Results from a trial in Western Australia’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 produced the interesting result that retaining residue in a continuous cereal rotation significantly increased grain protein levels over the long term.
CSIRO researcher Phil Ward will speak this month at the Grains Research and Development Corporation (GRDC) Grains Research Update, Perth, where he will summarise the results of a trial investigating the impacts of rotation diversity and residue handling on wheat yield and protein content.
Factors influencing protein levels in wheat are an area of high importance and interest to WA grain growers.
The trial has been conducted with GRDC investment on red loamy soil at Cunderdin, where a long-term rotation trial was first established in 2007. The research has involved collaboration between the WA No-Tillage Farmers Association, The University of WA and CSIRO.
It has compared four rotations – continuous wheat; continuous cereal (wheat, wheat, barley); district practice (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 (N) was applied to each crop in response to seasonal conditions and expected N requirements. No extra N was applied to legumes or fallows.
Dr Ward said 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," he said.
"Preliminary estimates suggest that these benefits are sustainable.
"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."
