Written by:  Peter Newman | Farm Business Consultant | 0427 984 010

I had a call from an agronomist mate of mine from Western Victoria last year asking “once we have glyphosate resistant ryegrass, will we ever get glyphosate back?”

They were facing a situation where most paddocks had glyphosate resistant ryegrass, which had caused “the fastest practice change” my agronomist friend had ever seen in his thirty years of agronomy.

The practice change? Double paraquat?

Yes, that’s right. Growers in this region, in one year, had moved from using a double knock of glyphosate followed by paraquat, to paraquat followed by paraquat.

How common is glyphosate resistance?
About one in six paddocks in Australia has glyphosate resistant ryegrass.

One in eight in WA & SA and about one in Five in Vic and NSW. There’s not a whole lot of ryegrass in Queensland, so they’re off the hook for now, and somehow Tassie escaped! This table shows the percent of ryegrass populations where glyphosate resistance was found (> 20% survivors to the herbicide).

This data is from the GRDC funded resistance survey. A team of researchers from around the country randomly sample about 1500 paddocks every 5 years. They found ryegrass in about 90% of the paddocks, so ryegrass is still the King.

Will we ever get glyphosate back?
Going back to the original question from my Victorian Agronomist friend, unfortunately the answer is probably a resounding NO. Generally speaking, resistance is permanent. We may see the level of resistance in a population drop a little if the herbicide isn’t used for a while, but one application and we’ll probably be back to where we started.

We have seen a couple of anecdotal observations where growers had some radish resistant to SU herbicides, and after 10-15 years of avoiding the SU’s and keeping radish numbers very low, they saw radish become susceptible again. But this isn’t the norm.

Is double paraquat a good idea?
Yes, in the short term, but there are two problems.

• Clearly there’s the risk of paraquat resistance. There’s a handful of populations of paraquat resistant ryegrass around the country, and Dr Roberto Busi from AHRI has observed that often glyphosate resistance happens first and paraquat resistance follows shortly thereafter. There’s no documented link between the two resistance mechanisms, but there is something fishy going on.
• There’s a storm brewing over a link between Parkinson’s disease and paraquat. I’m not sure how strong the evidence here is, but regardless we could be at risk of this herbicide being banned one day. It is already banned in some countries.

How do I avoid paraquat resistance?
Keep mixing. Mixes with group 14 (G) herbicides such as Voraxor, Terrador, Hammer, Butafenacil etc. are a good idea. Also mixes with pre-em ryegrass herbicides helps.
Keep using the glyphosate – paraquat double knock where you can, and as soon as you see something dodgy, get a resistance test. Knowledge is power!

The best thing you can do is all of the Weedsmart Big 6 (and more) to run down your ryegrass seedbank. If you don’t know what the Big 6 is, you can check it out here www.weedsmart.org.au Part of my role includes working with the Weedsmart team, and the Big 6 is our short list of tools that you should be using if you are keen to keep your cropping future alive and well.

Summary
I’ve been banging on about glyphosate resistance for a long time now, and as with most resistance issues, it has taken longer to happen than we thought it might, but it is well and truly here now. The most recent survey really woke us up, and guess what, those resistance samples were taken at the end of 2020 and we’ve had two wet years with plenty of glyphosate applied since. It’s not doom and gloom, there’s plenty we can do to manage this problem, but just be aware that glyphosate resistance is now widespread, it won’t go away, and we need to take action.