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III. Break Crops for Nutrient Management

III. Break Crops for Nutrient Management

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392



ROBSON et al.

Table VII

Agronomy of Organic Break Crops—Fertilizer Requirements



Agronomic

characteristic



Faba bean

(spring)



Sugar beet



Carrot

b



Fertiliser

No N necessary, Low. Excessive N Low

requirements—N

can reduce

reduces sucrose

nodulation.a

concentrations.b

Phosphorus

Noneb

Low j

Medium to highb

Potassium

Potash may be

Low j

Medium to highb

b

required



Other



FYM equivalent



Manure

may help

establishment

and meet K

requirement.b



Mg and B are

likely to be

deficient in

sandy soils and

those with

high pH.m

None



Hemp

Medium to high



Linola

c



Mediumc

Mediumc



Low



d



Lowd

Mediumd



Magnesium may

The role of trace

need to be added

elements in

on low index

hemp has never

h

soils

been clarifiedn



Manure additions

to supply

phosphate and

potash may be

necessary.b



Manure additions

(3–15 t ha−1)

may be

necessary if

hemp does not

follow a fertility

building phase.n



Best grown on a

low input basis.q

P requirement

mean soil

reserves need to

be significant.b



a



McEwen et al. (1989) and Roughley et al. (1983).

Lampkin (1990).

c

Bosca and Karus (1998).

d

Roughley et al. (1983), Lampkin (1990), Ramans (1993), and Norman (1993).

e

Archer (1985).

f

Lampkin (1990) and Holmes (1980).

g

Lockhart and Wiseman (1978).

h

MAFF (1993–1994).

i

Tanner and Hume (1978).

j

Lampkin (1990) and Bray and Thompson (1985).

b



1990). Winter and spring genotypes are available, with winter types being more

common in Europe due to their generally superior yields and yield stability (Fox

and Milford, 1996). Conventional global production is around 4 Mt, with China

being the largest producer, and Germany the largest European producer (DeBoer,

1995). There are currently no figures for the land area in Europe used to produce

organic beans. In the United States, 2110 ha of organic beans was produced in

2000 (Economic Research Service, USDA, 2001).



393



AGRONOMIC AND ECONOMICAL POTENTIAL



Lupin

No N required



Lowe

Lowe



Oilseed rape

e



High



f



Potato

Medium to high



Swede

g



Soybean



h



Medium



None recommendedi



None recommendedi

None recommendedi



Low to mediumk

Medium. Very

low amount

removed at

harvestl

Sulfur and

magnesiumo



Mediumg

Highg



Mediumh

Mediumh



Potato crops are

used as an

opportunity for

FYM addition,b



Swedes are

especially

sensitive to

Boron

deficiencyp



Should follow a

fertility

building crop.

In addition,

manure will be

needed to meet

the high

nutrient

requirements.b



An opportunity for

FYM addition also

benefits from soil

improvements. In

the UK different

manures composts

and seaweed

extracts are used.

Most used rates

6–25 t ha−1, and up

to 75 t ha−1.r



15–25 t ha−1 if

crop is not

following a fertility

building crops



k



Lampkin (1990), Holmes (1980), and Perkin (1981).

Holmes (1980) and Orson (1995).

m

Bray and Thompson (1985).

n

Bosca and Karus (1998).

o

Orson (1985).

p

Archer (1985) and Rhodes (1972).

q

Turner (1993).

r

Lampkin (1990), Lockhart and Wiseman (1978), and Ginger (1987).

s

Lampkin (1990) and Simpson and Stopes (1991).

l



Faba beans are relatively tolerant of low soil N status, and if no inorganic N is

available, they rely on N-fixing nodule-forming bacteria of the species Rhizobium

leguminosarum to provide N for their growth and development (Roughley et al.,

1983). Rhizobia associated with faba beans fix more N in 2 months than those on

any other crop provided that the temperature is below 15◦ C (DeBoer, 1995). Fixation rates are about 170 to > 330 kg ha−1 N under careful management (DeBoer,

1995; Dyke and Prew, 1983; Mytton, 1997; Patriquin et al., 1975). Rhizobia will

fix about 87% of the plants’ N requirements in unfertilized soils, or about 42% in



394



ROBSON et al.

Table VIII

Agronomy of Organic Break Crops—Soil Requirements



Agronomic

characteristic

Soil pH,

optimum



Faba bean (spring)

6.5–7.0a



Soil type



Wide range of

suitable soils.

