Wednesday, 31 July 2019

What is a Weed?

What is a Weed?

What  is  a Weed?


There have been numerous definitions of weeds. Older ones include ‘a plant not valued for its use or beauty’ and ‘a plant whose virtues have yet to be discovered’. However, most modern definitions convey an opinion that the plants are considered undesirable in some way. This is reflected in the German term unkraut and the French Malherbe.  A great variety of reasons can account for a plant being unwanted but most encompass a view that the plant is a nuisance and in some way hinders or interferes with human activity. This leads to the understanding that crops may at some time also be weeds. For example, the grass plants growing in the domestic garden lawn is acceptable and encouraged, but when they spread to the adjacent flowerbed they are considered weeds. Similarly, crop seeds which are shed in the field can grow in subsequent crops in following years and contaminate them. The definition of weeds adopted by the European Weed Science Society is ‘any plant or vegetation, excluding fungi, interfering with the objectives or requirements of people’. Similarly, the Weed Science Society of America has adopted the definition ‘a plant growing where it is not desired’.

Reasons for classifying a plant as a weed

The definitions above emphasize that there is nothing special about the biology of weed plants but hey merely have to interfere with the activity of humans. We generally think of weeds as being a nuisance because they interfere with agricultural activities, but Table  1.1  summarises some of the other reasons for considering plants to be weeds and therefore for managing their occurrence. The examples given are from northern Europe and are considered in more detail below.

What  is  a Weed?

What  is  a Weed?

Reduction of crop yield is the major reason for attempts to reduce weed populations in arable crops, but effects on crop quality are almost as important for horticultural crops. The first attempts at controlling weeds used manual labour and hand-pulling or hand-hoeing. A major advance was the mechanisation of the process, permitting a greater area to be covered in a day. The technology required the development of a machine to sow the crop in rows so that the weeds in the spaces between the rows could be easily removed by an implement drawn behind a power source (animal or mechanical). This was the main method of weed management from the early 19th century up to the middle of the 20th century in developed countries and is still practised successfully today, around the

A major revolution was the development of herbicides in Britain, the USA and Switzerland. The ability to reduce weed populations growing in crops has been an important component of the increased food production by western agriculture. The recorded increases in crop yields in the UK over the  50  years since 1940, of 1 tonne every ten years (Fig. 1. l), contain contributions from a number of sources, including improved varieties of higher potential yield, improved crop nutrition and improvements in all aspects of crop health, including weeds. In trials, unweeded control plots provide a comparison with plots on which the weed population has been severely reduced (Fig. 1.2). Clearly the impact of weeds on crop yield can be considerable, or more accurately, some weed species have a large impact on crop production. This leads to consideration of the relative impact of different weed species which then allows the prioritization of weed management options. The most important weed species which have the largest impact on crop yields in northern Europe are given in Table  1.2.  In organic systems, where herbicides cannot be used, management of weeds is of greater concern than management of pests and diseases. In conventional systems, the spend on herbicides in the  UK  is nearly half the spend on all agrochemicals.

What  is  a Weed?
Fig.  1.1  Trends in  UK  wheat yields in the 20th century.

Investigations into how weeds achieve the reduction of crop yield show that shading can often account for much of the effect. This is particularly important when the weed seedlings emerge at the same time as, or earlier than, the crop because then the weed has the opportunity to intercept more light and shade the crop. Selection of crop species has tended to favour those lines which emerge early and show rapid seedling growth because then the crop will shade the weed. The earliness of complete crop ground cover used to be an important feature of husbandry which is receiving increased attention now that the desire is to minimise herbicide applications.

What  is  a Weed?
Fig.  1.2
adjacent weed-free plots for three varieties of wheat (data for 1968-1990). Yield loss  (YO) on weedy plots of continuous winter wheat crops compared to.

Table  1.2 Europe’s worst weeds: the results of an informal survey of weed specialists in December  2001.
What  is  a Weed?

Weeds may also interfere with the below-ground functioning of crop plants. Uptake of water and of nutrients by weeds represents the use of resources which we would prefer to be incorporated into crop plants. Recent research indicates that the roots of many weeds are concentrated in the surface layers of the soil, in contrast to more deeply rooted crop species. This gives the weeds an advantage when scavenging for fertilizers but puts them at a disadvantage when water is limiting.

