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VI. Break Crops for Pest and Disease Management

VI. Break Crops for Pest and Disease Management

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Although total elimination of weeds is never the aim in organic systems (see

Section II.D), it is important to point out that weed populations have frequently

been observed to be higher after the carrot crop than before (Lampkin, 1990; Rose,

pers. comm.; Halder, pers. comm.).

Carrots can be grown at several points in a rotation including after cereals, as they

do not have a high N requirement (Lampkin, 1990). They are increasingly sown

with precision seeding equipment, and uniformly sized seed is used to promote even

germination (Lampkin, 1990; Rubatzky and Yamaguchi, 1997; Sanders, 1996).

Since harvesting also requires specialized topping and lifting machinery, carrots

are not mechanically compatible with a cereal rotation. Carrot harvest usually

begins in late summer and can continue into winter in areas where severe frosts are

unlikely (Wiseman et al., 1993). Alternatively, the roots can be covered with straw

or straw and polythene and left to over-winter in the ground. They are then sold

according to market demands throughout the winter and early spring (Lampkin,

1990; Rose, pers. comm.; Rubatzky and Yamaguchi, 1997). Late harvests may

cause problems if a winter cereal is to follow the carrot crop. Earlier harvest dates

can be achieved by early sowing, by promoting faster growth through soil warming

(through the use of polythene on carrot beds prior to sowing), and by sowing at

high densities to produce “baby” carrots rather than full size roots.

Organic carrots, grown with high and low mechanization give the highest net

margins of all the crops analyzed (Table X). The high net margins reflect the

demand for organic carrots for human consumption and justify the use of hand

weeding and/or specialized equipment for growing carrots. There is a high and

increasing demand for organic carrots (Caspell and Creed, 2000; Fowler and

Lampkin, 1999) for fresh sales and for processing, although as with all products in rapidly changing markets, marketing channels should be established before

the crop is grown.



B. SWEDE (Brassica napus VAR. napobrassica)

Swedes are among the most commonly grown and widely adapted root crops

(Sanders, 1996) and are an essential part of rotations on many traditional mixed

organic farms in Europe (Blake, 1990). Traditionally they have been grown as

winter fodder, e.g., in the Norfolk four-course rotation (Wiseman et al., 1993);

however, more recently farmers have been growing shopping swedes which have

been bred for human consumption (Michaud, 1997). Most countries do not publish

production figures for swedes separately from those for organic vegetables in

general, although it is known that around 3000 t of swedes with a market value of

£1.13 M was produced in the UK in 2000 (Soil Association, 2000a).

Like many noncereal crops, swedes can provide a valuable break from a range of

cereal pests and diseases. Along with many root crops, swedes are poor competitors



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with weeds and require weeding on at least one or two occasions throughout the

season. If time and money are available for rigorous weed control, then swede can

be regarded as a cleaning crop, but weed populations are often higher after swede

crops (Lampkin, 1990; W. Rose; F. Halder, pers. comm.). Swedes make a positive

contribution to soil organic matter, and leave a residual biomass of between 1.3

and 1.5 t ha−1 of dry matter (Lampkin, 1990). This assumes that the tops are

incorporated into the soil at harvest.

Swede crops would ideally follow a ley in rotation due to their moderate N

requirement. However, they can also follow a cereal with a possible application of

manure (Lampkin, 1990). Crops are usually direct drilled using a specialized swede

drill. At harvest, they are either eaten off the field by sheep or topped and lifted in

one pass using specialized root harvesting machinery (Wiseman et al., 1993). In

most districts of the UK, swedes are harvested in October–November (Lockhart

and Wiseman, 1978). This makes it difficult to establish a winter cereal after

swedes. If winter cereals are to be planted after a swede crop, the swedes should

be planted early and harvested from late July (Michaud, 1997). Alternatively, the

swedes can be lifted early and allowed to ripen in a clamp (Lockhart and Wiseman,

1978).

The high net margin of £3319 ha−1 makes organic shopping swedes a profitable

break crop (Table X) and may justify the expenditure on machinery not used in

cereal production as well as providing grade outs for livestock feed. As with most

organic edible horticultural crops, there is a high and increasing demand for organic

swedes (Caspell and Creed, 2000; Fowler and Lampkin, 1999).



C. SUGAR BEET (Beta vulgaris)

Sugar beet has been grown for sugar production since the mid-18th century in

eastern Europe and is now a valuable conventional crop throughout temperate areas

of the world. Around 37% of the world’s sugar comes from beet (the remainder

being extracted from sugar cane) and around 40 Mt of sugar is produced annually

from beet on a global basis (Winner, 1993). At present, sugar beet is rarely grown

in organic systems because of the high weeding costs and the absence of markets

and organically certified processing plants (Lampkin, 1990), although a market is

now available in the UK (Lampkin and Measures, 2001) and some other countries.