Drought prone

and water-logged

soils unsuitablek



Soil temperature



Germinate and grow

well in cool soilu

251–300 mm during

growing seasonx



Water

requirements



Sugar beet



Carrot



Hemp



Linola



6.5–7.0b



5.5–7.0c



>5.0d



6.0–7.0e



Lighter, wellDeep, friable, fertile, Rich loam with high

structured soils

well–drained

levels of organic

l

are optimal

sandy loams or

matter; well

Uncompacted root

peats.

structuredo

zone, freely

Uncompacted root

drainingm

zone and stone

freen



Above 5–8◦ Cv

250–330 mm during

growing seasonm



Prepare, if possible,

in autumn. On

heavier soils, a

coarsely broken

and level furrow

slice is preferable

to allow for frost

action. On lighter

soil, a furrow

pressm

Loose, medium,

Level, surface with

coarse with no

some coarse

obstructions in top

aggregates to

∗∗∗

90 cm

prevent cappingm



450–600 mm during

the growing

seasonc



Land preparation Loosen compacted

soil prior to

planting, avoid

surface

compactionk



Carrots can be

planted on raised

beds or ridges†



Seedbed



Fine, firm and

level∗ ∗



a



At least 670 mm per

annum, with at

least 250–300 in

vegetative periody

Ploughed in autumn

to 20–23 cm, then

harrow and roll in

springo



Droght-resistant

cropz



Firm and moisto



Fine, even, firm, and

moistz



Anon. (2001) and Lockhart and Wiseman (1978).

Bay and Thompson (1985).

c

Rubatzky and Yamaguchi (1997).

d

Bosca and Karus (1998).

e

Lockhart and Wiseman (1978) and Milford and Shield (1997).

f

Wiseman et al. (1993), and Milford and Shield (1997).

g

Pouzet (1995), Weiss (1983), and Ward et al. (1985).

h

Burton (1989).

i

Lockhart and Wiseman (1978) and Anon. (1998).

j

Tanner and Hume (1978).

k

Hebblethwaite et al. (1983).

l

Bray and Thompson (1985) and Allison et al. (1996).

m

Bray and Thompson (1985).

n

Rubatzky and Yamaguchi (1997) and Wiseman et al. (1993).

o

Bosca and karus (1998).

p

Ramans (1993a)

q

Lockhart and Wiseman (1978), Wiseman et al. (1993), and Shield (1999).

b



Wide range of soils,

not very lightp



Compacted land

loosened. Heavy

land cultivated in

autumn to allow

maximum

weathering. Light

land ploughed and

pressed in spring‡



395



AGRONOMIC AND ECONOMICAL POTENTIAL



Lupin

<7.0. Inoculation is

necessary above pH

6.5 f

Deep well-drained

loams. Can be

grown on shallow,

sandy soils. Poor

germination in

crusted soilsq



Oilseed rape



Potato



Swede



>5.0h



>6.0i



6.0–6.8 j



Well drainedr



Loose, friable

well-drained, and

aerateds



Well drainedt



Well drained.

Production is least

stable in sandy soil j



Above 5◦ Cw



Compacted land

broken up to allow

free drainage†



Firm, with a good

structurel



r



700 mm for maximum

seed development,

450–500 mm in

growing seasonw

Compacted land

broken up



Fine, firm, well

structured, and

moist



10◦ C minimum j

450 mm through the

growing seasonh



200–300 mm through

growing season∗



Deep cultivated to

30 cm. Raised beds

allows for easier

management. Beds

formed to create a

stale seedbed and, if

necessary, destoned.

Beds remade just

prior to planting¶



In very wet areas

swedes may be

grown in ridges.

Planting in beds

makes for easier

management∗ ∗



After ploughing,

minimum tillage

possible j



Deep and fine, except

on light soils



Fine, firm, and moistu



Quite fine and firm j



Wiseman et al. (1993), and Pouzet (1995).

Burton (1989) and Harris (1992).

t

Sanders (1996), Anon. (1998), and Michaud (1997).

u

Lockhart and Wiseman (1978).

v

Ramans (1993a) and Turner (1993).

w

Weiss (1983).

x

Anon. (2001).

y

Bosca and Karus (1998) and van der Werf et al. (1996).

∗ Sanders (1996).



Wiseman et al. (1993) and Sanders (1998).



Raman (1993) and Turner (1993).

Đ

Ward et al. (1985).



Lockhart and Wiseman (1978) and Whitney and McRae (1992).

∗∗ Lockhart and Wiseman (1978).

∗∗∗ Anon. (2001) and Hebblethwaite et al. (1983).

