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Sons.

Problem	Mechanism
Reduced crop yield	Interference with access to light, water and nutrients
Reduced crop quality	Admixture of contaminating seeds in arable crops Contamination of vegetable crops
Delayed harvesting	Conservation of moisture may delay ripening and increase moisture level when harvested
Interference with harvesting	Climbing plants making combining more difficult Vigorous, late‐growing weeds interfering with harvesting of potatoes and sugar beet
Interference with animal feeding	Plants with spines or thorns inhibiting animal foraging
Poisoning	Poisoning either through ingestion or through contact
Tainted animal products	Imparting an undesirable flavour, e.g. to milk
Plant parasitism	Competing for nutrients and water
Reduced crop health	Acting as an alternative host for crop pests and diseases Increasing the amount of vegetation at the base of the crop, increasing moisture and disease
Reduced animal (and human) health	Acting as an intermediate host or a vehicle for ingestion of pests and parasites Photosensitivity Teratogens Carcinogens
Safety hazard	Reducing vision on roadsides Causing a risk of fire under electricity lines and on garage forecourts
Reduced wool quality	Hooked seeds reducing the value of fleece
Water flow prevented	Plant mass blocking ditches and irrigation channels
Allelopathy	Releasing substances toxic to the growth of crop plants
Impacted crop establishment	Vegetation preventing the establishment of young trees Competing for space with establishing crops

       1.4.1 Yield losses

Crop losses approaching 100% are recorded in the literature (Table 1.3; Lacey, 1985). Such yield losses will, of course have a profound effect on a national economy in terms of both the need to import foodstuffs and the costs of weed control. Despite the many methods of weed management that are now available worldwide, it is estimated that approximately 13% of crop losses are still due to weeds alone (Table 1.4). Indeed, in 1974 the annual cost of weeds to agriculture in the USA was estimated at US$10 billion, with 50% owing to yield reductions and 50% owing to the cost of weed control (Rodgers, 1978).

      In the tropics, parasitic weed species from the genera Cuscuta (dodders), Orobranche (broomrapes) and Striga (witchweeds) can have a profound effect on a range of crops. They absorb nutrients directly from the crop plant, which may not set seed at all in the case of cereals such as sorghum.

      Weed control techniques are therefore aimed at the reduction in the competitive ability of weeds in a crop and the prevention of weed problems in a future crop. The former is increasingly based on chemical use, and the latter also requires suitable cultural and agronomic practices.

Yield loss may be usefully related to the number of weeds per unit area causing a defined yield loss in a defined crop, that is, as a Weed Threshold (Table 1.5) or as a Crop Equivalent (the amount of resource an individual weed uses expressed as the number of crop plants this resource would support, although in practice it is the biomass of the weed and the crop which is measured). Generally, these figures have only been determined for weed interaction with major crops, but they give a good indication of the ability of a particular species to compete with all crops.

Table 1.3 Examples of yield losses owing to weeds.

      Source: Lacey, A.J. (1985) Weed control. In: Haskell, P.T. (ed.) Pesticide Application: Principles and Practice. Oxford: Oxford University Press, pp. 456–485. Reproduced with permission of Oxford University Press.

Crop	Yield loss (%)	Country
Cassava	92	Venezuela
Cotton	90	Sudan
Groundnuts	60–90	Sudan
Onions	99	UK
Rice	30–73	Colombia
Sorghum	50–70	Tanzania/Nigeria
Sugar beet	78–93	Texas, USA
Sweet potatoes	78	West Indies
Wheat*	66	UK
Yams	72	Nigeria

^* From Moss (1987).

Table 1.4 Estimated percentage crop losses owing to weeds, 1988–90 (from Oerke et al., 1995).

      Source: Oerke, E.C., Dehne, H.W., Schonbeck, F. and Weber, A. (eds) (1995) Crop Production and Crop Protection: Estimated Losses in Major Food and Cash Crops. Amsterdam: Elsevier.

	Estimated loss owing to weeds (%)
Africa	16.5
North America	11.4
Latin America	13.4
Asia	14.2
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