R&D Edition: Controlling herbicide resistant ryegrass in onions with pre-emergents

Vegalogue is a regular podcast from vegetable, potato and onion industry peak body AUSVEG, where we examine the pressing issues and latest developments in our sector.

Annual ryegrass is one of the most costly weeds for the Australian ag industry. It’s a major problem in grain crops, and it causes significant issues in horticulture as well.

For many years, farmers have relied on herbicides to control ryegrass, but in Australia the weed has now evolved resistance to multiple chemistries, particularly post-emergents.

The prevalence of herbicide resistance in ryegrass populations varies around the country; across southern Australia the Grains Research and Development Corporation cites a range of 1-93 percent for post emergent herbicide resistance, but only 0-32 percent for pre-emergents.

That lower resistance to pre-emergent herbicides prompted Tim Groom of Tasmanian onion grower Wynyon to explore which pre-emergents might work well for an onion crop, in concert with an integrated weed management approach.

Learn more about the project discussed in this episode: Accelerating the adoption of best management practices for the Australian onion industry

Thanks for listening to Vegalogue! You can find out more about AUSVEG and the Australian vegetable industry at ausveg.com.au. Subscribe to our newsletter, or follow us on Facebook, LinkedIn, YouTube, Instagram, or Tik Tok.

Transcript

There had been enough ryegrass go to seed in that period of time that when we prepared the field ready, there was plenty of ryegrass across the field that had come up.

So not a good outcome, but a good place for a trial.

Welcome back to the Vegalogue podcast, a dialogue about the Australian vegetable industry from AUSVEG.

I'm Tom Bicknell.

Annual ryegrass is one of the most costly weeds for the Australian ag industry.

It's a major problem in grain crops, and it causes significant issues in horticulture as well.

For many years, farmers have relied on herbicides to control ryegrass, but in Australia, the weed has now evolved resistance to multiple chemistries, particularly post-emergence like glyphosate.

The prevalence of herbicide resistance in ryegrass populations varies around the country.

Across southern Australia, the Grains Research and Development Corporation cites a range of 1-93% for post-emergent herbicide resistance, but only 0-32% for pre-emergence.

That lower resistance to pre-emergent herbicides prompted Tim Groom of Tasmanian onion grower Wynyon to explore which pre-emergence might work well for an onion crop in concert with an integrated weed management approach.

Thank you very much for joining me, Tim.

Thank you, Tom.

Could you maybe talk us through the challenge that onion growers are facing with ryegrass at the moment?

So ryegrass has been used as a pasture species and also as a green manure crop.

So there's ryegrass in our cropping system that over a period of years where it's been exposed to what are now called group one herbicides, has developed some resistance.

It varies from farm to farm and from paddock to paddock, but it's certainly increasing.

And group one herbicides are currently the only tool we have in our armory to control ryegrass in onion crops.

They're also the same tool that's used in a variety of other crops on the same farm in different parts of the rotation.

So ryegrass is getting exposed to these herbicides over and over again.

There's a very low level of resistance in the natural population and what we're doing is a selective breeding program where the susceptible grasses get killed out, the not susceptible grasses survive, they go to seed, that seed spreads.

It spreads from various means and we end up with a bigger problem.

So you've done some trials yourself.

You started these a few years ago to try and explore some options with ryegrass control.

Could you maybe tell us about how that trial project got started and what you were looking for?

Yeah, so that started from having a crop failure where we basically lost a paddock of onions due to ryegrass, effectively lost it.

That was very poor yield because of ryegrass.

So I was determined to see whether there was solutions out there that we could find.

And I guess from my point of view, from the herbicide path initially, I came across a guy by the name of Michael McPherson, who at the time was working for InTrade.

He's back working for InTrade again now, who had a lot of experience in broadacre control of ryegrass.

And his advice was that we needed to try and find a pre-emergent herbicide in onions that was active on ryegrass that didn't hurt the onions.

So I think that the first sort of set of trials I did was back in 2018.

Nothing sort of looked that promising.

Had another go at it the following year.

Similar thing.

What I thought might have worked, that it's on the label to work in cereals for ryegrass, didn't control the ryegrass well enough.

