Australian Herbicide Resistance Initiative (AHRI)

Stacked rotations in the low rainfall zone

Stacked rotations in the low rainfall zone

Below is the output from a RIM run we developed here at AHRI to demonstrate how the Ryegrass Integrated Management (RIM) computer model can be used to put theories to the test. Have your own theory? Click here to contact us or follow the link below.
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by Peter Newman


This week’s RIM run was suggested by grain grower Bob Nixon in the low rainfall zone of Western Australia. He uses double stacked canola rotations to hit ryegrass hard then grows as many consecutive cereals as he can before rotating back to canola. He wanted to know how many cereal crops he could grow before ryegrass numbers blow out.
Note: RIM wasn’t happy with the double stacked canola (due to disease risk) so we had to fudge it by using a legume:canola rotation. Legume was the same price as canola with the same herbicides.
Let’s set it up:
  • Low rainfall zone of Western Australia
  • No resistance to glyphosate or the pre-emergent herbicides
  • If we hit the ryegrass hard for two years in a row then use the Rolls Royce ryegrass herbicides in the first wheat crop, can we get the ryegrass seed bank low enough to grow several wheat crops in a row without a blow out?
C:C:W:W:W:W:B vs. same rotation + windrow burn canola


Screenshot 1 rework



What happens if we do more windrow burning or use another form of harvest weed seed control such as a chaff cart every year?



Screenshot 2 rework

What about if we reduce the length of the rotation?


Screenshot 3 rework



This RIM run essentially shows that the grower can choose the ideal rotation for his/her farm and add enough weed control to make it work. In this case, using harvest weed seed control every year is enough to beat the ryegrass and keep the rotation going. We used a chaff cart every year, however any of the harvest weed seed control tools will do the same job.


RIM is a great tool to compare some weed control strategies using computer simulation before making the changes on farm. RIM simulates the ryegrass seedbank based on years of research data to give realistic results.


Download RIM for free and give it a try.


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Some of the assumptions used in this simulation were:
  • Loam soil in low rainfall zone of northern cropping belt of Western Australia.
  • Yields. Wheat 1.8 t/ha; Canola 0.9 t/ha.
  • Costs: Canola seed cost estimated to be $70/ha (this was included in RIM by increasing glyphosate post-em in canola cost).
  • Herbicide control %. Trifluralin 80%; Propyzamide 90%; Sakura 90%; Glyphosate 99%.
  • Windrow burn / Chaff Cart – removed 70% of ryegrass seed in crop.


The picture below shows some of the assumptions used for these scenarios. All of these assumptions can be adjusted to suit a particular farm.


nixon assumptions rework