As featured in Arable Farming Magazine
Weed control without glyphosate
by Arable Farming
Spring barley volunteers have so far proved the biggest problem in AHDB-funded trials established last autumn to look at weed control without glyphosate. Alice Dyer gets an update.
With glyphosate’s years in the weed control toolbox potentially numbered, AHDB is carrying out an autumn black-grass trial to determine if a pre-drilling alternative can be found to the herbicide.
In the project’s first year, the tramline trials laid out this autumn compare the farm’s standard approach to pre-drilling weed control using a power harrow and glyphosate, with a Vaderstad Cultus heavy duty spring tine with narrow points and levellers and a Bomford pigtail tine with wide duck-foot points, both without the use of glyphosate.
Heavy soil types
The trial field has large areas of different and heavy soil types, which were reflected in the weed populations and pre-treatment, black-grass was only found on the headland of one tramline with 41 plants, but not in any of the treatment tramlines.
ADAS’ Dr Sarah Cook, who is leading the trials at AHDB’s Strategic Farm West in Warwickshire, believes the main reason for this is the vast number of spring barley volunteers (400/square metre) outcompeting the black-grass and changing the temperature and moisture levels of the soil.
Cultivations were carried out on September 18, 2020, and repeated on September 30 on the plots without glyphosate.
All plots were then drilled on October 1.
At a cultivation depth of 3-8cm, the farm standard approach of a power harrow and glyphosate kept seed in the top layer of the seedbed and spring volunteers were ‘well destroyed’, Dr Cook says.
She adds: “The [Bomford] spring tine with no glyphosate was at 4-7cm [depth], so still shallow and did not bring up any mysteries from the deep or populations in the seedbed.
It left soil in a mini ridge and it didn’t manage to remove all the spring barley volunteers and some survived.” The Cultus was used at 4-8cm deep, but up to 10cm on light parts of the field and only 2cm on heavier soil.
This variability could bring in problems if used as a stand-alone treatment without glyphosate, Dr Cook says.
Although the tilth produced better than with the Bomford spring tine, the Cultus left large, 15-20cm sized clods on the heavier soil.
Spring barley removal was also better with the Vaderstad, but still left a lot of barley volunteers intact.
On October 26 the crop was assessed at growth stage 12-13 where good establishment was seen across all of the treatments.
Black-grass remained minimal but cleavers were found in high numbers on parts of the field with medium loam soils.
Four months post-drilling, at the end of January, volunteer barley remains a problem across the whole trial but is particularly bad in the plots where glyphosate was not applied.
Cleavers and some charlock and groundsel are also still present, but farm manager Rob Fox says black-grass is his biggest concern.
“Broad-leaved weeds in cereal crops don’t concern me as much because we have plenty of options like Ally and Starane to take it out.
Being an aggressive plant, you have to get cleavers early enough but they’re fairly easy to control.
It’s the volunteer cereals and grass-weeds where our options are hugely limited.” Once fields dry up in spring, spring barley volunteers will be treated with Topik (clodinafop).
Mr Fox adds: “My long-term worry is any black-grass legacy in there.
It’s a very varied field for black-grass and there’s some very bad areas in it and some very clean areas, so it will be interesting to see what’s there come June.” The farm’s standard method of attack against black-grass is glyphosate pre-drilling or a spring crop following glyphosate.
Currently the trials are not showing much variation between the different machinery passes and how many weeds are present, Mr Fox says.
“The issue with cultivating two or three times more than we normally would is expense, fuel use, carbon release and creating too fine a seedbed which is not weatherproof.
If we get rain before drilling, then the top two-three inches [of soil] turns to mush.” The same trial was attempted two years ago, but the hot, dry September meant soil was too hard and then it did not stop raining, highlighting the challenges of potentially losing glyphosate as a vital pre-drilling tool.
In a wet autumn, attempting cultivations could exacerbate black-grass problems, Dr Cook adds.
