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02 Mar 2022

All eyes on weed control

As featured in Arable Farming Magazine

All eyes on weed control

by Arable Farming Magazine February issue

Difficulties obtaining glyphosate and propyzamide left weed control in some combinable crops starting on the back foot last autumn. Martin Rickatson asked agronomists if there had been widespread issues as a result of this.

It may not have been a bone-dry winter, but compare December-January rainfall records for 2021-22 with those of a year before and it is clear there has been a much lower level of precipitation across the key arable areas of the UK.

While that may have held some crops back a little, it has done much the same for weed growth with temperatures also a little colder, the former have largely got the upper hand over the latter going into spring.

This is in spite of the fact some crops faced challenges from the off thanks to difficulties for some supply lines in sourcing herbicides, including glyphosate for pre-crop stubble hygiene and propyzamide for early season weed control in oilseed rape.

However, while this proved a problem for some, it has not been an issue common to all.

Combinable crops Chris Bean, Zantra technical director and agronomist for a number of farms in south east England, says glyphosate and propyzamide sourcing issues had negligible effect on early season weed control in his areas combinable crops.

We werent especially affected by product shortages and managed to secure what we required.

The cereals I look after all received pre/early post-emergence herbicides onto good, moist seedbeds, so weed control programmes got off to a good start.

By late January there were a few cleavers and some brome visible in a few crops, but most looked fairly clean in weed terms by midwinter, he says.

Crops have grown and developed well and the success of weed control programmes is well illustrated by the clean crops we have compared to the weed burden that has established on non-cropped ground over winter.

In the Midlands, Warwickshire based independent agronomist Jeremy White also found industry worries about product shortages did not translate into sourcing difficulties for him and his clients.

A combination of forward planning and a good working relationship with key suppliers allowed me to source sufficient product for the jobs required, he says.

The supply issues certainly affected product costs, but I was able obtain what was needed to ensure a good standard of weed control.

Assessing crops in late January, Mr White notes oilseed rape crops looked much stronger than at that point in the last two years.

AstroKerb has worked well on grass- and broad-leaved weeds and OSR crops generally appear to be in good health.

On that basis, Im optimistic for the prospects for this years crop.

Black-grass burden In cereals, though, the picture is a little different, says Mr White.

While broad-leaved weed control has been very good, the black-grass weed burden remains a huge concern, with flufenacetbased pre-emergence programmes having struggled to control the weed in my problem fields.

Mid-autumn rainfall was a key issue.

Having received more than 60mm through the second half of October and with December also wet I recorded 85mm and dull, I fear some herbicides that had been applied were diluted out of the rooting zone or moved down through the soil profile too quickly for optimal uptake by the weeds.

In Suffolk, Prime Agriculture independent agronomist Marion Self also reports that her farmer clients managed to secure sufficient stocks of herbicide for autumn-sown combinable crops.

On-farm deliveries of propyzamide arrived just in time to meet the desired field conditions and application dates, but some juggling of propyzamide products on-farm was necessary to ensure there was enough product for oilseed rape either straight propyzamide such as Kerb Flo, or propyzamide plus aminopyralid, as in AstroKerb plus propyzamide for use in beans.

With regard to glyphosate, we again planned early to secure supply, but did have to sometimes ration applications to fit available stock to the area requiring treatment.

This meant keeping spray timings close to drilling so a single application pre-drilling was sufficient.

That was a challenge when snatching spray opportunities ahead of drilling, but with a bit of patience and nerve, the autumn weather and field conditions meant this worked.

Seedbed preparation Ahead of spring, though, securing sufficient glyphosate for cover crop destruction and spraying-off stubbles ahead of seedbed preparation has become more of an issue, as initial stocks have been depleted while spring requirements are coming through in dribs and drabs.

Cereal weed control pro grammes appear to have so far worked well, says Ms Self, with residual herbicides in stacks and sequences having provided good levels of broad-leaved and grass-weed control.

