HRZ trial yields lessons in resistant ryegrass management

Author: Alistair Lawson | Date: 15 Jan 2015

University of Adelaide's Dr Chris Preston in a trial at Southern Farming Systems' Westmere, Vic, site.

Dr Chris Preston at Southern Farming Systems' Westmere, Victoria, site during AgriFocus 2014 where he presented findings from a long-term integrated weed management trial in southern Victoria.

Getting mouldboard ploughing right, the advantage of an expensive herbicide strategy and how not to narrow windrow burn are just some of the initial learnings from a long-term trial in southern Victoria’s high rainfall zone.

The project was set up to address the problem of herbicide resistance in the high rainfall zone (HRZ), particularly southern Victoria. It is being led by the University of Adelaide’s Dr Chris Preston in conjunction with Southern Farming Systems (SFS).

Five years ago, Dr Preston surveyed paddocks in southern Victoria and found some worrying results.

“We discovered that 60 per cent of paddocks between Geelong and Hamilton had ryegrass resistant to both group A and group B herbicides,” Dr Preston said. “Essentially, all the post-emergent herbicides for cereals were gone, so the conversation turned to, ‘what are we going to do about this?’”

From 2012, a trial at SFS’ Lake Bolac site, which has a history of resistant ryegrass, assessed the effectiveness and applicability of cultural control practices before seeding, in combination with pre-emergent herbicides on management of herbicide-resistant annual ryegrass in the Victorian HRZ.

The cultural control practices include mouldboard ploughing, stubble burning, stubble incorporation with light cultivation and retained stubble with direct sowing. These were followed up with low cost (such as trifluralin mixtures), medium cost and high cost (such as Sakura + Avadex Xtra mixtures in wheat) pre-emergent options.

Lessons learnt from the trial so far include:


Mouldboard ploughing

Although expensive, early results from mouldboard ploughing were promising despite some wild radish germinating in the area ploughed.

“We pretty much got the ryegrass out but we didn’t get 100pc of it, which came back to haunt us because now (2014) the ryegrass numbers are back to what they were when we started (2012),” Dr Preston said. “That comes back to the HRZ environment. In a long-season scenario where there is plenty of rain, any ryegrass that is germinating late after treatments have been applied will produce a lot of seed.

“In dry environments we don’t see that so much, but in the HRZ it’s a problem. If we’re not stopping the seed set of ryegrass it will reset the seedbank pretty quickly.”


Pre-emergent herbicides

Dr Preston says the biggest lesson learned from using pre-emergent herbicides was not to incorporate stubble.

“If you’ve got too much stubble and you want to get rid of it, burn it,” he says. “By incorporating stubble we’re moving the ryegrass away from where the herbicides are and they just don’t work as well.

“We also learnt that if you’ve got post-emergent resistant ryegrass and you think you’re going to manage your way out of that by growing wheat and barley, it’s not going to happen. Even with our best treatments, we kept a lid on the ryegrass, but numbers are still going up. We haven’t actually driven the ryegrass down, we’ve just kept it stable by putting in a lot of pre-emergent herbicides.”

Unsurprisingly, the cheapest pre-emergent herbicide strategies were the least effective. Dr Preston says the mid-cost strategy was better but the expensive strategy was best.

“In the cereal side of your rotation, if you really want to keep a lid on the ryegrass you’re going to have to go for some pretty expensive herbicide options to do that in the HRZ,” he said. “A lot of that is about needing the length of persistence we get out of that product, particularly the Sakura® + Avadex® Xtra mix.”

Dr Preston says they used RT canola in 2014 because of its flexibility – they are able to grow it as a triazine tolerant (TT) canola, a Roundup Ready® canola and use an array of treatments on it.

“That has been Rustler® up front; we’ve got two applications of Roundup Ready® herbicide, we’ve got atrazine in there and we’re going to crop top it with Weedmaster DST®. With that, we’re going all-out to try and fix the ryegrass problem, with the question being, how important is that seed set control component in the system in southern Victoria?

“In my belief, it is, and this will give us some data around that and how important that component is. If we don’t get on top of it with canola this year we’ll go in with a pulse next year and brown manure that.”


Narrow windrow burning

Dr Preston has some important advice for growers in the HRZ who want to have a go at narrow windrow burning.  Or, how not to windrow burn.

“Last year we got a pretty big barley crop and we thought we’d set it up to windrow burn,” he said. “Unfortunately, part of the trouble we had was lodging ryegrass. In this environment we get a lot of ryegrass that lodges, so you don’t actually get it going through the header, it just lies on the ground, so we had all these windrows with ryegrass in between.

“We tried to burn it but we ended up burning everything. That was fine for the ryegrass between the rows, but the burn got too fast and didn’t burn the windrows all the way down to the ground, so there are streaks of ryegrass across the site.

“The lesson there is that if you’re going to windrow burn, don’t start practising with barley because it’s probably the hardest crop to do it in. Start with something easy like canola. Learn how to do it and do it well, and don’t make the mistakes we did.”

More information

Dr Chris Preston,

GRDC Project Code UA00113

Region South