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Weeds culture

In many agricultural systems around the world, competition from weeds is one of the major factors reducing crop yield and farmers’ income. In developed countries, despite the availability of high-tech solutions (e.g. selective herbicides and genetically-modified herbicide-resistant crops), the share of crop yield loss to weeds does not seem to reduce significantly over time (Cousens and Mortimer, 1995). In developing countries, herbicides are rarely accessible at a reasonable cost, hence farmers often need to rely on alternative methods for weed management.

Worldwide limited success in weed control is probably the result of an over-simplification in tackling the problem. Too much emphasis has been given to the development of weed control tactics (especially synthetic herbicides) as ‘the’ solution for any weed problems, while the importance of integrating different tactics (e.g. preventive, cultural, mechanical, and chemical methods) in a cropping system-based weed management strategy has long been neglected.

Integrated weed management is based on knowledge of the biological and ecological characteristics of weeds to understand how their presence can be modulated by cultural practices. Based on this knowledge, the farmer must first build up a global weed management strategy within her/his cash crop sequence, and then choose the best method of direct weed control during crop growing cycles. Besides this, it must be remembered that weed management is always strictly embedded in crop management itself. As such, the interactions between weed management and other cultural practices must be duly taken into account. For example, the inclusion of cover crops in a crop sequence is an interesting way to integrate weed management with nutrient management in low-external input systems, with additional benefits on other important agro-ecosystem properties (e.g. soil fertility, soil moisture retention, biodiversity, etc.).

HOW TO IMPLEMENT AN EFFECTIVE WEED MANAGEMENT STRATEGY

A long-term effective weed management strategy is based on the practical application of the ecological concept of ‘maximum diversification of disturbance’, which means diversifying crops and cultural practices in a given agro-ecosystem as much as possible. This results in a continuous disruption of weed ecological niches (Liebman and Davis, 2000) and hence in a minimized risk of weed flora evolution towards the presence of a limited number of highly competitive species. Besides this, a highly diversified cropping system also reduces risk of the development of herbicide-resistant weed populations.

In practice, weed management strategies should integrate indirect (preventive) methods with direct (cultural and curative) methods. The first category includes any method used before a crop is sown, while the second includes any methods applied during a crop growing cycle. Methods in both categories can influence either weed density (i.e. the number of individuals per unit area) and/or weed development (biomass production and soil cover). However, while indirect methods aim mainly to reduce the numbers of plants emerging in a crop, direct methods also aim to increase crop competitive ability against weeds.

Preventive methods include crop rotation, cover crops (when used as green manures or dead mulches), tillage systems, seed bed preparation, soil solarization, management of drainage and irrigation systems and of crop residues.

Cultural methods include crop sowing time and spatial arrangement, crop genotype choice, cover crops (when used as living mulches), intercropping, and crop fertilization.

Curative methods include any chemical, physical (e.g. mechanical and thermal) and biological methods used for direct weed control in an already established crop. A list of the main methods that can be used in an integrated weed management strategy is shown in Table 1.

Hereafter, the main effects on weeds of preventive and cultural methods are described, trying especially to highlight their possible interactions, which are not always easy to predict in the field. Curative methods are not treated here; however, it must be stressed that the effectiveness of any of them can be expected to increase if preventive and cultural methods are concurrently applied.

Differentiation of crops grown over time on the same field is a well-known primary means of preventive weed control. Different crops obviously bring about different cultural practices, which act as a factor in disrupting the growing cycle of weeds and, as such, preventing selection of the flora towards increased abundance of problem species (Karlen, 1994). In contrast, continuous cropping selects the weed flora by favouring those species that are more similar to the crop and tolerant to the direct weed control methods used (e.g. herbicides) via repeated application of the same cultural practices year after year.

In addition, continuous cropping can negatively interact with tillage systems and shift the weed flora towards a troublesome composition. For example, in continuous winter cereal cropping in temperate regions, minimum tillage can cause the dominance of grasses with low-dormant seeds, such as Alopecurus myosuroides and Bromus spp., to occur after a few years. (Froud-Williams, 1983). In these cases, the consequent higher use of graminicides acts as an additional selection factor for the weed flora and can also accelerate the selection of herbicide-resistant biotypes. To recover highly degraded floristic situations such as the one just pictured, it is imperative to rotate cereals with crops having a different growing period, as well as to plough the soil from time to time to disadvantage low-dormant grass species whose seeds are usually unable to emerge from deep soil layers. If there is a long fallow period between the cereal and the next crop, this can be exploited to cultivate the soil to stimulate the emergence of problem weeds, which are then destroyed by additional cultivation or by herbicides.

Rotation between crops having the same growing period, although certainly preferable to continuous cropping, is not as successful as rotation between crops with different growing cycles in reducing the number of weeds emerging in the field. Compared to weed-density reduction, the effect of crop rotation on weed biomass reduction is less systematic because it depends on factors such as the following:

the competitive ability of crops included in the rotation;

the effectiveness of direct weed-control methods (e.g. herbicides), and

the frequency of tillage and cultivation.

Table 1. Classification of cultural practices potentially applicable in an integrated weed management system, based on their prevailing effect.