Reasonable field conditions enabled well-timed use of Avadex and allowed us to apply robust stacks where required, tailored to weed profile and pressure, with minimal crop phytotoxicity effects.

Weather and field conditions also allowed us to apply planned follow-up early post-em herbicide treatments or to top up areas with additional residuals where black-grass was emerging, despite previous applications.

While there have been some frosts, winter cold periods have not been extensive, and crop and weed growth have continued throughout winter, she says.

Fortunately, in most situations weed populations are relatively low thanks to the success of autumn herbicide programmes, although some cleaning up of broad-leaved weeds may be necessary in early spring.

Oilseed rape-wise, in more dirty crops, AstroKerb, with or without Belkar, has worked well at controlling a broad spectrum of weeds.

Contact herbicides In cereals, as soon as the weather and field conditions allow, significant grass-weed populations which have pushed through autumn programmes will need contact herbicides while the weeds are small but actively growing, assuming that, based on field history, sulfonylurea-based products are likely to give worthwhile control in these situations, she says.

Autumn residuals have generally given good broadleaved weed control but, in some situations, weeds such as groundsel, cranesbill, cleavers and umbelliferae species can be found and are growing well.

Programmes that included Liberator and Proclus look to have controlled Italian ryegrass really well, illustrated by headland edges where the sprayer has not quite reached that are full of the weed.

Both earlier- and later-drilled wheats appear robust, says Ms Self, although the latter are characterised by fewer tillers and some crops that went into wetter seedbeds are less forward and in some cases a little pale.

While these should improve as soils dry, cereals on lighter land and soils more prone to manganese deficiency starting to show these symptoms will be treated with manganese as soon as conditions allow travel without making mess.

Corteva Agriscience suggests that, despite favourable autumn and winter conditions for cereals, many crops will still require a contact herbicide this spring.

Alister McRobbie, the firms cereal herbicides product manager, says: Good weather for crop establishment and autumn residual herbicides, followed by a mild December and January, gave residual chemistry the best chance to dampen the weed burden and a drier winter also enabled sprayers to travel late into the year.

With early indications that the burden in cereal crops is limited, some might question the need for a contact herbicide for grass-weed control this spring.

But the warmer autumn will have had an effect on residual herbicide stacks, meaning they will run out of steam earlier, allowing grass-weeds to germinate later in the season.

Rising spring temperatures will bring grass weed and broad-leaved weed flushes, and with wheat still highly valued, there is a strong incentive to invest in weed control.

Control programmes He adds that the foundation of some weed control programmes may have been affected by the reduced availability of glyphosate pre-drilling, with a knock-on effect of greater weed burdens going into winter.

A spring application of a contact graminicide such as Broadway Star will control brome species, ryegrass and wild oats in winter wheat, and is also effective against a wide range of broad-leaved weeds.

Used in conjunction with a residual programme, it provides robust control going into the key period of the growing season.

Best results are achieved with a recommended adjuvant and weeds should always be targeted when they are small and actively growing.

Spraying when its cold will reduce efficacy.

A good rule of thumb is that if lawn grass is growing and needs cutting, conditions are probably suitable for applying Broadway Star.

Early applications for sterile brome control will benefit from tank mixing with pendimethalin to provide some residual control where there is the potential for further germination, he says.

Broadway Star will also control difficult broad-leaved weeds such as cleavers, groundsel, brassica weeds, pansy, poppy and speedwell.

In addition, at 265g per hectare, Broadway Star plus an adjuvant will control umbelliferous weeds such as bur chervil and shepherds needle, provided the weeds are small at application.

And for those still planning to drills some wheat, the product also now has approval for use in spring types, in which it can be applied at 200g/ha with an adjuvant, and used to target wild oats and broadleaved weeds.

Herbicide active ingredients

  • AstroKerb Propyzamide + aminopyralid
  • Avadex Tri-allate
  • Belkar Arylex/halauxifenmethyl + picloram
  • Liberator Flufenacet + diflufenican
  • Proclus Aclonifen
  • Broadway Star Florasulam + pyroxsulam

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