Effects Of Weed Type

The morphological habit of a weed will substantially affect its impact on the growth and yield of the crop. For tall-growing weeds such as C. album (fat hen), there is a linear relationship between the extent of yield reduction and weed

Model Yield Loss Weed Density Cousens
  1. 6.1. The rectangular hyperbolic model for relating yield loss to weed density, illustrating its parameters A and I. YL = percentage of yield lost because of weed competition; d = weed density; I = percentage of yield loss per unit weed density as d^ 0; A = percentage of yield loss as d ^ ». After Cousens (1985).
  2. 6.1. The rectangular hyperbolic model for relating yield loss to weed density, illustrating its parameters A and I. YL = percentage of yield lost because of weed competition; d = weed density; I = percentage of yield loss per unit weed density as d^ 0; A = percentage of yield loss as d ^ ». After Cousens (1985).

density; as few as 3 plants/m2 were sufficient to cause statistically significant reductions to the yield of cabbage (B. oleracea var. capitata) crops. Low growing weeds such as S. media (chickweed), Poa annua (annual meadow grass) and Urtica urens (nettle) caused smaller reductions in cabbage yield. Detailed studies by Rohrig and Stutzel (2001) demonstrate the effects of three levels of weed competition on the growth of cauliflower as determined by changes to aerial dry weight, leaf area index, height and curd diameter (Fig. 6.2).

Effects Weed
Time from transplanting (months)

^ nil weed g

moderate weed ^

severe weed




Cabbage Leaves Area Index

Time from transplanting (months)

Fig. 6.2. The effect of competition from Chenopodium album (fat-hen) on the growth and curd size of spring- and summer-transplanted cauliflower (Brassica oleracea var. botrytis) cv. Fremont. Simulated (lines) and observed values (symbols) for (a) aerial dry weight, (b) leaf area index (LAI), (c) height and (d) curd diameter of cauliflower (after Rohrig and Stutzel, 2001).

The presence or absence of a Brassica such as cabbage (B. oleracea var. capitata) does not normally affect the relative proportions of weed species. An exception to this seems to be shepherd's purse (C. bursa-pastoris), which when treated with the herbicide trifluralin (a,a,a-trifluro-2,6-dinitro-N,N-dipropyl-p-toluidine) was then suppressed by the crop. Freyman et al. (1992) studied the competitive effect of shepherd's purse. It is one of the most common and difficult weeds to control in vegetable cole crops, because of its botanical similarity to brassicas. Consequently, this is where the use of herbicides would be most likely to cause damage to the crop plant. Results indicated that intra-row competition from the weed could be reduced by use of closer spacing within the rows. Closer intra-row spacing is normally associated with the use










ai I





^ nil weed moderate weed competition competition

^ severe weed competition


Time from transplanting (months)


Time from transplanting (months)

Fig. 6.2. Continued.

Time from transplanting (months)

Fig. 6.2. Continued.

of wider inter-row spacing. Reducing the intra-row spacing diminished competition and made cultivation easier because of the increased distance between rows. Chickweed (S. media) was the main weed affecting winter cabbage in Lawson's (19 72) studies; this was capable of surviving winter frost and then accelerating into rapid growth in the spring, becoming the dominant weed species and eventually shading the crop. Treatment with the herbicide propachlor (2-chloro-N-isopropylacetanilide) delayed the capacity of S. media to cause competition. Combining this with the earlier use of trifluralin controlled this weed and allowed the crop to dominate the competitive relationship. The dominance of weed species will change as a result of altering crop husbandry systems, including changing the spectrum of herbicides and other control techniques used and also resulting from several biological factors. This is what Lawson termed 'the ever moving target for weed control' (H. Lawson, personal communication).

In California (USA), for example, cultivated radish (R. sativus) and the weed (R. raphanistrum, i.e. wild radish) are both introductions from Europe. Continual interspecific hybridization since their arrival in America has converted cultivated radish into a weed and the climatic range has been enlarged continuously (Panetsos and Baker, 1986).

The outcome of competition is determined by the timing of emergence of weed seedlings relative to those of the crop Brassica and the extent to which environmental conditions during the early stages of growth favour either of the competitors. Difficulties arise where herbicides fail to be effective against the entire spectrum of weed species present in the crop. Under these circumstances, those weeds that are uncontrolled are given a selective advantage and become predominant, producing populations with strong competitive advantages in comparison with the crop. These will provide major problems during crop harvesting and can act as reservoirs for pests and pathogens. They will also return substantial numbers of viable weed seeds into the soil, causing an escalating problem for future years especially where the opportunities for crop rotation are limited (Roberts et al., 1976).

The use of transplanted brassicas provides the crop with a considerable growth advantage since it is capable of competing with weed seedlings more quickly. The crop canopy closes rapidly, inhibiting the further growth of weed seedlings. This is usually linked to the use of a pre-planting soil-incorporated selective herbicide capable of destroying germinating weed seedlings. There is a defect in this strategy caused by the presence of cruciferous weeds within the soil flora that are unaffected by such selective herbicides. This leads to an upsurge in weeds such as shepherd's purse (C. bursa-pastoris) and of volunteer crop Brassica plants such as the increasingly problematical oilseed rape (B. napus) seedlings that are residual from preceding crops. There may also be other volunteer groundkeeping crops such as potatoes, which by virtue of their presence as resilient vegetative tubers have a growth advantage even when in competition with transplanted brassicas.

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  • elisabetta
    What is the effect of weed on maize prodcution?
    8 years ago
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    What are the effects on weeds on a crop?
    6 years ago
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    What is the four effects of weeds on crop growth?
    4 years ago
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    What is the effect of weeds on plant yield?
    4 years ago
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    What type of weed plants affect the crop ?
    4 years ago
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    How weeds affect crop production?
    3 years ago
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    What are the common ten weeds and ho it affect the crop in zambia?
    2 years ago
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    What types of weed plants effect wheat crops?
    1 year ago
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    What is the effect of weed on crop production & their management?
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    What is the major effect of weed on crop production?
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    How can too many weeds affect crop yield?
    1 year ago
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    What is the impact of weeds on crop production?
    12 months ago
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    What the effects of weed control on agricuitural production?
    11 months ago
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    What are the 6 ways in which weeds affects plant growth?
    6 months ago
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    Which weeda affect brassicas?
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