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Chapter 1
An Introduction to Weed Biology
1.1 Introduction
The human race has been farming for over 10,000 years. Weeds would have been an unwelcome presence alongside crops ever since the first farmers saved and planted seeds in the region that is now present- day Turkey and the Middle East. Indeed, when these early farmers noticed a different plant growing, decided they did not want it and pulled it up, they were carrying out a form a weed control that is still used today: hand roguing.
But what are weeds? Weeds are all things to all people, depending on the viewpoint of the individual. To some they are plants growing where they are not wanted; to others they are plants growing in the wrong place, in the wrong quantity, at the wrong time; and to some they are regarded as plants whose virtues have yet to be fully discovered! The need to control weeds only arises when they interfere with the use of the land, and this is usually in the presence of a crop, such as in agriculture and horticulture. Weed control may also be necessary in other situations including amenity areas, such as parks and lawns, in water courses, or on paths and drives where the presence of plants may be regarded as unsightly. It should not be overlooked, however, that weeds contribute to the biodiversity of ecosystems and should only be removed when financial or practical implications make their presence unacceptable. With this in mind an appropriate definition of a weed is:
Any plant adapted to man- made habitats and causing interference of the use of those habitats
(Lampkin, 1990)
1.2 Distribution
On a global basis only about 250 species are sufficiently troublesome to be termed weeds, representing approximately 0.1% of the world’ s flora. Of these, 70% are found in 12 families, 40% alone being members of the Gramineae and Compositae. Interestingly, 12 crops from 5 families provide 75% of the world’ s food and the same 5 families provide many of the worst weeds (Table 1.1). This implies that our major crops and weeds share certain characteristics and perhaps common origins.
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.
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 both in terms of 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% 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 $US10 billion, with 50% due to yield reductions and 50% due to the cost of weed control (Rodgers, 1978).
Table 1.2 Problems caused by weeds (from Naylor and Lutman, 2002).
| Reduce crop yield | Interference with access to light, water and nutrients |
| Reduce crop quality | Admixture of contaminating seeds in arable crops Contamination of vegetable crops |
| Delay harvesting | Conservation of moisture may delay ripening and increase moisture level when harvested |
| Interfere with harvesting | Climbing plants can make combining more difficult Vigorous, late - growing weeds can interfere with harvesting potatoes and sugar beet |
| Interfere with animal feeding | Plants with spines or thorns inhibit animal foraging |
| Cause poisoning | Poisoning either through ingestion or contact |
| Taint animal products | Impart undesirable flavour, e.g. to milk |
| Act as a plant parasite | Competing for nutrients and water |
| Reduce crop health | Act as an alternative host for crop pests and diseases Increase vegetation at base of crop increasing moisture and disease |
| Reduce animal (and human) health | Act as intermediate host or a vehicle for ingestion of pests and parasites |
| Photosensitivity |
| Teratogens |
| Carcinogens |
| Are a safety hazard | Reduce vision on roadsides |
| Risk of fire under electricity lines, on garage forecourts |
| Reduce wool quality | Hooked seeds reduce value of fleece |
| Prevent water flow | Plant mass blocking ditches and irrigation channels |
| Exhibit allelopathy | Release of substances toxic to the growth of crop plants |
| Impact on crop establishment | Vegetation prevents establishment of young trees Competition for space with establishing crops |
Table 1.3 Examples of yield losses due to weeds (from Lacey, 2001, by permission of Oxford University Press; *from Moss, 1987).
| 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 |
Table 1.4 Estimated percentage crop losses due to weeds, 1988–90 (from Oerke et al., 1995).
| | Estimated loss due to weeds (%) |
| Africa | 16.5 |
| North America | 11.4 |
| Latin America | 13.4 |
| Asia | 14.2 |
| Europe | 8.3 |
| Former Soviet Union | 13.0 |
| Oceania | 9.6 |
| Average | 13.1 |
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.
Yield loss may also occur in addition to direct competition for resources. Allelopathy is the production of allelopathic chemicals by one plant species that may inhibit (or, in the case of positive allelopathy, stimulate) the growth of other species. Anecdotal evidence of negative allelopathic effects has been reported for a number of weed species, although supporting research is often lacking. Recent findings have been reviewed by Olofdotter and Mallik (2001) and others (see Agronomy Journal vol. 93). Given the ample evidence of allelopathy exhibited by crop species, it is highly likely that many weed species will also display these effects, and that it is only a matter of time before research demonstrating this becomes readily available.
