Do a little, or do a lot – when it comes to weed control and cultivation choices, it all depends on the individual scenario at hand. CPM takes a look at what to consider when the preference is to reduce tillage intensity.
“The biggest danger is complacency; we’ve had a good season for control but there’s still blackgrass in the seed-bank.” CHRIS DICKINSON
By Rob Jones and Janine Adamson
Without cultivation, crop competition is crucial for weed control in low disturbance systems, which can often depend on soil conditions – also the basis for good establishment and herbicide efficacy.
“A high-yielding crop without doubt has a good rooting system to support it, so I don’t agree with the idea that zero till means accepting significantly lower yields,” says independent soils and cultivations adviser, Philip Wright.
He believes the long-term aim is a resilient soil that allows unrestricted movement of roots, along with water and air for aerobic conditions that support biological activity and crop growth. Good drainage is also must.
However, moving to less intensive cultivation isn’t a simple process. “It requires more time in the field with the spade assessing the situation, anyone going down this route should take that detailed approach.”
According to Phillip, low disturbance and zero tillage keep weeds in a certain stratum, which should make them easier to manage. “But we still see problems because a small number of weeds make a lot of seed. As a principle, I don’t see a problem with using the plough to reset weed seed numbers because it doesn’t undo the work improving soil, but use it sparingly.
“More generally, whenever a farming system is in operation for consecutive years, nature adapts and can instigate problems. Strategic tillage is shown to work – if you have to do it, fix the problem, and the soil will recover.”
A trial at Agrovista’s Lamport AgX site further demonstrates the interaction between cultivation approach and weed control, but this time from a pre-emergence herbicide efficacy perspective.
Technical agronomist, Chris Dickinson, says the overall picture is that like many, blackgrass control has been better this season. “The chemistry has worked well, even on rough seedbeds, and the dry spring suppressed blackgrass which thrives in wetter conditions.
“In both seasons, the pre-em programme based on Proclus (aclonifen) performed better than the Luximo (cinmethylin) programme on shallower tilled plots. However, on deeper cultivated plots, the cinmethylin has the edge.”
In year one of the trial (drilled 15 October 2023) there were six cultivation depths ranging from direct drilled to a Sumo Trio. This season (drilled 1 November 2024), there were two cultivations: two sequential passes with a straw rake to tickle the surface versus an X-Press disc cultivator at 90mm.
On farm, Chris suggests cultivations around 40mm deep are a good balance between creating tilth but not moving too much soil. “In year one, the 40mm cultivation was where both herbicide programmes delivered equal levels of control.
“The more soil we move, the more blackgrass we mix into the profile leading to increased germination. We could see that effect in the untreated plots in 2023-24 – we harvested 4t/ha in the no-till but only 2t/ha from the most intensively cultivated plots in the untreated part of the trial.”
With it being dry this season, Chris advises leaving soil undisturbed until there’s enough moisture to cultivate. “With an early harvest, the weed seed will be on the surface that much longer providing more time for predation and degradation of the seed.
“The biggest danger is complacency. We’ve had a good season for control but there’s still blackgrass in the seed-bank. If a field has a long-standing problem, we could still see significant autumn germination after one year of good control.”
Bayer technical manager, Ben Giles, points out that while keeping grassweed seed on the soil surface is effective, care is required in terms of herbicide use, particularly in reduced tillage situations. “Your aim is to decrease the grassweed population before using in-crop herbicides. Without cultivation, a lot rests on how you use Roundup (glyphosate).”
According to WRAG guidelines, only one application of glyphosate is recommended before drilling if there’s no cultivation, to reduce the risk of repeat applications to the same weeds. “The first UK case of glyphosate resistance in Italian ryegrass was announced earlier this year, so it’s a real threat.
“Be patient, wait for sufficient moisture in the seedbed to get a good chit before spraying off and then drilling. Ensure that you use a dose of glyphosate that’s effective for the size and species of the targets present in the field,” adds Ben.
Where there’s high blackgrass or Italian ryegrass pressure, a two-spray herbicide programme is the best option to maximise control, he suggests. “Create a programme which allows you to include a diverse range of actives. You could start with a metribuzin co-form such as Alternator Met, Cadou Met or Octavian Met (all metribuzin+ flufenacet+ diflufenican), plus Proclus, to give four modes of action at pre-em timing.
“The ideal situation would be to follow up around two weeks later. However, using a strong pre-em with good longevity is sensible insurance in case the second application is delayed or not possible because of the weather.”
As seen during recent years, with reduced tillage comes the potential risk of brome. And as this becomes a more significant weed, more knowledge is required, says ADAS’ John Cussans. “The species mix, germination patterns and responses to cultivation are all things that we don’t understand as well as we thought,” he adds.
The relative abundance of different brome species is shifting over time. In the Bromus group (soft, rye and meadow brome), rye brome is now the main problem, whereas in the Anisantha group, there’s a significant shift to great brome from sterile brome.
John believes the shift is likely to have been caused by weed management – selecting for the harder species to control.
“Great and sterile brome are winter annuals – most of their germination is in autumn, so focus control around the pre-em timing, much as you would do with blackgrass. Just make sure the chemistry you use has activity on brome.
“Rye, soft and meadow brome germinate 50/50 between autumn and spring. They have high potential seed return in spring crops so any herbicide programme requires activity in both autumn and spring,” explains John.
He adds that cultivation is an area which is starting to be approached differently. “There was an understanding that species had different responses to light so you could control sterile and great brome by cultivating after harvest, and delayed cultivation for the other species.
“But, recent work following a brome survey and a re-evaluation is calling this into question. There are populations where we see a light response, but the vast majority don’t have this.”
Regardless, he believes focusing on conditions post-harvest should be the priority. “If it’s dry after harvest, leave the weed seed on the surface to increase mortality and predation. Once there’s some moisture in the seedbed and soil surface, the optimum approach is a very superficial cultivation to increase seed-to-soil contact and weed germination, which can be controlled before drilling,” he concludes.
This article was taken from the latest issue of CPM. Read the article in full here.
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