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    deep ripping
    fact sheet
                                                                                                                                                        2009




         Deep ripping not appropriate
         for all soil types
         A comprehensive understanding of the cause and location of high soil strength is
         required before undertaking deep ripping as research has shown it is does not provide an
         economic solution in some soil types.
                                                             PHOTO: BrAD COLLiS




          KeY pOints
          ■   deep ripping of compacted soils
              is most likely to improve grain
              yields on sandy soils and where
              compaction has occurred on
              upper parts of the soil profile
              through machinery traffic or
              livestock trampling.
          ■   deep ripping is less effective on
              heavy clay soils unless combined
              with gypsum on sodic soils prone
              to waterlogging.
          ■   deep ripping will provide little
              benefit if other subsoil constraints
              such as salinity, sodicity or acidity
              are also present.
          ■   recent advances in machinery,
              such as ‘slotting’ and deep
              placement equipment to
              simultaneously introduce
              ameliorants at depth with ripping,
              could increase the financial and
              agronomic effectiveness of this                                      The greatest yield responses
              approach to managing subsoil                                         following deep ripping have
              constraints.                                                         been recorded in sandy soils.

         Soils with high strength can occur                                       By breaking up the soil, deep ripping      of ameliorants. Nutrients can be
         naturally or due to compaction, but                                      can free the way for roots to penetrate    deep placed to ameliorate nutrient
         either way, left untreated, these soils can                              the soil and access water and              deficiencies and toxic concentrations
         significantly limit on-farm profitability                                nutrients, leading to yield increases.     of aluminium, deep placement of lime
         by reducing grain yields and quality.                                                                               can ameliorate acidity and gypsum
                                                                                  However, it is only effective on certain   can ameliorate sodicity.
         Soil compaction can occur in many                                        soil types and is only likely to be
         cropping soils of southern Australia                                     financially viable when combined with
         and may be traffic or livestock-                                         strategies to ameliorate other subsoil     What is deep
         induced or naturally occurring.                                          constraints such as nutrient deficiency

         By limiting the ability of crops to gain
                                                                                  or toxicity, or sodicity (see Sodicity).   ripping?
         access to water and nutrients, soil                                      How the soil is managed after ripping      Deep ripping, sometimes known as
         compaction can reduce crop growth,                                       also affects the return from ripping.      subsoiling, involves disturbing the soil
         grain yields and quality.                                                Converting to stubble-controlled traffic   below the normal cultivation layer, often
                                                                                  farming can help retain the value of       to 40cm, with strong, narrow tines
         Deep ripping involves disturbing the                                     deep ripping.                              without inverting the soil. it is not the
         soil below the normal cultivation layer,                                                                            same as delving, also called clay delving,
         often up to 40 centimetres, without                                      Deep ripping also presents an              where wide-bladed tines are used to mix
         inverting the soil.                                                      opportunity for deep placement             deep clayey subsoil with sandy topsoil.


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                                                                                             PAGE 2




                         Why deep rip?                                    When to deep rip?
                         Deep ripping breaks up traffic-induced           Deep ripping is slow and costly. Timing         often result in large soil clods being
                         or naturally occurring layers with high          of ripping is critical but the results          brought to the surface. Conversely,
                         soil strength. With deep ripping, roots          can last for many years if appropriate          if soils are too wet, smearing can
                         can penetrate the soil faster and deeper         management practices are used.                  result. A good rule of thumb to assess
                         to absorb more soil moisture, capture                                                            appropriate soil moisture condition
                                                                          For maximum yield response, the                 is to roll the soil (at different depths
                         more soil nutrients and improve yield.
                                                                          depth of ripping must be below                  where the ripping will occur) between
                         These benefits vary with soil type and
                                                                          the traffic pan and this may mean               your hands. if a long ribbon (>10cm)
                         are more likely to be observed where
                                                                          penetration to at least 30cm. Moist             forms easily then the soil is still too
                         there are no other chemical constraints,
                                                                          soil throughout the ripping depth               moist. if no ribbon can be formed or if
                         such as sodicity, present.
                                                                          is necessary to reduce the power                the soil is too hard to reform then the
                         Care must be taken to avoid immediate            requirement and wear of points and to           soil is probably too dry.
                         re-compaction by minimising wheel                obtain efficient softening.
                         traffic on newly-ripped paddocks.                                                                When suitable soil moisture conditions
                                                                          The main difficulty in accommodating            exist, deep ripping should be done
                         Some growers move to controlled                  deep ripping within the farming system          prior to the cropping phase but
                         traffic to reduce re-compaction. By              is the availability of suitable soil            sufficient time allowed for subsequent
                         matching all implement widths and                moisture conditions.                            seedbed preparation. in many
                         tractor wheel spacing, controlled traffic
                         ensures all vehicle traffic is confined to       if soils are too dry, draft and                 environments/seasons this is not
                         the same tracks, minimising the area of          fuel consumption are increased                  always possible.
                         re-compacted soil.                               significantly. ripping very dry soils can
                                                                                                                          in Western Australia, in the areas
                                                                                                                          where ripping has been adopted
                                                                                                                          fastest, responsive soils are often
                                                                                                                          cropped continuously with wheat and
                                                                                                                          lupin rotations. ripping can be done
                                                                                                                          after substantial summer rains, or at
PHOTOS: KELLiE PENFOLD




