annua (L.)
1.3 The importance of weeds
Most plants grow in communities consisting of many individuals. If the resources available (such as space, water, nutrients and light) become limiting then each species will be forced to compete. Weeds are often naturally adapted to a given environment and so may grow faster than the crop, especially since the crop species has been selected primarily for high yield rather than competitive ability. A unit of land may therefore be regarded as having a finite potential biomass to be shared between crop and weeds, the final proportion being determined by their relative competitive ability.
1.4 Problems caused by weeds
The most obvious problem caused by weeds is the reduction of yield through direct competition for light, space, nutrients and water. Weeds can have many further effects on the use of land, as illustrated in Table 1.2.
Table 1.2 Problems caused by weeds.
Source: Naylor, R.E.L. and Lutman, P.J. (2002) What is a weed? In: Naylor, R.E.L. (ed.) Weed Management Handbook, 9th edn. Oxford: Blackwell Publishing/BCPC. Reproduced with permission of John Wiley & 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 |