For many years plant breeders, especially those working with cereals, have ignored the ability of their crops to compete with weeds, and as a result, have developed cultivars that have erect leaves that are good at trapping radiation but not good at shading lower-growing plants (weeds). However, there is now some interest in the identification of cultivars which are more suppressive of weeds or at least tolerate their presence better. Such so-called ‘competitive varieties’ are an important contribution to the development of organic systems in which herbicides are not used.

Until the introduction of statutory seed testing, a major means of spreading weeds was with crop seeds. Now, the seeds of crops traded internationally must meet minimum quality standards of purity as defined by either ISTA (International Seed Testing Association) or AOSA (Association of Official Seed Analysts). Despite these regulations, seeds of many weed species still occur in crop seed samples (Table  1.3),  and of course, there is no check on farm-saved seeds. Transport of straw can also move weed seeds from farm to farm.

What  is  a Weed?

The presence of a large volume of weeds in a crop canopy can alter the water relations of the crop. This may be particularly important where the product is a seed and needs to be harvested relatively dry (e.g. cereals, oilseed rape). The mat of weeds transpiring in the canopy can reduce the loss of water and hence delay crop seed ripening. Such a delay may make harvesting riskier. In addition, the presence of weeds can make the operation of harvesting machinery more difficult and slower, particularly when climbing or choking weeds are present.

In grassland, the presence of weeds can present problems additional to the reduction of forage yield. Spiny species such as thistles may inhibit animals from utilising the pasture species in their immediate vicinity. Some species, such as ragwort  (Senecio jucobeu)  are poisonous. Although most stocks avoid the growing ragwort plants, when incorporated into hay or silage these plants still maintain their poisonous properties although they are dead. Stock are unable to avoid the plants in such circumstances. Most buttercup species are poisonous if consumed to excess. The presence of some aromatic species, e.g. wild species of garlic (Allium spp.) in forage may lead to milk becoming tainted.

The effect of weeds on human activities may not be direct. Many wild species act as hosts for crop pests and diseases. Many cruciferous weeds (related to oilseed rape, sprouts, cabbages etc.) are also susceptible to the same diseases, e.g. clubroot  (Plasmodiophora brassicae),  and can therefore be described as alternative hosts. An important part of the control strategy for this disease is to avoid planting brassica crops in the same location for the following five years. However, the presence of weeds of the same family may allow the pathogen to maintain high inoculum levels. The common chickweed  (Stellaria media)  is the source of the mosaic virus for a number of crop species. Many weed species harbour nematodes which may infect crop species and cause significant yield loss. The
weed fat hen  (Chenopodium album)  is a host for the black bean aphid which damages broad and field beans. Some plant diseases require more than one species as hosts at different stages in the life cycle. It is important to reduce the occurrence of such alternate hosts if the disease is to be contained. An example of this is the need to control  Berberis  (barberry bushes) to restrict the sources of the cereal rust  Puccinia graminis.

Particularly in the pasture, plants may act as a source of ingestion of animal parasites. There is not always a specific relationship between the animal disease organism and the plant species; often the grass crop itself is a passive agent leading to ingestion, e.g. of lungworm. Weed seeds with hooks or spines are often cited as examples of seeds that are dispersed by animals when they become entangled in animal coats, but the presence of such seeds may reduce the value of the fleeces and thus lead to a desire to reduce the occurrence of that species in the pasture.

It should be clear that the definition of a weed is not just related to agricultural situations. Tall plants which grow on roadside verges may obscure motorists’ vision. This creates a need to manage the vegetation height, usually by mowing. Similarly, scrub species growing beneath power lines may create a fire hazard and require control. The tall vegetation along railway lines was a distinct fire hazard in the age of steam trains when sparks from the smokestack could set vegetation alight.

Aquatic plants can be a nuisance where their growth impedes the flow of water in drainage ditches or irrigation channels. An example in Europe is Canadian pondweed  (Elodea canadensis),  while worldwide the water hyacinth  (Eichhornia crassipes)  is particularly troublesome on many lakes and waterways. Introduction of exotic species into aquaria and ornamental ponds (e.g. Canadian pondweed) poses increasing problems resulting from their escape into natural water systems.

