Critical Periods Of Competition

There are critical periods of growth when weeds provide competition with crops and other times when they are not exerting depressing effects on yield (Nieto et al., 1968). Soils used for Brassica production usually contain more than 2000 viable seeds of annual weeds/m2 (20 million/ha). Only a small proportion of these germinate into seedlings at any one time, but this can still lead to populations of 100-300 weed plants/m2 competing with the crop for resources. Yield losses vary according to the habit of the predominant weed species and the crop growth stage at which they begin competing with each other for resources. In drilled summer cabbage (B. oleracea var. capitata), for example, losses varied from 25 to 100% with a weed density of 100 plants/m2. The scale of losses depended on the weed species present and the degree to which environmental conditions favoured either the crop or its competitors (Roberts et al., 1976; Rohrig and Stutzel, 2001).

In vegetable crops, the concept of 'critical periods' is highly developed to indicate times during which any competition from weeds will cause irreversible yield loss. A 'critical period' of weed competition is defined as 'the minimum time over which weeds must be suppressed to prevent yield loss'. There are two separate components to such critical periods: (i) the time a crop must be weed free after drilling or planting so that emerging weeds do not reduce yields; and (ii) the subsequent time when weeds which emerge within the crop can remain before they begin to interfere with crop growth and productivity.

These factors define the optimum timing for weed removal to prevent yield loss rather than stopping intense competition. Use of this concept is limited to timing of cultivation and other measures, and is dictated by crop tolerance and the stages susceptible to weed growth. The critical period varies with crop type and cultivar, location, spectrum of weed species and their density in any particular season (Weaver, 1984). In cabbage (B. oleracea var. capitata), the length of the weed-free period, weed density and ambient light reduction caused by competing weeds were important factors governing yield loss (Miller and Hopen, 1991). Maintaining the plants free from weed competition in the first 4 weeks after sowing was crucial for the establishment of yield and quality (Table 6.5).

The concept of critical periods can be criticized for failing to take into account the influence of the full range of ecological and agronomic factors specifically related to a particular site. These simulation models of competition tend to be generally empirical regression models based on one or more parameters such as weed density, relative leaf area or relative time of weed emergence. These parameters vary widely between sites and over seasons. Such models also tend to lack physiological values dependent on the interaction of the crop and its surrounding weeds competing for resources.

Table 6.5. Critical periods of weed competition in cabbage (Brassica oleracea var. capitata), related to yield.

Weeks weed-free after emergencea

Cabbage yield kg/m2













LSD (0.05)


After Miller and Hopen (1991). aWeed = velvetleaf (Abutilon theophrasti). LSD = least significant difference.

This competition is process oriented and defined as the distribution of growth-limiting factors between species in a vegetation canopy and the efficiency with which each species uses resources for growth (Spitters, 1990).

The resources for which the crop and weed compete are radiation, nutrients and water. The rate of use of these resources depends on the structure and growth pattern of crop plants and interactions with the weeds. Cauliflower (B. oleracea var. botrytis), for instance, has a relatively constant resource allocation pattern throughout the vegetative and reproductive growth stages and does not respond to competition for radiation by significantly changing dry matter distribution. Since it is a relatively tall plant susceptible to damage from wind and heavy rainfall, it requires support from an appropriately sized stem which is responsible for plant stability. The lack of side shoots and branches in cauliflower ensures that the size of leaves primarily determines the lateral expansion of the plant.

Radiation is the most crucial resource for which there is competition between Brassica crops and weeds. Crops are usually planted in widely spaced rectangular patterns, thus developing ecologically as single plants rather than 'closed' or row canopies. The influence of weeds on light availability in direct-sown cabbage (B. oleracea var. capitata) is shown in Table 6.6.

Good husbandry ensures that ample irrigation and fertilizer are provided so that water and nutrients are not limiting either for the crop or for competing weeds. Hence the spatial development of competing plants and their morphological adaptation to unfavourable growing conditions strongly influence the distribution of radiation within the canopy.

The growth of spring- and summer-planted cauliflower as affected by the weed Chenopodium album (fat hen) was studied by Rohrig and Stutzel (2001). It is evident that this weed had a much greater impact on crops planted later in the season. This is due to higher temperatures and the accelerating growth rate of the weed in summer.

Table 6.6. Effect of weed growth on light availability in direct-sown cabbage (Brassica oleracea var. capitata).


Cabbage yield kg/m2

Light availability to cabbage as a percentage of full sunlight

Weed - free control



Weed - free 6 weeks



Weed - free 4 weeks



Weed - free 2 weeks



Weed - free 1 week



Unweeded 2 weeks



Unweeded 4 weeks



Unweeded 6 weeks



Unweeded 14 weeks



LSD (0.05)



After Miller and Hopen (1991). aWeed = velvetleaf (Abutilon theophrasti). LSD = least significant difference.

