Trial results back crop rotation

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The trial also showed that retaining residue in a continuous cereal rotation significantly increased grain protein levels in the long term.
Camera IconThe trial also showed that retaining residue in a continuous cereal rotation significantly increased grain protein levels in the long term. Credit: WAFarmers

Results from a trial in WA’s central grainbelt suggest the highest sustainable wheat crop yields and grain protein levels will be achieved from employing a diverse crop rotation — comprising wheat, a legume and canola — when compared with other rotations tested at the site.

The trial also showed that retaining residue in a continuous cereal rotation significantly increased grain protein levels in the long term.

CSIRO researcher Phil Ward spoke about the topic at the GRDC grain research updates in Perth this week, summarising a trial investigating the impacts of rotation diversity and residue handling on wheat yield and protein content.

The trial was held with GRDC investment on red loamy soil at Cunderdin, where a long-term rotation trial was first established in 2007. The research involved WA No-Tillage Farmers Association, UWA and CSIRO.

It compared four rotations: continuous wheat, continuous cereals of wheat, wheat, barley, district practice of wheat, wheat, legume, and wheat, wheat, fallow), and diverse (wheat, legume, canola).

Starting with the 2010 harvest, sub-plot treatments were established, comprising areas where residue was retained and spread behind the harvester, or windrowed and burned before sowing the next crop.

Nitrogen was applied to each crop in response to seasonal conditions and expected nitrogen requirements. No extra nitrogen was applied to legumes or fallows.

Dr Ward said the diverse rotation increased wheat yields by about 0.2t/ha and wheat grain protein by 0.9-1.6t/ha.

Differences between wheat yields in the various rotations were initially small for the first few years of the trial.

“However, over the long-term, the diverse rotation – in which the frequency of cereal crops was 33 per cent – increased wheat yields by about 0.2 tonnes per hectare and wheat grain protein content by 0.9 to 1.6 per cent, when compared with continuous cereal or continuous wheat rotations,” Dr Ward said.

“While the causes of the yield and protein benefits are associated with including grain legumes in the rotation, there may be other benefits to do with soil nitrogen cycling that have not yet been quantified.”

Dr Ward said the district practice rotation – in which the frequency of cereal crops was 67 per cent – also resulted in higher grain yields (by about 0.4t/ha) and higher grain protein levels (by 1.2 to 1.9 per cent) when compared with continuous cereal or continuous wheat rotations. However, he said the sustainability of these benefits was yet to be determined.

“Without adequate replacement of soil nitrogen reserves in the district practice rotation, the high yields and protein benefits in this rotation might not last for much longer,” he said.

Dr Ward said there was no consistent effect of residue management on crop yield.

“However, retaining residue in the continuous cereal treatment significantly increased grain protein levels – from 11.2 per cent to 11.8 per cent from 2011 to 2017,” Dr Ward said.

“There was no significant effect of residue retention on average wheat grain protein in the diverse rotation.

“The retention of cereal stubble, with a generally low N content, was previously thought to tie up soil N, and was expected to lead to a decline in wheat grain protein.

“Therefore, the increase in grain protein with residue retention in the continuous cereal rotation was an interesting result.”

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