Pig waste found best left to lie

School of Agriculture, Food and Wine research officers Chris Penfold and Jake Howie led the three-year trial, which began in 2017.

Mr Penfold had previously undertaken trials on cost-effectively using biosolids and compost, which had inspired the idea of looking into pig waste - a manure readily available from the Roseworthy Campus piggery.

"The benefits of using animal manures and composts in farming systems have been established in numerous trials in Australia and overseas, with gains in yield coupled with improved soil quality over time," he said.

"In the past, I had looked more into increasing the nutrient content of the manure to justify its transport.

"For this study, I wanted to look further into better utilisation of those manures once they were applied to the ground.

"There had been successful trials into chicken manure and injecting them into the soil profile.

"But that can be capitally expensive and even slow, so it made me wonder whether farmers needed to go to that effort.

"And whether there was a loss of nitrogen when broadcast manure was left on the soil surface prior to seeding."

The trial compared three control strips of no fertiliser to single strips with treatments of manure at 5 tonnes a hectare, 10t/ha, 20t/ha and 50t/ha using a commercial spreader in early May 2017; the 'conventional' application of DAP at 80 kilograms/ha at seeding and urea at 60kg/ha in mid-August.

Three treatments were then overlaid in replicated strips, with the manure incorporated with a mouldboard plough to a depth of 16 centimetres (above the heavy clay subsoil); with a conventional cultivator to 10cm and just left on the surface.

Compass barley was then direct drilled across the site in early June at 80kg/ha.

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In the first year, 5t/ha (1650kg dry weight) of manure produced an extra yield of 1.07t/ha or 214kg/t of manure applied.

Higher rates of manure application however received lower rates of return (68.7kg, 65.4kg and 38.4kg of grain per tonne of manure for 10t/ha, 20t/ha and 50t/ha), with 20t/ha required to achieve a similar yield to the conventional fertiliser treatment.

Grain protein was higher in the 50t/ha manure and DAP plus urea treatments (10.5pc) compared with the others (average 7.7pc).

In 2018, lentils were sown, with only the conventional plot receiving fertiliser, but the trial was not harvested due to the poor season.

To determine any residual value of the manure, Compass barley was again sown in 2019, with only the conventional plot receiving 50kg/ha of DAP.

No urea was applied due to a forecast dry spring.

In 2019, despite no additional fertiliser being applied to the manure treatments, yields still increased by an average 35kg/t of manure applied per hectare (or 350kg/ha for the 10t/ha application rate).

Mr Penfold said this suggested the manure provided an ongoing benefit, even though yields were constrained by the harsh season.

More interestingly, the research found no yield benefit in either method of incorporation compared with leaving the manure on top.

"In fact in 2017, direct drilling resulted in a slightly higher yield than conventional cultivation and mouldboard plough (71kg/ha and 118kg/ha respectively)," Mr Penfold said.

"The results in 2019 remained consistent with this pattern with direct drill having a marginal benefit of 68kg/ha over incorporation.

"Therefore, the anticipated nutrient loss through volatilisation from the soil surface was insignificant and is an important finding given our desire to utilise nutrients as efficiently as possible."

Mr Penfold believes manures are "underrated" in the general knowledge about their benefits.

"Manures provide more than just nitrogen and phosphorous," he said.

"They also provide trace elements and organic matter, which many people don't account for."