There are two species of wild oat now prevalent across the UK – the familiar spring germinating avena fatua and the winter germinating avena sterilis ssp. ludoviciana, or sterile oat.
The sterile oat is now widespread, particularly across the Midlands and southern England, and it is generally perceived to be more tolerant to cold and drought conditions.
Both wild species may occur within a single field, sometimes in mixture but sometimes in discrete, separate patches.
Challenges to controlling the weed are exacerbated by the fact its large seed size means it is able to germinate from as deep as 15cm, so cultural controls, such as those employed to combat black-grass, are not appropriate.
This is according to Dr Bill Lankford, Adama herbicide technical specialist, who says it is a well-known fact that wild oats can develop resistance to ACCase modes of action including fops and dims, but less so to ALS-inhibiting herbicides.
Growers may be led to believe that low efficacy of certain herbicides last year was down to resistance, but it could be the result of a number of other factors.
He says: “The incidence of herbicide resistance reported by institutes doing resistance testing is actually quite low.
Bear in mind samples sent in will have a degree of bias because they are sent in as a result of a failure by a herbicide.
Only around half of those samples sent in have actually suffered from resistance.
Resistance is an important consideration, but it is still quite low.”
It is also complex in wild oats, with genetic research showing there are five mutations to the target enzyme in the ACCase mode of action.
“That means in the UK and Ireland you can get a wild oat resistant to a fop but not a dim, or a den but not fops.”
Reliant Last season’s cold and dry spring weather also meant active ingredients such as clodinafop were less effective because they are reliant on active growth to move the active into the meristem of the target species.
Dr Lankford says: “That is the rationale behind needing vigorous spring growth of the weed to get maximum efficacy.
When you have cold, dry weather, that is not happening and even an application targeted at the right growth stage of the weed may not have worked quite as well, particularly in spring 2021.
“Another option is you may go a bit later when conditions are better – that is fine, but the crop canopy will be more vigorous, which can make good coverage of the target more problematic and your target species has more biomass. Application efficacy becomes really important for those later applications.”
Keeping application speed down to 12kph will improve the efficacy of the herbicide (see photo, below).
Dr Lankford says: “For early applications before GS31 this year, we advise using 100 litres/hectare and keeping to 12kph.
Nozzle choice is more flexible [early on] and current practice is pretty good as long as speed is reasonable.
“If you do go at the latter end of the Topik timing, which is up to GS41, really consider whether you can up the volume. You are going to have target plants considerably bigger than in early spring, smothered by a crop canopy. Anything you can do to improve spray penetration, such as using different nozzles, will be to your advantage.”