Australian Herbicide Resistance Initiative (AHRI)

Ryegrass management in pasture

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.
Contact us for a RIM run
by Peter Newman

Mixed farms with a pasture phase generally don’t want to eradicate ryegrass, they want to manage it. They want low ryegrass numbers in crop and some ryegrass in their pasture phase. Most growers in the Darkan, Western Australia are keen to keep their pasture rotation, but they are having trouble with Select® resistant ryegrass and they are worried if they simply switch to Roundup Ready® canola they may quickly break Roundup® as well. This RIM run was suggested by Darkan consultant Andrew Ritchie and will feature in an upcoming RIM workshop in the area.

Let’s set it up

  • Mixed farm in high rainfall zone in the south west of W.A. (great sheep country)
  • 40% Select® (clethodim) resistance
  • Three years of clover pasture, grazed and spray topped in spring followed by Canola : Barley rotation, then rotating to wheat (or Oats) in fifth year of cropping phase before returning to pasture

We are looking to answer the question, how can we manage the ryegrass seed bank and minimise Select® and Roundup® resistance?

PPPCBCBW rotation minus and plus clethodim resistance

Screenshot 1

One of the problems with the rotation above is that while there are relatively low ryegrass numbers in the crop phase, there are still ryegrass in crop that are being sprayed with herbicides such as clethodim and the pre-emergent herbicides. Resistance is inevitable in this situation. So begs the question, can we manage the ryegrass seed bank so we have fewer ryegrass in the crop phase, and still have adequate ryegrass in the pasture phase?

No pre-em herbicide in last crop

Screenshot 2

No clethodim vs. switching to RR canola

Screenshot 3

Chaff carts are very compatible with mixed farming situations. The chaff dumps provide an excellent source of sheep feed over summer and into the autumn feed gap (these benefits have not been taken into account for the RIM runs above). They are one of the rare practices that are good for the sheep and good for the crop. We can manipulate the use of the chaff cart and herbicides to manage our ryegrass seed bank depending on what phase of the rotation we are in. This gives us the ability to have low ryegrass numbers in crop which limits the evolution of resistance, and adequate ryegrass numbers in our pasture.

These RIM runs show the benefit of one of the oldest forms of resistant weed management – the pasture phase. Running a seed bank down with a pasture phase followed by a phase of crop is a tried and tested formula for success.

 

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.

Download RIM

Assumptions
  • High rainfall zone of Western Australia.
  • Herbicide control %. Trifluralin 80%; Propyzamide 90%; Sakura® 90%; Boxer Gold® 85%; Select® 97% (60% for resistance scenarios); Glyphosate 99%
  • Low intensity grazing – 8 DSE with 80% ryegrass seed set control
  • High intensity grazing – 11 DSE with 95% ryegrass seed set control
  • Chaff cart – removed 70% of ryegrass seed in crop.
  • Spray topping pasture – 90% ryegrass seed set control
  • Sheep gross margin – $35/DSE

 

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

darkan assumptions