Published on 22/07/2022
It controls an aggressive, potentially costly competitor before it can affect crop yields; and it provides another mode of action (MOA) to help manage weed resistance to commonly used post-emergence herbicides.
Some populations of wild oats in New Zealand are known to be resistant to Group 1 Fop herbicides; resistance to other Group 1 (Dim/Den) or Group 2 (SU) herbicides, which are widely used, has not been officially confirmed but is suspected.
Avadex Xtra is a residual thiocarbamate (Group 15) pre-emergence herbicide which has been used for selective wild oat control in NZ for many years.
Nufarm territory manager Jeff Hurst says even though it’s an ‘old’ product, it’s arguably even more important today than it ever was because of the development of resistance in wild oats to commonly used post-emergence herbicide MOAs.
“Avadex Xtra is an important tool for managing the risk of resistance on your farm.”
That’s on top of the inherent yield benefits with pre-emergence control of a weed that suppresses cereal yields right from the time that wheat and barley crops germinate.
“Potential yield loss from wild oats can be absolutely phenomenal, up to 75-90%. The beauty of a pre-emergence herbicide like Avadex Xtra is that you’re cutting that risk of yield penalty before it can even take effect, as opposed to relying solely on spraying post-emergence, and effectively waiting until wild oats have already impacted your yields.”
Avadex Xtra will not control germinated or emerged oat seedlings, or oat seeds sitting on the soil surface. Correctly applied, however, it can significantly reduce wild oat pressure in wheat and barley, with a further benefit being that oat plants which survive treatment may be sensitized to subsequent herbicide application.
Application technique and timing is very important.
It should be applied to soil just prior to, or up to three weeks, before drilling wheat or barley. After application, it must be immediately incorporated to a depth of 5 cm, using light harrows.
When this is done, Avadex Xtra creates a vapour layer below the soil surface.
“As the germinating wild oat seedling grows through this layer, the vapour is taken up by the elongating oat shoot, and it interferes with cell division and elongation.”
Jeff says without proper incorporation – which entails two passes over the paddock – results will be disappointing at best.
“If it’s not incorporated, the herbicide just sits on top of the surface of the soil. The vapour created when Avadex Xtra comes into contact with moisture disappears into the atmosphere, and wild oat control is minimal, because the coleoptiles don’t come into contact with it.”
If applied to very dry soils, Avadex Xtra remains inactive until rain or irrigation is applied, and it should not be applied to soils that are too wet to achieve good incorporation.
Wheat or barley seed needs to be drilled below the depth of incorporation to prevent injury to shallow planted crops, and growers are advised to drill seed at right angles to incorporation.
Use of multiple MOA groups in weed control programmes is recognized best practice to minimise development of resistant populations, and prolong the effective life of existing chemistry.
Pre-emergence soil residual herbicides like Avadex Xtra play a key role in this effort, Jeff says.
Which method works best?
A new field trial that has recently started in South Canterbury aims to find out how Avadex Xtra might fit into growers’ strategies for managing herbicide resistant grass weeds.
The trial has been initiated via FAR’s Growers Leading Change (GLC) programme, and will compare the results of three different incorporation techniques at one site with suspected Group 2 active resistance in annual ryegrass.
The crop is Voltron second year wheat, and the three treatments are tyne harrow, stubble rake and maxi-till, each used as the first incorporation for Avadex Xtra, followed by a second pass with the seed drill at right angles.
FAR senior field officer for vegetable and herbage crops, Ben Harvey, says recent surveys have shown grass weed resistance to Group 2 herbicides is quite common in different parts of New Zealand, and South Canterbury growers have experienced this as well.
FAR has conducted trials in recent years to find alternative herbicides to Group 2, particularly for removing grass weeds from cereals.
South Canterbury GLC members asked for the new trial to explore ways of improving the herbicide’s efficacy, and maximising its effectiveness when used as part of a sequence.
The trial will be taken through to harvest, after which results will be communicated to growers.
Growers Leading Change is a knowledge exchange programme which encourages arable farmers to consider, develop and introduce new ideas, technologies and ways of working.
It is co-funded by FAR and the Ministry for Primary Industry’s Sustainable Food and Fibre Futures Fund (SFF Fund).