And then in 2023, again following ongoing crop failures or part failures due to ryegrass, Bree Howe and I had a more serious trial of our own bat that we put down looking at a range of pre-emergent herbicides, some which are already registered as ryegrass on the label, but they don't work well enough.

And other products that had the potential to control ryegrass in onions because they were used overseas.

So I knew they should be safe on onions.

They had activity on grass.

And then a couple of long shots where there was some new ryegrass herbicides registered in cereals and thought, well, let's see what they do.

So, 2023, that was a replicant of block trial at Caprona at Ellenton Pastoral.

And the only thing that had activity on ryegrass to any decent degree was too hard on the onions.

And everything else that we tried was basically ineffective at the end of the day.

So, the next step there was AUSVEG through the VegNET program, and it held a field day at Boat Harbour, at George Griffin's property, where we talked about ryegrass.

Zarmeen had already introduced me to Peter Boutsalis, who's a specialist in ryegrass as a weed in Australia.

I showed Peter and Zarmeen the trial at Caprona.

And following some ongoing discussions, Peter suggested that we should try and look at whether we could mix one of the group ones, which is clethodim, with a range of pre-emergence, but put it on very early post-emergence, because it should be safer to the onions, and it may sort of spike up the clethodim.

So that's what we've done in 2024, 25.

So the first step of that was Peter screened a whole range of different herbicides that were active on ryegrass in pot trials and onions to see whether they were safe to onions at the sort of flag to half a true leaf stage.

There was a list of herbicides that didn't cut it because they weren't safe enough.

The ones that looked good enough is what we took to the trial.

So at David and Nadine Vertigan's property in 24, we put down a trial where we had a range of what we would normally call pre-emergent herbicides, but applied early post-emergence.

They were running in one direction and then we had clethodim running in the other direction to see whether we would see any positive interactions.

What was the paddock that you were doing the trial on?

What was the history there?

So that had had onions in, I think the last time it had onions was in 2018 from memory.

There was a small amount of ryegrass on the edge of the paddock that we suspected to be resistant, but it wasn't really a problem.

And then since then, it's had other crops in it, it's had potatoes, it's had poppies, it's had wheat, it's had canola, it had just come out of canola.

And clearly there had been enough ryegrass go to seed in that period of time that when we prepared the, when David prepared the field, ready, there was plenty of ryegrass across the field that had come up.

So not a good outcome, but a good place for a trial.

I know from a broad-acre perspective, they put a lot of thought and effort into controlling the weed seed load in the soil.

Is that as prominent for onion growers, that thinking?

Yeah, it has to be.

I mean, I think, if you look at the work that GRDC have done, the take-home message is, at every opportunity, try and prevent ryegrass going to seed in crops.

So that means where there's opportunities to control ryegrass, because you do have effective herbicides that are group ones, you need to use them.

And then you need to use any other measures that you can come up with to all produce or prevent ryegrass going to seed.

Unfortunately, the problem with onions are that they have a very low tolerance to weeds, and including grass, because you don't need many ryegrass plants per square meter to end up with a bit of a mess.

So our tolerance was fairly low.

Having said that, we've got to improve on what we're doing.

In the trials in Penguin in Northwest Tas, what did you find out?

So we did have some, I think you could say, promising results.

So yeah, so all the pre-emergent herbicides that we applied early post-emergence were saved to the onions.

There's only really two pre-emergent herbicides on their own that had any decent activity on ryegrass.

That was Prasulfocarb, which is sold as part of many things, but it's Arcade is what we used.

And Secura, where the ryegrass pressure, because the ryegrass pressure sort of varied across the trial sites, very difficult in a natural situation to get complete uniformity of grass seed pressure.

Where it was lighter, Secura looked really good.

I think if you look at the magazine when it comes out, there's a photo there of what is basically a clean plot.

Where the ryegrass pressure was higher, it did a job commercially useful, but not what we want to be longer term.

Yeah, so I think there's a direction there that we've got a couple of candidate herbicides that need more work.

This is one trial in one year, one season in one location.

