RESULTS from a long-running trial have provided new insights into the profitability of different crop rotations and how they interact with seasonal conditions and crop residue.
As part of the Grains Research and Development Corporation (GRDC)-funded, long-term no-till farming systems project, monoculture wheat has been compared with more diverse rotations for nine years at Cunderdin.
The three main partners of the project are the WA No-Tillage Farming Association, the CSIRO and The University of Western Australia.
GRDC Western Region Panel member Bill Ryan said that, overall, the continuous cereal rotation had the highest gross margin in the trial, followed by monoculture wheat and the "maximum crop diversity'' and "maximum profit'' rotations.
The maximum diversity rotation was a disc sown, cereal-legume-brassica rotation and the maximum profit rotation was a tyne seeded, cereal-cereal-fallow/legume rotation, with windrow burning and shallow tillage.
Dr Ryan said the lower gross margin in the maximum diversity treatment was because of the cover crop grown in the first three years, along with poor canola yields in the relatively dry years of 2007, 2010 and 2012.
"The profitability of the maximum diversity and maximum profit treatments was reduced by the inclusion of less profitable legumes, despite higher wheat yields," Dr Ryan said.
"The fallow also reduced the profitability of the maximum profit treatment from 2013 to 2015."
Dr Ryan said the long-term nature of the trial was valuable as it allowed WA growers to see the effects of crop management practices over time.
School of Plant Biology and UWA Institute of Agriculture lecturer Ken Flower, who presented the findings, said it was realised that growers did not follow a fixed rotation, but rather moved in and out of different crop sequences as determined by factors such as seasonal outlook, previous yield response, weeds and grain price.
"In the context of this trial, growers would, at different times, be moving in and out of all of the different treatments - for example growing a number of wheat crops in a row (monoculture wheat), then switching to another cereal such as barley or oats (continuous cereal) and then a break crop or two, perhaps including a fallow (maximum diversity/maximum profit)," he said.
Dr Flower said the gross margins presented in the research reflected those of a fixed rotation and were useful as they showed the more reliable gross margins of cereals, the disadvantage of cover crops and fallow on gross margins, as well as the potential for "boom or bust'' with break crops, as influenced by factors such as rainfall and grain price.
"The rotation giving the optimum gross margin would not in reality be monoculture wheat or continuous cereals, but would largely be based on cereals and also contain break crops like canola.
"The frequency of the canola break crops would be higher under conditions that maximise their benefits - such as high grain price, low weed pressure and good soil moisture/rainfall.
"It should also be remembered that this trial is being conducted on relatively heavy soil. It might well be expected that similar trends would occur on lighter soil, however, changes may be more rapid."
The trial results also suggested there were no significant yield differences between monoculture wheat and wheat in more diverse rotations for the first five years.
After this period, lower yields in some years with monoculture wheat compared with the more diverse rotations was thought to be due to one or more factors including disease associated with lack of rotation and heavy residues from the previous year affecting crop establishment.
