Key Points
- Seasons played bigger part in yield then rotation
- Stubble management critical for seeding after high residue cereals
- In wet years, diverse rotations yielded better, dry years showed no significant difference
- No significant yield differences in first 5 years between continuous cropping and diverse rotation
- After five years continuous cropping yields suffered
- Diverse rotation better off in average or above average seasons
- Crops benefit from having some residue in dry years
Since 2007 WANTFA has been investigating the long-term effects of different cropping systems including continuous cereals through to more diverse rotations. Other system effects that are being investigated include fallow, tramlines, stubble retention, windrow burning and shallow tillage. This year the trial has been approved for another three years of GRDC funding, which those involved say will be important to see the longer term impacts of each system.
Manager of the project, Dr Ken Flower from UWA, said that while the past nine years have had several below average seasons, there were still some interesting trends. The first year of the trial was a set up period where cover crops where grown, sprayed out and knife rolled to get a high base level of residue at the beginning. Plots were all then treated as different farming systems, with some windrow burnt each year and others fully retained and seeded with discs.
‘There has been a whole series of dry years, with the 70 year average for the area being 360mm, the average over the last 20 years is 300mm and we have received less than this for the last 9 years, apart from 2014.’ Ken said.
‘Rainfall is the main driver of wheat yields and there is a strong relationship between the two in this trial, however, we see there are now other system impacts on yield,’ Ken said.
For the first five years there were no significant differences in yield between the diverse rotations and continuous cropping. However, Ken said they were starting to pick up interesting differences in the sixth year onwards. With higher rainfall, such as 2014, the positive effects of crop rotation were much greater (due to better fertility and less disease) compared with the dry years where differences between the continuous cereals and diverse rotations were small.
High residue and uneven cereal residue spread behind the harvester was shown to create issues with seeding and crop germination of following crops.
‘We are measuring anything from about 1 to 4t/ha more residue behind the harvester wheels compared to the edge of the cutter front. That itself has caused problems because it’s impacting on the establishment of following crops,’ Ken explained.
‘We are also measuring about 2-3t/ha more residue in the retained stubble plots compared with the windrow burn plots.’
When comparing wheat yields, Ken found an interaction between stubble retention and the residue type (rotation), with some crop residue types, especially cereals, reducing yield of following crops when residue amounts were high and not spread properly; whereas other crop residue types (like canola) had a more positive effect, even with high amounts of residue.
‘Residue burning had a positive effect in a dry year following a good year (i.e. with high residue carryover into the following season), however in years after poorer seasons where residue was low, the fully retained residue plots yielded better than the burnt plots,’ Ken said.
Due to residue impacts they have now put residue managers on the seeder and 2015’s crop residue was spread more effectively behind a John Deere header.
‘One strong message is if we have a good year and we can’t spread the residue properly behind the harvester and/or manage it at seeding, it can significantly reduce yields of the next crop,’ Ken said.
Weeds have also been monitored in the trial, and for the first few years there weren’t any notable differences in burdens. However, more recently, we have measured less weeds in the windrow burnt plots compared to the retained residue treatments.
Overall, wheat yields following a fallow where highest but they didn’t make up for the profit lost from having a fallow in one year. Rotations and stubble management have had no effect on soil organic carbon so far. The biggest effect is still seasonal, with lower soil organic carbon following dry seasons with low rainfall. Over the nine years of the trial the continuous cereal had the highest gross margin, even though this treatment had lower yields in wet years.
What’s next?
Additions to the trial in the next three years include Albus lupins as these tolerate higher pH soils and have an improving market.
One of the treatments, which has been a continuous pasture since 2008, is being reseeded as there isn’t much clover or medic left in it. The wheat rotation is being sown to a Clearfield® variety (Impress) this year to provide more weed control options.
The next three years in this study will focus on identifying the long-term changes due to crop rotation and crop residue management under no-tillage. Particular emphasis has been placed on improving the understanding of soil nitrogen availability where crop residue is being retained and where cereals dominate the crop rotation. Changes in the weed seed bank and leaf and root diseases due to crop rotation are also areas of interest, as well as an economic analysis of the drivers of crop rotation profitability.
WANTFA research continues to focus on the fundamentals of no-tillage systems (i.e. groundcover, rotations and weed control options).
Organisations involved:
WANTFA – Neil Cordingley, David Minkey and Matthew McNee
UWA – Ken Flower and Nathan Craig
CSIRO – Phil Ward and Shayne Micin