                                                                                                                          the break of the season before sowing
                                                                                                                          wheat.

                                                                                                                          ripping at the break conflicts with
                                                                                                                          lupin seeding or with early sowing
                                                                                                                          of wheat. Delaying wheat or lupin
                                                                                                                          planting can reduce yields. However,
                                                                                                                          this must be weighed against potential
                          The block clods of the compacted layer following ripping (right) compared to the small          yield advantages from ripping,
                          friable aggregates in an uncompacted soil (left).                                               including its residual value.


                         high sOiL strength                               from deep ripping depend on soil type,          FIGURE 1 AFFECT OF TYRE WIDTH
                         Many of australia’s cropping soils are
                                                                          rainfall and crop species.                      ON THE SHAPE OF COMPACTION
                         pre-disposed to compaction because the           an audit of agricultural land in australia by   IN UNDERLYING SOIL
                         proportions of sand, silt and clay particles     the national Land and Water resources
                         in these sandy or loamy soils are ideal for      audit (2001) suggested that once the bulk
                         tight packing. also the types of clays in        density exceeds 1.6 – 1.8g/cm3 in sandy
                         these soils do not often promote strong          soils and above 1.4g/cm3 in silty and clay
                         swelling and shrinking, which helps break        soils, root penetration, and consequently
                         up compaction.                                   plant growth, are affected.

                         soil compaction can reduce crop growth           compacted or hard layers can also
                         and yield as it limits the ability of crops      cause water logging within the root zone,
                         to gain access to water and nutrients.           often resulting in a perched watertable.
                         compacted layers can occur naturally due         if this occurs, the soil profile above the
                         to the chemical and physical characteristics     hard layer can become saturated, oxygen
                         of the soil or may be induced by traffic (as     can be excluded and plant roots can
                         seen in figure 1) or livestock.                  eventually die.
                                                                                                                                                                          Topsoil
                         these layers may be visible or indicated         although no comprehensive surveys have
                         by distorted root growth. When measured          been undertaken to estimate the extent
                         they will record a penetration resistance of     and severity of compacted layers in
                         at least 2 Mega pascals (Mpa). generally         australian agricultural soils, the available
                         a subsoil strength of 1.5Mpa is the              evidence suggests compacted layers are a
                         accepted threshold at which root growth          major problem in some soils.
                         is restricted. at a strength of 3Mpa root
                         growth is significantly reduced.
                                                                           Tyre width can change the shape, position       Compaction zone
                         deep ripping is one method of reducing            and degree of compaction beneath the
                                                                                                                                SOURCE: Subsoil, kirby, et al, Soiil and Tillage Research,
                         soil compaction, however yield responses          tyre tracks but compaction still occurs.                                         Elservier, Netherlands, 1991
                                                              PAGE 3




