen and nutrients.”

There is unfortunately no silver bullet for solving waterlogging.

However, waterlogging is highly variable in terms of its effects on individual paddocks and farms. “We saw in 2021 that waterlogging occurred through the winter period in the HRZ, but in 2022 it occurred from September through to November. That would have affected crops and yields differently in 2022 than in 2021.

“There is unfortunately no silver bullet for solving waterlogging. But there are certainly tools in the toolkit that can be utilised. The project showed that in the Hagley trial applying nitrogen was a benefit, but there are also other things that growers can look at, such as drainage and crop rotation.”

GRDC has made significant investments to help growers reduce the impact of waterlogging. Over the past few years, it has invested more than $2 million in a suite of projects to help find solutions including those related to subsurface water management and into the future of plants that are better adapted to waterlogged conditions.

More information: Greta Duff, gduff@sfs.org.au