Reduction of weed emergence

Alternation between winter and spring-summer crops

Cover crops (used as green manures or dead mulches)

Reduction of weed emergence

Cover crop grown in-between two cash crops

Reduction of weed emergence

Deep ploughing, alternation between ploughing and reduced tillage

Seed bed preparation

Reduction of weed emergence

False (stale)-seed bed technique

Reduction of weed emergence

Use of black or transparent films (in glasshouse or field)

Irrigation and drainage system

Reduction of weed emergence

Irrigation placement (micro/trickle-irrigation), clearance of vegetation growing along ditches

Crop residue management

Reduction of weed emergence

Sowing/planting time and crop spatial arrangement

Improvement of crop competitive ability

Use of transplants, higher seeding rate, lower inter-row distance, anticipation or delay of sowing/transplant date

Crop genotype choice

Improvement of crop competitive ability

Use of varieties characterised by quick emergence, high growth and soil cover rates in early stages

Cover crops (used as living mulches)

Improvement of crop (canopy) competitive ability

Legume cover crop sown in the inter-row of a row crop

Reduction of weed emergence, improvement of crop competitive ability

Intercropped cash crops

Reduction of weed emergence, improvement of crop competitive ability

Use of slow nutrient-releasing organic fertilizers and amendments, fertilizer placement, anticipation or delay of pre-sowing or top-dressing N fertilization

Killing of existing vegetation, reduction of weed emergence

Post-emergence harrowing or hoeing, ridging

Killing of existing vegetation, reduction of weed emergence

Pre- or post-emergence spraying

Thermal weed control

Killing of existing vegetation, reduction of weed emergence

Pre-emergence or localized post-emergence flame-weeding

Biological weed control

Killing of existing vegetation, reduction of weed emergence

Use of (weed) species-specific pathogens or pests

Cover crops (used as green manures or dead mulches)

Inclusion of cover crops in a rotation in the time frame between two cash crops is another good preventive method to be used in a weed management strategy. Cover crops do not give a marketable yield but, by extending the period in which the soil remains covered by vegetation, exert a series of beneficial effects on the agro-ecosystem, such as optimization of natural resource use (solar radiation, water, soil nutrients), reduced water runoff, nutrient leaching and soil erosion, and, last but not least, weed suppression (Lal et al. 1991).

Cover crop effects on weeds largely depend upon cover crop species and management, following cash crop, and weed community composition (Bàrberi and Mazzoncini, 2001). Weed suppression is exerted partly through resource competition (for light, nutrients and water) during the cover crop growing cycle, and partly through physical and chemical effects that occur when cover-crop residues are left on soil surface as a dead mulch or ploughed down and hence used as green manure (Mohler and Teasdale, 1993; Teasdale and Mohler, 1993). Interference with weeds, including competition, physical, and allelopathic effects, is generally higher when grasses or crucifers are used as cover crops than when legumes are used (Blum et al. 1997). Interference from cover crops and their residues is related to their occupation of ecological niches otherwise available for weeds. This is mostly a result of the sequestration of soil nutrients (especially N), to the release of allelochemicals (e.g. glucosynolates from crucifers and sorgoleone from Sorghum spp.) and to modifications of the soil microenvironment (Gallandt et al. 1999). Examples of highly weed suppressive cover crops are rye (Figure 1), sorghum, kale, rocket and mustard. In contrast, although direct weed suppression by legumes can be significant, their residual weed control effect is usually lower because the high quantity of N released from their residues after cover crop destruction stimulates weed emergence, especially when legumes are used as a green manure (Blum et al. 1997).

When cover crops are used as dead mulch (i.e. they are left to decompose on soil surface), weed suppression seems mostly to be the result of the physical effects of mulch, rather than to nutrient- or allelochemical-mediated effects (Teasdale and Mohler, 2000). In particular, weed suppression seems directly related to the Mulch Area Index (mulch area divided by soil unit area), which influences light extinction through the mulch and consequently weed seed germination. Small-seeded weed species appear to be more sensitive than large-seeded species to mulch physical effects as well as to allelochemicals (Liebman and Davis, 2000). Timely sowing of cover crops is very important to enhance biomass production and hence to increase their weed suppression potential.

Cover crops can also interact with other biota; for example, they promote the establishment of vesicular-arbuscular mychorrhizae, which in turn may shift weed flora composition by favouring mychorrhizal plant species at the detriment of non-mychorrhizal species (Jordan et al. 2000).

Figure 1. High weed suppression exerted by a rye cover crop

The effect of primary tillage on weeds is mainly related to the type of implement used and to tillage depth. These factors considerably influence weed seed and propagule distribution over the soil profile and therefore they directly affect the number of weeds that can emerge in a field.

Mouldboard ploughing is very effective in reducing weed density and hence it is an important preventive method where farmers are forced (or are willing) to use partially suppressive direct weed control methods (e.g. mechanical weeding), and reduces the labour needed for subsequent hand-weeding. In contrast, with non-inversion tillage (especially with no tillage) weed seeds are only partially buried and therefore they are mainly distributed in the upper soil layer, from which they can easily germinate and give origin to established plants.

