As featured in Arable Farming Magazine
New survey aids growers in fight against wild oats
by Arable Farming Feb 2021 Issue
With some growers perceiving a rise in wild oats, new survey results and revisiting control methods will provide a clearer picture of how to tackle the weed. Marianne Curtis reports.
The first wild oat survey for 20 years has shown an increasing incidence of the winter wild oat (avena sterilis ssp. ludoviciana) in some regions and higher occurrence of resistance to herbicides in this species than in the common wild oat (avena fatua).
Ruth Stanley, country manager at off-patent crop protection manufacturer Life Scientific, says wild oats have become the forgotten enemy.
Most competitive
“It’s actually our most competitive grass-weed, on a potential yield loss/plant basis.
Just one wild oat plant/square metre can reduce yields by as much as one tonne/hectare in winter cereal crops, and 0.6t/ha in spring cereals.”
With increasing numbers of growers asking about wild oats, NIAB weed biology specialist John Cussans says: “We felt it was really time to provide an updated picture of wild oat herbicide resistance and also a general picture of where we are with wild oats currently across the UK, so we asked for wild oat samples to be submitted after harvest, along with a detailed questionnaire.”
NIAB and Life Scientific collaborated on the survey which saw 105 samples submitted by growers or agronomists – 97 of these were viable for testing for their susceptibility to pinoxaden (for example Axial Pro) and iodosulfuron/mesosulfuron (for example Niantic).
Of the samples, 70% contained the common wild oat and 30% the winter wild oat or both, says Mr Cussans.
“When people have looked at the winter wild oat in the past they have found about 10% of wild oat populations containing it so it was surprising to find 30%.
Distributed Samples were distributed in a band across central England, with winter wild oats accounting for half the population in some areas, he adds.
“Why it seems to have gone under the radar is difficult to tell.
However, it is difficult to tell the two species apart – you can only discriminate between them when the seeds are being produced on the adult plant.” (See Identifying wild oats panel) According to the survey, common wild oats tend to be more prevalent in rotations containing more spring and break crops, whereas winter wild oats tend to be seen more in rotations where there is a lower proportion of spring crops and break crops, says Mr Cussans.
“We had 11 samples that were from continuous cereal rotations and either 100% or 99% of wild oats in these scenarios were winter wild oats.” Reasons for an apparent increase in wild oats are both cultural and chemical, says Ms Stanley.
“There are several factors. Min-till and direct drilling have become more popular.Burial [ploughing] increases the dormancy of wild oat seeds.
With min-till and direct drilling, seeds are left on the soil surface which increases germination, particularly when soil is moist.
“Also, there has been a reduction in the use of ALS chemistry, traditionally used to control black-grass, but growers worried about reliance on this chemistry for black-grass control have forgotten that these herbicides are actually very effective at wild oat control.”
Resistance
Examining herbicide resistance in winter and common wild oats, Mr Cussans found that 10-15% of populations showed resistance to ACCase inhibitor pinoxaden, with no great difference between wild oat species.
Five to 10% of common wild oats showed resistance to ALS herbicides compared with 10-15% of winter wild oats.
“The initial findings point to cases of resistance in both species to both Axial Pro and Niantic, with the study confirming the occurrence of resistance is indeed higher in the winter wild oat.” Ms Stanley adds: “Although we’re getting more and more reports of wild oats as a problem weed, it’s reassuring to know from these survey results and the resistance testing, that ALS chemistry such as Niantic and Cintac still work and have a valuable place in the herbicide programme to provide efficient control.
“It’s important to ensure correct product application to prevent decreased sensitivity in the field, so we continue to get the best performance from these herbicides.”
Identifying wild oats
Common wild oats (avena fatua) have single seeds with awns. Winter wild oats (avena sterilis ssp.
ludoviciana) have distinctive twin seeds, each seed with an awn, which are shed together and difficult to pull apart.
Looking in the centre of the pairs of seeds it is sometimes possible to see a third or fourth seed which do not have awns.
Source: John Cussans, NIAB
Effective control
Now is the time to check the wild oat situation on your farm as treating them early makes them easier to kill, says Syngenta grass-weeds technical manager Georgina Wood.
“Look out for germinated wild oats, particularly winter wild oats – the waxier and more hardy they are, the more difficult they are to control.
Also, they are larger when you can get on the land to make an application and will need a higher rate of chemical to control them than those that germinate in spring.” The challenge is that wild oats can germinate from autumn to spring and growers are concerned that if they go early, they will miss the later ones, says Ms Wood.
“Those that germinate in autumn are more damaging to yield as they are bigger and more competitive with the crop.
They are likely to have more tillers and produce more seed which can be an issue for the seedbank.” It is easier to hit weeds with a finer spray when the crop is more open, says Ms Wood.
Stem elongation “As crops go into stem elongation, the crop shades them more so larger spray droplets, produced, for example, by changing to an Amistar/air induction nozzle, are needed to get into the canopy to target the weeds.” This year, failure to apply pre-ems in autumn because ground conditions were too wet in some areas could have had a negative impact on wild oat control, says Ms Wood.
