Background

An assessment of the potential yield loss that will occur if weeds are not controlled is essential in any weed management decision making process that has the aim of optimising use of herbicides and other weed control techniques. Spraying routinely, without considering the need, increases the risk of resistance evolving, is bad for biodiversity and the environment and potentially wastes money.

Some weed species are more competitive than others, but quantifying these differences is a challenge as many factors influence the effect weeds have on crop yields. These include:

  • Weed density – the size of the infestation
  • Distribution pattern within the field – weeds are rarely evenly distributed, but are often ‘patchy’
  • Crop density and vigour – competitive crops are better able to suppress weeds
  • Number of weed species present – weeds will compete amongst themselves
  • Soil and environmental conditions – different species have different preferences
  • Emergence patterns – earlier emerging plants tend to be the most competitive

Much research has been conducted to compare the competitive ability of different weed species in winter wheat and the table below is based on data published in: Marshall EJP et al., (2003) The role of weeds in supporting biological diversity within crop fields. Weed Research 43, 77-89.

Relative competitive ability of 26 weed species in winter wheat crops

Listed from most to least competitive on an individual plant basis

These data relate to winter wheat crops but should be relevant to other autumn sown arable crops too

Common name

Latin name

Competitive Index* (CI)

(% yield loss per weed plant/m2)

Nos. weed plants/m2 that result in a 5% yield loss

(= 5 ÷ CI value)

Severely competitive

Cleavers

Galium aparine

3.0

1.7

Wild-oats

Avena spp.

1.0

5.0

Italian rye-grass

Lolium multiflorum

1.0

5.0

Sterile brome

Bromus sterilis

1.0

5.0

Black-grass

Alopecurus myosuroides

0.4

12.5

Highly competitive

Charlock/mustard

Sinapis spp.

0.4

12.5

Oilseed rape

Brassica napus

0.4

12.5

Scentless mayweed

Tripleurospermum inodorum

0.4

12.5

Common Poppy

Papaver rhoeas

0.4

12.5

Moderately competitive

Black bindweed

Fallopia convolvulus

0.3

16.7

Chickweed

Stellaria media

0.2

25.0

Field Forget-me-knot

Myosotis arvensis

0.2

25.0

Fat-hen

Chenopodium album

0.2

25.0

Redshank

Polygonum maculosa

0.2

25.0

Knotgrass

Polygonum aviculare

0.1

50.0

Annual meadow-grass

Poa annua

0.1

50.0

Sow thistles

Sonchus spp.

0.1

50.0

Fumitory

Fumaria officinalis

0.08

62.5

Speedwells

Veronica spp.

0.08

62.5

Red Dead-nettle

Lamium purpureum

0.08

62.5

Crane’s Bill

Geranium spp.

0.08

62.5

Groundsel

Senecio vulgaris

0.06

83.3

Fool’s Parsley

Aethusa cynapium

0.06

83.3

Weakly competitive

Scarlet Pimpernel

Anagallis arvensis

0.05

100.0

Field Pansy

Viola arvensis

0.02

250.0

Parsley Piert

Aphanes arvensis

0.02

250.0

Notes:

  1. A 5% yield loss is often used as a threshold at which the cost of control very roughly equates with the cost of control (herbicide + application).
  2. These are average values – in 50% of cases there are likely to be bigger reductions than those given here. Hence, with severely and highly competitive weeds especially, an insurance approach is sensible and it may be advisable to treat lower populations.
  3. The actual weed population density is important – a high infestation of even weakly competitive species can substantially reduce yields.
  4. Spring emerging weeds (e.g. Black bindweed, Fat-hen, Redshank, Knotgrass & Fumitory) are likely to be more competitive in spring-sown crops than the figures here would indicate, as they will emerge at the same time as the crop, rather than into winter wheat crops, already well-established.

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