Theoretically, if direct weed control was effective enough to reduce weed seed shedding (S), non-inversion tillage systems should reduce weed density over time to a greater extent than plough-based systems. This should happen because of higher weed seed bank depletion (D) in non-inverted soil, driven by higher emergence rates and environmental conditions (related to lack of seed burial) not conducive to seed secondary dormancy; and by higher seed predation by soil fauna. In terms of weed population dynamics, a reduction in population size occurs if D > S, a situation that is very rarely encountered with non-inversion tillage because on-field weed control is rarely complete, therefore weeds have a very good chance of setting seeds and replenishing the soil seed bank. For this reason, weed densities in minimum- and no-tillage systems are invariably higher than in plough-based systems (Froud-Williams, 1988; Cardina et al. 1991; Spandl et al. 1999). Weed seed bank data taken in a long-term experiment in which four tillage systems were used for 12 consecutive years in a continuous winter wheat or a pigeon bean-winter wheat rotation showed that total weed seedling density was higher in no tillage, minimum tillage (i.e rotary harrowing at 15 cm depth), and chisel ploughing (at 45 cm depth) in the 0-15, 15-30, and 30-45 cm soil layers respectively (Bàrberi and Lo Cascio, 2001). Density in the whole (0-45 cm) layer did not significantly differ among tillage systems, but in no tillage more than 60 percent of total seedlings emerged from the surface layer, compared to an average 43 percent in the other tillage systems (Figure 2). Crop rotation did not influence either weed seed bank size or seedling distribution among soil layers, and had a small influence on major species abundance. The weed seed bank was dominated (> 66 percent of total density) by Conyza canadensis and Amaranthus retroflexus , which thrived with chisel ploughing and no tillage, respectively. Among other major species, Bilderdykia convolvulus and Chenopodium album were mainly associated with mouldboard ploughing, Papaver rhoeas and Portulaca oleracea with minimum tillage, and Lolium multiflorum and Veronica spp. with no tillage. Results suggest that, although substitution of mouldboard ploughing by non inversion tillage (especially by minimum tillage) may not result in increased weed problems in the long-term, use of no tillage is likely to increase weed infestations because of higher seedling recruitment from the topsoil, and consequently an increased requirement for herbicide application. Use of no tillage can be desirable in the tropics because these conditions would exacerbate weed-control problems.

Figure 2. Percent weed seedling distribution over soil layers in mouldboard ploughing at 45 cm depth (P 45), chisel ploughing at 45 cm depth (CP 45), rotary harrowing at 15 cm depth (RH 15), and no tillage (NT) after 12 consecutive years’ application of the different tillage systems (after Bàrberi and Lo Cascio, 2001, modified; data are means of two crop rotations). For each layer, bars labelled with the same letter are not significantly different at P £ 0.05 (LSD test).

Disturbance posed to weeds by tillage is dependent more on the type of implement than on tillage depth. Tools that do not invert the soil (e.g. chisels) increase weed density and shift weed flora composition towards an increased presence of biennials, perennials, and non-seasonal annuals. Most of these species are characterized by wind-dispersed seeds with reduced longevity and dormancy and are unable to emerge from deep soil layers (Zanin et al. 1997). Examples of species usually favoured by non inversion tillage or no tillage are Agropyron repens , Calystegia sepium , Lolium perenne and Plantago spp. (perennials), Digitaria sanguinalis , Lolium multiflorum , Setaria viridis and Thlaspi arvense (annuals).

Relative abundance of perennial species in a weed community is also favoured by reduced tillage frequency over a crop sequence. For example, the inclusion of a perennial ley in a crop rotation means that the soil is not tilled yearly. Lack of soil disturbance, coupled with higher control of annual weeds by repeated mowing in the ley, can shift weed community composition towards a higher presence of biennials and perennials. In contrast, plough-based systems seem to encourage some annual dicots such as Chenopodium album , Papaver rhoeas and Polygonum spp., although this effect is always modulated by the effectiveness of direct weed-control methods (e.g. herbicides) used in the crop rotation (Légère et al. 1993; Liebman et al. 1996).

In a given cropping system, weed density can be reduced to a greater extent when tillage methods change than when the same tillage system is used year after year. A long-term trial carried out at Pisa (Italy) in a soybean-winter wheat two-year rotation showed that by alternating mouldboard ploughing at 50 cm depth with minimum tillage (rotary harrowing at 15 cm depth) it was possible to reduce weed density in wheat compared with chisel ploughing (at 50 cm depth), minimum tillage or shallow ploughing (at 25 cm depth) when used every year (Bàrberi et al. 2001, Table 2). Use of minimum tillage for winter wheat and of deep ploughing for soybean was better than the opposite system because in the first case the weed community was mainly composed by less competitive species ( Anagallis arvensis and Papaver rhoeas vs. Lolium spp., Polygonum aviculare and Veronica spp. in the second case). A very simple way to diversify the tillage system is to include in a rotation crops that require different tillage operations (e.g. cereals and root crops).

SEED BED PREPARATION

Cultivation for seed bed preparation has two contrasting effects on weeds: (i) it eliminates the emerged vegetation resulting from after primary tillage; and (ii) it stimulates weed seed germination and consequent seedling emergence, thanks to soil mixing and reallocation of seeds towards shallower soil layers. Together, these two effects can be exploited through application of the false (stale) seed bed technique, a preventive method with the specific aim of reducing weed emergence in the next crop cycle.

The false seed bed technique consists in the anticipation of cultivation time for seed bed preparation, in order to stimulate as much as possible the emergence of weeds prior to sowing. Emerged weeds are then destroyed by the next cultivator pass or by application of a total herbicide (e.g. glyphosate), the latter being useful especially where perennial weeds are present. At sowing time, the seed bank of those weed species able to emerge together with the crop is then already partially depleted and their emergence in the crop is reduced. Cultivation can be performed with any mechanical tools, but spring-tine harrows (Figure 3) are preferable because of their high working capacity and relatively low cost. Application of the false seed bed technique can reduce weed emergence > 80 percent compared to standard seed bed preparation (van der Weide et al. 2002). Obviously, application of this technique implies that there should be enough time (at least 2-3 months in temperate climates) between harvest of the previous crop and sowing of the next crop to allow weeds to emerge. For this method to be effective, the soil must have enough moisture to sustain weed-seed germination. Therefore, this method is useless where soil water availability is limited. Where farmers expect high rainfall events to occur between primary tillage and crop sowing, they must evaluate whether anticipation of seed bed preparation could increase the risk of damaging soil structure or delaying crop sowing because the soil cannot be timely worked: both effects may counteract the benefits of the false seed bed technique and therefore require careful evaluation.

