The Effects Of Weeds Competition

The agronomist defines weeds as 'plants which interfere adversely with the production aims of the grower' (Spitters, 1990). Weeds affect crops by:

  • reducing crop growth and yield, mainly due to competition that limits resources such as light, water and nutrients;
  • reducing the financial value of the harvested product, either by contaminating the produce or by diminishing its quality; and
  • hampering husbandry practices, especially harvesting operations, thereby increasing costs.

Weeds reduce the financial profits either directly by lowering output (kg yield per ha X price per kg), or indirectly either by reducing the market price achieved through the diminution of quality attributes or by increasing husbandry and harvesting costs. Each of these penalties is imposed on Brassica crops by weeds. Increasingly, it is their effects on crop quality that are of prime consideration. These effects and their interactions are summarized in Table 6.2.

Factors that interact influencing the degree of mutual interference (competition) between weeds and crops are as follows.

Table 6.1. List of angiosperm families whose species account for the majority of the 700 weeds introduced into eastern North America.


Number of species















After Hill (1977).

Table 6.2. The effects of competition by

an aggressor (A) species

on a suppressed (B) species.

Competition for

Competition for

Competition for both


light only

nutrients only

light and nutrients


(a) Intrusion of A into

(c) Intrusion of A

B suffers:

the light environ-

into the nutrient

(a) reduced light

ment of B:

supply of B:


reduced light

reduced nutrient

(c) reduced nutrient

supply for B

supply for B



(b) As a result of

(d) As a result of

(b) reduced capacity to

reduced light

reduced nutrient

exploit the nutrient




B has reduced

B has reduced

(d) reduced capacity to

capacity to exploit

capacity to exploit

exploit the light

its own nutrient

its own light supply




Interaction of (a)

Interactions of (c)

Interactions of ab, ac, bc,

and (b)

and (d)

bd and cd plus any

higher order interactions

Source: Donald (1958).

Source: Donald (1958).

  • Crop factors: type of crop, time of sowing and/or transplanting, time of thinning, and population density which is a function of seed rate, germination and thinning.
  • Weed factors: species of weed(s), time of germination, start of growth and population density controlled by seed population in the soil, percentage germination and seed importation from other areas.

The level of competition resulting from these crop and weed factors will be modified further by weather conditions, soil type, soil fertility and cultivation practices.

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