Cropping
Soil management
Unravelling the benefits of crop rotation
Break crops underpin sustained yield improvements in dryland wheat crops throughout the world. In this article, CSIRO
Plant Industry scientist John Kirkegaard reviews the impact of break crops on the productivity of wheat and explains how
advanced methods are helping researchers more fully understand how crop rotation can be used to lift crop yields.
cross the diverse environments of
Australia, Europe and North America
dryland wheat crops yield a surprisingly similar
average of 20 per cent more following
appropriate break crops.
Across southern Australia, 26 trials also
delivered an average yield benefit to wheat
following a broadleaf break crop of 20%
although the impact on yields varied from
–16% to 197%, depending on seasonal
conditions and crop management.
While the reasons for the improved yields are
usually associated with reduced disease
or increased nitrogen or soil water, in some
cases the reasons underlying the yield response
are unclear or difficult to measure and these
inexplicable ‘rotation effects’ continue to puzzle
researchers worldwide.
Much of the mystery surrounding rotation
effects could simply arise because it is not
possible to measure all of the well-known
factors influencing yield such as disease,
nitrogen and soil water.
Another reason could be that crop rotation
also can influence soil structure and soil biology
in ways scientists are only just starting to
understand.
To fully understand and measure these
impacts the paddock soil and roots need to be
kept intact.
In the past, this has not been possible but
with new genetic and microscopy methods
researchers are now able to achieve a real-life
snapshot of how soil microbes, plant roots and
soil structure interact in response to
crop management.
At a glance
• Rotating wheat with a break crop
remains the main control strategy
for several wheat diseases.
• Additional benefits of crop rotation
include increased soil nitrogen
levels and improved soil structure
and soil microbiology.
• In some cases, the reasons
underlying wheat yield response
to rotation are unclear and
scientists continue to explore how
rotation can be fine-tuned to
increase crop production.
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Photos: CSIRO Plant Industry
A
Rotating wheat with broadleaf break crops such as canola or legumes can reduce the incidence of wheat
diseases and increase soil nitrogen levels. Additional benefits, which are more difficult to measure, include
improved soil structure and stimulation of the soil microbial population which, in turn, lifts crop productivity.
Disease control
Plant diseases such as take-all and crown rot
can be reduced substantially by growing a nonhost crop in sequence with cereals.
The value of break crops to disease
suppression depends on the diseases
present in the cropping system, the host status
of the proposed break crop and the availability
of other disease control strategies such as crop
tolerance or resistance and chemical control.
Break crops remain the primary control
strategy worldwide for several wheat diseases
including take-all.
In southern Australia much of the break-crop
benefit under well-fertilised dryland wheat
crops has been attributed to the control of takeall.
In north-west United States, the average
response of winter wheat to soil fumigation was
7% in paddocks cropped no more than every
third year to wheat, 22% in paddocks cropped
every second year and an astounding 70% in
those cropped every year to wheat.
The yield responses to the fumigation
treatments were primarily due to the control of
root diseases such as take-all, Rhizoctonia and
Pythium. In some cases, the benefits of disease
reduction can extend into the second
wheat crop.
For many diseases, including take-all,
seasonal conditions influence the extent to
which the disease will impact on crop growth.
As a result, the level of disease present
immediately before a cereal cycle is not
necessarily an indication of the break-crop
benefit.
For example, in south-eastern
Australia, CSIRO researchers found wheat
following wheat could achieve similar yields to
wheat following a break crop if conditions for
take-all development were poor and crop
nutrition was adequate.
Recent Canadian research showed seasonal
conditions explained 59–75% of wheat disease
severity while crop rotation played only a minor
role in the incidence and severity of disease,
despite overall reductions in pathogen
populations and higher grain yields in more
diverse rotations.
In other words, similar disease levels at the
start of the season can result in either yield
reductions or increases, depending on how the
disease interacts with seasonal conditions.
Residual nitrogen
Cereals derive yield benefits from legume
break crops due to both the disease break and to
additional soil nitrogen available following
legumes The relative contributions of the
nitrogen and disease-break benefits will vary.
A review of 135 site-years of Australian
data showed a 40–50% yield benefit to wheat
following legumes where no nitrogen was
applied to the wheat, which dropped to
about 10–17% when economically optimum
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January 2005
Soil management
Residual water
Most of the negative effects of broadleaf
break crops on following cereals relate to their
impact on residual water in areas where
complete recharge of the soil water profile
might not occur before, or during, the growth of
the subsequent wheat crop.
Under conditions of low water availability,
low disease pressure and low yield potential, the
amount of pre-sowing soil water will usually
dictate the yield of the following wheat crop,
A
B
CSIRO Plant Industry
nitrogen rates were applied.
The extra nitrogen available to the first crop
following a legume break crop in temperate
areas averages about 37 kilograms per hectare,
although a further 30kg/ha could remain in the
soil.
