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New Group 14 herbicides expand broadleaf weed control options

There has been a gap in the pre-emergent herbicide options for growers to control broadleaf weeds, particularly in pulses, but also in cereals.

Dr Chris Preston, Professor, Weed Management at The University of Adelaide, says the suite of Group 14 [G] herbicides now on the market is helping growers to control brassica and thistle weeds, and to rotate away from the imi chemistry that has been heavily used in recent years.

“These newer Group 14 herbicides are of particular value in pulse crops,” he says. “Until recently, Group 14 products have only been used in small quantities, predominantly as a knockdown spike ahead of planting, but several newer products have pre-emergent herbicide use patterns.”

The Group 14 mode of action inhibits the plant’s ability to produce chlorophyll, and the leaves die. Uptake of the herbicide is usually through the leaf surfaces – either through contacting emerged weeds or taken up from the soil as the weed seedlings break through the soil surface.

“The introduction of Terrain (flumioxazin) as a pre-emergent broadleaf herbicide in front of faba beans offers a helpful use pattern in the pulse phase,” Chris says. “Terrain is also a good fence line weed control option on heavier soils, provided all surface vegetation is removed before application.”

Another option in pulses is Reflex (fomesafen), which offers growers pre-emergent control of broadleaf weeds that might resist other mode of action (MOA) groups. This product can be used either pre-emergent incorporated by sowing (IBS) or post-sowing, pre-emergent (PSPE), and provides an alternative to imi herbicides for the control of weeds like sowthistle and prickly lettuce in pulse crops. Lentils are less tolerant than other pulse crops, so Reflex can only be used IBS in lentils.

A third recently released Group 14, Voraxor (a mixture of saflufenacil and trifludomoxazin), provides a pre-emergent control option for broadleaf weeds in front of cereal crops.

As these products are residual in the soil, Chris says it is essential that growers check labels for appropriate rates and plant backs, both in front of the crop sown and for following crops.

He also reminds growers and agronomists of the importance of ensuring that herbicide product choice is always based on addressing the likely weeds present that could cause economic loss or produce large quantities of seed.

Mixing and rotating herbicide modes of action is one of the WeedSmart Big 6 tactics, and crop competition is just as important,” he says. “Pulses are particularly sensitive to competition from weeds in the early crop stages.

Using these Group 14 herbicides in a tactical way to control early germinating broadleaf weeds could be an excellent way to keep pulses profitable in our southern farming systems.”

The WeedSmart Big 6 strategy embraces herbicide and non-herbicide weed control tactics to keep weed numbers low in cropping systems. The latest weed control tactics and technologies will feature at WeedSmart Week in Port Lincoln on 29–30 July 2024.

For long-term sustainable use of these herbicides, look for opportunities to mix and rotate herbicides within and between seasons and crops. For example, Terrain offers broad-spectrum weed control in tank mixes with TriflurX, Terbazine, Avadex Xtra, Kyte, Simagranz and Rifle. Terrain has a narrow weed spectrum for the rate registered in-crop for faba beans.

With some clever planning, these products can help ‘bring back’ some previously lost chemistry using the mix and rotate tactic, extending the effective life of a broader range of herbicide options. When Group 14 herbicides are coupled with some non-herbicide tactics, the grower can regain control of herbicide resistance on their farm and operate in a low-weed situation.

Although the Group 14 mode of action has been commonly used for over 15 years, these products have generally been used in small quantities. There are no recorded cases of weeds resistant to Group 14 herbicides in Australia.

However, resistance to Group 14 is becoming widespread in North America. Chris urges Australian growers to practice the WeedSmart Big 6 tactics to minimise weed seed production and stave off resistance in our cropping systems.

Until recently, Group 14 herbicides were used mainly as a spike to enhance the knockdown effect of other herbicides. As glyphosate resistance becomes more common in weeds present in Australian cropping systems, spikes are an even more critical component of double-knock strategies.

“Spikes on their own do not control glyphosate-resistant ryegrass”, says Chris. “However, adding a spike like Terrad’or (tiafenacil) to the paraquat application makes the double-knock more robust. Spike application rates also select for resistance to Group 14 herbicides, so including other weed management practices is vital.”

Resistance profile of Group 14 mode of action (CropLife)

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