Australian Herbicide Resistance Initiative (AHRI)

Rotation plus

Ryegrass growing boxes on tables

Wearing a seat belt reduces the risk of dying in a car accident. The seat belt is not perfect (people wearing seatbelts can still die in car accidents), however it does reduce the risk, particularly when used in combination with other safety technologies (like airbags).

In the same way, rotating herbicides is not perfect, but it does reduce the risk of resistance evolving, especially when combined with other weed management tools.

We received a lot of feedback and more research data on this issue since the last edition of AHRI insight, and we decided to follow up with a more complete summary.

There is more to the story.

If we combine herbicide rotation with other weed management tools, including non-herbicide weed control, we are still in the game.

What do South Africa and South Australia have in common? Dual Glyphosate and paraquat resistant ryegrass populations. Research conducted on these populations confirmed that multiple resistance involves one gene for glyphosate resistance and a different gene for paraquat resistance.

This is why herbicide rotation and the double knock are still a good idea.

In the last edition we reported that ryegrass can develop resistance to both glyphosate and paraquat despite rotation between the two herbicides.

Let’s set the record straight. We strongly believe in herbicide rotation. The message from the last edition was that if we only rotate herbicides, we are in a lot of trouble.

The bottom line

Glyphosate and paraquat multiple resistance has occurred several times in the past in ryegrass where there was over-reliance on these two herbicides without diversity and without follow up control.

In a nutshell – summary of research

  • Dr Chris Preston from the University of Adelaide has studied two populations of glyphosate and paraquat resistant ryegrass from the pasture seed industry in South Australia. Chris has found that resistance evolved to glyphosate and paraquat independently of each other and the resistance plants crossed to form multiple resistant individuals (report available below).
  • A glyphosate and paraquat multiple resistant population of ryegrass from a vineyard in South Africa was discovered by Andy Cairns from the University of Stellenbosch and studied by Dr Qin Yu and Prof Stephen Powles of AHRI in Western Australia. They too found that the ryegrass evolved resistance to these two herbicides independently with distinct resistance mechanisms (see below)
  • Dr Roberto Busi and Prof Powles from AHRI did a laboratory based study using low doses of glyphosate to select for resistance and found that the ryegrass were multiple resistant to glyphosate and paraquat (see AHRI insight 17). Resistance to the two appear to be linked in this study. However, the level of resistance found was relatively low and this occurred under controlled laboratory conditions. It is unlikely that this is what has caused resistance in the field populations but it is a very alarming and surprising result.

Table 1. Summary of four populations of glyphosate and paraquat multiple resistant ryegrass.

AHRI insight 18 table (new)


There was no double knock

In all situations where glyphosate and paraquat multiple resistance has been found, the populations evolved resistance through being sprayed almost exclusively with glyphosate and paraquat with no follow up weed control in any form.

Cross pollination

As ryegrass is cross pollinated, multiple resistance can occur through crossing of a glyphosate resistant plant with a paraquat resistant plant to result in offspring with resistance to both herbicides.

Resistance evolution

Given that the researchers found two separate genes endowing resistance to glyphosate or paraquat there are two possibilities as to how multiple resistance occurred;

  1. Sequentially – that is a given plant evolved resistance to one herbicide and later evolved resistance to the other, or
  2. Cross pollination – a plant in one part of the field evolves resistance to paraquat and another plant nearby evolves resistance to glyphosate. The two plants cross pollinate and voila, multiple resistant ryegrass is the result. This appears to have occurred in one of the populations studied by Chris Preston where glyphosate resistance evolved in an irrigation channel and paraquat resistance evolved in the adjacent field.

So what do we do with this information?

One thing is very clear, herbicide rotation is good but alone is not enough. All of these multiple resistant populations are from situations where herbicides were the only form of weed control and herbicide rotation was the only form of IWM.

If the double knockdown was used every year at lethal rates we are confident that resistance would have been a low probability. However, this ‘ideal world’ situation is difficult to achieve in real life.

The double knock of herbicide followed by another form of weed control e.g. mowing in pasture seed production, sheep grazing among grape vines etc. is achievable and sustainable.

Rotating between glyphosate and paraquat, then following up with other weed management is likely to significantly reduce / delay the evolution of resistance. Perhaps the bigger benefit of this system is the reduction in the weed seed bank that will result.

The bad news

There is more paraquat resistance around than you may wish to believe. Back in the 1980’s Prof Powles documented many paraquat resistant barley grass populations in Victoria. Recently, Dr Chris Preston took 39 samples of ryegrass from pasture seed growers and found 9 populations that were resistant to paraquat. What is possibly even more alarming is that Chris showed that 50% of barley grass samples collected from Lucerne fields in South Australia in 1999 had resistance to paraquat.. Barley grass has evolved this resistance to paraquat in the pasture seed industry where paraquat is used as a winter cleaning spray. The reasons for this high frequency of resistance are due to mowing being less effective on early seeding barley grass and there are also genetic reasons due to barley grass being a self-pollinator. This will feature in a future AHRI insight.

The good news

Fortunately we have only seen paraquat and glyphosate multiple resistance evolve in ryegrass in Australia. Indeed, paraquat and glyphosate still work in most parts of Australia. With diversity, these knockdown herbicides will still be very effective on a broad spectrum of weeds for many years to come. However, we must develop robust, diverse systems and be vigilant to avoid disaster in the weeds that are prone to multiple resistance.

Keep rotating, but with diversity.

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