Table 2. Relative density (percentage) of the main weed species and total weed density (plants m -2 ) observed in winter wheat just prior to post-emergence herbicide application (data pooled over two years and three nitrogen rates). Data are shown as arcsine-transformed (relative density) or square root-transformed (total weed density) means to allow direct interpretation of SEDs. DP = deep ploughing (50 cm); SP = shallow ploughing (25 cm); TLP = two-layer ploughing (shallow ploughing + subsoiling at 50 cm); CP = chisel ploughing (50 cm); MT = minimum tillage (rotary harrowing at 15 cm). Significance of the F tests (indicated as superscript): *,**,***, ns = P £ 0.05, 0.01, 0.001, and non significant, respectively. After Bàrberi et al. (2001), modified.

Weeds culture In many agricultural systems around the world, competition from weeds is one of the major factors reducing crop yield and farmers’ income. In developed countries, despite the

Cultural weed controls

Organic farmers recognise that every element of farming is inter-linked, and that good rotational design produces healthy soil, healthy plants and good yields. Crop rotation is the cornerstone of organic farming practice. Rotation and forward planning are also important for managing weeds. In this section we provide information on cultural methods aimed at preventing weed problems arising in the first place and which farmers and growers can plan to incorporate into their rotations.

The underlying principle of a preventative approach is to produce a constantly changing environment to which no single weed species can adapt and become dominant and unmanageable. In practical terms this means as diverse and long a rotation as possible consistent with the farm system and which prevents the weeds returning seeds to the soil seed bank.

We have provided sections on:

  • crop rotation issues including fertility building, the use of manures, the use of intercrops and undersowing as well as the strategic use of fallowing.
  • crop management including choosing varieties, seed rate and crop spacing and crop establishment
  • the use of mechanical methods such as tillage and cutting regimes
  • the use of livestock
  • the importance of on-farm hygiene.

Crop Rotation

Crop planning is a cornerstone of organic farming practice and it has important implications for weed management. It can be designed to positively influence weed control and to make a useful contribution to the whole farm management strategy. Typically rotation cycles extend over several years with often only an annual change of crop, but the inclusion of cover crops, intercrops and green manures increases the crop diversity in a rotation. In horticultural systems there may be sequential cropping where short-term crops follow each other in succession.

Weed population density may be markedly reduced using crop rotation but there has been little experimentation. Success depends on the use of crop sequences that create a diverse pattern of competition, allelopathic interference, soil disturbance, and production needs (such as the time of sowing and harvesting). There should be regular changes between spring and autumn-sown crops, and between annual and perennial crops, between dense leafy crops and those with an open habit, and between crops that require a long growing season and others that mature quickly. Rotation may also allow the use of a range of cultivations and direct non-chemical weeding methods that may be applicable to the different crops. The aim is to provide an unstable and inhospitable environment that prevents the proliferation of a particular weed species.

Choosing crops and their sequence

The length of the rotation, the choice and sequence of crops will depend upon individual farming circumstances that will include factors like soil type, rainfall, topography and enterprises. However, the aim is to produce an unstable environment in which no single weed species is allowed to adapt, become dominant, and therefore difficult to manage. No one rotation can be recommended, but ideally in terms of weed control rotations should include:

  • alternation of autumn and spring germinating crops,
  • alternation of annual and perennial crops (including grass),
  • alternation of closed, dense crops such as oats which shade out weeds, and open crops such as maize which encourage weeds,
  • a variety of cultivations and cutting or topping operations that directly affect the weeds.

Various suggestions and observations include:

  • putting sensitive annual crops after perennial leys. Research has shown that in the third cropping year after a grass/clover ley there is twice as much weed emergence as compared to the first
  • include a row crop that allows the use of one or more cultivations to kill emerged weeds and encourages the germination of others, so reducing the soil seedbank and hence potential weed numbers in future crops. Cultivations may also reduce the problem of perennial weeds by disrupting growth and smothering regeneration in the growing crop. Typical cleaning crops include turnip, sugar beet, and potato
  • uncultivated leys provide a completely different habitat for weeds and may be used to reduce or eliminate particular weed species. Few studies have been made of the effectiveness of leys for controlling weeds but trials suggest that there is little advantage for weed management in leaving leys down longer than 3 years. The species composition, and the mowing and grazing regimes are important in the weed dynamics. Management of the weeds at the time of ley establishment is critical as is the method of ending the ley to avoid a flush of weeds due to the release of seed dormancy by cultivation. A greater proportion of ley in the rotation usually results in lower seed numbers in the seedbank in comparison with arable crops. It was a traditional way to deal with land infested with wild oat but does not eliminate the weed completely
  • where a long grass break does not form part of the rotation weed problems are likely to be more severe. The problem will be greater where less vigorous and therefore less competitive crops are grown. Among the cereals, oats and winter rye are the most competitive followed by triticale
  • canopy development and shading are important for weed suppression and choice of cultivar can influence this
  • higher seeding rate and narrower row spacing increase the level of weed suppression
  • competitive cereals like rye may be grown as short duration

Fertility building

The fertility-building period, or ley, will influence the weed population. If it is well managed it can act as a weed suppressing phase. It is important to choose the right species and ensure they establish quickly, especially for grassland systems where the ley may last for several years. Establishment of leys can be easier in the autumn period than in the spring because sowing in spring coincides with the main spring flush of weeds. The seedbed needs to be well prepared, and good contact made between seed mix and, ideally, moist soil to achieve good establishment. The choice of fertility building crop is also important. Rotations with grass leys have been shown to be beneficial in reducing weed seed numbers compared with rotations that do not include a grass phase. Grassland systems, which have temporary leys rather than permanent pasture, will provide the opportunity to control perennial weeds during the cultivations between ploughing and reseeding.

