As featured in Arable Farming Magazine
Diversity key to sustainable weed control
by Arable Farming August 2020
A multi-pronged approach to weed control that focuses on biology rather than chemistry can help tackle resistant weeds and promote diverse arable landscapes. Alice Dyer reports.
Integrated weed management is how weeds were tackled before the introduction of chemical solutions and while herbicides still have an important place in the arable rotation, the challenges of relying on ‘bag and bottle’ farming are becoming increasingly apparent.
Dr Jonathan Storkey, plant ecologist at Rothamsted Research, says: “When herbicides came along they were, and they continue to be, very effective.
“Because of that effectiveness we’ve become over-reliant and tend to assume any solution can be found in a can. We need to be thinking about taking a more
This means moving away from the ‘sledgehammer’ attack on weeds conventional herbicides bring, he says.
“We normally hit weeds really hard once in their life cycle with one [type of] weed control. If individual weed species manage to escape that single control, there’s not a lot we can do about them and they set seed.”
Herbicide resistance in the UK
The UK does not fare too badly on the world stage for resistant weeds, but the resistant species that are present here are causing considerable problems in the
In the UK there are 16 known resistant weed species that have developed resistance to nine herbicide modes of action, with most cases coming from cereal cropping situations, according to Dr David Comont, research scientist at Rothamsted Research.
In broad-leaved species, resistance levels are still relatively low, but resistant grass-weeds, including wild oats, ryegrass and particularly blackgrass, are now becoming very prevalent, particularly across England, says Dr Comont.
“Black-grass is reported as the number-one issue on arable farms. In part, what is causing that is some of the grass-weed populations have evolved resistance to multiple modes of action simultaneously.
“Recent studies have found UK brome populations may be developing herbicide resistance and populations of ryegrass are showing the first signs of reduced sensitivity to flufenacet.
Black-grass is also now showing the first signs of reduced sensitivity to glyphosate. We know from other countries that glyphosate resistance can evolve from other species.”
Dr Comont says the situation is down to a lack of diversity.
“There are fewer chemicals, fewer mixed farms and narrower arable rotations, with mainly autumn-sown crops. This helps autumn germinating weeds such as black-grass to flourish. We then become reliant on using herbicides alone.”
Integrated weed management tackles weeds throughout the life cycle using a variety of tools to suppress population with ‘little hammers’ says Dr Storkey.
“But key to this is understanding the biology of the weed,” he says.
“A much more diverse management approach leads to a more diverse weed community – but we need to understand the biology and ecology of each weed species to understand how they will respond to any given weed management tool.”
For example, spring cropping works for black-grass because it mainly emerges in autumn, but that would not work for a different weed species, with a different life cycle, he says.
“If you have a spring germinating weed such as fat hen, spring cropping will select for it and you will get more fat hen if you have more spring crops. Weeds with a persistent seed bank, such as poppies, which can survive in the soil for up to 50 years, would not benefit from rotational ploughing or tillage as black-grass might.
“When you think about what an integrated weed management approach is, the central idea is to keep weeds guessing so we’re not designing a simple system, which is probably what we tended to do in the past.
Mechanically weeding cereal crops
Inter-row hoes are becoming more widely used in row crops to control competitive weeds, but detriment to yield is often cited as a barrier to their use in cereal crops.
NIAB’s Will Smith, who is studying the use of inter-row cultivations for weed control for a PhD, says there could still be a yield benefit with wider rows if weed pressure is reduced.
Another concept NIAB is looking to explore further is how inter-row cultivations can be used alongside herbicides.
Mr Smith says: “We’re segregating herbicide and cultivation, so the herbicide is sprayed in a narrow band over the crop row, while the hoe will then cultivate the remaining space between the rows at a suitable later timing.
“On the spacing we’re looking at, this is reducing herbicide use by about 60%.”
More on integrated weed management
To find out more about integrated weed management, listen to episode two of our Crop it Like it’s Hot podcast, The future for sustainable arable weed management in the UK.
To listen, visit croptecshow.com
“We’re trying to mix up the tools and our approaches, so we have a much more diverse farming system, so any given weed does not know what is coming next.”
This would also result in more diverse weed communities that are not dominated by a single species, says Dr Storkey.
“One of the effects of integrated weed management is you end up with a more diverse weed flora. Science is indicating that if you have the same number of individual weed plants but a more even and diverse community the competitiveness of those weeds in any given crop will be lower.”
This is going to improve on-farm biodiversity, with studies showing weed species preferred by pollinators and birds tend to be less competitive species such as early flowering broad-leaved weeds like chickweed, and meadow-grass, says Dr Storkey.
“I believe weeds will not always be bad all of the time – there might be some opportunities to tolerate some beneficial weeds.”
Getting spring cropping right
Spring cropping has become a well-used tool in black-grass management, but delaying drilling until soils are warming is an important factor for adequate control.
Will Smith, trials manager at NIAB, explains: “Our work last year showed that drilling date throughout spring is as significant as during autumn, with a similar pattern of reduced black-grass emerging the later you go in the season. Earlier drilling before February can result in as many seedlings as found in autumn-sown crops.”
The study found the best balance is to delay drilling until the beginning of March, when warming soils can help drive the competitiveness of the crop, Mr Smith says.
“We formulated a matrix trial to see how the overall advice we typically give for black-grass control relates to cultivation and spring drilling.
“Direct drilling seemed to be less effective for [control of] Italian ryegrass. It also seems that being such a competitive weed, ryegrass plants tiller more profusely with greater seed return and therefore, spring cropping overall is less effective. However, spring cropping is still an effective option at fighting black-grass problems.”