Crop and weed competition in arable crops

The composition and sequence of arable crops in the rotation influences the weeds present and the opportunities for control. Discover which species are the most competitive to help you plan integrated weed management strategies.

Weed damage in crops

The risk of unacceptable levels of crop damage due to weeds depends on:

Weed species
Weed density
Weed growth stage
Crop species (and rotation)
Crop density
Crop growth stage

Weeds can also delay ripening and harvesting (e.g. cleavers in oilseed rape) or impair produce quality (e.g. weed seeds in harvested grain).

How competitive are arable weeds?

The following table shows the typical density (weed plants/m²) that causes a 5% wheat yield loss.

Information source: Marshall EJP, Brown VK, Boatman ND et al. (2003). The role of weeds in supporting biological diversity within crop fields. Weed Research 43, 77–89.
Competitive ability	Weed
0–5 Very competitive	Barren brome, cleavers, Italian rye-grass and wild-oat.
12–17 Competitive	Barren brome, cleavers, Italian rye-grass and wild-oat.
18–25 Moderately competitive	Chickweed, fat hen, forget-me-not and redshank.
> 50 Relatively uncompetitive	Common fumitory, scarlet pimpernel, shepherd’s-purse, dove’s-foot crane’s-bill, red dead-nettle, annual meadow-grass, knot-grass, groundsel, common speedwell and field pansy.

How competitive are arable crops?

The competitive ability of common arable crops is indicated in the tables, below.

Autumn-sown crops

Well suited to heavy soils
Usually higher-yielding
Provides overwinter crop cover

Competitive ability with weeds	Autumn-sown crop
++++ (most competitive)	Barley, oats, rye and oilseed rape*
+++	Wheat
++	Beans
+ (least competitive)	N/A

Spring-sown crops

Less suited to heavy soil
Usually lower-yielding
Helps spread workloads
Good for biodiversity

Competitive ability with weeds	Autumn-sown crop
++++ (most competitive)	Barley, oats, rye, oilseed rape* and potatoes**
+++	Wheat, triticale and sugar beet***
++	Beans
+ (least competitive)	Peas

*Although relatively few herbicide active ingredients are available for oilseed rape, resistance management is less of an issue. However, the autumn crop has a relatively limited pre-drilling weed-control window. Shed seed can also lead to the development of oilseed rape volunteer weed populations.

**In potato crops, non-specific herbicides are available. The crop is a good competitor with weeds, but can result in volunteer potatoes in subsequent crops.

***Sugar beet has multiple low-dose systems for weed control and can be mechanically weeded. Initially, it is poorly competitive. It is also associated with very late harvests – with the potential to cause issues in subsequent crops.

Rotational weed management

Choice of crop affects many aspects of weed management, including the time of drilling, the type and timing of cultivations and the range of herbicides available. Variety choice is also likely to be influential, but less is known about this aspect.

A diversity of crops – including autumn-sown and spring-sown crops – across the rotation increases the variety of practices and herbicide options available. Although this can increase the range of weed species present, it usually reduces overall weed numbers.

How to manage weeds in arable rotations

AHDB

Topics:

Cereals & Oilseeds Wheat Barley Oilseed Rape Weeds

Sectors:

Cereals & Oilseeds

Tags:

weed management