Grassland or clover/grass leys are an important part of the organic farming system in the UK. On livestock farms grassland forms the basis of the production process, in arable systems the ley is used primarily for maintaining or restoring soil fertility. The grass may be managed as a short, medium or long-term crop and this may determine the composition of the desirable sward species and the nature of the associated weeds. The seed mixture for a ley may include a relatively simple mixture of grasses and legumes or may be more complex and contain a range.

Choosing varieties

Varieties that consistently suppress weeds are generally more desirable in organic systems (although this might be outweighed by marketing necessities) as opposed to varieties that tolerate weeds (and which potentially allow weeds to develop and return seeds to the soil seed bank).

Organic varieties: should show quick germination and establishment, rapid early and vigorous growth, and the ability to rapidly cover the soil and shade it (prostrate or tall varieties) to out-compete weeds at an early a stage in the crop cycle as possible. Varieties are well known to differ in architecture and competitive ability and whilst those that out-compete weeds are preferred it should also be borne in mind that those with erect foliage or that can tolerate some degree of mechanical weeding are also likely to be useful. Some crop types or varieties might produce allelochemicals that prevent weeds from developing or germinating although information on this is generally lacking.

In grassland systems the choice of variety may be dominated by forage value, but if there is opportunity the most vigorous species should be selected, as these will determine the productivity of the whole ley period. The trend in conventional cereal production has been to grow the taller stemmed varieties for their weed suppressing ability. Some farmers have stayed with the shorter stemmed varieties and employed a weed topper/cutter which will remove and, ideally, collect weed seed-heads so long as there is a difference in height between crop and weed. New research in wheat is investigating leaf angle development, height and speed of development on weed suppression to aid farmer variety choice (see below).

Seed size: varieties with a larger seed size have been shown to exhibit greater initial vigour of emergence and growth, which may subsequently provide extra competitive ability. If there is a choice available then the most vigorous species should be selected, as these will be more likely to out-compete weeds and suppress their development.

Clean seed: It is important that the crop seed is free from contamination by weed seeds. Organic farmers are obliged to use organically produced seed and this should be clean. It is important if saving seed that it is taken from weed free crops, and ideally, professionally cleaned. Tolerance levels of contamination should be low although they are not generally well defined as of yet. Tolerance levels of dock in Switzerland are one seed per 100g.

Seed rate and crop spacing

Spatial distribution of the canopy foliage and rooting system will be important for weed suppression. In drilled or transplanted crops the proximity of the plants to one another will determine the competitiveness of the plant stand as a whole. The principle is that the greater the amount of space taken up by the crop in the rows, the less space there is available for the weeds to invade. However, it should be borne in mind that closely spaced crop plants compete with each other and that it is also expedient to allow sufficient space between plants to allow for efficient mechanical weeding should weeds develop and threaten the crop.

Seed rates tend to be higher for organic than conventional crops. There is also an allowance for potentially lower germination rates and loss of the crop by mechanical weeders.

There has been much work in cereals on row spacing, pattern, direction of sowing and seed rates (typically 10% higher in organic cereals). Results are varied and interactions between the factors are often significant. For example in narrow widths an E-W sowing was favourable whereas in wide row widths a N-S sowing showed better response. Findings from the EU funded WECOF trials are awaited to give more definite recommendations.

In vegetable crop market size specifications are often the main driver in determining the crop row spacing.

Establishing the crop

The ability of the crop to get off to a good start ahead of the weed flora is critical. Good soil management practises are important to provide the best possible seedbed in which to plant a crop. The impact of a poor compacted soil can soon be seen on crop establishment and subsequent weed invasion.

In some systems sowing can be aided by the use of primed seed, or by transplanting an already established plant into a freshly prepared weed free seedbed.

Transplanting is a popular technique in organic horticultural systems. Bare rooted transplants can be raised on holdings or modular plants raised or bought in then planted out in the field. Advantages include the benefit of accurate spacing i.e. not having to rely on germination that can sometimes lead to uneven establishment with subsequent yield and quality penalties. It also accentuates the difference in size between crop and weed, which can be vital for mechanical weeding at later stages.

Intercropping and undersowing

Intercropping (or mixed cropping) and undersowing involves growing two or more different crop species in the same area. The advantage for weed control is that the crops cover more ground, so there is less space available for weed emergence. Intercropping can involve purely cash crops or a mixture of cash crops and fertility building crops.

Intercropping: is widely practised in certain countries and an enormous variety of intercropping systems have been developed. Both component crops may be taken to yield or one may be there as a living mulch to improve weed control. In successful intercrops weed suppression is usually superior to that of either of the component crops when grown alone. Crop density, crop diversity, crop spatial arrangement, choice of crop species and cultivar will all affect weed growth in intercropping systems. If water or nutrients are limiting then growth of one or both intercrops will suffer. Improved weed control alone is unlikely to justify their use and there must be other obvious benefits if the change in cropping practice is to prove economic.

