Competition, weed-crop

Table I. Estimated average annual losses due to weed competition in crops, United States, 1975-1979...

In summary, weed competition with crop plants and their control constitute some of the highest costs in the production of food, feed, and fiber. In the United States, losses caused by weeds and the cost of their control totals about 15.2 billion annually. Total losses from weed competition with major crops produced in the United States is approximately 8.9 billion. 

Zimdahl, R. L. "Weed-crop competition. A review." Int. Plant Prot. Center, Oregon State Univ., Corvallis, OR. 1980. 

Leaves and whole plants added to the soil caused the greatest inhibitions to weeds in turnip, radish, and cabbage plots (Figure 7). This effect might be due to 1) a selective allelopathlc effect of water hyacinth and vegetables upon weeds, and/or 2) competition with crops. Water hyacinth has been widely used as a fertilizer in the chinampas because it improves the physical and chemical properties of soil and it exerts a certain control of weeds through itsdecomposition in the soil. 

Early crop establishment Objective prevent weed-crop competition... 

Herbicides reduce fertilizer costs. Weeds are in direct competition with crop plants for nutrients from the soil. Without weed control, farmers would be fertilizing the crop and the weeds. 

Herbicides reduce irrigation costs. Weeds are also in direct competition with crop plants for water. Thus, irrigation water used by weeds is not available for the production of a crop. 

Herbicide activity is either selective or nonselective. Selective herbicides are used to kill weeds without significant damage to nearby plants. They are used to reduce weed competition in crops, lawns, and ornamental plantings. Nonselective herbicides are chemicals that kill all plants present if apphed at an adequate rate. They are used where no plant growth is wanted, such as fence rows, ditch banks, driveways, roadsides, parking lots, and recreation areas. 

Nonchemical or traditional practices, such as weed seed removal, optimal crop seeding rates, crop selection, enhanced crop competitiveness, crop rotation, and mechanical weed control are all important components of an effective weed management program (458,459). In the context of modern intensive chemical herbicide appHcation, nonchemical practices may even represent an innovative approach to weed management and should receive careful consideration. 

In organic systems the number of hours of hand weeding needs to be reduced, as labour, particularly in the developed countries, is expensive. This can be achieved by competition, the timing of cultivations, adaptation of the rotation nutrient management system and by crop density (Kropff, et al., 2000). Barberi, et al. (2002) experimented with a system of soil disinfection using hot steam to sterilise weed seeds. 

Weeds directly compete with the crop for water, nutrients, light and other growth factors. Competition for water begins when root systems overlap as they absorb water and nutrients (3). Competition for water depends on the rate and completeness with which a plant utilizes the soil water supply (4). Competition for water usually occurs with other forms of competition. For example, competition between weeds and peas (Pisum spp.) centered on light and water depending on weed height (5). 

Estimated Crop Losses from Weed Competition... 

The total monetary loss resulting from weed competition in the cereal crops was 1.3 billion annually. The most frequently reported weeds were mustards (Brassica spp.) followed by wild oats (Avena fatua L.), bromes (Bromus spp.), and wild garlic (Allium vineale L.) (11). Losses in vegetables was 5% of the total while in fruit and nuts the loss was 7% of the total. Crabgrass, bermudagrass [Cynodon dactylon (L. )... 

How weeds produce their effect is the subject of continuing research. As rather recently defined, the negative effect of weeds on crop plants includes both competition and allelopathy (1.) and has been termed interference. Previous weed science research considered the competition component foremost and, usually, solely (e.g. 2). Allelopathy, however, has received some attention recently, with a number of symposia and reviews devoted to allelopathic effects. Some of these cover the effects of weeds on crops (3, 4, 5). In this chapter, I review the possible role of allelopathy in weed interference in crop production in the Mid-South. A second objective of this review is to indicate where research is needed and to suggest potential lines of future research, especially with respect to the developing role of conservation tillage practices in this region. 

Weed management in organic agriculture use preventive methods such as an appropriate crop rotation, precise soiled preparation before sowing crops, narrow seed spacing, etc. (Labrada 2003). Many of them include ways in which allelopathy (often together with competition) could play an important role (Fig. 14.1). 

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