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II. Crop Rotations as the Central Management Tool in Organic Farming

II. Crop Rotations as the Central Management Tool in Organic Farming

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or elimination of fallow periods and the inclusion of roots and clover into the

rotations (Bullock, 1992; Lampkin, 1990). Economic pressures and increasing demand for certain agricultural products hailed the end of the Norfolk four-course

rotation at the beginning of the 1900s.

In conventional agriculture, chemical alternatives have allowed the intensification and specialization that have taken place over the last century. Economic

pressures have led to the simplification of cropping systems and the substitution

of cultural methods of weed, pest and disease control, and fertility management

with synthetic chemical alternatives. The reliance on crop rotations in conventional

agriculture is now much reduced and continuous monocultures and short rotations

are now common in temperate parts of the developed world (Karlen et al., 1994).

In organic farming systems, the use of temporally and spatially diverse crop rotations remains fundamental to the success of organic production systems; positive

management of biological and ecological systems replaces inputs of synthetic pesticides and soluble NPK mineral fertilizers. While crop selection must, inevitably,

be market driven to provide efficient economic production, a well-balanced sequence of crops should be chosen that requires minimum external inputs, nutrients,

machinery, and energy to maintain soil fertility and quality, health, and yield

(Jordan and Hutcheon, 1996; Vereijken, 1995). Due to economic constraints, a

farmer may be unable to maintain high plant and animal biodiversity at any one

time. However, this may be compensated to a certain extent by the rotation of

principal crops over a defined period (Zettel, 1995).

Crop rotations influence plant production in many ways. Early studies concentrated on their effects on soil fertility and survival of plant pathogens, but crop

rotations also influence soil physical and chemical properties (e.g., structure, nutrient levels), soil erosion, soil microbes, and larger fauna (e.g., nematodes, insects,

mites, earthworms). Increased N supply is a major benefit where legumes form part

of the rotation, but other factors and mechanisms responsible for increased yield

due to crop rotation are not completely understood (Bullock, 1992; Karlen et al.,

1994). Factors such as improved utilization of other soil nutrients, decreased weed,

insect and disease pressure caused by growing a succession of crops with different

root systems, changes in disease and pest resistance patterns, and the ability to

compete with weed species also contribute to the rotation effect. Application of

chemical fertilizers and pesticides in conventional systems does not always completely compensate for the rotation effects noted in organic and low-input systems

(Bullock, 1992; Karlen and Sharpley, 1994).



A. TYPES OF ORGANIC ARABLE ROTATIONS

Crop rotations always have to serve multiple objectives, which often conflict

(Olesen, 1999). In all organic rotations, however, there will be a fertility building phase and a cash crop or income-generating phase. In terms of time, the way



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in which these are partitioned will depend on the climate, soil type, topography,

available markets, and the presence/absence of livestock on the farm. Organic

rotations should ensure sufficient nutrients for the crops and minimize nutrient

losses for both environmental and economic reasons. Depending on the type of

rotation, a major proportion of the required nitrogen should be provided through

leguminous crops. Organic standards also demand that farmers should aim to minimize pests, diseases, and weeds through rotations and careful choice of break

crops (UKROFS, 2001). Soil structure and organic matter levels should be maintained, and a profitable output of organic cash crops and/or livestock should be

achieved (Soil Association, 1998, 2000a, 2000b). The principles of rotation design

are described in greater detail in Table I.

Traditional, mixed organic farms produce a range of cereals, fodder crops, and

livestock products and operate what are known as “stocked” rotations. These

constitute the majority of organic farms in Europe (Foster and Lampkin, 1999),

Table I

Key Principles of Crop Rotation Designa

Principle

Rotate deep and shallow rooting crops

Alternate crops with large and

small root biomass

Rotate N2-fixing and N-demanding crops

Alternate weed-susceptible and

weed-suppressing crops

Grow crops with different pest and disease

susceptibilities

Grow catch crops, green manures, and

undersow crops

Alternate autumn and spring sown crops

Use appropriate crops, suited to climate

and soils

Balance forage and cash crops

Balance labor requirements and availability

Good timing of tillage operations



a



Lampkin (1990).