In terms of mixed cash cropping there have been investigations into organic winter wheat and beans that reduced weed growth and gave better yields than sole cropping. Leeks and celery intercropping has also been shown to increase weed suppressing ability, and reduce reproductive capacity of late emerging groundsel. There are probably a wide range of combinations that could be designed to suit the farmer’s rotations and marketing needs although some practical experimentation is required

Undersowing: aims to cover the ground with a quick-growing dense layer of vegetation underneath the crop. The undersown species is prostrate, usually leguminous, and adds to or maintains fertility. It also suppresses weeds. Combining cash cropping with fertility building in this manner potentially produces an economic return, and it may mean there is either no need for an isolated fertility period, or that the length of that phase can be reduced. Undersowing cash crops with fertility building crops has other advantages apart from weed suppression, but so far the technique has only been widely used in cereal growing where it helps to re-establishing leys and avoid bare ground after harvest. Short-strawed cereal varieties can be difficult to manage, and straw difficult to save, due to the undersown crop growing up into the cereal. Further research into variety choice and sowing rates is needed to ensure that competition between the two crops is kept to a minimum.

Using cutting regimes for weed control

Cutting and topping weeds will have an impact on the type and weed flora in a field and can be invaluable in preventing return of weed seed to the soil seed bank. Cutting and topping are important for weed management in pasture, grass and leys. Topping can also be used as a remedial measure in vegetable or other crops to prevent weeds from seeding.

Pasture systems: good management involves maintaining the condition of the sward by cultural means. In particular reducing weed intrusions by chain harrowing in spring and topping regularly during the growing season. Mowing during the seeding year must be carefully judged and close cutting avoided. Spring sown stands should be cut no later than mid-August to allow recovery before winter. Summer sowings should be left unmown until November. Undersown lucerne should be left to grow into the winter. Where companion grasses are growing strongly, light winter grazing may be desirable. In grazed pasture weeds that are not eaten by livestock, will need to be topped to prevent seed shed. The established crop may be cut up to four times per year starting in mid-May. The crop is quickly weakened by defoliation, either by grazing or cutting, at too young a stage especially in spring or autumn. Before entering the dormant stage the crop must be allowed to make sufficient growth to replenish the food reserves in the root.

Grassland systems: cutting for hay or silage will have an impact on the weed flora. Silage tends to be cut early in the season when the sward is young and fresh, whilst hay is cut at a later stage. There can be both advantages and disadvantages associated with the timing of cutting depending on the weed flora and the ultimate requirements of the system. Cutting late may allow weeds in the pasture to grow to maturity and set seed. The ripe seeds may contaminate the hay and remain viable when passed through livestock. Dock seeds should not survive low pH silage, however they will survive in a later cut of hay. This mature seed may also shed on the ley surface and find opportunities to germinate in situ or be transported by livestock to other locations. In contrast, cutting early for silage in fields, with for example an infestation of creeping thistle, may encourage the spread and growth of this weed. Hence, there has to be a balance between the requirements of the farming system and weed control implications.

Horticultural and stockless arable systems: ley management will include topping at intervals during the summer to a height of around 10-15 cm. Ideally in fertility building leys the sward should not be allowed to get higher than 40 cm (or knee height). If the vegetation gets higher than this, then topping will create a mat of vegetation that will act like a mulch. This can create dead spots in the ley where clover may be excluded by the more vigorous grasses, or which weeds may colonise. Topping the ley regularly will also ensure that tall weeds that may have germinated will not be able set seed.

Using manures

The use of raw manures and slurry has often been associated by farmers with increased weed problems. The problems can arise in various ways; either as a result of weed seeds in the manure, as a result of the way in which it is applied or due to the stimulatory effect of the nutritents on weeds already present in the soil.

Weed seeds in manure: some manure contains weed seed, either seed that has passed undigested through animals or from bedding materials like small-grain straw and old hay. High-temperature aerobic composting (recommended under organic standards) can greatly reduce the number of viable weed seeds as long as the temperature is maintained at higher than 60°C for more than three days. Operationally compost will need to be regularly turned to achieve even heating through the whole heap and to get material from the outside (where seeds are likely to survive) to the inside (where the highest temperatures are likely to be generated. In a similar way, aeration of slurry can reduce the number and viability of weed seeds (see information below).

Applying manure: when applying manure or slurry try not to create conditions which stimulate weed seeds to germinate (excessive soil disturbance, creating bare patches etc.). Applying slurry to stubble after silage cuts can provide optimum conditions for weed seed germination. A nutrient-rich bed of cattle slurry will produce a high potassium environment which will favour weeds such as docks rather than grasses. Dock seeds should not survive low ph silage but will survive in a later cut of hay.

Some research has shown that placing manures and slurry more accurately on crops can benefit the crop rather than the weeds. Crop plants are generally sown fairly deeply and they germinate from a lower level in the soil profile than weeds, which tend to dwell on the surface and germinate from 0-3 cm. Crop plants also root more deeply. This tendency can be exploited for weed management. In arable/horticultural systems manure placed 10 cm below the soil surface encourages the crop seeds to grow down into the nutrient-rich layer before the surface-dwelling weeds can reach it. This technique can also be used with broadcast spread slurry. If it is ploughed in rather than left on the surface it will be available to the crop before the weeds can reach it.

