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07 Sep 2021

The wild oat challenge

As featured in Arable Farming Magazine

The wild oat challenge

by Arable Farming

After a bumper year for wild oats, Alice Dyer finds out why their control is getting harder.

With cold and dry conditions hampering herbicide activity and many crops sown in less than ideal conditions last autumn, wild oats had a perfect opportunity to take hold this season, with the weed able to complete its life cycle in just a few weeks.

NIAB weed biology specialist John Cussans says: It is an ever-present weed but this year has been particularly bad. Wild oats will come and go seasonally but, crucially, if we do not react to years like this where we have had a failure of control and got a lot of seed return, we are building up a problem over time.

There are two types of wild oat species in the UK avena fatua, the UKs native wild oat which is spring emerging, and winter wild oat, avena sterilis ssp. ludoviciana.

Winter wild oats were first recorded as an arable weed in 1926, but their prevalence continues to grow. Last year, studies revealed that across the central southern band of England, where the weed is most prevalent, 60% of samples contained winter wild oat, growing alongside the spring germinating species.

This mix poses enormous control challenges when germination can effectively take place from September until June or July, says Mr Cussans.

Do not be fooled by the names for spring oats, around 30-40% will germinate in autumn and the remaining 60-70% in spring. For winter oats, 70-80% emerge in autumn and germination is protracted.

Seed return

However, there are a number of integrated weed management (IWM) methods that can be used to get on top of seed return.

Mr Cussans says: A true no-till scenario, where you are leaving wild oats on the soil surface, can be fantastically effective. As a grass-weed, oats are one of the new eaten by seed predators like carabid beetles, mice and birds because they are big and nutritious and found in large patches.

Studies have found that up to 50% of wild oat seeds can be removed through predation in the seven days after harvest. However, the large nature of the wild oat seed also means it can germinate from much further down the soil profile compared to black-grass. Studies show that even at 17.5cm depth, around 45% of seed is capable of germinating.

So, conversely to no-till, if tillage is being used then it must be deep and inverted to be effective, Mr Cussans says.

Harvest is also a major cause of seed movement, with seeds being spread within and between fields ‘like wildfire, so biosecurity is key.

Looking ahead to the next crop, wild oats are incredibly competitive, but also very sensitive to crop density and competitiveness.

You can have massive reductions in wild oat seed production by increasing crop density. With wild oats it is a double whammy if youve got good crop canopy

closure and very little light getting to the soil surface, youll massively suppress these late germinating weeds as well.

We are using later drilling as a key part of IWM, but were in an environment where it just doesnt suit late drilling to get a suppressive crop, and thats the elephant in the room that will need to be addressed at some point, Mr Cussans says.

Although the signs are there that resistance to contact herbicides is developing, in contrast to many of its grass-weed cousins, wild oats are both in-crossing and hexaploid, with complicated genetics, meaning resistance is very slow to develop.

Resistance development to contact herbicides in wild oats is incredibly slow and nowhere near the levels we see in black-grass and ryegrass, despite being exposed to these herbicides just as much and for as long. The majority were also only resistant to one group of herbicides, unlike black-grass where if a fop doesnt work, a dim wont work either, says Mr Cussans.

However, the results did reveal that the winter species appears to be developing herbicide resistance quicker than its native counterpart.

Avadex (triallate) has been applied to wild oat populations for 60 years and there are no signs of insensitivity or resistance to it, Mr Cussans adds.

Barrie Hunt, technical manager at Gowan Crop Protection, says: Triallate is considered a multisite herbicide in contrast to a majority of other herbicide products, making it a valuable tool in resistance management.

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