Take-no-prisoners approach to resistance ryegrass
The advantage of using a robust herbicide package has been highlighted during a trial in the border region of South Australia’s south-east and western Victoria aimed at managing clethodim-resistant ryegrass.
The research, by the University of Adelaide along with the MacKillop Farm Management Group, indicates that using an appropriate herbicide regime involving Roundup® and triazine-tolerant (RT) canola (Table 1) followed by wheat in the second year improved control over resistant ryegrass by up to 70 per cent compared with standard grower practice strategies (first row in Table 1 – simazine incorporated by sowing (IBS) followed by atrazine and clethodim post-emergence).
||Post-emergent|| Ryegrass seed set spikes/m2
| Pacific Seeds Hyola® 525RT®
||Simazine incorporated by sowing (IBS)
| Simazine IBS
1. Glyphosate at cotyledon
2. Atrazine + glyphosate at six-leaf (separate applications)
| Propyzamide + Avadex Xtra® IBS
1. Glyphosate at cotyledon
2. Atrazine + glyphosate at six-leaf
3. Crop-top glyphosate
The resistant ryegrass problem in southern Australia’s high-rainfall zone is well-documented, with many paddocks estimated to be harbouring populations of the weed resistant to post-emergent chemistry, particularly Select® (clethodim).
Ryegrass populations from Frances, SA, at the trial site on the Victorian side of the border were tested for resistance. They came back as eight times more resistant to clethodim than a susceptible population.
As a result, University of Adelaide researcher Dr Sam Kleemann says it was unsurprising that the standard grower treatment during the canola phase gave poor ryegrass control. He says they observed marked differences between treatments especially in regard to ryegrass seed count.
“Where we should be coming out of canola and reducing the ryegrass seed burden, the standard grower treatment actually doubled it,” he says. “We went from 3000 ryegrass seeds per square metre at the beginning of 2014 to 6000 seeds/m2 at the beginning of 2015.
“Conversely, in the other two treatments we dropped seed counts substantially — by about 70 per cent.”
The benefits of good crop competition with weeds were also on display during the trial’s canola phase. Dr Kleemann says that hybrid canola can give a 50 per cent greater reduction in weed seedset than open-pollinated varieties and that late applications of glyphosate also played an important role in the canola phase.
“In a more favourable spring, you can’t go past an over-the-top or under-windrow application of glyphosate,” he says. “It provides a great opportunity to prevent seed production of late germinations.”
However, Dr Kleemann says even in the high-level control treatments, there still remained reasonable numbers of ryegrass seeds in the 2015 wheat crop, which he put down to possible seed carryover from the dry winter of 2014. Given similar conditions in 2015, Dr Kleemann says he would expect to see the same thing this year.
Dry conditions at sowing in 2015 inhibited the effectiveness of some pre-emergent herbicides in the 2015 wheat crop.
“Sakura® on its own really struggled because we didn’t have enough moisture on heavy soil types to get the herbicide from the surface down to the root zone. Predominantly, Sakura® is taken up by the roots and what quite often happens is you get enough moisture to get the ryegrass to germinate, but not enough to get the herbicide down where it’s required, and subsequently it struggled a bit, particularly with a high population of ryegrass.”
However, where Avadex® Xtra (triallate) was included in the mix as well, ryegrass control improved by up to 60 per cent over Sakura® alone. Dr Kleemann believes the reason for this was that as the ryegrass was germinating, it was making contact with the Avadex® Xtra, facilitating improved control.
He says the dry springs in 2014 and 2015 have been the best for herbicide impact for many growers, with plenty of ryegrass plants struggling to set seed.
“It’s not often you see ryegrass struggle to set seed, but you can pull out plenty of plants in this trial that are functionally dying.”
More information:Dr Sam Kleemann,
University of Adelaide,
0418 256 475,
In the last issue of Ground Cover in a report on the Hocking family’s grains enterprise in South Australia (issue 119, page 41 – southern edition) the reference to faba beans should have read broad beans. The Hockings trialled one paddock of PBA Samira faba beans in 2015, but Aquadulce broad beans provide the main legume break in their system. They export broad beans, with 70 to 78 per cent meeting the 14-millimetre screening, and also finish lambs on the broad bean stubble. The Ground Cover team apologises for this error. The corrected article can be read here.
GRDC Project Code UA00113