In many cases, the growth of weeds that follows manuring is a result of the stimulating effect manure has on weed seeds already present in the soil. This can be due to the flush of nutrients (e.g. supply of nitrates), enhanced biological activity in the soil, or other changes in the fertility status of the soil. Some work has indicated that excesses of potash and nitrogen in particular can encourage weeds but in any case it is prudent to monitor the nutrient content of your soil and manure, and spread manure evenly to reduce the incidence of weed problems.

Don’t give the docks an advantage: applying slurry to stubble after silage cuts can provide optimum conditions for weed seed germination. A nutrient-rich bed of cattle slurry will produce a high potassium environment which will favour weeds such as docks rather than grasses. Dock seeds should not survive low ph silage but will survive in a later cut of hay.

Weed Management and Livestock

In mixed systems, where grass/clover leys are used for fertility building, livestock can make good use of the nutrients and they also produce manure, a resource which can be used around the farm to fertilise cash crops. Apart from leys, pastures will also need weed management and increasingly conservation of old or rough pastures requires specialist grazing. Animals can also be used to consume cut weeds or other plant material like chaff or screenings that are likely to contain some weed seeds.

Animals have different grazing habits and it is even recognised that different breeds or individuals are likely to have different tastes and habits. The species, breed, age and individuality of animals will all affect what they will eat and therefore what effect they will have on both weeds and pasture. Variability within the feeding site (e.g. vegetation, topography) can also be important as can other factors such as the weather. In general terms:

  • goats are browsers and have a reputation for enjoying tough and woody plants
  • sheep are recognised as being useful for weed control as they graze close to the ground and will eat a wide range of plants. They can be used early and some breeds are hardy
  • cattle can be used for early grazing but there are a large number of different breeds and types with different grazing requirements including beef, dairy and traditional breeds. Grazing strategies appear to be related to plant energy content and digestibility and this will affect how plants are eaten (leaves or stems or other parts of plants), which plants are eaten (species) and size plants eaten (young or more mature plants). Cattle tend to avoid longer coarser grass and hairy, spiny or poisonous plants. The selection of certain plant species and plant components as well as the location of these plants is based on the previous experience of the animals or learned from their mothers when they are calves
  • pigs are good at rooting and have been recommended at various times for digging out perennial weeds like dock and couch when fenced within fields (and tightly stocked)
  • geese consume grassy weeds and have been used to weed in between rows of well established crops
  • horses and ponies are grazed on ever increasing areas of land and can be used as part of a grazing rotation. They prefer frequent small amounts of fibrous grass or other high roughage material. They have been known to dramatically increase the number of docks in a rotation

It is important to get the right grazing balance over the year to get the maximum benefit for the animals and also to prevent damage to the sward or soil. For example, stocking more lightly in the winter months and in wet periods prevents poaching. So think about the right season to graze, how long to graze, how many animals to graze and how long the grazing area will need to recover. Things to consider include:

  • timing grazing to benefit the pasture and promote competition with weeds
  • timing grazing to damage the weeds, e.g. to remove flowers or seed heads before seed production
  • allowing time for the pasture or forage to recover between grazings
  • making sure that livestock that have been grazing on weedy land feed on weed seed free forage for 4- 5 days before introducing them to weed free areas or pastures (some seeds will remain viable after passing through animals which may take a few days)

Suggestions for rotating livestock, depending on situation, include:

  • alternate the grazing of sheep and cattle from year to year or to use mixed grazing for better weed control. Mixed grazing in the same field may be detrimental to the cattle
  • exploit animals

Fallowing

It is usually not desirable to have to plan a fallow period into a rotation, but it may be necessary if weeds cannot be controlled during cropping or fertility building. It may not be necessary to stop cropping for a whole year, but instead to employ a bastard fallow i.e. no crop for part of the year. Tillage without a crop for a season is sometimes referred to as a black fallow. Fallowing the land for part of the growing season, as a bastard or summer fallow, can be as effective as a full fallow, is more suitable for lighter land and can be fitted into most rotations.

Fallowing is often best during the summer when cultivations can take place and the drier periods allow for root desiccation. This technique is more useful in plough-dominated systems rather than grassland management. One aim is to cultivate the soil progressively deeper over time, exposing underground plant parts to desiccation at the soil surface but in this case dry weather conditions are essential. Ploughing begins in June/July allowing time for an early crop to precede it. A bastard fallow is often used after a ley to reduce perennial weeds before sowing a winter cereal. There is also an opportunity for birds to feed on wireworms exposed during soil disturbance.

Fallowing has been shown to reduce perennial weeds within a rotation. The aim is to kill the vegetative organs of the weeds by mechanical damage and desiccation. For a full or bare fallow, heavy land is ploughed in April to give the weeds time to start into growth. It is cultivated or cross-ploughed 10-14 days later to produce a cloddy tilth. The soil is cultivated or ploughed at frequent intervals to move the clods around and dry them out. By August the clods should have broken down and the soil is left to allow the weed seeds to germinate. In September/October the weeds are ploughed in and the land prepared for autumn cropping. If a cereal is to follow the fallow, wheat bulb fly may be a problem because it lays eggs on bare ground in July. This can be overcome by sowing a green manure such as mustard to cover the land during this period.

Although there is the benefit of reduced weed control costs in subsequent crops after an effective fallow, the economics of taking land out of production for a full year together with undesirable effects on the soil and the environment, make the use of a bare fallow unlikely for weed control in the organic system. There is no financial return during the fallow period while labour costs accumulate during the fallowing operations. As an alternative to fallowing, cleaning crops such as potatoes and turnips allow repeated hoeing for weed control (but are not suited to heavy land).