The additional nitrogen arises from reduced
use of mineral nitrogen by the legume (spared
nitrogen), subsequent decomposition of
legume residues or from reduced
immobilisation of existing soil mineral
nitrogen due to the lower carbon to nitrogen
ratio of legume residues.
Non-legume break crops also leave nitrogen
behind in the soil but can differ in the amount
of nitrogen spared.
For example, CSIRO research found linseed,
with its shallower rooting system, produced less
biomass and left 30–50kg/ha more nitrogen in
the profile at harvest than canola or mustard.
Cropping
A wheat root (A) one month after harvest, in close association iwth a root of a previous canola crop (B) within a structural
biopore at a depth of 600 millimetres. Cryo-SEM allows visualisations of such intact associations of soil structure, biology
and roots in field grown samples providing insights which are lost in disturbed samples. Note residual root hairs extending
from the old canola root to the biopore wall (at least 12 months since crop harvest) and the new wheat root emerging
from, and in close association with the remnants of the old canola root. Recent estimates suggest 40-80 per cednt of
subsoil roots are confined to these biopores.
which can be relatively unresponsive to other
inputs such as fertiliser.
Break crops also can influence water use
during the growth of the following wheat crop.
For example, CSIRO research during the 1994
drought showed wheat yielded better than
expected based on the amount of pre-sowing
water.
The higher wheat yield was apparently
related to deeper infiltration and more efficient
use of limited rainfall during the grain-filling
period.
Such ‘in crop’ impacts of previous crops on
wheat water use also can result from wheat
roots being healthier in rotation systems
and therefore more effective at extracting
soil water.
‘Mystery’ rotation effects
While a proportion of the 20% average
increase in wheat yield following a break
crop can be attributed to disease reduction,
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Cropping
Soil management
nitrogen benefits, improved water supply or a
combination of these factors, some rotation
effects remain difficult to explain.
Many scientists now suspect that soil
microbiology and soil structure play a much
bigger role than previously thought in
enhancing crop growth and grain yield.
Since farming systems are increasingly
adopting a no-tillage approach to crop
production, it is likely the effects of rotation on
soil structure and soil biology will become more
obvious.
By using novel methods to understand
how crop management influences soil
microbiology, root growth and soil structure,
researchers believe crop rotation could be
fine-tuned to lift grain yields further.
For example, cyro-microscopy has been
used to capture the image in the photo on page
43.
Soil microbiology
Break crops can influence the population of
micro-organisms living close to the root surface
(the rhizosphere) — and depending on the
species, rhizosphere microbes can stimulate or
suppress plant growth, or influence the
availability of soil nutrients.
The results of two recent CSIRO projects
highlight how much scientists have to learn
about soil microbiology and how it can
influence plant growth.
The first project set out to determine if canola
break crops reduce mycorrhizal fungi in the soil
and in doing so reduce the phosphorus
nutrition of following wheat crops.
The results showed the fungi were reduced
in wheat following canola but surprisingly
wheat nutrition and yield were unaffected.
The researchers concluded the fungi could
even be parasitic to wheat (and not beneficial as
previously thought) during early growth stages
under commercial field conditions across
southern Australia.
The second project has been investigating the
impacts of hydrogen gas which is released (up
to 5000 litres per day) by the nodules of some
legumes as they fix nitrogen.
The hydrogen gas stimulates the growth of
specific soil organisms, which use it for growth.
These organisms can promote plant growth
as soil taken from near nodules or treated with
hydrogen gas increased the growth of both
legumes (14%) and non-legumes (18–32%).
This mechanism could explain some of the
rotational benefits of legumes which are not
related to nitrogen or disease.
Soil structure
The roots and residues of break crops
can influence soil structure by exuding
or releasing stabilising or destabilising
substances into the rhizosphere, breaking up or
enmeshing roots and their associated
fungal hyphae.
In addition, deep-rooting perennial pastures
such as lucerne leave behind stable root
channels which subsequent crops use to
penetrate deeper into the soil profile.
The root channels are known as biopores and
contain living roots along with root residues of
previous crops.
Biopores improve water infiltration rates and
could alleviate transient waterlogging in duplex
soils and provide crops with better access to
subsoil moisture. The biopores become zones
of concentration for roots and soil organisms in
no-tillage soil as the roots of new crops
repeatedly grow down the low resistance
biopore pathways into the soil.
The discovery of biopores has forced a
rethink about the nutrient status of the subsoil,
which traditionally has been thought of as being
low in organic matter but in fact could be
biologically active where most of the roots are
located.
The agronomic significance of biopores
remains uncertain and scientists continue to
unravel their significance in farming systems.
For more information contact John Kirkegaard
on [email protected], phone (02) 6246
5080 or fax (02) 6246 5000. Visit the
web site www.cropscience.org.au to
download the full review from which this
article was summarised.
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