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Feathertop rhodes grass: don't let it get away from you

Feathertop Rhodes (FTR) grass is high on the list of serious weeds in the Australian cotton industry, particularly in parts of Central Queensland and in dryland farming areas.

This weed is difficult to control and is high-risk due to it naturally not being overly susceptible to glyphosate. With the dry conditions that we've experienced in recent years, FTR has also been tough to manage, which may have allowed for a building seedbank. 

In this CQ enewsletter, we have a look at ensuring an integrated weed management (IWM) plan for FTR, particularly for areas of the region that have yet to plant and are looking to get on top of this weed in the next few months. 

What makes FTR so difficult to control: 

·       It is a small-seeded surface germinator that favours no-till systems. 
·       Individual plants can produce 6000-40,000 seeds. 
·       Several cases of glyphosate resistance have been reported. 
·       It is naturally tolerant of glyphosate. 
·       It germinates after small amounts of rain (<10mm). 
·       It germinates across a wide range of temperatures; more than one cohort per year. 
·       It is capable of producing seed just six weeks after germination 
·       It quickly becomes stressed under dry conditions. 
·       It establishes quickly on lighter textured soils. 
·       These lighter soils are not suited to cultivation. 

What are my options

No single practice alone will manage FTR but using several strategies as part of an IWM plan can be highly effective. Key practices to consider within an IWM strategy include: 
·      Use of knockdown herbicides 
·      Use of residual herbicides 
·      Crop rotation 
·      Spot spraying/WeedSeeker®/chip hoe 
·      Strategic tillage 
·      Burning. 

Knockdown herbicides should be seen as a last resort rather than as a preferred option. They should only be used on small, unstressed plants; and also used in conjunction with a well-timed double knock. Resistance in both Group (1) A and Group (9) M herbicides is a real threat. 

Residual herbicides require planning and an understanding of the plant-back restrictions of the product before use. Crop competition will improve the efficacy of some products, and these products work best with minimal plant residue, but efficacy will vary from product to product. After application, most residuals need incorporation by rainfall or machinery into moisture to activate. Splitting applications can extend the residual life of some products. 

Spot spraying/WeedSeeker®/ chip hoe can be very effective for control of escapees or survivors. This can be a good option when the seed bank is low and you have sporadic germinations. If mature plants have dropped or are about to drop seed, remove them from the paddock and burn the plants and seeds. 

Strategic tillage will ‘reset’ the weed problem, not completely remove it. However, it can be very effective if used in conjunction with residual herbicides to take out future germinations. It can also be an effective option as part of a ‘double-knock’ strategy. 
 
Burning is also a good way to remove dead plant material prior to residual herbicide application and/or tillage. Burning may have some effect on surface seed viability if the temperature of the fire is hot enough, but this is very hard to achieve. Burning single or small clumps is time consuming but effective in allowing herbicide deposition onto otherwise protected growing tips. Burning can be effective in reducing seed numbers but will not destroy all seed. 
 
Research indicates that a Group A (Group 1) herbicide followed 5-7 days later with Group L (Group 22) paraquat provide much better control than a glyphosate followed by paraquat. Weeds must be at the three-leaf stage to early tillering. Translocation of Group A (1) herbicide decreases as plants move to reproductive stage. Extra punch can be added to the paraquat with a residual such as a Group (6) C or (27) H herbicide. FTR seed is short lived. One or two years of seed set control can bring the problem under control. Best control is achieved with a diverse approach, attacking the plant at various stages of its life-cycle.  

Grower observation: Brendan Swaffer 

 

Brendan Swaffer runs a zero-till and controlled traffic farming system near Clermont, growing a range of cereal crops and dryland cotton if there’s an early break in the summer. This hasn’t been the case since 2015 but he is keeping his options open for 2021/22 if the seasonal conditions allow for planting in December.
“We have feathertop patches in a lot of our paddocks,” he told CottonInfo last week. “We are keeping it in check, but I wouldn’t say we are really getting on top of it, either.”
Using Roundup Ready cotton varieties in his dryland system has been one option in his arsenal to help control FTR, but he added that it need to be used with a range of strategies. Brendan also said that he was mindful of the risk of herbicide resistance and, when he does use glyphosate, he tries to ensure he is using high-grade products.
He has also used targeted cultivation of affected areas, keeps a chip hoe in the ute to target specific patches as he spots them, and he uses a range of different herbicide groups. 
In the past, he has done targeted cultivation and followed up with Group K residuals (metolachlor) for short-term residual control of any new plants that germinate after the cultivation.
“My most recent strategy has been to use a Group B residual (Imazapic). The area I treated last year looks exceptional now, eight months later,” he said. “The drawback of this option is the plant-back period with the residual. We are locked out of some crops (including cotton) and are restricted to options like chickpeas and tolerant sorghum varieties.”
He has also used targeted boom spray applications in the fallow, flicking the boom at problem areas, but this approach is something he's not entirely happy with.
“That strategy requires covering every acre. As well as being time-consuming and expensive, it doesn’t kill mature plants.”
He is keen to see what new technology may bring. With his current self-propelled spray rig getting due for replacement, Brendan said he would investigate emerging technology around camera-quipped spray rigs to target problem weeds.
An example of this is the John Deere See and SprayTM  technology, which featured in the winter 2021 edition of Spotlight magazine, and was developed from initial work on vision-based plant detection technology funded through a combination of industry research projects with CRDC, Sugar Research Australia, Hort Innovation and the University of Southern Queensland. He said it could be another useful tool in his toolbox to control FTR.
“You need to look at a range of options to control feathertop," Brendan said. "It’s something that can get away from you if you don’t treat it seriously.”

Photo by Cindy Benjamin, WeedSmart.

 
For more information contact CottonInfo Technical Lead for Weed Management, Eric Koetz on eric.koetz@dpi.nsw.gov.au or 0413 256 132. 
 
Read the CottonInfo information sheet on Feathertop Rhodes grass and read all our resources on weed management
Read the Queensland Department of Agriculture and Fisheries information sheet, and the Grains Research and Development Corporation information on integrated weed management for FTR. 
Videos: 
What our video on minimising glyphosate resistance in the cotton farming system. 
An extra note on herbicide mode of action classification changes: 

Herbicide mode of action (MoA) classifications have been updated internationally to capture new active constituents and ensure the MoA classification system is globally relevant.

The global MoA classification system is based on numerical codes, which provides infinite capacity to accommodate new herbicide MoA coming to market, unlike the alphabetical codes currently used in Australia. Read more from CropLife Australia

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