It is claimed that the performance of crop species can be reduced by the release of inhibitory chemicals by neighbouring species. This is termed allelopathy. Many laboratory experiments have shown that aqueous plant extracts can reduce crop seed germination. This can be shown by soaking grass clippings from the lawn in water for  24  hours and using the filtrate to water seeds of any crop species in a germination test in Petri dishes. Fortunately for the grower, in many situations, the effect disappears when the same experiment is modified by applying the filtrate to the soil in which the seeds have been sown. There is thus serious doubt as to whether allelopathy can account for any yield reductions of crops in the field in northern European conditions. However, there is increasing evidence that allelopathy can play a role in crop-weed interactions in warmer and wetter tropical growing conditions.

Pride is a very major influence in deciding on weed control because growers want their own crops to look cleaner than their neighbours’. Anecdotal evidence suggests that about half of all herbicide applications may not be economically justified and therefore lead only to cosmetic benefits. However, financial pressures are forcing a re-evaluation of all the components of a crop management system and the proportion of cosmetic applications should decrease.

Beneficial effects of weeds

It seems paradoxical that weeds may be also be considered valuable. Part of the resolution of the paradox is in the definition of a weed as a plant out of place. Clearly, in the right place, a plant species may have properties which are beneficial to man or his activities.

The man-managed arable and grassland areas of Europe generally have a reduced diversity of flora and fauna. Reduced biodiversity has been used as a measure of the intensity of management. Current EU policy is to encourage farmland biodiversity through less intensive farming, achieved either by reducing the intensity of management overall or by reducing the managed area. Weeds are perceived as valuable indicators of biodiversity because of their role in providing food or shelter for animal species. Much of the decline in farmland birds have been linked to the reduction of weed occurrence in arable crops. The rarer farmland birds can be encouraged to visit and to breed by accepting a certain level of weeds in a crop. A delay in weed management activities may permit chicks to be reared before the food supply is removed, and weeds in stubbles can provide some winter feed. Weedy strips are planted to encourage cover for game birds. While birds are particularly prominent and well recorded, the same arguments can be used for other, less well-studied organisms.

Rotational grassland is very dependent on relatively few species of grasses, especially  Lolium  (ryegrass) and  Festuca  (fescue) species. The seed mixtures sown now contain only a small number of species of grasses, sometimes with red or white clover, and are very different from the seed mixtures sown in the first half of the 20th century. A typical mixture then might have contained additional species of grasses (e.g.  Phleum pratense, Dactylis glomerata, Poa trivialis, Cynosurus cristatus),  legumes (e.g.  Trifolium incarnatum, Onobrychis viciifolia, Anthyllis vulneraria)  and many other species which were included to improve the mineral nutrition of the sward (e.g.  Achillea millefolium, Plantago lanceolata, Potentilla anserina  ,  Chicorium in the  us,  Symphy  tumoffic inale).

Bare soil is very vulnerable to erosion by wind and water. The presence of a plant cover helps to lessen the momentum of impacting raindrops and the plant roots help to bind the soil to reduce its ability to move. There is considerable interest in limiting weed management measures to allow populations of beneficial organisms to develop. Insects which pollinate crops (e.g. bumblebees) need a source of nectar and pollen when the crop is not in flower, and weeds can provide this. The natural predators of aphids include ground beetles, spiders and hoverfly larvae. Weed flowers may provide a food source for these natural predators and thereby reduce the reliance on aphicides. Weeds themselves may be more attractive to crop pests; thus aphid infestations on sugar beet in weedy fields have been reported to be lower than those on weed-free crops.

Similarly, intercropping of carrots with clover has been shown to reduce infestations of carrot root fly. Some weeds may actually be valuable plants happening to grow where we do not want them. Around the world, many wild plants are used as herbs for flavouring, and others are used for their medicinal properties. There is a resurgence of interest in ‘herbal’ remedies which are perceived as safer than synthesised pharmaceutical drugs even though the latter has undergone a rigid approval process. Many drugs have their origins in plant secondary metabolites, and indeed some are still derived by extraction from plantings of such medicinal plants. Foxglove  (Digitalis purpurea)  is a poisonous plant and a source of the glycosides digitalin and digitoxin, which are used as cardiac stimulants and vasoconstrictors.