identifying high-strength or compacted soils
When talking about high-strength            capacity (usually in winter when the                   Soil and land type
soils, the terms ‘high bulk density’        soil profile has wet-up to at least 50cm               Sandy soils tend to have higher bulk
and ‘compaction’ are often used             depth) as soil water content can have                  densities compared to clay soils, so
interchangeably.                            a bigger influence than compaction                     awareness of soil type will help in
                                            on penetration resistance. readings                    diagnosing high bulk density. Soil
Bulk density is the weight of soil in a     of more than 1.5MPa should be                          strength is generally independent of
given volume. Many subsoils naturally       investigated further. if the soil is not               soil type and so provides a unifying
have high bulk density and sandy soils      wet, field capacity readings from the                  measure of compacted soil.
are more prone to high bulk density.        suspect compacted site should be
                                            compared with those from a known                       Subjective assessment of
When soil compaction occurs the bulk                                                               soil structure
density of the soil in the compacted        uncompacted site. Both sites should
                                            have the same soil type and the same                   The presence of physical constraints
layer increases. Compaction typically                                                              can be assessed visually by digging
occurs to a depth of about 30 to            soil water content.
                                                                                                   a shallow soil pit (see box below)
40cm due to traffic by machinery            Plant symptoms                                         to observe crop rooting depth, the
and/or animals. The type of tyre, the       identifying plant symptoms that are                    presence of compacted soil layer and
air pressure in the tyre, the weight of     directly caused by compacted subsoil                   fracture planes. The occurrence of wet
the vehicle and, most importantly, soil     horizons is difficult because other                    soil within the rooting zone at harvest
moisture, all have a bearing on soil        factors can cause similar symptoms.                    also indicates the presence of subsoil
compaction by vehicles.                     Overall, compacted subsoils impede                     constraint/s.
Compaction is typically measured by         root growth, causing reduced plant
                                            vigour and poorer water-use efficiency                 Signs of compaction are large peds (or
the amount of force (in megapascals)                                                               clods) with a platy (horizontal) shape
required to penetrate a layer; bulk         as roots have limited access to water
                                            and nutrients, especially in the subsoil.              or massive (featureless) look. in poorly
density is measured as the mass of soil                                                            structured cracking clays, ped faces
per unit volume (g/cm3).                    Other symptoms include a lack of roots
                                            present below a certain depth (shallow                 are dull rather than shiny. The soil may
                                            root system) and, in some cases,                       feel puggy when wet and peds will tear
Penetrometer                                                                                       apart like raw pastry. When dry, peds
An accurate and rapid method for            roots turning or branching to grow
                                            horizontally above the compacted                       are not friable; they break where you
determining soil strength (to depths                                                               apply force rather than parting along
of more than 45 centimetres) is             layer. This horizontal growth can often
                                            be easily observed in pulse and oilseed                natural fracture faces.
to use a cone penetrometer. High
penetration resistance may reveal a         crops by carefully excavating a few
compacted layer but the measurement         plants with a shovel. Plant roots can
is strongly affected by soil moisture.      penetrate compacted layers through
The measurements should ideally             cracks and macropores in structured
be taken when the soil is wet to field      soils causing very little impact on yield.




soil pits                                       FIGURE 2 ROOT ZONE SOIL CONSTRAINTS
in-crop soil pits allow the rooting depth                – PHYSICAL, CHEMICAL AND BIOLOGICAL
of a crop to be determined. they can
help identify potentially limiting soil
layers that negatively impact on root
growth. these can include compacted
                                                Soil borne
layers or hard pans, carbonate layers,          pathogens
ferricrete or calcrete. this method             - Fungi         Surface stubble                                       Surface
is useful in diagnosing the presence                                                                                 cultivation A Horizon
                                                - Nematodes
of chemical and physical subsoil
constraints. Once identified, detailed          Deleterious                                                         Compaction
                                                bacteria           Al toxicity Fe, Zn
testing is required to determine the
extent of any limitations and whether                               (low pH) deficiencies
                                                                     Acidity                                          Sodicity B Horizon
it is feasible to reduce or remove                                            Ion toxicities Transient                 Low
them. aerial photographs, eMi                                               (Na, B, Al, HCO3) salinity              permeability
(electromagnetic induction) maps that                          Macro and
indicate soil moisture, salt and clay
                                                              micronutrient                  Alkalinity              Reduced
                                                              deficiencies                   (high pH)               leaching
content and maps of yield or biomass
can be used to determine the best
locations to dig soil pits and to take
soil samples.
                                                                      PAGE 4