Reasoning

Improve soil structure, aeration,

water-holding capacity, and drainage

High biomass crops increase the organic

matter remaining in the soil for soil

microbial and macrofaunal populations

Attempt to meet farm’s N demands from

within the system

Interrupt weed life cycle to reduce

populations

Break pest and disease life cycles, reduce

host plant presence in rotation

Maintain soil cover to protect for erosion

and leaching

Combat weeds and distribute workload



To make rotation economically as well as

ecologically viable

Prevent over- and understaffing

Balance positive results of tillage such as

weed control, with negative impacts such

as disruption of macrofaunal activities

and decrease in soil organic matter



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although there are increasing numbers of specialized organic units throughout

temperate areas in Europe and North America that are operating without livestock.

Such farms implement “stockless” rotations, or in some cases they are defined as

“semistockless” rotations if manures are imported from nearby livestock holdings.

Due to different land capabilities, farmers may operate more than one type of

rotation on the farm.

1. Stocked Rotation

In a mixed ley/arable rotation, a short- or medium-term ley would typically be

used, occupying 30–50% of the rotation (Anon., 1991; Lampkin, 1990). A grass

clover ley with 30% clover could accumulate 120–180 N kg ha−1 y−1 (Kristensen

et al., 1995; Scholefield and Smith, 1996). This will accumulate sufficient nitrogen

for the exploitative phase, while additionally providing fodder for livestock through

both grazing and silage. The livestock return nutrients, importantly P and K, which

may be distributed to soil around the farm and provide an income during the fertility

building phase. The ley period also provides opportunities for weed control by

mowing, hand weeding, or cultivations prior to sowing (Anon, 1991).

Examples of typical mixed ley/arable rotations are shown in Table II. There

is a high proportion of graminaceous species present in many organic ley/arable

rotations, and this seems to contradict organic farming guidelines on rotation design

(Table I). Crops often lack variation in rooting depths, rooting habits, and root

biomass production (Litterick, 2001). Cereals in the rotation are susceptible to

similar pests and diseases, and if several years of cereals are grown with only a

single year break, the similar crop life cycles can increase the predominance of

problem weeds, in particular graminaceous species such as blackgrass (Alopecurus

myosuroides) and volunteer cereals. Most cereals are susceptible to diseases such as

septoria (Septoria tritici and S. nodorum), take-all (Gaeumannomyces graminis),

Table II

Two Examples of Typical Ley/Arable Rotationsa

Example 1



Example 2



2- to 3-year short-term ley (red clover or

lucerne on calcareous soils)

Wheat (or potatoes), then green manure

Potatoes/roots

Wheat

Rye or oats (undersown)



4- to 5-year grass/clover ley

Winter wheat

Winter oats or barley

2-year red clover/Italian ryegrass ley

Winter or spring wheat

Cereal/grain legume mix

(or cereal undersown with ley)



a



From Lampkin (1990).



AGRONOMIC AND ECONOMICAL POTENTIAL



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eyespot (Pseudocercosporella herpotrichoides), rusts (Puccinia spp.) and mildew

(Erisyphe spp.), and pests such as aphids (various species). Take-all and eyespot can

reduce yield by up to 30% if uncontrolled (Wibberley, 1989). Oat is an exception

as it is not susceptible to take-all, and rye has fewer pest and disease problems

than other cereals (Agrios, 1997; Lampkin, 1990).

Although most organic crops suffer from less serious infestations of soilborne

pests and diseases than conventional crops (van Bruggen, 1995), the fact remains

that organic crop yields are often significantly less than those obtained from their

conventional counterparts (Lampkin and Measures, 2001; SAC, 2000). There are

many reasons for such yield differences; however, the difference may be partly

addressed through further diversification of organic rotations. It may be possible

to reduce dependence on cereals and increase production of novel break crops,

which bring different benefits and characteristics to the system, thereby improving

the performance of the individual components and the system as a whole.