“If you move wet clods around, black-grass is very good at surviving, wheat isn’t so good and black-grass will just re-root.”
To combat this, Dr Cook advises striving to reduce black-grass populations and seedbank now, while glyphosate is still a tool that can be widely used and targeting weeds at the white thread stage when they are their smallest and most vulnerable.
Further trials by AHDB found that much lower levels of blackgrass are typically seen in both winter and spring crops where an autumn cultivation is adopted, regardless of whether glyphosate is used (see graph).
However, the jury is still out when it comes to cover crops and more work is needed to truly understand their long-term impact on weed populations, says Dr Cook.
“Cover crops definitely suppress weeds and alter the atmosphere, conditions and surface of the soil and discourage other seeds from germinating, but we don’t have a lot of information on what happens to those seeds – are they waiting for opportunity to germinate? Cover crops also bring in more predators to eat seeds.
A good crop will also compete against black-grass, so a competitive crop is probably the key thing here.” Loss of glyphosate would also severely impact cover crop management, she adds.
“Often a cover crop can turn into a weed. If you can’t get rid of it, you have competition against your crop.”
Perennial weed challenge
Organic farming is a widely considered goal in Denmark as pesticide use, particularly glyphosate, comes under increasing government restrictions.
And while organic area is fast-growing in Denmark, perennial weeds, namely Canada thistle (creeping thistle), are forcing a small percentage of farms back into conventional systems.
Poul Henning Petersen, senior crop specialist at agricultural knowledge and innovation centre, SEGES in Denmark, says: “Canada thistle is a serious problem for organic farmers in Denmark and it will become a problem for conventional crops [if glyphosate use is reduced].”
Field trials this autumn as part of a Danish Agriculture and Food Council voluntary initiative to reduce the use of glyphosate found disc harrowing three times at 5-6cm depth was as good as glyphosate pre-sowing.
“However, it must be said this autumn was very dry so all the volunteers have killed very easily,” Mr Petersen says.
“Use of glyphosate in Denmark has increased a lot in the last 15-20 years, so we know politicians are concerned about the size of consumption.
Using preharvest in cereals is the most important use for control of weeds but also to make harvest easier.
It is also important for cover crops desiccation and stubble in autumn.
“In Denmark we are also doing work into intercrop hoeing.
It’s used in maize in combination with chemical control and as a last treatment in sugar beet after two herbicide treatments.
We also tried to use hoes in cereals, but I don’t think it will be used by farmers as long as we have herbicides.”
In the field: Rob Fox, Warwickshire
Over the past seven years, Rob Fox has made a concerted effort to reduce the high numbers of resistant black grass at Squab Hall Farm, Warwickshire.
Upping seedrates from 300 seeds/sq.m to more than 400/sq.m, growing more competitive varieties and using weed mapping has all shown marginal gains, but the most valuable tool has been an increase in spring cropping, he says.
“In seconds wheats we couldn’t do anything about the seedbank before going into OSR.
The level of black-grass return going from a first wheat and into a second wheat and beyond was just completely outrunning us and there was very little we could do about it.”
Second wheats have been swapped for spring barley, giving an extra opportunity to spray off weeds before the crop is established.
“Black-grass is not completely eliminated but certainly where we’ve had small patches in the past, we’re getting those completely gone and where we’ve had very dirty fields we’re being able to put those back into a decent winter wheat.
“We’ve also been in the situation where we’ve spring cropped particularly bad fields two, three or four years in a row.
One particular field had three spring barleys and spring beans before we were happy to put that back into a winter wheat situation.” Although ploughing is not the farm standard, on particularly dirty ground, a plough is occasionally recruited to get the seedbank to a more manageable level.
Mr Fox says: “This year we got a decent pre-em on and on a lot of wheat ground we have a follow up residual in October as well.
I think on the whole, where we’ve managed to get a decent stale seedbed it’s going to be an average year weed-wise.