A similar effect to that of fallowing can be achieved with rapidly developing crops like radish (Raphanus sativus) that are harvested before the onset of weed competition. The short interval between crop establishment and harvesting in this crop encourages weed seed germination but does not allow the weeds time to set seed or reproduce vegetatively.

Download our leaflet on fallowing.

Farm Hygiene

Weeds are, by their nature tenacious and almost impossible to eradicate once established. The best form of management is preventing their establishment in the first place. Weeds are easily spread between fields and between farms and it is worth taking some trouble to try and prevent this with some basic hygiene measures. Ask yourself the following questions:

Have you got a system for detecting weeds early?

  • Managing a particular weed will be easier if it is detected early and prevented from spreading
  • Ensure that all people who work on the farm or visit it are alert to the possibility of spreading weeds and weed seed and ask them to tell you if they notice any particular areas of weeds.
  • Keep records of problems weeds and their spread or otherwise. A digital camera can be a useful tool to record presence of weeds and monitor changes over time.

Is your farm machinery spreading weeds?

  • Weed seeds are easily carried in soil, crop residues and on machinery so these should be regularly cleaned down.
  • If there is a serious weed infestation in a particular field, or machinery is moving through fields where weeds are flowering, then washing down machinery should be a serious consideration.
  • Hygiene is particularly important at harvest time. In crops like cereals weed seeds may be scattered in the field or caught on the machinery and dislodged later some distance from the original source. It may be necessary to add screens to combines to catch weed seeds at harvest. Older models may already have these features.

Tillage

Soil cultivation or tillage in its various forms has long been the mainstay of weed control and is the most effective way to reduce the weed seed bank. Seeds are encouraged to germinate and then the soil is cultivated mechanically to kill off the plants. The mouldboard plough is the traditional implement for burying weeds and crop residues as ground preparation for establishing a new crop. One piece of research showed that the annual loss of seeds from a natural soil weed seedbank (with no addition of fresh seed) was 22% with no cultivation. When the soil was cultivated twice a year the annual loss was 30%, and when cultivated four times it was 36%. However, it is not just the cultivations associated with the post-harvest incorporation of crop and weed residue that have weed control benefits. The method, depth, timing and frequency of cultivation may influence the composition, density and long-term persistence of the weed population.

Tillage is often divided into three types: primary, secondary and tertiary. However there are many operations that do not easily fall into these categories or span them all. We have provided an outline of the main tillage types below (see our topic sheet as well).

Primary tillage: is the principal cultivation operation before crop establishment. The main choice is between plough or non-plough systems (reduced or minimal cultivation, conservation tillage, no-till, direct-drilling etc. (see below)). A range of machinery is available for primary tillage and some combinations can even work a stubble down to a seedbed in a single pass. Ploughing is seen as a method by which weed seeds can be buried below the depth from which they are capable of germinating, and it is sometimes said that ploughing can be used to bury a weed problem. This is particularly useful for small-seeded or annual grass weeds, which are often short-lived and may survive being buried. But this short-term solution to poor weed control in a previous crop often leads to long term problems due to the persistence of the buried weed seeds in the soil seedbank as viable seeds may be brought to the surface by ploughing in subsequent years and will germinate if conditions are suitable.

  • Reduced tillage: the concept of direct drilling crops without resorting to ploughing became popular after the development of the non-residual herbicides but recently, there has been renewed interest, primarily out of concern for soil conservation. Under reduced tillage there is better control of soil erosion, conservation of soil moisture and more efficient use of fossil fuel. However wind disseminated and perennial weed species can increase and volunteer weeds are also likely to be a problem. However, not all soils are suitable for reduced tillage and less nitrogen may be made available to crops where cultivation is reduced to a minimum.

Many organic farmers find it impossible to dispense with the plough routinely. Each method of cultivation has its advantages and disadvantages and the principle should be to use as many different types of cultivation as possible over the rotation which prevents any one type of weed getting the upper hand. Finer seedbeds produce more weed seedlings but a smooth surface makes direct weed control easier. Larger clods of soil produce fewer weed seedlings but the rough surface gives emerged weeds protection against direct weeding operations.

Secondary tillage: is used to prepare seedbeds and leave a level surface for drilling. Typically it involves disking or harrowing to a depth of 10 cm. Rotivators and power harrows are also used and are able to prepare seedbeds even when ploughing has not been carried out. Implements are available that can combine shallow seedbed preparations with some deeper cultivations in a single pass. Others can loosen the soil below the surface while leaving the preceding crop debris on the soil surface.

  • Stale or false seedbed: is a seedbed prepared several days, weeks or even months before planting or transplanting a crop. The technique is recognised as a strategy suitable for organic farming and has been widely used for many years. The stale seedbed is based on the principle of flushing out weed seeds ready to germinate prior to the planting of the crop, depleting the seedbank in the surface layer of soil and reducing subsequent weed seedling emergence. The soil is cultivated about four weeks before drilling to stimulate germination and encourage the first, and usually biggest, flush of weeds. If irrigation is available it could be used to help.

Tertiary tillage: is the soil cultivation that is used directly as a means of physical weed control. It is dealt with in some detail under mechanical weed control.

Cultural weed controls Organic farmers recognise that every element of farming is inter-linked, and that good rotational design produces healthy soil, healthy plants and good yields. Crop