Redefining fenceline weed control options

Weeds in non-crop areas have free access to moisture and nutrients, and control is often delayed until after planting, allowing these weeds to produce vast quantities of seed.

Some new herbicide registrations may help growers regain control and reduce the risk of herbicide-resistant weed seed moving from the borders into paddocks. If used alongside cultural practices, these residual herbicides may solve what has become a significant problem for growers.

Dr Peter Boutsalis from The University of Adelaide and Plant Science Consulting says glyphosate resistance is of particular concern in cropping paddocks. In the 2020 random weed survey in Victoria, one-third of the paddocks sampled had annual ryegrass populations that tested positive for glyphosate resistance. Nationally, it was 16 per cent of cropping paddocks, not including fencelines.

“Paraquat resistance is also on the rise, with at least 50 ryegrass populations confirmed from cropping paddocks nationwide. This rise in paraquat resistance demonstrates that treating large weeds on fencelines with knockdown herbicides in spring is unsustainable,” he says. “Although the number of paraquat-resistant populations is still relatively small, many carry resistance to both glyphosate and paraquat.”

Peter is strongly urging growers to be proactive in taking a new approach to fenceline weed management.

“The best time to undertake fenceline weed control is in autumn, before seeding, when the weeds are small and the weather is still warm,” he says. “One significant advantage of using residual and knockdown chemistry at this time is that growers can achieve effective, long-term control in one pass.”

Bayer Crop Science and Nufarm Australia have recently brought products to market with new registrations for fenceline use patterns. Alion (indaziflam, Group 29 [O]) from Bayer and Terrain Flow (flumioxazin, Group 14 [G]) from Nufarm both have strong residual properties, control a broad spectrum of weed species and have a good margin of safety when applied near trees following label instructions.

If weeds have emerged in the area to be treated, include an effective knockdown in the tank mix to eliminate these weeds while they are small and at the same time applying a uniform application of the residual herbicide on the soil surface.

Both products can be applied to dry soil several weeks before being incorporated into the soil by rainfall. The labels provide all the necessary details about rates, mixing partners and mixing order.

Uragan (bromacil, Group 5 [C]) from Adama has been on the market for a decade and is registered for long-term residual weed control on fencelines and other non-cropped areas. It controls many important weed species, including multiple-resistant ryegrass, brome, barley grass, summer grasses and many broadleaf weeds, including wild radish. This product is not safe for use under trees and requires moist soil at application for best results.

The use of Alion and Uragan in broadacre cropping situations is for fencelines only, with no in-crop use permitted, whereas Terrain Flow has both in-crop and fenceline registrations. Rotating between these three herbicides with diverse modes of action for residual weed control along fences can reduce selection pressure for resistance development. Field trials indicate that growers can expect at least six months of complete weed control with these products.

To preserve these herbicide modes of action, it is essential that growers adopt the WeedSmart Big 6 ‘mix and rotate’tactic on non-crop areas. For example, apply Alion in years 1 and 2, Terrain Flow in year 3, then Uragan in year 4 before returning to Alion in year 5 if needed. Use these herbicides in combination with a knockdown such as glyphosate, paraquat or amitrole if emerged weeds are present.

John Stevenson, southern regional manager for Warakirri Cropping, has been addressing the threat of resistant weeds on fencelines and firebreaks for many years, often feeling that it was a losing battle.

He believes these new residual options, along with cultural practices, will be very beneficial in the weed management program on Warakirri properties.

“Wherever we can, we are trying to put pressure on weeds along our fences and firebreaks by increasing crop competition and stopping weed seed set at all costs,” he says. “To start, we are sowing our crops as close to the fences as possible. Before harvest, we spray-top, mow, condition and bale two passes of the outside border of the paddocks to ensure no weed gets through the spring.”

This practice has the added benefits of providing excellent, weed-free access to the paddocks for harvest and a reliable firebreak. They also monitor these areas for any weeds that re-grow after the mower has been through. John believes the new long-term residual options will further strengthen this management strategy.