Biological features of weeds

What makes an aggressive and successful weed? A species may become a weed because of a chance combination of circumstances that make its attributes particularly advantageous to its growth and survival. For example, during the late  1970s  and  1980s  the increased use of minimum tillage techniques to establish winter cereals, combined with the absence of rotations, provide a niche that favoured the increase of barren brome  (Anisantha sterilis)  in the  UK.  This weed remained a serious problem in monocultures of cereals until rotations were reintroduced and ploughing replaced minimum tillage.

One can speculate that certain attributes of plants will predispose them to weediness. Clearly, such attributes as high seed production, a short growing season and effective seed dispersal will tend to make a species a successful weed. Further, the species should possess a varied genotype that equips it to accommodate a wide range of environmental conditions. An interesting approach to describing the features of weediness was made by Baker  (1965),  who tried to define the ‘design features’ of an ‘ideal weed’. For both annuals and perennials,
these were:
0  The ability to germinate in many environments
0  Discontinuous, self-controlled germination and great longevity of seed
0  Rapid seedling growth
0  Early-onset of seed production in a range of environments
0  A long period of seed production
0  Self-compatibility
0  Easy cross-pollination
0  High seed output in favourable circumstances
0  Some seed production in adverse conditions
0  Long and short-distance dispersal
0  Special means of competition
and, if perennial:
0  Vigorous vegetative reproduction
0  Brittleness of lower nodes or rhizomes
0  Ability to regenerate from fragments
This list is particularly helpful when considering the potential weediness of new weed species, but the absence of these attributes does not necessarily mean that a species will not become a weed. Indeed, important weeds often possess only a proportion of these attributes. The agricultural ‘environment’ in which the species exist, and the way farmers manage their land, are just as important.

Weed  biology

The biology of weeds, as highlighted by ‘Baker’s Rules’, plays a very important role in determining the success of individual species as weeds. This may be due to the behaviour of the seeds, the competitive ability of the plants or their seed production. The biology then interacts with the crop or land management, whether that crop is winter wheat, apples or amenity grassland.

The behaviour of weed seeds is particularly important to the potential of a species to become a weed. One of the earliest observations on the number of seeds in the soil was made by Darwin (1859). He placed mud from a pond in a cup and counted the seedlings emerging over six months. He obtained 537 seedlings from 210 g (dry weight) of mud. The weed seed population is frequently large, considerably larger than the typical sowing rates of crop species. The sowing rate of barley is about 3 million seeds: in contrast, seed populations per hectare of poppy species  (Pupuver spp.) have been measured as 279million, of brome  (Bromus spp.) as 24 million and of blackgrass  (Alopecurus myosuroides)  as 55million. Estimates of the total weed seed population in arable soils are usually between 1000 and 10  000  seeds/m2.

The practical significance of weed seed populations in the soil is that they impose a need for continued weed control over a number of years. This is because not all weed seeds germinate at the same time and because most herbicides do not affect dormant weed seeds. The weed seed bank is the primary method of ensuring the longevity of a plant species and, in agricultural terms, the appearance of new seedlings in future crops. Thus, the numbers of seeds produced and their longevity in the soil are of prime importance. Seeds with a short persistence, such as those of barren brome  (Anisantha sterilis),  will have their primary effect on crops grown in the following year, whereas seeds with long persistence, such as poppy  (Papaver rhoeas),  will continue to have the potential to infest new crops for many years. The distribution of seeds is also important, as those with attributes facilitating long-distance transport will have greater potential to infest new areas than those that simply drop their seeds close to the parent plant. The behaviour of seeds is influenced by agricultural practice; for example, ploughing tends to increase seed persistence compared to surface cultivation.

The ability of weeds to produce seeds for future generations will depend on the intrinsic productivity of the plant and on its ability to compete with other vegetation, either crops or other wild plants. Some species produce large numbers of small seeds, whilst others produce fewer, large seeds. Both strategies can be successful. Poppy  (Papaver rhoeas,  many small seeds) and wild oats  (Avenafatua, few large seeds) are both common in agricultural landscapes. Species also differ in their competitive abilities, which is important in relation to their effects on crops but also significant as far as seed production is concerned.  A  vigorous competitive weed will have an appreciable effect on crop production and is also likely to be successful in producing seeds.

Many of the other components of weed population dynamics can affect the success of weeds. However, there is often a close link between the significance of the biology and the crop or land management imposed by farmers. For example, the persistence of volunteer oilseed rape depends on the induction of secondary dormancy in the seeds, which is influenced by the post-harvest cultivation regime used by the farmer. Similarly, the success of cleavers  (Galium aparine)  in winter wheat can depend on the timing of herbicide treatment and the pattern of seedling emergence, which can occur in both autumn and spring.