2. Stockless Rotation

The trend toward specialization in conventional farming has meant that there

are large areas of countryside in Europe, e.g., Eastern and South Eastern Denmark,

Eastern Germany, and East Anglia in the UK, where livestock have not been farmed

for many years (Høgh-Jensen, 1999). The cost of bringing livestock into previously

stockless farms during conversion (to organic) is frequently prohibitive (Lampkin,

1990). An increasing number of European farmers are operating stockless organic

rotations for this reason (David et al., 1996; Stopes et al., 1996; von Fragstein,

1996).

With no livestock to generate an income, medium-term fertility-building leys

become less economically viable. The fertility-building stage cannot be omitted

and typically, short-term leys (1 year) are used in conjunction with green manures and catch crops. Currently, European regulations allow the fertility-building

phase of a rotation, for example, 2-year red clover green manures, to qualify for

set-aside payments, which considerably improves the economics of stockless rotations (Lampkin and Measures, 2001). The absence of livestock means that all

nutrients must be imported from outside, fixed by leguminous crops (in the case

of N), or returned as crop residues. Some stockless rotations do not use imported

manures. Only through creative and informed use of the different characteristics of

catch crops and green manures, can stockless systems ensure their sustainability.

Work to develop viable stockless rotations has been carried out in the UK by

CWS Agriculture (Leake, 1999) and by ADAS (Cormack, 1999). Both found that

the rotations tested performed well in the short time in which they have been established (10 and 9 years from establishment to publication of results, respectively).

The crop sequence used within these rotations is shown in Table III. Crop yields

were good; pest, disease, and weed problems were generally manageable; and the



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Table III

Stockless Arable Rotations as Reported by Leake in

Leicestershire (1999) and Cormack in Norfolk (1999)

Leicestershire



Norfolk



2-year red clover



1-year red clover cut and left

as mulch (2 for conversion)

Potatoes

Winter wheat

Spring beans

Spring wheat (undersown)

Barley (undersown)



Wheat

Oats

Beans

Wheat

Oats



rotations were economically viable. However, both experiments were carried out

on deep, fertile clay loam soils. Further work is required to determine whether such

stockless rotations would be equally viable elsewhere and over a longer time period.

Research in several countries including France, Germany, Sweden, Denmark,

and the UK is currently looking at the economic and agronomic potential of different stockless rotations and the implications of these for long- and short-term soil

fertility, and pest, weed, and disease incidence (ADAS, 2001). Several workers

are currently examining the potential for composts as a source of nutrients for

stockless systems, where both on-farm waste and waste from the local community

can be composted and used as additional organic fertilizers during the arable phase

of the rotation (von Fragstein and Schmidt, 1999). The use of nutrient inputs other

than those fixed from legumes and reincorporated in crop residues are particularly

important on stockless rotations on lighter soils.

Some European farmers are currently operating a semistockless system in that

they own no livestock, but import manures from surrounding stocked farms. Farmers tend to stack the manure according to organic regulations, prior to application,

or mix it with other materials and compost it (Soil Association, 1998; Rose, pers.

comm.). Manure from extensive conventional farms can be used; manure from

intensive livestock units or from farms growing genetically modified crops is

forbidden (EC,1991, 2092/91). The importation of up to 170 kg ha−1 N is currently

allowed under European organic regulations, although the rules governing such

practices are likely to become more restrictive in the future (EC, 1991, 2092/91;

Brenman and Haward, pers. comm.). Farmers operating stockless rotations including one or more high value horticultural crops often need to import nutrients

to meet crop requirements, particularly on light soil (Lampkin, 1990; W. Rose,

pers. comm.). Two examples of semistockless rotations operating in North East

Scotland are shown in Table IV.