1

Wiseman et al. (1993).

2

Wisemand et al. (1993), Pouzet (1995), Weiss (1983), Ward et al. (1985).

s



Soybean



5.5–8.0g



396



ROBSON et al.

Table IX

Agronomy of Organic Break Crops—Pests, Diseases, and Weeds



Agronomic

characteristic



Faba bean

(spring)



Sugar beet



Carrot



Hemp



Linola



Pests and disease Chocolate spot

problems and

greater on acid

soils.a Long

their control

rotations

recommended

for the general

control of many

bean pests and

diseases.b



Long rotations for

Plant end of

control of beet

May-mid-June

cyst and other

to avoid the first

nematodes and

hatch of carrot

wireworms.

root fly. Careful

Repeated

harvesting

cultivation for slug

reduces rots in

control. Removal

storec

of debris and host

plant weeds for

aphid and downy

mildew controlk



Hemp is relatively

pest and disease

free. It is

intolerant of

many biocides.d



Linola is more

prone to disease

problems, such

as Altenaria,e in

wet climates



Weed problems

and their

control



Sugar beet is a poor Carrots are a poor

competitor with

competitor with

weeds. Crops need

weeds. Crops

to be kept weed

need to be kept

free by mechanical

weed free by

means.k

mechanical

means.k



Hemp is a natural

suppressor of

weeds.l



Linola is a poor

competitor with

weedsm



Beans are a poor

competitor with

weeds. Crops

need to be kept

weed free by

mechanical

means.k



a



Hebblethwaite et al. (1983).

Anon. (2001).

c

Rubatzky and Yamaguchi (1997).

d

Bosca and karus (1998).

e

Ramans (1993b).

f

Carter and Faut (1984).

g

Weiss (1983), Ward et al. (1985), NIAB (1993), and Ekbom (1995).

b



fertilized soils (Richards and Soper, 1979). Inoculant containing the bacterium can

be added to the seed or the soil, although many soils naturally contain sufficient

numbers of the required bacteria. For example, soils at Rothamsted had populations

of 106 g−1 irrespective of inoculant addition or previous bean crop (Roughley et al.,

1983).

Nitrogen fixation by grain legumes can contribute to the overall N balance of

the organic rotation in stocked and stockless systems (Cormack, 1996; Prew and

Dyke, 1979; Rochester et al., 2000). On stockless farms, where the grain is sold off

the farm, a significant proportion of the available N is removed from the system.

Beans can leave a net negative N balance (e.g., Jensen, 1989, 1995; Patriquin,

1986), but other studies have reported a N residue of between 45 and 100 kg ha−1



AGRONOMIC AND ECONOMICAL POTENTIAL



Lupin

Susceptible to damage

from birds and

rabbits, also pea and

bean weevil. Netting

and fleecing are

options for control.f



Oilseed rape



Potato



Swede



397



Soybean



Clean seed and long Main problem for potato Clubroot and mildew

Many of the diseases

rotations to control

is blight. Use high

are the primary

can be controlled

Sclerotinia and

quality seed. Destroy

disease. Good

by using clean

Phoma. Soil pH

ground keepers,

drainage, removal of

seed, long

over 6.5 reduces

infected debris, and

infected debris, and

rotations, and

clubroot. Stem flea

potato dumps to limit

long rotations can

where possible,

beetle and pollen

the spread of disease.

be effective.

resistant varieties.

beetles are serious

Potatoes host wide

Maintenance of pH

There are few

pests. Pyrethroid

range of other

above 6.5 can also

pests of soybean

sprays are the only

diseases and pests

help avoid clubroot.

that are of

alternative to

such as aphids, slugs,

Flea beetles and root

economic

chemical control.g

and potato cyst

fly controlled by

importance with

nematode. Removal

fleecing. Delayed

the exception of

of host plants, clean

planting avoids the

nematode pests j

fields, and long

first generations of

rotations used for

root fly i

control.h

Swedes are a poor

competitor with

weeds. Crops need

to be kept weed free

by mechanical

meansk



h



Raman and Radcliffe (1992).

Lockhart and Wiseman (1978), Sanders (1996), and Rose (1998).

j

Tanner and Hume (1978).

k

Lampkin (1990).

l

Bosca and Karus (1998).

m

Turner (1993).

i



(Stopes et al., 1996). For example, Prew and Dyke (1979) found that bean residues

could contribute 45–50 kg ha−1 of N to the soil in one season. Sprent and ’t

Mannetje (1996) found that soils following bean crops at 35 sites in a European

study had positive N balances after seed removal.