Interaction between weed biology and crop management

Knowledge of weed biology, particularly in relation to reproduction and to population dynamics, is a necessary prelude to successful weed management. There is a need to balance the detrimental aspects of weed growth against any beneficial aspects, particularly their role as food or hosts for crop pests and diseases against the harbouring of natural predators and food or shelter for valued wildlife.

In developing weed management strategies and in planning the detailed tactics it is crucial to emphasise the necessity for a long-term approach. Often the strategies will incorporate a sequence of crop management decisions, which may involve the planned use of herbicides but should also incorporate non-chemical methods. Clearly, it is important to prevent the introduction of new weeds, so the cleanliness of seed and reduction of the spread of weed seeds by way of straw, manure or machines are important.

Some weed species can be regarded as indicators of particular soil conditions and therefore attention to ameliorating these conditions has a major role in weed management. For example, the occurrence of spurrey  (Spergula arvensis)  can be taken to indicate an acid soil with low pH, so liming is an important tool for control as well as benefiting the growth of most crops. Similarly, perennial sow thistle  (Sonchus arvensis)  can be taken to indicate poor soil structure and so remedial ploughing, subsoiling or draining may improve crop growth as well as reducing the weed occurrence.

The rotation of crops has an important part to play. The avoidance of continuous cultivation of a field with the same crop is crucial to prevent the selection of well-adapted weed flora. The use of winter-sown and spring-sown crops, alternating between annual and perennial crops (i.e. including a grass ley which may have other benefits) and alternating between close, dense crops which shade out weeds (oilseed rape, rye) and more open crops (maize, many vegetables), all help in preventing the preponderance of particular species. The variety of crops permits the use of a wider range of herbicides and a variety of soil cultivation methods both before the sowing of the crop and while it is growing. In addition, cutting or topping operations in leys can suppress tall perennials. The choice of soil cultivation technique has an effect in selecting the weeds which survive the disturbance best. Decisions on the use of ploughing versus minimum tillage and the timing of cultivations are important. The time of sowing, the use of cross- drilling and the incorporation of a ‘stale seedbed’ into the cropping system are all critical.

The selection of competitive varieties of crops can play a role in suppressing the growth of weeds. Currently, few growers of arable crops take this into account when selecting cultivars, apart from those intending to grow crops organically, when the grower needs to optimise all aspects of weed suppression. Of particular importance is the ability of the variety to produce a completely closed crop canopy as early as possible. This can lead to the successful suppression of weed growth through shading. Crop species which germinate and establish quickly (e.g. cereals) are notably more suppressive of weeds than others, such as many vegetables, which may take a long time to produce a closed canopy. There are important differences in the earliness and leafiness of different varieties, and the information merits incorporation into the weed management strategy. Crop competitiveness can also be increased by other features of agronomy such as plant nutrition, closer row spacing, higher sowing rates and pre-germination of seeds.

Although it is feasible to develop a weed management strategy for a single farm or individual field, the implementation of the strategy is more difficult because the weather plays an important part in determining the ability to work on the land and to perform particular activities. Thus the strategy will always need to be flexible and incorporate a number of approaches in order to be resilient.

The incorporation of biological control of weeds is attractive. The idea of encouraging natural enemies is sound  so  long as the introductions are closely confined to the target weed and do not transfer to other species when the target species is reduced to a rare food plant or host. Biological control is expensive to set up but can be cheap to continue. There are relatively few successful examples. The control of prickly pear cactus  (Opuntiu  spp.) in Australia has been a major success. There is the possibility of using a moth from southern Africa for bracken  (Pteridium uquilinum)  control in the UK and we are close to controlling musk thistle  (Curduus  nutuns)  in the USA. Recent advances in biological technology have led to the development of mycoherbicides, i.e. genetically engineered fungi capable of controlling specific weeds.

The options for mechanical control of weeds are many. The grower can decide whether to choose an overall treatment which is usually fairly expensive, as opposed to targeted operations (now using machine vision) for which the equipment is expensive. Nevertheless, there are often other benefits from mechanical weeding, e.g. soil aeration. The difficulties associated with mechanical control are that optimum timing may be difficult because of the weather, and the extra traffic gives the risk of soil compaction.

