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VI. Break Crops for Pest and Disease Management
ROBSON et al.
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
AGRONOMIC AND ECONOMICAL POTENTIAL
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,
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.
ROBSON et al.
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
AGRONOMIC AND ECONOMICAL POTENTIAL
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,
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
ROBSON et al.
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.).
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
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
AGRONOMIC AND ECONOMICAL POTENTIAL
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
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
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