Where to deep rip
The success of deep ripping in                      and high-pressure injection to                          to control (non-ripped) treatments
improving crop growth and yield is                  simultaneously apply ameliorants                        on lighter textured soils. in 46 trials
strongly dependent on the soil type                 at depth with deep ripping, could                       conducted between 1981 and 1989
and presence of other soil chemical/                increase the effectiveness of this                      in WA on yellow loamy sands, all but
physical constraints to crops.                      approach.                                               two demonstrated increased yield with
                                                                                                            ripping.
Generally, ripping is most successful               Once deep ripping and amelioration
on sandy soils and least successful                 are completed, strategies such as                       in other areas and on different soil
on heavy clay soils (see summary                    stubble retention and controlled traffic                types deep ripping has been much less
compiled by John Kirkegaard, CSirO,                 should be considered to maintain soil                   reliable and benefits persist for shorter
Table 1). its effectiveness in improving            structural improvements, otherwise re-                  periods. it is not clear whether this is
subsoil structure is also strongly                  compaction will quickly occur in most                   due to the compacted layers having
dependent on implement design,                      soil types.                                             less impact on crop productivity or that
soil water content and the depth of                                                                         the deep ripping approaches have not
ripping as well as the concurrent use               Generally, sand-over-clay soils will not                been fully effective.
of amendments. The primary aim of                   respond to ripping unless the depth of
deep ripping should be to maximise                  the sand is deeper than the depth of                    A review of crop responses to deep
disturbance (loosening) in the subsoil              ripping. Sand-over-gravel soils have                    ripping to 40cm in southern NSW
whilst minimising the draft.                        responded to ripping if the gravel is not               from 1980 to 2005 showed yield
                                                    a completely restrictive layer.                         improvements in only five of the 15
Deep ripping appears to be more                                                                             crops measured despite deep ripping
successful where compacted layers                   in WA deep ripping has increased                        reducing soil strength to less than
induced by tillage are present rather               cereal yields in areas with more than                   1MPa. ripping had no effect on yield
than where the subsoil is sodic and                 350 millimetres rainfall by more than                   at five sites and significantly reduced
has inherently high soil-strength,                  600 kilograms per hectare in deep                       crop yield/biomass at four sites.
for example sodosols. Sodosols                      sandy soils that have a compacted
are common in the cropping areas                    layer less than 40cm deep and where                     The current published evidence does
of Victoria and South Australia. On                 the subsoil is not highly acidic.                       not support deep-ripping of soils in
sodic soils, ripping is unlikely to                                                                         south-eastern Australia, except in
                                                    A study in the northern Mallee of                       combination with gypsum on sodic
have significant long-term beneficial               Victoria recorded yield responses in
effects unless the structure of the soil                                                                    clay soils, where winter water-logging
                                                    wheat of up to 40 per cent by deep                      can cause significant yield reductions
is simultaneously stabilised through                ripping on non-sodic sandy loam soils.
amelioration with gypsum (calcium                                                                           in wet seasons.
                                                    Studies in WA have also demonstrated
sulphate) or organic matter (see Soil               yield increases of 25 per cent in wheat
pits, page 3). recent advances in                   and 30 per cent in chickpea compared
machinery, such as slotting equipment



TABLE 1 SUMMARY OF DEEP-RIPPING RESPONSES
  RELIABLE RESPONSES                                               VARIABLE RESPONSES                                                    FEW RESPONSES
  Soil type and     Deep acid               Neutral/           Sodic clay         Duplex-                   Duplex-        Red loams             Black
       depth          sand               alkaline sand   (grey/brown vertosol)     deep                     shallow        (Kandosol)          vertosols
  Cultivated 0cm                                                                                                                                 Self
    zone 10cm                                                                     Waterlogging                                                 mulching
  Compact
    layer                                                        Strength >1.5 – 2.0 MPa; Distorted roots
             25cm   Acid layer                                                    Waterlogging                              Acid layer
                                         Low nutrient
                                                              Sodic clay            Sodic clay
  Associated                              availability                                                  Sodic clay
                                                              Anaerobic
    subsoil 100cm      High density        Salinity                                 Anaerobic           Anaerobic         High density
                                                             High density
   problems             Low water           Boron                                  High density        High density      well structured
                                                            Poor structure                                                                      Salinity
                      and N holding                                               Poor structure      Poor structure
                                                               Salinity                                                                          (Cl-)
                        N leaching                                                   Salinity
           200cm
    Mean crop         WA 20–37%           Vic/SA 23%        NSW 33% (0-300)          WA 22%              4%                Few              Few
    response                                 (0–43)              WA 47%           NSW 0–20%
  range (wheat)                                                  Vic 25%
     Best bet        Rip with lime if        Rip with        Rip with gypsum   Rip if clay >30cm      Do not rip      Few responses Few responses
  management           acid claying          nutrition            Avoid           Possibly add                                         Self repairing
                                                              re-compaction       gypsum/lime
   References         Jarvis (2000)     Sadras et al (2005) Chan et al (2003)   Crabtree (1989)    Crabtree (1989) Kirkegaard (W’shop) Dalal (W’shop)
                    Davies et al (2006) Wilhelm (W’shop) Armstrong (2001)        Davies (2006)    Davies et al (2006)
                      Hamza (2003) Armstrong (W’shop) McKenzie et al (1990) Jarvis (Various)
                      Hall (W’shop)                          Hamza (W’shop)     Ellington (1986)
                                                                              Kirkegaard (W’shop)
                                                               PAGE 5




