How to tackle weeds after a tough start to the season

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 yields 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.

See also: Pigs in the arable rotation – benefits and what to consider

Many autumn-sown crops did not receive their usual pre-emergence herbicide programmes, while others established poorly and therefore lack competitiveness.

This means large, vigorous, overwintered weeds could be more yield-damaging and harder to control this spring.

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 and try to avoid exacerbating future problems by allowing high weed seed return or increasing  resistance pressure.

A big flush of any weed, especially blackgrass, that is allowed to set seed, could easily set back years of hard work.

Spring weed control advice

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

Here are some pointers for winter cereals, spring crops and oilseed rape.

1. 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 or Hatra, 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 (such as when attempting to combine herbicides with T0 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 T0 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, Mr Coombs 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.

Glyphosate

In severe weed pressure situations, there may be no other option than to spray off areas of crop with glyphosate to avoid a large weed seed return.

This is a hard decision given the difficulties of the season, but should be made sooner rather than later – ideally before applying spring fertiliser and fungicides.

Tips for applying glyphosate:

  • Target small plants
  • Check plants are not shaded out
  • Avoid applying glyphosate during stem extension of blackgrass
  • Consider using two well-timed sprays
  • Use an effective dose
  • Follow best-practice advice https://ahdb.org.uk/knowledge-library/glyphosate-resistance

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

“Ideally, Broadway Star should always 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 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)

2. 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.

This is too long to wait before drilling cereals such as spring barley, says 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.

He recommends sowing spring barley at 450-500 seeds/sq m, whereas spring wheat may need to be nearer 600/sq m due to its lower tillering capacity.

“You must be realistic about establishment, especially on heavy land or sub-optimal conditions.”

Also consider halving seed rate and drilling twice, at a 30-45deg angle, he suggests. “It takes twice as long, but can be a very effective way of increasing 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 Arylex (halauxifen-methyl), 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.

Actives

  • Ally Max SX metsulfuron-methyl + tribenuron-methyl
  • Astrokerb aminopyralid + propyzamide
  • Atlantis iodosulfuron-methyl-sodium + mesosulfuron-methyl
  • Broadway Star florasulam + pyroxsulam
  • Crawler carbetamide
  • Kerb propyzamide
  • Liberator diflufenican + flufenacet
  • Monolith mesosulfuron-methyl + propoxycarbazone-sodium
  • Pacifica Plus amidosulfuron, iodosulfuron-methyl-sodium + mesosulfuron-methyl
  • Starane Hi-Load HL fluroxypyr

Spring weed control in OSR

Where growers have struggled to apply autumn/winter herbicides to oilseed rape, weed pressure should be monitored closely.

Crawler can be applied to the end of February for grassweed control, providing a useful alternative mode of action for populations resistant to other actives, says independent agronomist Andrew Cotton.

Many rape crops in his area received Kerb or Astrokerb over the autumn.

He says Astrokerb has controlled thistles and mayweed, but does not control cleavers, and a follow-up treatment may also be required where straight Kerb was used.

Galera is one option that can be applied from the four-leaf stage of the crop up to just before flower buds are visible above the canopy. Belkar can be applied earlier, but is slightly less effective on mayweed, he says.

Another option where cleavers aren’t an issue is Dow Shield, which offers relatively cheap control of mayweed and thistles.