Using residuals to keep irrigation infrastructure and borders weed-free
Everyone knows that non-crop areas on farms are the main breeding ground for weeds and herbicide resistance. Even so, timely weed control is often compromised as other important work gets prioritised.
Eric Koetz, NSW DPIRD researcher, says that residual herbicides can take some of the pressure off knockdown herbicides, which have been the mainstay control tactic for non-crop areas on farms for decades.
“Resistance to the key knockdown herbicides, particularly glyphosate, glufosinate and paraquat, has the potential to compromise herbicide-tolerant traits in cotton and other crops,” he says. “Using other herbicide and non-herbicide tactics to manage these areas is the best long-term strategy to protect crop yield.”
In addition to being a source of weed seed that can readily move into paddocks, weedy channels and borders also provide a green bridge for insect pests and disease inoculum and impede irrigation water flow.
“Implementing a robust residual herbicide program on non-crop areas buys growers time in the peak periods,” says Eric. “It is then less time-consuming to tidy up any survivors with other tactics, such as chipping or blow-torching, which can be done on-the-spot as weeds appear.”
In a CRDC-supported trial on the IREC field station at Whitton in the Riverina, Eric tested the efficacy of five herbicides with residual activity and glyphosate against grass and broadleaf weeds. Each herbicide has its own set of constraints and benefits.
Controlling weeds in non-crop areas fits in the ‘mix and rotate’ and ‘stop seed set – take no prisoners’ pillars of the WeedSmart Big 6 integrated weed management program to minimise the risk of herbicide resistance.
This year, Griffith is hosting WeedSmart Week on 18 and 19 August, showcasing how growers can implement the Big 6 tactics to tackle hard-to-control weeds across both the irrigated and dryland landscapes.
Which residual herbicides were tested?
The herbicide treatments were all applied in winter to provide a weed-free start to the cotton cropping season. The recommended registered herbicide rate for each herbicide was applied using a water rate of 100 L/ha.
The glyphosate control treatment was compared with treatments of:
- Valor (flumioxazin, Group 14) for knockdown and residual control of broadleaf weeds and grasses.
- Diuron (diuron, Group 5) for knockdown and residual control of broadleaf weeds and grasses. Requires moist soil at the time of application.
- Stomp Xtra (pendimethalin, Group 3) for residual control of most annual grasses and certain broadleaf weeds. No knockdown activity.
- Arsenal (imazapyr, Group 2) for knockdown and residual control of broadleaf weeds and grasses.
- Arsenal + Stomp Xtra, (Group 2 and 3 tank mix partners).
- Cavalier (oxyfluorfen, Group 14) for knockdown and residual control of broadleaf weeds and some grasses.
Six months after the herbicide treatments were applied, the demonstration strips were assessed for efficacy against the weed species present and the longevity of residual control.
How can I make the most of these herbicides?
Adding residual herbicides to the program for non-crop areas must focus on managing weeds that are tolerant of or resistant to glyphosate, such as annual ryegrass, fleabane and feathertop Rhodes grass.
For too long, a reliance on glyphosate for weed control in non-crop areas has compromised its efficacy in-crop and fallow areas. Every effort needs to be made to reduce the potential for herbicide-resistant weeds to spread from field edges and irrigation delivery channels into cropping fields.
The trial showed differences in how long the residual herbicide provided effective control and the effect of each herbicide on the weed spectrum present at the site.
Eric says that the Arsenal treatments provided the best long-term control, keeping channel banks clean for over six months. Valor, Diuron, Cavalier and Stomp Xtra (only grasses) provided good initial control, but the residual effect was short-lived. Consequently, using these later herbicides will require follow-up weed control.
He says that although Arsenal provides the best long-term control, it should be applied in the non-cropping season and, if used on irrigation channels, the irrigation water should be retained on-farm.
Valor offers medium-term control, but follow-up weed control tactics must be included in the program. The WeedSmart principle of mixing and rotating herbicide modes of action needs to be applied to non-crop areas. Neglecting to do so is a major driver in selection for herbicide resistance.
Residual herbicides can do much of the heavy lifting, vastly reducing the number of weeds present when a follow-up knock-down herbicide is applied. The WeedSmart Big 6 ‘mix and rotate’ pillar applies to all herbicide programs. To minimise herbicide resistance risk, consider mixing two or more compatible residual herbicides and avoid regular use of any single mode of action.
Are there non-herbicide weed control tactics suitable for irrigation infrastructure?
Residual herbicides can provide respite during peak periods in the cropping calendar, but they are not a one-off or routine solution for weed management on non-crop areas of the farm.
Site-specific weed control tools such as burning, spot spraying, and chipping are the most effective follow-up tactics to employ in conjunction with residual herbicides. Having the equipment always on-hand and including monitoring for weeds in everyday operations will pay off in the long term.
Keeping weed numbers low in non-crop areas will reduce the chance of tolerant or resistant individuals being present when herbicides are applied.
Commercial development of robotic and drone systems capable of navigating complex environments to detect and spot-spray small weeds also has great potential.
Where practical, vegetative ground cover is a set-and-forget option for borders and irrigation infrastructure. Some initial results from the Area Wide Management project support non-herbicide tactics such as establishing kikuyu on channel banks to suppress weeds.
WeedSmart provides a practical planning framework and farming systems approach to integrated weed management to keep weed numbers low, maximise crop yield and improve long-term farm profitability.