It's going to need more trials over a range of sites, over a range of seasons to get a level of confidence that one, it's got to work, and two, it's not going to hurt the crop.

And you're looking at combinations of the two?

Yeah, I guess where we hopefully will be at is that Hort Innovation will come up with a project that will go out to tender.

It's not something I'll be doing.

I'll happily hand it over to someone who does this, as their full-time role.

But there's a few leads there I think we can look at in terms of working with the Cura more closely and potentially some of the other things could be put on over of a low dose, a number of applications, just to try and, basically to try and explain it, that some of these herbicides are fatty soluble in water, so every time it rains, they can leach away and lose their activity.

So one way around that, rather than putting one dose on, is to put a number of smaller doses on just to try and keep the herbicide where you want it, in that top layer to control grass, without leaching into the roots of the onions.

So I think that's one of the tools that we need to look at.

It's not the only tool.

There's also, we talk about controlling ryegrass and the rest of the rotation.

I think the next step beyond that will be some of this new technology that's coming out, spot spraying machines that will detect grass in onions, and be able to spot them with something that's either, yeah, non-selective without touching the onions, or something that's selective that if I do touch the onions, it doesn't hurt the onions very much.

So that will be the cleanup process, potentially.

Talking about rotations, the site that you ran the trial on, the grower there, David, has employed a stale seedbed technique.

What's the benefit of that in this context?

Yeah, David's been doing that for a few years, not necessarily designed to control ryegrass.

It's just very effective at controlling weeds, generally.

So basically, he prepares the ground in the in the autumn, basically ready to sow, but leaves it, lets it sit and settle.

You get weeds germinate over the winter, and you can plant into that.

So you've already got a decent weed germination before the onions are planted.

You plant into that, you spray it off with like round up prior to the onions emerging, and you've given the onions a head start basically on the on the weed, because I'd always already germinated.

Obviously, more keep on coming afterwards, but you're not disturbing the ground much when you plant.

So it's a really useful method when done correctly.

But one of the keys to it is you have to prepare the soil in the right condition.

If you don't prepare the soil in the right condition, if it's too wet particularly, you can end up with problems.

So it's got to be done right.

But when it is done right, it's very useful.

And are there particular rotations or break crops that you think can help control the ryegrass seed load that will give you some options to really knock it down without harming the onions?

Yeah, I don't think we know enough about what the best rotational mix is.

I think that's work in progress.

Clearly, there's crops that can be grown that, again, you don't just grow them to control ryegrass.

Obviously, farmers are going to try and make money, but there are crops that can be grown that you can use herbicides that control ryegrass, that are group one.

So you're taking the pressure off the group ones in the rotation.

So is the trick for a lot of these weeds that have resistance to certain chemistries is you've got to mix up the chemistries so they're not overexposed to a single product and then try every other avenue to reduce the numbers at the same time?

Yep, that's it in a nutshell, basically, yes.

So in cereals, there's wheat and barley grown here.

There's a number of options in cereals that could be used and possibly the opportunity to even spray the cereal crop several times to get as best to control of ryegrass as you can.

Any crop that dries off is an opportunity for the ryegrass to go to seed.

So in our cropping rotation, that's cereals, it's poppies, it's pyrethrum, but I guess it's onions as well because they get lifted and the grass dries.

Whereas say green veg, you're talking broccoli, cauliflower, that kind of thing, green peas, there's less of an opportunity for that to happen.

So what's next?

Are you going to be doing any more trials?

I will happily handball it on to someone else and again, happily provide my advice if they're prepared to listen to what I think that we've learned.

So yeah, I think that element, we need to continue with the work that we've started, get it to a commercial outcome.

And hopefully by that time, there'll also be other tools like some of this new technology coming through that is able to spot grass within an onion crop or other crops for that matter.

Yeah.

Well, thanks very much for your time, Jim.

My top?

No worries.

Thank you.

Part of Jim's trial was conducted in collaboration with Dr. Peter Boutsalis of Plant Science Consulting as part of the Accelerating the Adoption of Best Management Practices for the Australian Onion Industry Project, which is funded by Hort Innovation using the Onion Research and Development Levy and contributions from the Australian Government under project code VN21000.

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