In all ley/arable rotations, there is a clear need for a break crop. In a stocked rotation, the break is needed predominantly for pest, disease, and weed management.



AGRONOMIC AND ECONOMICAL POTENTIAL



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Table IV

Examples of Semistockless Rotations Used in North East Scotland

Rotation 1



Rotation 2



2-year red clover

Potatoes (+15 t ha−1 compost)a

Carrots

Wheat

Barley



2-year red clover

Potatoes (+15 t ha−1 compost)

Winter wheat

Carrots

Barley

Leeks (+5 t ha −1 compost)



a



Compost made from cattle manure, straw, and grass clippings.



In the stockless rotations, the break crop is needed to provide a pest, disease, and

weed break, and for nutrient addition. Soil physical and structural problems can

occur in both stocked and stockless rotations and particularly in certain soil types

(Shepherd et al., 2000). Some break crops can be used to help alleviate such problems, either because of the nature of the break crop itself or as a result of cultivation

methods used during production.



B. SOIL FERTILITY

The aim of nutrient management in organic systems is to optimize the use of

on-farm resources and minimize losses (Kăopke, 1995). Organic agriculture often

has to deal with a scarcity of readily available nutrients, in contrast to agricultural

systems which rely on soluble fertilizers. Maximum use should be made of crops

that can contribute toward building soil fertility (Jordan and Hutcheon, 1996). The

supply and management of N are more complex in organic than in conventional

agriculture. The major challenge for N management in organic systems is to synchronize the availability of N mineralized from manures and crop residues with

crop demand. Apart from N imported with manures, composts, and seeds or as

atmospheric depositions, organic agriculture relies mainly on symbiotically fixed

N2. Cereals and many horticultural crops are very demanding in terms of their

N requirements. It is therefore important in organic ley/arable systems to maximize the symbiotic N2 fixation in legumes, maximize the cycling of N (and other

nutrients) from the entire soil profile, and minimize N losses through leaching,

volatilization, and denitrication in organic ley/arable systems (Kăopke, 1995).

Ensuring an adequate supply of P, K, and trace elements can also be difficult in

organic systems (particularly on stockless farms) due to the restrictions on acceptable nutrient sources in the organic standards (UKROFS, 2001). Organic materials

such as FYM and some types of compost are valuable sources of P and K. Various



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naturally occurring P, K, and trace-element fertilizers including rock phosphate,

basic slag, potassium sulfate, and seaweed extracts are approved for restricted use

in organic agriculture; but many are costly and may have to be imported (Simpson

and Stopes, 1991).

Break crops can contribute to the nutrient status of the soil in three main ways.

First through biological N2 fixation; second, through reincorporation and recycling

of nutrients and organic matter; and third by reducing nutrient losses. Each crop

species has slightly different characteristics, e.g., N demanding or N2 fixing, shallow or deep rooting, amount and quality of crop residue returned. The appropriate

choice of crops within the rotation and their sequence are crucial if nutrient cycling

within the farm system is to be optimized and losses minimized over the short and

long term (Lampkin, 1990; Stockdale et al., 2001; Vereijken, 1995).

1. N Fixation

The ability of legume–rhizobium symbioses to fix N2 enables many organic

farming systems to be self-sufficient in nitrogen. The amount of N2 fixed in any

one year is dependent on the climate, soil type, and the crop species and variety grown (Ledgard and Steele, 1992): average values of 200 kg N ha y−1 have

been recorded from grass/clover leys in temperate climates (White, 1987). Many

temperate leguminous crops do not return large quantities of N to the soil, since

almost all of the N fixed is removed in the grain at harvest, e.g., dry beans and peas

(Fisher, 1996). In several cases it has been shown that soils may be depleted in

N as a result of legume production (Peoples et al., 1995). Where the inclusion of

legume green manures is not possible in a rotation, legumes must be chosen which

combine reasonable grain yield with relatively high residue returns. For example,

when peas are harvested green (e.g., for freezing), much of the fixed N is left in

unharvested plant parts which can be incorporated into the soil or used for animal

feed (Sprent and ’t Mannetje, 1996). The appropriate choice of leguminous crops

for organic rotations will allow the farmer to maximize the amount of N fixed and

optimize the balance between N offtake and return in crop residues.

