Ways to tackle weeds after wet start to season – Farmers Weekly – Dick Neale

High rainfall as the 2019-20 season kicked off is causing headaches for weed control in autumn-drilled cropping. ...

Paul Spackman seeks advice on how to manage the situation for cereals and oilseed rape.

It is shaping up to be a tough spring for controlling weeds, as the legacy of the wet autumn and winter means many crops are at greater risk of yield losses if weeds are allowed to get established in the coming weeks.

Significant areas were left uncropped in the wettest parts of the UK, resulting in more unplanned spring cropping on land that will take time and effort before it’s ready for drilling.

What’s more, many autumn-sown crops did not receive their usual pre-emergence herbicide programmes, while others established poorly.

This means large, vigorous, overwintered weeds could be more yield-damaging and harder to control this spring, while spray decisions are complicated further as spring-germinating weeds emerge, presenting a wide spectrum of growth stages to tackle.

Growers are advised to manage their expectations, although they should try to prevent a high seed return or reduce resistance pressure, which would exacerbate future problems.

A big flush of any weed, especially blackgrass, that is allowed to set seed, could easily set back years of hard work. Here are some pointers for winter cereals, spring crops and oilseed rape.

 

Weed control in winter cereals

While some crops in drier regions and those drilled early received an effective residual pre-emergence spray, others have gone untreated where spraying was impossible.

Independent agronomist Andrew Cotton says flufenacet-based pre-emergences worked well where applied, thanks to moist soils and slow blackgrass emergence last autumn.

However, weed pressure is significant where no pre-emergence was applied, especially on later-sown crops in wetter areas.

“Crops aren’t as advanced and are more open, so lack the usual competitiveness.”

Well-timed post-emergence herbicides will be important, and success requires applying products early to small, growing weeds, which will more easily take up actives and are less able to detoxify them than larger weeds.

“With blackgrass, for example, spraying before it starts tillering gives a much better chance of control,” says Mr Cotton.

He suggests combining a sulfonylurea-based product, such as Atlantis, with the residual chemistry in Liberator to control established and emerging grassweeds. Alternatively, Starane Hi-Load, with or without Ally Max SX, offers good broad-leaved weed activity.

For any weed, timing herbicide sprays correctly is a balancing act. Avoid spraying too early, as weeds may not have emerged and residual activity could run out by the time they come through, he advises.

Equally, delaying too long (like when trying to combine herbicides with TO fungicides) risks larger weeds becoming harder to control and makes good spray coverage more difficult.

“Assess every field individually and prioritise those to treat first. If there is an opportunity to combine herbicides with the TO fungicide, be aware of label restrictions. For example, Atlantis cannot be mixed with tebuconazole.”

Acknowledging the issues with herbicide resistance in post-emergence chemistry, Bayer herbicide specialist Ben Coombs insists good results are still achievable. Reducing weed populations to prevent seed return is the most effective anti-resistance strategy, he says.

If grassweeds are the priority, he recommends applying Monolith at 0.33 kg/ha, whereas the three sulfonylureas in Pacifica Plus, applied at 0.4-0.5 litres/ha (depending on weed spectrum), may be more suitable where more broad-leaved weed control is needed. In either case, include 1 litre/ha of the adjuvant Biopower.

Another option against brome, ryegrass and wild oats is Broadway Star, says Alister McRobbie, herbicide product manager at Corteva.

“Ideally, Broadway Star should be used with a residual partner, such as pendimethalin, as part of a programme based on a combination of actives that includes a pre-emergence residual.

That’s been harder to do this season, so anyone applying Broadway Star must do so carefully for optimum control and resistance management.”

Tips for maximising post-emergence performance include:

  • Apply to small, actively growing weeds
  • Use robust rates and an appropriate adjuvant, if needed
  • Ensure environmental conditions are right (moist and warm – 6C+ – soils either side of application)
  • Tank mix with a residual partner where label permits (such as pendimethalin/flufenacet).

 

Maximise competition in spring crops

Unplanned spring cropping is a challenge for weed control, especially if soils need more work to create a seed-bed, thereby stimulating weed germination. Growers should minimise soil disturbance before and during drilling, and spray off overwintered and newly emerged weeds with glyphosate beforehand.

It may be tempting to wait for the main flush of spring-germinating weeds, but in the case of blackgrass, this doesn’t usually occur until the first week of April, which is too long to wait before drilling cereals such as spring barley, advises Dick Neale, technical manager at Hutchinsons.

Drilling late leaves less time for barley to establish and build yield, and risks conditions being too dry for residual chemistry to work.

Mr Neale says other crops better suited to late drilling – or even managed fallow or a cover crop – may be better in some circumstances.

For spring cereals, growers are better off drilling as soon as soil conditions allow, applying an effective flufenacet-based pre-emergence herbicide and maximising crop establishment and competitiveness, such as by including starter fertilisers and using higher seed rates, he adds.

Also consider halving seed rate and drilling twice, at a 30-45deg angle, he says. “It takes twice as long, but can be an effective way to increase crop competition and establishment.”

Liberator is the main residual pre-emergence option for grassweed control in spring barley, and Mr Coombs says it is best applied to moist soils within 48 hours of drilling, at 0.3 litres/ha.

It can also be applied in spring wheat, before growth stage 14.

For broad-leaved weeds such as poppy, fumitory, fat hen, cranesbill, cleavers and chickweed, Mr McRobbie recommends products based on halauxifen, applied to small, growing weeds.

He points out that where crops have gone into cloddy seed-beds, there could be protracted germination of grassweeds and broad-leaved weeds as clods break down, exposing fresh seeds.

 

Spring weed advice
  • Identify weeds present, including species and growth stage
  • Target herbicides on field-by-field basis
  • Prioritise treating fields with an established crop
  • Use robot rates to tackle a range of growth stages
  • Maximise the competitive vigour of spring crops
  • Minimise seed return to avoid creating longer-term problems.

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