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 Brassicacrops. 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.
density; as few as 3 plants/m2were sufficient to cause statistically significant reductions to the yield of cabbage (B. oleraceavar. 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 Röhrig and Stützel (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).
0 100 200 300 400 500 600 700 800 900 1000
1 2 3
severe weed competition nil weed
competition
moderate weed competition
Time from transplanting (months) Dry weight (g/m2)
(a)
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
1 2 3
Time from transplanting (months) LAI (m2/m2)
nil weed competition
moderate weed competition
severe weed competition (b)
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 oleraceavar. 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 Röhrig and Stützel, 2001).
The presence or absence of a Brassicasuch 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 (␣,␣,␣-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
0 100 200 300 400 500 600 700 800
1 2 3
Time from transplanting (months) (c)
Height (mm)
nil weed competition
moderate weed competition
severe weed competition
0 50 100 150 200 250
1 2 3
nil weed competition
moderate weed
competition severe weed competition
Curd diameter (mm)
Time from transplanting (months) (d)
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 (1972) 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 Brassicaand 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 cropBrassicaplants 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.