2. Effects on Nutrient Cycling and Soil Organic Matter

An important feature of organic systems is that crop residues are returned to

the soil (directly or indirectly) after harvest. Due to their long roots, certain break

crops (e.g., lucerne, Medicago spp.; hemp, Cannabis sativa) can access nutrients

from soil depths inaccessible to cereal roots. Therefore nutrients from deep soil

layers present in the nonharvested break crop parts eventually become available in

the upper soil layers following microbial decomposition (Bosca and Karus, 1998;

Lampkin, 1990).

Soil organic matter (SOM) is important due to its stabilizing effects on soil

structure and because it acts as a reservoir for plant nutrients (Brady and Weil,



AGRONOMIC AND ECONOMICAL POTENTIAL



379



1999; Cresser et al., 1995). Some break crops, particularly grass/clover leys and

leguminous and nonleguminous green manures, can make a positive contribution

to SOM levels (Karlen et al., 1994). For example, work in Illinois showed that a

conventional rotation of maize, oats, and clover increased SOM more than continuous maize, regardless of fertility inputs, probably because the rotation used

tillage less frequently and produced more root residues (Odell et al., 1984). Similarly, Drinkwater et al. (1998) found that SOM levels increased over 15 years

in two maize/soybean organic rotations which used legumes for fertility building.

They found that SOM levels decreased over the same period in a conventional

maize/soybean rotation which relied on synthetic fertilisers for fertility. In the UK,

Clements and Williams (1964, 1967) showed that SOM tends to accumulate under grass clover leys. The nutrients accumulated within the SOM, in particular,

available forms of N, then become available to following crops when the ley is

ploughed in.

3. Reduction of Nutrient Losses

The choice of crops and cropping sequence influences the movement of soluble

N through the soil profile and ultimately into the groundwater (Karlen and Sharpley,

1994). Fertilizer spread on ploughed agricultural land causes the greatest release

of nitrate into the environment in European countries (Powlson, 2000). Nitrate

leaching is particularly severe where the ground is left with no crop following

harvest. Losses of other nutrients including P and K through leaching and soil

erosion can also be a problem where soil is left bare for long periods. Therefore

the practice prescribed by organic standards of balanced rotations with judicious

use of catch crops and green manures provide an opportunity to significantly reduce

losses of nutrients, in particular, nitrate (Smith et al., 1996; UKROFS, 2000). Good

manure management within the crop rotation is also important if nutrient losses

are to be minimized (Smith and Shepherd, 2000).



C. SOIL PHYSICAL CHARACTERISTICS

Break crops and rotations in general can have a great impact on soil physical

characteristics. The most significant benefit results from the incorporation of SOM

(Sumner, 1982). Soil organic matter content strongly influences the soil’s structural

stability: it aids soil aggregation, leading to a more stable structure with improved

aeration and drainage. Microbial decomposition of SOM produces organic materials, such as polysaccharides, that cement soil aggregates, promoting their stability.

Aggregates are also cemented by soluble salts, oxides, calcium carbonate, and

oxyhydroxides of iron and aluminium (Marshall et al., 1996).

As plant roots die, they decompose to provide energy and nutrients for the

microbial population (Cresser et al., 1995). Crops with different root biomass and



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architecture will deposit organic matter in varying amounts at different depths in

the soil profile. This encourages microbial activity at different rooting depths and

means that nutrients will be available for other plants at those depths. The greater

the root biomass left in the soil, the greater the supply of substrate and nutrients

(Killham, 1994).