N mineralized from faba bean residues, as with all crop residues, is susceptible to

leaching during the winter, depending on the C : N ratio of the residue and whether

aboveground debris was incorporated into the soil with the root material (Justus

and Kăopke, 1995; Mitchell and Webb, 1996).

There has been substantial documentation of the positive effect of bean crops

on a subsequent cereal. Prew and Dyke (1979) found that wheat and barley crops

needed 45–50 kg ha−1of additional N to achieve the same yield after an oat break

crop than after a bean break crop. This translated to cereal yield advantages of



398



ROBSON et al.



1.1 t ha−1when no fertilizer was applied to the wheat or barley, and 0.6, 0.3, and

0.3 t ha−1 when 50, 100, and 150 kg ha−1 fertilizer N were added, respectively.

The Rothamsted classical experiments at Broadbalk and Hoos Barley conducted

between 1969 and 1978 illustrated further the positive impact of beans on the

subsequent cereal. Without N fertilizers, wheat yielded 47% more grain after

beans than after wheat, and barley yielded 52% more after beans than after barley

(Rothamsted Experimental Station, 1970). These results may not be caused entirely

by N2 fixation, because beans also provide an efficient break from take-all (Prew

and Dyke, 1979). The cause of the yield increases on the subsequent cereal may

therefore be multiple. This is often referred to as the “rotation effect” (McEwen

et al., 1989).

Beans fit well into a cereal rotation as they can be drilled, harvested, dried, and

stored with only minor modifications to existing equipment (Dyke and Prew, 1983),

an important factor in minimizing fixed costs. The faba bean’s capacity to grow

in N-poor soils and produce high yields with only an annual application of FYM

increases the attractiveness of faba bean as a break crop in organic agriculture. A

problem with the crop is the wide variation in yields that can be obtained from

season to season: average yields of organic crops in Europe are around 3.5 t ha−1

(Lampkin and Measures, 2001), but yields of 50 to 200% of that figure are not

uncommon in conventional farming systems (Halder and Taylor, pers. comm).

On heavier soils, there may be difficulty in planting the seed deep enough, as

the seeds need a covering of at least 7–10 cm (Dyke and Prew, 1983; Wilson,

1997). The seeds can be broadcast and ploughed in to achieve sufficient sowing

depth (Wilson, 1997). If the beans are to be harvested dry, this may cause a conflict

of timing between harvesting and sowing a winter cereal (Table VI). In the UK,

faba beans can be harvested green in early September, but can also be left on

the field until early November. In the latter case, a winter cereal would not be a

suitable following crop. In Canada and the United States, faba beans are harvested

between late August and early October (Aldhouse and Patriquin, 1985; DeBoer,

1995). Harvesting the crop green has the possible advantage that it removes less

N from the soil (Sprent and ’t Mannetje, 1996).

Organic faba beans have the highest net margin of the legumes assessed in

this review (Table X). The net margin for faba beans (£747 ha−1) is higher than

that for organic winter oats (£621 ha−1) (Lampkin and Measures, 2001). Partly

due to difficulties in sourcing GM-free soya beans, there is a good market for

beans for organic livestock feeds in the UK. In-conversion beans can also attract

a considerable price premium (Lampkin and Measures, 2001).



B. LUPINS (Lupinus albus)

Several species of lupins are grown in temperate agriculture. White lupins

(Lupinus albus) are the most common and provide the greatest benefits



Table X

Summary of Economic Evaluation (£ha−1 Unless Otherwise Indicated) of Break Crops in Organic Agriculturea



Crop

Carrot

Carrot (low

mechanisation)

Swedes

Potatoes

Potatoes (low

mechanisation)

Faba bean

Winter oats

Sugar beetb

Linola

Lupin

Oilseed rape

Hemp

Soybean

a

b



Yield (t ha−1)



Price (£t−1)



38.3

33.8



250

300



28.0

28.0

28.0

3.5

4.3

44.0

1.4

2.4

2.4

5.5

2.3



Output



Variable inputs



Gross margin



Contractor cost



Casual labor



Net margin



0

0



9,563

10,238



805

2,858



8,758

7,379



1,947

1,403



460

1,932



6,351

4,045



225

250

300



0

0

0



6,335

7,035

8,470



268

1,803

3,151



6,068

5,233

5,319



1,668

1,858

1,593



1,081

253

851



3,319

3,122

2,875



200

180

45

250

175

195

60

250



340

236

0

456

340

254

500

254



1,043

1,110

1,980

806

756

717

832

824



149

125

182

115

90

79

225

195



1,043

985

1,799

691

666

638

607

629



295

353

1,128

317

308

353

356

390



0

12

230

0

0

0

0

23



748

621

441

374

358

285

250

216



Fowler and Lampkin (1999, unpublished).