This encourages more vigorous growth of the weed and an increasing capacity to shade the cauliflower, thereby reducing radiation use efficiency in the crop. When this weed was growing at high density, it substantially reduced cauliflower total dry weight, leaf area index and curd diameter; crop height was reduced more in the late summer crops compared with earlier maturing types.

With spring and summer Brassica crops grown from direct seeding or using transplants, provided the weeds are removed effectively in a single weeding, then there are few adverse effects of competition. The crucial period for weeding Brassica crops is in the earliest stages of establishment and as subsequent growth develops. The crop canopy closes soon after weeding and prevents further competition. Brassica crops, however, show significant effects of row width and weed removal treatments on yield and quality. Yields tend to rise as row spacing is reduced, up to critical values, and thereafter they diminish. Weed removal becomes essential once competition between the individual crop plants and weed species commences after planting or drilling. Provided weeds are removed before this critical time, usually by a single weeding operation (using either mechanical or herbicidal methods), then yield and quality losses are prevented. This requires the use of herbicides applied pre- or post-planting, or mechanical methods to remove the developing competition. The period when Brassica crops are establishing is especially crucial in preventing the development of weed competition. This is because there are few, if any, environmentally acceptable herbicides that can be applied to established stands of Brassica crops. Single, thorough mechanical weed removal must normally occur between 2 and 3 weeks after transplanting, depending on the crop row spacing employed. Weed competition commences more quickly at narrow row spacings compared with wider ones.

Weeds have limited effects in Brassica crops grown during the autumn and winter provided they are removed before active crop growth recommences in spring. Those weeds remaining into the spring compete severely with the crop, resulting in smaller marketable cabbage (B. oleracea var. capitata) heads, reducing internal head quality, decreasing numbers of plants forming heads and in very severe cases weeds can cause death of the crop plants (Lawson, 19 72). Particularly successful weed species in Lawson's northern European (Scottish) studies included chickweed (Stellaria media), annual meadow grass (Poa annua), shepherd's purse (Capsella bursa-pastoris) and knotgrass (Polygonum spp.). A major factor in the competitive fitness of these weeds was an ability to grow rapidly in spring, outpacing and overtopping the crop and causing shading effects. Weed competition was analogous to increasing the density of a crop planting up to a point where cabbage population densities became so high that normal development was prevented. Conversely, comparison of cropped and uncropped plots indicated that the crop could exert considerable competitive pressure on the growth and development of weeds, particularly where their capacity for expansion had been retarded by treatment with herbicides.

The management decision of whether or not weed control is economically worthwhile depends on a costs and benefits analysis. This identifies that at a given level of weed infestation, crop yield and/or quality are likely to be damaged if the weeds remain uncontrolled. Models of weed and crop competition are an essential part of both short- and long-term crop management planning.

The most favoured model that describes the relationship between weed infestation and crop yield loss is a rectangular hyperbolic curve (Cousens, 1985) (Fig. 6.1). Many discussions of the relationship between crop yield and weed densities have used either sigmoidal or quasi-sigmoidal models meaning that there is a threshold weed density below which there is no yield loss. In this relationship, there is virtually no competition between weed and crop at low weed density despite the plant size being at a maximum. This ignores the fact that the competitiveness of each individual weed plant is greatest when it has achieved maximum size and is exploiting nutrient and water resources from the soil. Support for the use of a sigmoidal relationship is based on the observation that at very low weed densities, statistically significant reductions of yield are difficult to demonstrate in crops such as cereals. In vegetables such as the brassicas, however, losses may result from the damage to product quality that is difficult to quantify by using field experiments. Also, experimental designs are often incapable of detecting differences in yield of the magnitude expected at low weed densities; this does not mean that such differences do not exist and, especially with Brassica vegetable crops, exert an indirect influence on their profitability. A hyperbolic model of the relationships between weed density and yield loss indicates, however, that wherever weeds are present in a crop there will be competition for resources such as nutrients and water. This occurs even when there is very limited or no shading effect reducing the light available to the crop plant. The hyperbolic model assumes that weeds are distributed at random in relation to crop plants. Consequently, as weed density increases, the mean distance from the weed to crop plant remains constant.

Several practical studies emphasize the influence of cultivar selection, row width and planting date on weed competition for Brassica crops. These vary substantially in husbandry systems using a wide range of crop spacings, population densities and methods of establishment (direct seeding versus transplanting). Efficient weed management is essential because of the high intrinsic values of the products, limited availability of herbicides and high labour costs.

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  • uffo
    What is the critical period of weed competition in cabbage?
    4 years ago
  • anna
    What is the critical period of competition in plant and what it is use?
    2 years ago
  • Thomas
    What is the magnitude of loss due to weed competition with crops?
    3 months ago

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