Further examples of yield loss caused by weeds include the effects on non- plant organisms. One example of this is the presence of dandelion (Taraxacum officinale) in fruit orchards. Dandelion flowers are preferentially visited by insect pollinators and so pollination of fruit blossom (and therefore fruit yield) is reduced.
1.4.2 Interference with crop management and handling
Some weeds can make the operation of agricultural machinery more difficult, more costly, or even impossible. The presence of weeds within a crop may necessitate the need for extra cultivations to be introduced. This often leads to crop damage, reduced yields and increased pest and disease occurrence, although in sugar beet crops, where inter-row cultivation is often carried out and has previously been associated with yield loss, recent findings suggest that careful implementation can result in no loss of root yield or sucrose content (Dexter et al., 1999; Wilson and Smith, 1999). This is possibly due to the development of tillage equipment that carries out more shallow cultivation and that is more carefully implemented, resulting in less seedling and root damage. Weeds can also affect the processes carried out prior to crop planting. For example, fat hen stems and leaves block the mesh of de- stoners, which are used prior to potato and other root crop planting. Species with rough, wiry stems that spread close to the ground (e.g. knotgrass, Polygonum aviculare) or are more erect in growth habit (e.g. fat hen, Chenopodium album) present major problems to the mechanical harvesting of many crops and can result in damage to machinery (e.g. pea viners) and subsequent harvesting delays. Other species can be troublesome when the crops are harvested by hand, such as the small nettle (Urtica urens) in strawberries and field bindweed (Convolvulus arvensis) in blackcurrants. The result of this is that fruit is not harvested and spoils on the plant.
1.4.3 Reduction in crop quality
Competition between crop and weed species can result in spindly leaf crops and deformed root crops which are less attractive to consumers and processors. A crop may have to be rejected if it contains weed seeds, especially when the crop is grown for seed, such as barley and wheat, and if the weed seeds are similar in size and shape to the crop, e.g. wild oats (Avena fatua) in cereal crops. Similar problems are encountered in the contamination of oilseed rape seed with seeds of weed species such as cleavers (Galium aparine). Where a proportion of the seed is saved for planting in subsequent seasons, this can cause a large increase in weed infestation. Contamination by poisonous seeds, such as darnel (Lolium temulentum) and corncockle (Agrostemma githago) in flour-forming cereals is also unacceptable and once led to vastly increased costs of crop cleaning. Such cleaning, however, has meant that these weeds are now probably extinct in agroecosystems in the UK. A further example that still causes major problems is black nightshade fruit (Solanum nigrum) in pea crops (Hill, 1977). In this case, the poisonous weed berry is of similar size and shape to the crop and so must be eradicated. Although grazing animals avoid poisonous species in pasture (e.g. common ragwort, Senecio jacobea), they may be difficult to avoid in hay and silage, and some species, notably the wild onion (Allium vineale), can cause unacceptable flavours in milk and meat.
1.4.4 Weeds as reservoirs for pests and diseases
Weeds, as examples of wild plants, form a part of a community of organisms in a given area. Consequently, they are food sources for some animals and are themselves susceptible to many pests and diseases. Because of their close association with crops, they may serve as important reservoirs or carriers of pests and pathogens, as exemplified in Table 1.6. Even where crop infestation does not occur, the presence of disease in weeds may cause problems, as is the case where grass weeds are infected with ergot (Claviceps purpurea), causing contamination of harvested grain with highly toxic ergot fragments.
Weeds may act as ‘green bridges’ for crop diseases, carrying the disease from one crop to another that is subsequently sown. Volunteer crops are particularly problematic in this case and can, in severe cases, negate the use of break crops as a cultural control measure for diseases. In addition, weeds can provide over - wintering habitats for crop pests, resulting in quicker crop infestation in the spring. Ground cover provided by weeds can increase problems with slugs and with rodents, as the weeds provide greater cover and therefore reduced predation.
In 1994 and 1995 there were several severe outbreaks of the disease brown rot in potato in several European countries, especially in Holland, which was possibly exported to other countries via infected seed potatoes. This extremely virulent pathogen (Pseudomonas solanacearum, syn. Burkholderia solanacearum, syn. Ralstonia solanacearum) causes a vascular ring rot in the developing tuber and causes a major loss of yield. Although often considered a soil- borne organism, it was not found to persist for long periods in the soil following the harvest of infected crops. However, it was found to survive in the aquatic roots of infected woody nightshade (Solanum dulcamara) growing at the edge of irrigation channels. Thus, it may be the case that the pathogen overwinters in this wild host and is le...