Amended 2001 (Lampkin and Measures, 2001).



Subsidies



400



ROBSON et al.



(Gladstones, 1998). Lupins have the highest protein content of any grain legume

(35–40%) and as such provide a valuable food source for humans and animals (Haq,

1993; Milford and Shield, 1997). There has been a recent resurgence of interest

in the potential of lupins as a human food and for soil improvement in marginal

tropical and subtropical areas of Brazil, Peru, and the Mediterranean (Haq, 1993).

Lupin flour has been used in bread, biscuits, cakes, pasta, baby foods, powdered

milk, snack foods, and fast foods. The inclusion of white lupins in rotation with

cereal crops is an established feature of conventional and sustainable agricultural

systems in several temperate areas of the world including southern Australia

(Hamblin et al., 1993) and Europe (Sprent and ’t Mannetje, 1996). Until recently,

they were rarely grown in temperate regions, because the available varieties had

an indeterminate growth habit and were unsuitable for cultivation. In rotations,

they are grown as either a grain crop or a green manure, although no figures are

available on the UK, European, or world acreage of organic lupins. Here, their

potential as an organic grain crop is examined.

New, florally determinate genotypes of lupins have recently made their arable

cultivation in the UK a realistic proposition (Shield and Scott, 1996). These are

autumn sown and have a greater and more reliable yield potential than spring

sown genotypes (typically 3–4.5 t grain ha−1). The crops are consistently ready

for harvest from late August to early September in the south of the UK (Shield and

Scott, 1996). This early harvest allows a subsequent autumn sown crop to be grown,

unlike the later harvested spring lupins, preventing an over-winter fallow between

crops. Lupins are combinable, therefore no specialized equipment is required for

their production. Once dried, the thick seed coat makes lupins relatively resistant

to losses caused by storage pests and diseases (Haq, 1993).

When well nodulated, the rhizobia associated with lupin (Rhizobium lupini) can

fix between 150 and 350 kg N ha−1 y−1, up to 79% of the crop’s N requirement

(Hardy, 1982; Milford and Shield, 1997; Smith et al., 1992). Hamblin et al. (1993)

reported that more N is fixed by a lupin crop than is removed in the harvested

grain, giving a positive N balance. Where soil pH is below 6 or where lupins

have not been grown before, inoculation of the soil or seed may be necessary with

the specific bacterium. The amount of residual N remaining following harvest

depends on the lupin species and genotype, but it increases with earlier planting

and higher sowing rates and varies between 7 and 113 kg N ha−1(Hamblin et al.,

1993). Lupins also possess specialized acid-secreting cluster roots which make

them particularly efficient at obtaining phosphorus from the soil (Milford and

Shield, 1997; Neumann et al., 2000).

The benefit of lupin in conventional rotations has been demonstrated in Australia

and South Africa where it is grown as a grain break crop, in the southeastern United

States, where it is grown in rotation with maize, and in northern Europe, where it

is grown as a green manure and grain legume (Hamblin et al., 1993; Haq, 1993;

Reeves 1984). In Australia, Reeves (1984) found that cereal grain yields increased



AGRONOMIC AND ECONOMICAL POTENTIAL



401



by 30–100% after lupins in a rotation, whereas Hamblin et al. (1993) found that

lupins in the rotation consistently increased wheat yields by around 45%. In the

UK, wheat yielded 3.1–6.7 t ha−1 after lupins (unfertilized) compared to a mean

of 2.0 t ha−1 after wheat (McEwen et al., 1989).

Lupins have a relatively high P and K demand, and this could be a limiting

factor in some soils under organic management, although yields of 2.9–4.8 t ha−1

have been recorded in organic systems (Milford and Shied, 1997). Conventional

producers apply high doses of phosphates, 300–500 kg ha−1, and up to 120 kg ha−1

of potash prior to lupin production (Haq, 1993). Organic growers would have to

pay careful attention to P and K budgets on the farm in order to ensure that their

availability was sufficient to allow economic lupin yields, since applications of

P and K fertilizers are restricted under organic regulations (e.g., EC, 1991, Regulation 2092/91).