rainfall                                     produce responses in crop growth, but                                   Negative responses to ripping can
On suitable soils, responses to ripping      grain yield responses may be curtailed                                  be observed in some seasons with a
have been least reliable in areas with       if there are poor finishing rains or soil                               ‘dry finish’ as crops growing on deep
less than 325mm annual rainfall.             moisture is restricted.                                                 ripped soils grow more vigorously.
Plant roots only grow where there is                                                                                 They produce more biomass, and use
subsoil moisture. With limited rainfall,     if the season has had good rainfall                                     up the available subsoil water more
soil may not be wet deep down in the         or significant amounts of soil water                                    quickly, leaving insufficient soil water
soil profile. ripping, therefore, will not   have accumulated during the previous                                    for grain filling.
provide roots any additional access          summer, there is more potential for
to water.                                    root growth, water and nutrient
                                             extraction from deeper in the soil
On responsive soils in the medium            profile when subsoil constraints have
rainfall area, ripping will almost always    been dealt with.



Deep ripping impacts
Following deep ripping, roots can            so they are potentially groundwater
penetrate the soil faster and deeper         recharge areas. They are also likely
to absorb more soil moisture, capture        to be susceptible to soil acidification,
more soil nutrients and improve yield.       partly through leaching of nitrate.
There are other impacts of deep
ripping, both positive and negative.         Apart from the productivity gains
                                             from deep tillage, the higher-yielding
residual value of deep ripping               wheat crops will use more water and
The response to deep ripping can last        nutrients. This will reduce the leaching
for many years. in responsive soils the      of nitrate, through deeper rooting and
residual benefit of the ripping in the       greater nitrogen uptake, and in the
second year is about half the initial        long-term will reduce flooding and
response and declines further over           salinity of lower landscapes.
time due to:
■ re-compaction by traffic;                  Deep ripping downsides
                                                                                             PHOTO: STEPHEN DAViES




■ natural soil settling and cementation;     Although deep ripping can increase
   and                                       water infiltration into the soil, it can also
■ the greater removal of nutrients and       destroy the natural soil aggregation
   water by the previous year’s higher-      and macropores of loamy or finer-
   yielding crop.                            textured soils, bring sodic subsoil
                                             to the surface (resulting in poor
The use of controlled traffic after deep     establishment of crops), and deplete
ripping has been shown to help retain        soil organic matter. it can also reduce
                                                                                                                      Differences in root growth are clear
the benefits of deep ripping for longer,     soil water content in the surface layers
                                                                                                                      between un-ripped soil (left) where very
especially in heavier soils.                 by disturbing and exposing the soil to
                                                                                                                      few roots penetrated below 30cm and
                                             evaporation, which can reduce crop
Other benefits of deep ripping                                                                                        the rip line (right) where roots were still
                                             yield in dry seasons.
The soils that are most likely to                                                                                     common at 40cm.
respond to deep ripping are sandy,