In soils such as clays with a large fraction of small pores, the addition of SOM

can decrease bulk density by creating larger pores, thus assisting root penetration,

water flow, and gas exchange. On sandy soils, SOM addition can improve the

profile’s ability to retain water by creating differently sized pores and through the

colloidal properties of the organic matter (Brady and Weil, 1999; Marshall et al.,

1996).

The cultivations required for the production of most crops reduce SOM levels

(Bullock, 1992; Cox and Atkins, 1979). Given its importance, most organic farmers

strive to conserve and enhance the SOM in their soils. Grass clover leys contribute

significantly to SOM levels in ley/arable rotations (Clements and Williams, 1964,

1967). Certain break crops including some grain legumes and green manures also

contribute small but significant amounts of organic matter to the soil following

cropping. The quantities of organic matter returned to the soil following production

of the less common organic crops such as hemp, sugar beet, and oilseed rape have

not been previously documented.

Some break crops have the potential to alleviate soil physical problems such

as compaction and reduced aggregate stability caused by cultivations carried out

in unsuitable soil conditions and/or with heavy machinery. These crops can also

aid soil structure because their root architecture, biomass, and nutrient requirements are different from those of cereals. Kirkegaard et al. (1993, 1994) suggested

that tap-rooted species, for example, oilseed rape, may improve subsoil porosity

through “biological drilling.” Angus et al. (1991) support this idea, but Cresswell

and Kirkegaard (1995) could find no evidence for it. Materechera et al. (1992)

suggested that a root’s penetration of dense soils is related to its ability to thicken,

thus increasing the radial pressure on the soil. Deep-rooted species, such as lucerne

(Medicago sativa), can penetrate up to 3 m over several years, leading to the development of extensive biopores. These would facilitate air and water movement

into the subsoil, and as roots follow the path of weakest mechanical resistance, the

pores would provide ready-made, low-resistance channels for new roots to occupy

(Cresswell and Kirkegaard, 1995).

Soil structural characteristics can be substantially affected by root systems

(Angus et al.,1991; Kirkegaard et al., 1993, 1994). Roots exude large quantities

of polysaccharides, which help to bind soil aggregates to form larger, more stable

aggregates. Axial pressure exerted by roots as they grow and move through the soil

compresses the area adjacent to the root channel, pressing aggregates together and

increasing their stability. It is by this means that the stable crumb structures found

under long-term pastures and prairies are formed (Marshall et al., 1996). Where



AGRONOMIC AND ECONOMICAL POTENTIAL



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the roots take up water, the soil in the vicinity dries and shrinks, compressing

aggregates and increasing their stability, and the soil can become cracked in all but

the sandiest of soils. These cracks, along with the root channels, create aeration

and drainage pathways in the soil (Brady and Weil, 1999; Marshall et al., 1996).

Not all break crops benefit the soil’s physical characteristics; soil compaction

and structural damage are potential problems wherever heavy machinery is

used. Certain crops, in particular roots such as carrot and swede, are often harvested using heavy machinery, late in the year, under wet conditions. It follows

that where the soil is less stable when wet, the damage from heavy machinery

will be more serious, and soil erosion and the creation of soil pans and compacted

layers below plough level are more likely (Bullock, 1992; Cox and Atkins, 1979;

Marshall et al., 1996). Agricultural soils, particularly in conventional systems, are

often left with no ground cover following harvest, and less stable aggregates are

more prone to destruction by raindrop action and erosion. They are carried away

by run-off or wind action, or remain in situ, clogging the remaining soil pores in

the surface layer, a phenomenon known as capping (Marshall et al., 1996). Organic

farming standards encourage the use of cover crops to minimize nutrient losses

and soil erosion (EC, 1991; UKROFS, 2001). In compacted soils there is a greater

risk of retarded mineralization caused by hypoxia, inhibition of soil fauna, injury

to root growth and function, and accumulation of CO2, ethylene, and organic acids

(Hansen, 1996; Rendig and Taylor, 1989).