The gross margin for organic lupins is low compared to that of faba bean, despite

its great potential for use as both a human and animal food (Table X). The demand

for UK organic animal feed is currently met by on-farm feed production and by

imports of grains such as soybean, mainly from outside the European Union.

However, the UK and European organic livestock industry is in a period of rapid

expansion, which is not being matched by increased production of organic animal

feeds (Soil Association, 2000a). There is, therefore, a strong, developing market

for high quality animal feeds, and some of this demand could be met by grain

legumes such as lupin.

Lupin seed contains 35–40% protein and 11–13% oil, making it a good replacement for soybean meal in the diets of most farm animals (Milford and Shield, 1997;

Rossetto, 1990; Shield et al., 1999). Lupin seed is already used in the production of

high quality livestock rations and in pet food biscuits (Haq, 1993). Africa, Europe,

and the United States all currently import lupins from Australia (Haq, 1993). If the

potential of lupins was more widely recognized by farmers and feed manufacturers, the crop could provide a financial opportunity for UK and European farmers.

With a 50% price premium, the net margin for lupins is £358 ha−1 (Table X), 57%

of the net margin anticipated from winter oats.



C. SOYBEAN (Glycine max)

Soybean is the most important oil and protein crop in the world (USB, 2000).

Soybean oil is used for cooking, to make margarine, and for a range of industrial

products including paints, linoleum, inks, and soap. Once oil has been extracted

from the seed, the residual protein cake is used to manufacture foods for animals and

humans. This crop provides 52% of the world’s conventional oilseeds (155.1 Mt in

1999) and is also the fourth biggest grain crop produced (6% of global production:

USB, 2000). The United States is the leading soybean producer at 46%, with



402



ROBSON et al.



around 1.1% being produced collectively in the E-15 countries, of which France

and Germany are the most important (USB, 2000). Soybeans are a subtropical crop,

but the breeding of varieties tolerant of conditions in northern latitudes now allows

their cultivation as far north as Canada and Sweden. Figures are not currently

available on the quantities of organic soybeans produced in Europe and the United

States, although it is known that soybean was grown on 37,338 acres of organic

land in the United States in 1997 (USB, 2000). The crop is increasingly being

grown in several European countries including France and Germany (Fowler and

Lampkin, 1999).

When effectively nodulated, soybeans can derive up to 40% of their N from

fixation (Tanner and Hume, 1978; Whigham, 2000). Soybeans are sensitive to

soil N status and on heavily fertilized soil may derive only up to 25% of their

N through fixation, between 60 and 95 kg N ha−1 y−1 (Burns and Hardy, 1975;

Criswell et al., 1976). Bradyrhizobium japonicum is the bacterium which infects

soybean, and inoculation of seed is recommended if soybean has not been grown

within the previous 5 years (Whigham, 2000).

Soybean crops grown for seed leave between 30 and 60 kg N ha−1as residue in

the soil (Mays et al., 1998; Tanner and Hume, 1978) and as such are a valuable

component of organic rotations. Soybean can also be used in sustainable systems

as a green manure. Sumner (1982) found that the highest vegetable yields were

obtained when soybean was used as a green manure every 4 years, with a winter

cover crop, in comparison with other green manures and cover crops in a rotational

experiment in Connecticut. Reports on yield of subsequent crops are mixed. Doll

and Link (1957) found increased yields of cereals following soybean. Bundy et al.

(1993) and Ding and Hume (1996) also found positive effects on following crops.

Crookston (1994) found no effect on yield, and Sarbol and Anderson (1992) and

Mays et al. (1998) found yield depressions in subsequent wheat crops, probably

due to allelopathic effects of the soybean residues.

Soybeans can be sown with a bean, beet, or grain drill, as long as it provides

good depth control to within 1 cm (Tanner and Hume, 1978). The crop is combinable and therefore is mechanically compatible with cereals. However, the capacity of the soybean crop to depress the yields of subsequent crops of wheat is a

major disincentive to place them in a wheat rotation as a break crop (Mays et al.,

1998).

There is already a substantial worldwide market for soybean and its products.

In 1999, conventional soymeal constituted 64% of the world protein meal consumption, and soy oil provides 28% of the global consumption of marine and

vegetable oil (Chomchalow et al., 1993; USB, 2000). Soybean meal is becoming

increasingly important in the production of high protein foods and drinks for human consumption (Hymnowitz, 1993). The UK imported 1.8 Mt of conventional

soybean seed and meal for the production of livestock feed alone in 1998 (Shield

et al., 1999).



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III. Break Crops for Nutrient Management

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