Competition

Weeds compete with desirable plants for space, water, nutrients and light, and so reduce the yield of desirable plants. Characteristics of the weed influence the outcome of competition. Relative rooting depths will influence competition for water and nutrients. Some weeds are particularly effective in the uptake of certain nutrients, e.g. black grass (Alopercurus myosuroides), and this may be due to selection pressure acting on the plant which came from a type that lived on disturbed land (low in nutrients). Mode of growth affects competition for light. For example, broad-leaved weeds spread out relatively close to the ground inhibiting light from reaching others at the soil surface. The Table below illustrates an example of the relationship between yield loss and weed density under competition.

It is during the very early stages of crop growth that weeds are particularly competitive. For example, in a field bean crop the first four weeks are the most critical; keeping fields free of weeds for five weeks can improve yield by as much as 300%.

Interference with crops by means of toxic exudates

Many plants produce chemicals which may, when released from their roots or leaves, adversely affect the growth of other plants. For example, a weed of flax, gold-of-pleasure (Camelina alyssum) has no effect on flax yield when both are grown together in pots watered from below. However, if water is allowed to fall onto its leaves a toxic material is leached (a phenolic compound) which can affect the growth of the flax.

A second example would be the extracts (breakdown products) of old root and rhizomes of couch grass (Agropyron repens) which are able to inhibit the germination and growth of oil seed rape seedlings.

Parasitic upon crop plants

This property is not of great significance in Australia, but extremely important in other parts of the world. Witchweeds (Striga spp.) can be a parasite on sorghum and maize (Africa, India and USA). Some forms of mistletoe (Amyema spp.) are locally important in tropical countries.

Summary 'Competition'

The total effect of the weed on the crop/pasture is often referred to as 'competition'. The balance of competition can favour the weed or the crop. It depends on the combined effect of all factors (See Figure below) including several important environmental parameters.

Influence of the crop, weed and the environment on the balance of competition.

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Steve Adkins