In the field: Colin Woodward, Oxon
Wild oats have been a historic problem on Colin Woodward’s farm near Chipping Norton, Oxfordshire, with most issues showing up in his spring barley crops.
He says: “The problem stems from the historic cropping and cultivation techniques used in the past.
And as wild oat seeds can lay dormant in soils for up to 20 years, this makes them quite a challenge for us.
“Although wild oat control is pretty good these days, there is always a chance the conditions could be right for historic seeds to germinate which if left unchecked further increases the seed return and weed burden.
“Because we grow spring oats in our rotation, we need to control the wild oats in our other cereal crops so they do not become a problem when we cannot control them.” Another challenge Mr Woodward faces is the lack of contact-acting products that can be used on barley.
“We do not use any residuals on spring barley, unless we are planting early, because I feel they are not effective if the spring turns dry and often knock the crop back, making it less competitive against black-grass,” says Mr Woodward.
Peace of mind
“In the areas where I know we usually have a problem I routinely apply Foxtrot EW, at the end of tillering, around GS27- 29.
This cleans up the crop, removes any wild oats and gives me peace of mind that I am keeping the burden low for the subsequent season.” Although the wild oats are a bigger problem in spring barley, Mr Woodward also sees them in winter cereals.
“In our winter wheat we control the wild oats at the same time as tackling brome and other grassweeds, by applying a contact herbicide in spring at GS30-31 when the weather warms up.
“In the winter barley I would hope our autumn residual herbicide stack will have prevented any wild oats from coming through, but if I do see an issue, I will follow up with a pinoxaden application later in the season,” he says.
Mr Woodward adds that cost is a big factor in decisions regarding his herbicide strategy.
“Because of the coming reduction in BPS payments, we need to count every penny and look at each variable input cost, reducing where we can without affecting outputs.
We do not want to compromise yield, but the crop must be profitable to be viable,” he says.
“Getting on top of these weeds is really important as any added competition for nutrients, water and light could cause a yield penalty.
Having a strategy in place that is both efficient and cost effective is really important to us,” he says.
How mapping and testing can help tackle wild oat problems
Where growers are experiencing higher levels of wild oats, this is most likely due to cutbacks in use of herbicides such as Atlantis (mesosulfuron + iodosulfuron), due to concerns over lack of efficacy for blackgrass control, says independent weed scientist Stephen Moss.
“This has taken the focus away from controlling wild oats [where these products are still effective],” adds Dr Moss.
Despite concerns 20 years or so ago that resistant wild oats would become much more prevalent, this has not materialised to the extent anticipated, he says.
“Resistant wild oats are widespread in the UK but did not build up as quickly as blackgrass.
However, that could change, so farmers should be on their guard and take action to keep populations in check.
Wild oats are one of the most competitive weeds and resistant types occur in 18 countries worldwide.” Where the resistance status of wild oat populations is unknown, plan ahead and consider having a resistance test done this summer, says Dr Moss.
“Black-grass is crosspollinating so resistance can be transmitted in pollen, but with wild oats this is not an issue as they are self-pollinating.
Seeds can move but populations do not build up as quickly and wild oats are often confined to patches.”
Rogueing
Mapping patches and rogueing or spraying off before shedding can help to get on top of the problem, he adds.
“If you know you have wild oats in certain fields do not ignore them – seed lasts quite a long time in the soil.
If you’ve had problems in the past, give priority to fields where more wild oats come up and treat with an appropriate herbicide in spring.
“In summer it is about mapping where the wild oats are.
More farmers should map where weeds are in June and July so they can focus management on those areas.
It is imperative with any grassweed that prompt action is taken to contain the problem, otherwise the patch becomes the field.” For a ‘low tech’ option, Dr Moss suggests using the what3words app, which allows mapping of three-metre x 3m patches and using a knapsack sprayer to treat affected areas.
“While some farmers do spray patches off, it can be haphazard and often they do not record the location.
Many farmers do not use GPS technology as well as they could do, particularly when weeds are confined to patches.
More accurate mapping allows weed patches to be targeted for control in successive years, which is the only way to drive down populations.
“If there is more pressure to reduce pesticide use and cut costs, targeting patches can prevent an epidemic in the whole field.” Where wild oat populations are low, hand rogueing can be very effective, says Dr Moss.
“Even in fields with patches that are too thick to hand rogue, it is still worth hand rogueing the rest of the field.
By doing so, you can stop most of the field becoming infested.”
Herbicides
When controlling wild oats with herbicides in spring, Dr Moss recommends not going too early.
“Make sure weeds have emerged and are actively growing.
This could be March, April or May depending on the product and weather.” He suggets products such as Broadway Star [pyroxsulam + florasulam] and Axial Pro [pinoxaden] in winter cereals.
“Avadex granules are a good choice in spring cereals and some other spring-sown crops.
In oilseed rape and beans, graminicides such as Falcon or Pilot can give good control – provided populations are not resistant.”