The economics of deep ripping
                                                                                                                        sodicity
Economics should be the overall              long-term beneficial effects unless the
deciding factor on whether an                structure of the soil is simultaneously                                    sodicity occurs naturally in
amelioration strategy is implemented         stabilised through amelioration with                                       soils that contain a high level
or not. The final strategy selected will,    either calcium or organic matter.                                          of sodium relative to calcium,
in turn, depend on the capacity of the                                                                                  potassium and magnesium.
landholder to afford these costs and         A sensitivity analysis can be a useful                                     it causes dispersion of soil
their attitude to risk.                      method of assessing the change in                                          particles, which leads to the
                                             crop yield by identifying a specific price                                 collapse of soil structure and
The benefit of deep ripping on crop          required to return the cost of deep                                        loss of pore spaces, which
performance varies widely. Most              ripping. in the example in Table 2 the                                     limits seedling emergence, the
studies indicate that deep ripping           total amelioration cost for deep ripping                                   growth and efficient function
alone provides little or no long-term        based on contract rates is $40/ha.                                         of plant roots and increases
impact on the physical condition of          With an average production cost of                                         waterlogging. gypsum
clay subsoils. Yield benefits have been      $165/ha the total cost of production                                       (calcium sulphate) application
recorded from deep ripping on non-           equals $205/ha. The breakeven points                                       is extensively used to counter
sodic sandy loam soils in the northern       for both wheat price and yield are                                         sodicity.
Mallee. On dispersive (sodic) soils,         highlighted in yellow over the page.
ripping is unlikely to have significant
                                                                                   PAGE 6




  TABLE 2 THE ECONOMiCS OF DEEP riPPiNG
                                                              Price ($/tonne)
     Yield (t/ha)
                          100               150               200            250             300             350
         0.5              50                75                100            125             150             175
          1               100               150               200               250          300             350
         1.5              150               225               300               375          450             525
          2               200               300               400               500          600             700
         2.5              250               375               500               625          750             875
                                                                                                                          an example of the impact of changes
          3               300               450               600               750          900            1050          in price and yield to achieve payback
         3.5              350               525               700               875         1050            1225          from deep ripping in the first year if the
          4               400               600               800               1000        1200            1400          total cost of production including deep
                                                                                                                          ripping is $205/ha.
         4.5              450               675               900               1125        1350            1575
SOUrCE: Identifying, understanding and managing hostile subsoils for cropping
- A reference manual for neutral-alkaline soils of south-eastern Australia.



Deep ripping equipment
Deep ripping can be achieved using a                        a dense subsoil, disturbance may
variety of implements.                                      need only be confined to relatively
                                                            shallow depths (for example, 20cm).
Various tine designs have been used                         Once deep ripping and amelioration
successfully in deep ripping including                      are completed, strategies such as
a ‘winged’ design (Spoor and Godwin                         stubble retention and controlled traffic
1978), shallow tines in front of the                        should be considered to maintain
deep tines (Hamza et al. 2005), and                         any improvements in soil structure.
the ‘Paraplough’, as well as more                           Otherwise, re-compaction will occur
traditional tines with straight shanks.                     quickly on most soil types.
Draft requirements increase rapidly                         recent advances in machinery,
with the depth of soil disturbance                          such as ‘slotting’ equipment to
so a good knowledge of where the                            simultaneously introduce ameliorants                         Where compaction is closer to the
principal soil physical limitation is in                    at depth with ripping, could increase                        surface a deeper pass with narrow
the soil profile is required. For example,                  the effectiveness of this approach to                        tillage points has been able to break up
on soils with a shallow but fertile                         managing subsoil constraints.                                thin compacted layers.
topsoil (for example, 10cm), overlying


Useful resources:
■	    A general description of various subsoil constraints and how to manage them for grain production (including deep
      ripping) can be found in Identifying, understanding and managing hostile subsoils for cropping - A reference manual
      for neutral-alkaline soils of south-eastern Australia, published by The Profitable Soils Group (2009) (ISBN 978-0-9806136-
      0-5) which can be downloaded from                        www.dpi.vic.gov.au/dpi/vro/vrosite.nsf/pages/soil_mgmt_subsoil
■	    Roger Armstrong, DPI Victoria                                              (03) 5362 2336, Email roger.armstrong@dpi.vic.gov.au
■	    Stephen Davies, Department of Agriculture and
      Food, WA                                                                   (08) 9956 8555, Email sdavies@agric.wa.gov.au
■	    John Kirkegaard, CSIRO Plant Industry                                      (02) 6246 5080, john.kirkegaard@csiro.au
■	    Western Australia                                                          www.agric.wa.gov.au/objtwr/imported_assets/content/lwe/land/
                                                                                 cult/fn088_1994.pdf

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Acknowledgements: Roger Armstrong, DPI Victoria; John Kirkegaard, CSIRO; and Stephen Davies;
Department of Agriculture and Food, WA.

				
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