Broad-leaved weed control in winter oilseed rape

Weed control in winter oilseed rape has always been challenging. Yield loss from poor broad-leaved weed control can range from 3% up to 73% depending on the vigour of the crop and this does not take into account contamination of harvested crop with weed seed which can reduce marketability of the sample.

For effective weed control the use of integrated weed management strategies are essential across the weed lifecycle including preventing seed return, depleting seed banks, effectively removing weed seedlings, stopping seed set and good on farm hygiene.

Integrated weed management (IWM) is the use of multiple weed control methods to sustainably manage a weed problem. It is a component of integrated pest management (IPM) and can include cultural, genetic, mechanical and biological weed control in conjunction with the use of herbicides.

See more on this in the AHDB research review “Weed control options and future opportunities for UK crops”.

Weed control is vital to produce high yields of good-quality crops. It also removes competition, reduces harvest difficulties and helps to prevent the spread of pests and diseases.

Weed control solutions such as Korvetto® or Belkar® herbicide give excellent control of a range of key broad-leaved weeds that would otherwise have a negative impact on the crop.

Yet with fewer active ingredients and a need to manage herbicide resistance, the weed challenge must be managed across the rotation. The use of Astrokerb®, or Kerb® Flo 500  (propyzamide), in oilseed rape is a key tool in managing blackgrass in the rotation.

Weed control is more than just using herbicides. Many factors determine weed incidence and effective weed management in arable crops requires integration of all these factors:

• Crop choice and rotation
• Managing the weed seedbank
• Cultivations
• Drilling date
• Crop competition
• Herbicide choice, application and timing
• Recent weed-control strategies
• Weather
• Agronomist/farmer weed tolerance threshold

The damage weeds cause depends on: 

• Weed species
• Weed density
• Competitive ability of the crops
• Growth stage when weeds compete

While some weeds are highly competitive, others pose little threat and may be valuable to wildlife. Weeds can delay ripening and harvesting, for example cleavers in oilseed rape. 

The competitive effects of weeds are well understood, with the weeds competing for light, nutrients and moisture. Early autumn is a time when competition for moisture is important and weeds, mainly grass weeds, can quickly smother the seedling crop.

The use of fop and dim grassweed herbicides at an early crop growth stage can be important to remove volunteer cereals from the establishing oilseed rape. 

Later in autumn and in the early spring when moisture is less limiting, then competition for light and nutrients is more important.

Some weeds, such as cleavers and chickweed, grow at lower temperatures having the potential to smother a crop and when nitrogen is applied, they have the ability to remove it faster than the crop. 

In general, it is assumed that broad-leaved weeds are less competitive than grass weeds in winter oilseed rape.

Scottish data (SAC trials) indicates a stronger relationship between weed ground cover in early spring and yield, with an approximate 1% decrease in yield for every 5% increase in weed ground cover. 

In summary, the impacts of poor weed control include: 

• Outcompeting the crop 
• Issues at harvest 
• Contributing to an admix penalty risk 
• Returning seed to the soil 
• Making oilseed rape fields look untidy 

Key broad-leaved weeds 

Broad-leaved weed species that can cause problems in UK oilseed rape crops include field poppy, chickweed, mayweed, cleavers and thistles. 

Other broad-leaved weeds that can be found in oilseed rape include cranesbill, fumitory, common field-speedwell, red dead-nettle, shepherd’s purse, field pansy, charlock, ivy-leaved speedwell and common hemp-nettle. 

See the weed biology section on the Corteva website for information on broad-leaved weeds in OSR. 

In summary, the impacts of poor weed control include: 

• Outcompeting the crop 
• Issues at harvest 
• Contributing to an admix penalty risk 
• Returning seed to the soil 
• Making oilseed rape fields look untidy