Like other noncereal crops, sugar beet can provide a valuable break from a range

of cereal diseases and pests. However, it should not be grown in the same rotation

as oilseed rape, since it suffers from shared pest and disease problems including

beet cyst nematode and alternaria diseases (Agrios, 1997; Cooke, 1993; Duffus

and Ruppel, 1993). A significant break between subsequent sugar beet crops is

also required to avoid build-up of weed beet, the seeds of which can survive in the

soil for several years.



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The beet crop could potentially have a positive effect on weed control through

the necessity and opportunity to control weeds (Lampkin, 1990). However, due to

the relatively low crop value, weed control is carried out only to prevent crop loss

due to competition, and weed populations can in many cases be higher after the

sugar beet crop than before (Halder, pers. comm.).

Sugar beet is usually sown with a swede drill and requires specialist root harvesting machinery to top and undercut the beets at or prior to lifting (Bray and

Thompson, 1985). The crop has a low N requirement, and high residual N concentrations in the soil can reduce the sugar content of the beet. It should, therefore, be

grown after a cereal or an N demanding crop (Lampkin, 1990). Where sugar beet

is grown before a winter cereal, the normal harvest date of the beet would be after

the ideal sowing date for the cereal. It may be necessary, especially on heavier

soils, to harvest the beet early. This would result in a yield loss, but it would allow

earlier sowing and, therefore, better establishment of the more profitable winter

cereal crop (Bray and Thompson, 1985). A beet crop sown after a cereal can also

cause problems within a rotation. Cereals are usually harvested in July/August and

the beet crop is not sown until the following March/April. This leaves the soil bare

for around 8 months during the winter which would make nitrate susceptible to

leaching (Allison et al., 1996). The risk of soil erosion is also increased on fallow

soils, particularly in winter. Organic farmers often choose to grow a cover crop

such as grazing rye or winter vetch over winter between crops which are harvested

early and those which are planted in the following spring. Such crops reduce the

likelihood and severity of soil erosion and nitrate leaching and can be ploughed in

prior to sowing the next crop (Soil Association, 1998).

When the crowns and tops of the sugar beet crop are ploughed back into the

soil after harvest, significant quantities of organic matter and nutrients are added.

For example, a sugar beet crop can contribute 0.6–1.0 t ha−1 dry matter to the soil

following incorporation of crop residues (Lampkin, 1990). Bray and Thompson,

(1985) estimated that 105 kg ha−1 N, 35 kg ha−1 P, and 145 kg ha−1 K were

returned to the soil from the crowns and tops of a 50 t ha−1 sugar beet crop.

Wheat planted after sugar beet needed less N when the tops had been incorporated

than after wheat (Sylvester-Bradley and Shepherd, 1997). However, the N released

from the incorporated tops is prone to losses through leaching, denitrification, or

volatilization (Sylvester-Bradley and Shepherd, 1997).

There is a small but growing market for the crop in Europe. Until recently

in the UK, the need for processing capacity was limiting interest in the crop

which can only be grown on contract. Without price premiums, sugar beet has

reasonable gross margins, comparable to field beans and winter oats (Table X),

but following trials in 2000 British Sugar plc are offering contracts in 2001 with

a price premium, bringing the gross margin to £1799 ha−1. However, the costs of

field operations reduces the net margin to below that for winter oats. Demand for

organic sugar, particularly for processing, is increasing, and processing capacity is



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415



likely to increase. No subsidies exist for sugar beet production in UK or European

agriculture at present. There is a small but growing market for the crop in Europe.



D. LINOLA (Linum usitatissimum)

Linola, a crop recently developed by CSIRO in Australia, is a low linoleic acid

linseed (Hocking, 1995). The genetic changes involved in reducing linoleic acid

from the original seed have no effects on any other part of the plant, therefore there

are no alterations to agronomic performance associated with the quality change.

So, although Linola can be regarded as a new crop, the fact that it was derived

from flax means that its agronomy and cultivation methods are already well known

(Anon., 1995). There are currently no figures available on the UK, European, or

world acreage of linola.

Unlike the other agricultural crops in this review, there is relatively little information available on the break crop effects of linola. It is not a cereal, therefore

it can be grown as a break crop in a cereal rotation as it is not susceptible to

cereal pests and diseases. For example,Kirkegaard et al. (1997) found that wheat

following Linola had a decreased incidence of rhizoctonia diseases (R. cerealis)

and complete suppression of take-all.