Where soil structural problems exist, root crops should perhaps be avoided, or

at least harvested early (or perhaps by hand in more intensive systems), allowing

the establishment of a green manure to minimize soil erosion, nitrate leaching, and

adding SOM (Lampkin, 1990).



D. WEED MANAGEMENT

Weeds are often regarded as the main problem for organic farmers (Leake, 1996;

Swisher et al., 1994). Weed populations can increase during conversion to organic

production (Albrecht and Sommer, 1998), although these can stabilize over

time, with appropriate organic husbandry (Davies et al., 1997). Weeds can make

cultivation and harvest operations difficult or impossible, and they compete with

the crop for resources. They may be parasitic on crop plants and can be poisonous

to livestock. Weeds may also act as hosts for pests and diseases during crop

growth and act as a bridge for pests and diseases to the following crop (Gwynne

and Murray, 1985). However, weeds also have positive roles that are recognized

in organic agricultural systems; elimination of all weeds should not, therefore, be

the goal of weed control in organic systems (Lampkin, 1990). Weeds can supply

ground cover to an otherwise bare soil, reducing the risk of erosion, leaching, and

soil crusting. The additional roots in the soil contribute to biological activity and



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soil structure. Weed species increase the plant diversity within the cropping system

and provide habitats for a wider range of insects and other invertebrates. Weeds

can also act as a reservoir for beneficial mycorrhizal fungi, which naturally occur

on most crop species (Atkinson et al., 2001). Plant roots infected with mycorrhizal

fungi have been shown to take up more soil nutrients and have greater resistance

to root pathogens (AzconAguilar and Barea, 1996). Weed control strategies can

be successfully targeted toward key problem species, such as couchgrass (Elymus

repens) or dock (Rumex spp.), and key growth periods (Bond et al., 1998; Welsh

et al., 1997) in order to minimize the impact of weeds on crop yield and quality.

The main weed control strategies used in organic farming often combine cultural

or husbandry techniques with direct mechanical and thermal methods (Lampkin,

1990; Stockdale et al., 2001). In conventional farming systems, total weed control

is often the aim. However, in organic systems, farmers aim to maintain weeds at a

manageable level using cultural means in order to ensure that direct weed control

measures are effective in preventing the loss of crop quality or farm profitability.

Husbandry practices include adjustment of soil conditions (e.g., by irrigation), various cultivation techniques, use of stale seedbeds, diverse crop rotations, preplant

mulches for high value crops, and use of cultivars particularly suited for organic

production. Mechanical and thermal intervention includes ridging-up in potatoes,

inter-row cultivation in root crops and cereals, postemergence harrowing to control

weeds in cereal crops, and heat treatment of weeds (infra-red or direct flaming)

prior to crop emergence and in between rows.

Weed control through crop rotation is more likely to be successful when the

growth habit and characteristics of the crop contrast with those of the previous

crop and the predominant problem weeds. Ideally the sequence of crops within

a rotation allows for the use of a range of different cultivation methods and soil

treatments. This should ensure that each weed association meets with competition,

or that its life cycle is disturbed (Klingman, 1961). The design and management

of an efficient crop rotation for weed control are complex, because a practice

that controls one weed may be ineffective or even encourage another species.

Diverse crop rotations are effective in reducing weed seedbank numbers and preventing highly adapted weeds such as blackgrass, wild oats, and volunteer crops

from becoming dominant (Karlen et al.,1994; Lampkin, 1990). These rotations

should

r alternate between autumn and spring germinating crops (and their respective

weed compliments).

r alternate between annual and perennial crops (e.g., cereals and leys).

r alternate between closed, dense crops which shade out weeds (e.g., field beans

or rye) and open crops such as maize which encourage weeds.

r provide a variety of cultivations and cutting/topping operations (in particular,

the traditional cleaning crops, leys, and green manures).



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