Recent developments have led to the development of thermal weeding, in which heat is used to kill weeds. Flame weeding can be used within the crop row. Its use before crop seedling emergence may indeed hasten the process; it is thus beneficial for slowly emerging crops, and is often used for vegetables. The development of chemical weed control has been a major success in terms of the additional food production that has resulted. There is much information on the compounds used. We can classify chemicals by usage in terms  of:
0  where they are applied (e.g. to the foliage or to soil);
0  when they are applied (pre-sowing or pre-planting, pre-emergence of seedlings
0  the extent of the application (overall, directed away from the crop or in a band
0  their mode of action (total or selective).
or post-emergence);
along the crop rows);

The weed scientist will also consider the selectivity of the compound because this will limit the crops on which it can be used (the action spectrum). How the molecule penetrates the weed plant and its mode of action are both important. After the weeds have died there are serious concerns about herbicide residues, especially their persistence and fate in the soil. Allied to this are the concerns of toxicity to non-target organisms (including humans), hazards and safe usage, and therefore the regulation of herbicide use. In order to save on application costs, weed management practitioners will want to know whether the compounds can be used in mixtures. Not least, the users will need to be convinced of the benefits in extra yield to be achieved for the cost of the herbicide and its application.

Weeds  in  the future

The most likely reason for changes in the species occurrence and distribution in agriculture and horticulture is any change in crop management. Current increased interest in early drilling and minimum tillage in winter wheat is already causing increases in grass weed problems, despite the use of stale-seedbeds. The arrival of herbicide-resistant crops, both conventionally bred and genetically modified, could help to solve current, otherwise intractable, weed problems, but may also bring their own difficulties, such as the control of herbicide-resistant volunteer crops. Crops present as weeds in other crops will remain important and the presence of ‘added’ herbicide resistance genes may make their control more difficult in some situations. The repeated use of single herbicides or single herbicide groups to control certain weeds has been causing increasing problems, due to the selection of resistant biotypes. In Europe, the main problems are associated with annual grass weeds resistant to substituted urea herbicides and/or the specific graminicides, but it is likely that grass and broad-leaved weeds resistant to sulfonylureas will become more common, as has already occurred in North America.

It is also possible that climate change may result in the appearance of ‘new’ weeds from southern Europe, but even here the most likely cause will be a climate-induced change in cropping. If northern Europe starts to grow large areas of grain maize or soybeans, then one would expect the appearance of weeds commonly associated with these crops. However, climate change could impact on weeds in a more subtle way. The occurrence of milder winters could influence emergence patterns in current weed species, changing their vulnerability to control techniques. Changes in climate might also affect the flowering of species requiring vernalisation to produce flowers.

A very different problem with weed management is likely to arise from the withdrawal of many older products from the market. Although an adequate range of products will be retained for the major crops, more and more smaller-area arable and horticultural crops will be bereft of suitable products. This will mean that integrated approaches to crop management, involving the increased use of physical weed control methods and changes in production systems, will have to be employed. This will result in a switch of weed species in these crops, away from those that are difficult to control with herbicides towards those that are difficult to manage with non-chemical techniques. Similarly, the increased interest in Europe in the production of organic crops will alter selection in favour of weed species that are not easy to control either by changes in crop management or by non-herbicidal methods of weed control. However, integrated crop management will become more important in many areas of crop production, switching the emphasis to favour those weeds adapted to such integrated methods.

Increasing concern about the impact of farming on rural diversity is prompting serious questions about the definition of a weed. Even if a weed in a crop is causing a reduction in crop yield it may be considered that, at a national level, its beneficial effects on populations of insects that are the food for birds outweighs this negative effect.  As  a consequence farmers may be asked to manage fields on a conservation basis, eliminating only the noxious weeds and not controlling those species that have other environmental benefits. Thus weed control may be targeted at blackgrass  (Alopecurus myosuroides)  and cleavers  (Galium aparine) but other weeds such as those in the Polygonaceae (redshank, knotgrass) or chickweed  (Stellaria media)  will be left to provide food for beneficial invertebrates and vertebrates. This approach will provide a severe challenge for farmers and growers in managing crops. It is not changing the impact that weeds have on crops but it is challenging perceptions of the main reason for controlling them, by changing the value placed on the non-crop species.

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