Linola is reported to have no effects (Thomas, 1996) or positive effects on wheat

yields (Angus et al., 1991; Kirkegaard et al., 1997). The increases may come from

reductions in take-all or rhizoctonia diseases in the following wheat crop. Linola

has a low nutrient requirement (COG Inc., 1992; Kirkegaard et al., 1997) and

is best grown on a low input basis (Turner, 1993) making it suitable for organic

systems. This is important in a rotation to contrast with cereals, which are nutrient

demanding. There is some evidence from trial work carried out in the UK that

the growing Linola crop may have allelopathic effects on weeds, but the work

will have to be repeated before conclusions can be reached (Robson and Litterick,

unpublished).

The production of Linola does not require different machinery or equipment

from that used for cereals. The crop fits well into a cereal rotation, and while

potentially increasing subsequent cereal yields, it also shows a yield increase when

grown after wheat or barley (COG, Inc., 1992). The harvest dates for linola range

from 170 to 210 days after sowing, and this may pose problems for their inclusion

in a cereal rotation, particularly in northern temperate latitudes, where growing

seasons are shorter (Flax Council of Canada, 1999).

Linola is better adapted to cooler environments than other polyunsaturated

oilseed crops, such as sunflower and maize, thus providing an opportunity to produce highly polyunsaturated oil in more northern latitudes, for example, northern

Europe (Anon., 1995). The low linolenic acid oil that is produced from Linola

is a high quality polyunsaturated oil similar in composition to sunflower oil and



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is suitable for widespread edible uses (Green, 1992). There is therefore a strong

interest in Linola for health food markets, but processing facilities are required

before large-scale production of the crop can be contemplated. The net margin for

Linola is £374 ha−1 assuming a 50% price premium (Table X). Although this is not

high in comparison to most of the other break crops discussed in this review, the

potential benefits from growing the crop within a rotation have created significant

interest from organic growers (Rose; Haward; Halder, pers. comm.).



VII. CONCLUSIONS

All the species discussed here have valuable characteristics if used as break crops

in organic arable rotations. Hemp, faba bean, and lupin have the greatest agronomic

potential as break crops, but with the exception of bean, they generate poor returns

for the farmer. Linola and soybean are also useful break crops, although soybeans

may have allelopathic effects on subsequent wheat seedlings. Swede, potato, and

carrot are the most profitable crops, but are less valuable in the rotation in terms of

soil fertility than hemp, bean or lupin. Sugar beet and oilseed rape are challenging

crops to grow organically, and there is currently a limited market for their produce.

If successfully grown, they could have some positive contributions to a rotation.

The benefits of organic farming for consumers, livestock, and the environment

are increasingly being demonstrated. Interest from consumers, environmentalists,

farmers, and policy makers is strong, and there is little doubt that the area of

land devoted to organic production in temperate areas will continue to increase

over the next decade at least. A great deal of work is required if organic rotations

involving novel break crops are to be optimized in terms of agronomy, economics,

and environmental impact.

There is a significant amount of valuable agronomic and market information

already available on the production of the more common organic crops, including

cereals such as oats and barley. However, the potential of a wide range of more

novel crops including pulses, oilseeds, vegetables, salads, fruit, fiber, and essential

oil crops, and the less common cereals such as spelt must be evaluated in order to

determine their break crop characteristics and the benefits and challenges which

they bring to organic systems. Many of the varieties developed for conventional

cropping are unsuitable for organic systems. Breeding work is urgently required to

develop crop varieties with characteristics particularly suited for organic systems.

Detailed agronomic studies concerning nutrition, crop husbandry, pest, disease,

and weed control are then required to optimize production systems for these

varieties.

The rapid expansion of the amount of land under organic husbandry is bringing a concomitant expansion in the range of crops grown. For many crops with



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established organic markets, and more especially for “new” organic crops, the

marketing and processing infrastructures are immature. Investment in the supply

chain will be necessary to allow the development of crops requiring processing.

Organic products with complex processing and marketing chains may be slower to

establish, but will provide entrepreneurial opportunities. Meanwhile, farmers aiming to solve agronomic problems in their rotations using the more nascent organic

break crops will need to use all channels of expertise to help them balance the

agronomic solutions and financial returns against possible practical and marketing

difficulties.



ACKNOWLEDGMENTS

The authors wish to thank the UK Ministry of Agriculture, Fisheries and Food for funding a 4-year

field- and desk-based project on the use of break crops in organic systems. We also wish to thank the

many farm staff, consultants, and scientists who helped us with this review. In particular, we wish to

thank Kate Barnard, Simon Brenman, Donald Clerk, John Fraser, Fred Halder, Rob Haward, Hugh

Ironside, Jan Redpath, William Rose, Alan Schofield, Andrew Skea, Dr. Dick Taylor, Paul Van Midden,

Dr. Robin Wood, and David Younie. We also wish to thank Feli Pomares and Frances Haldane for help

in preparation of the manuscript. SAC receives financial support from the Scottish Executive Rural

Affairs Department.



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