COVER CROPS IN SUSTAINABLE FARMING SYSTEMS

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COVER CROPS IN SUSTAINABLE FARMING SYSTEMS
DR. CAROL SHENNAN THE CENTER FOR AGROECOLOGY AND
SUSTAINABLE FOOD SYSTEMS UNIVERSITY OF
CALIFORNIA, SANTA CRUZ
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Why use cover crops?

• Cover crops are an important fertility management
  tool available to organic farmers
• Grown primarily for soil or ecosystem improvement
  rather than cash
• Can have negative consequences if managed
  incorrectly or the wrong species are chosen.

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Terminology

• Confusion about the terms
• cover crop, green manure, and catch crop.
• Frequently used interchangeably, but refer to
  different primary functions
• Cover crop used to prevent soil erosion by
  covering soil with living plants
• Green manure turned under for soil improvement
• Catch crop used to catch nutrients left after
  harvest of a cash crop and prevent leaching

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Goal Increase nutrient availability

• Cover crops can increase nutrient availability
  by
• residue breakdown - releases nutrients into soil
  solution or incorporated in soil microbes.
• As a source of readily available C cover crops
  stimulate microbial activity and increase the
  breakdown of soil OM.
• If cover crop is deeper rooted than preceding
  crops it can recycle nutrients from deeper in the
  soil and return them to the upper soil when
  residue is incorporated
• Cover crops rather than bare fallow reduce
  nutrient loss by capturing nutrients vulnerable
  to leaching and preventing soil erosion.

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Goal Increase SOM and improve nutrient
availability

• To build SOM look for a high biomass cover crop.
• Possible options for fall planting include
  non-legumes such as
• annual rye grass, cereal rye, triticale,
  legume/cereal mix,
• High biomass legumes provide N and build SOM
• sub-clover or woollypod vetch
• For summer planting options include
• sorghum/sudan grass, buckwheat
• tropical legumes
• cowpea, crotolaria, pigeon pea, lablab
  bean,sesbania

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Lablab beans
cowpeas
Summer cover crops - tropical species
Sorghum (C4 plant)
Pigeon peas
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Timing of nutrient release from the cover crops
depends on a variety of factors

• Soil temperature and moisture affect microbial
  activity (lower when cool, dry or waterlogged)
• quality of the cover crop residue also
  important
• C to N ratio of the residue determines whether
  there is net release of N (mineralization) or a
  net decrease in available N due to immobilization
  by the soil microbes.
• CN ratios around 22 or less lead to net
  mineralization,
• CN ratios above 22 lead to net immobilization of
  N.
• At low CN ratios microbes have more N than C
  available for growth, so they release N during
  decomposition.
• At high CN ratios soil microbes have more C than
  N available and take up N from the soil solution
  
• lignin, tannins and polyphenols in residue are
  resistant to microbial breakdown and slow the
  rate of decomposition.

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Effect of CN ratio on size and timing of peak
soil nitrate levels over 3 years - UC Davis field

(yr 1, yr2, yr 3)
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Goal Provide N

• Here the best choice would be a legume that is
  well adapted to your area.
• It is important to ensure that sufficient
  rhizobia bacteria are present in the soil to give
  good root nodule formation, and hence N2
  fixation.
• May need to inoculate with rhizobium if it is the
  first time a particular legume cover crop has
  been grown in a field.

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Timing of nutrient release

• Important to manage the timing of nutrient
  release from cover crops with times of high crop
  demand
• otherwise nutrients become vulnerable to loss
  through leaching and the crop may not receive
  adequate nutrition at key stages in the growth
  cycle.

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How to roughly estimate N contribution from a
cover crop

• Calculate above ground dry weight for a given
  area
• a) Take a number of samples from the field (clip
  at ground level, using a yardstick or frame to
  measure area to be sampled)
• b) Dry for a few days in sun, greenhouse or oven
  (140F) until "crunchy" or brittle.
• c) Calculate the dry weight produced in lbs/ac as
  follows
• Dry wt (lb/ac) wt of samples x 43,560 sq.ft
• of sq. ft sampled
• Multiply dry wt by the N content of the biomass
  to give total N in cover crop in lbs/ac
• Total N (lb/ac) dry wt x N 100

Adapted from Sustainable Agriculture Network.
1998. Managing Cover Crops Profitably. Second
Edition. Handbook Series Book 3. Sustainable
Agriculture Network. National Agricultural
Library. Beltsville, MD 20705-2351. Available
free online from SARE http//www.sare.org.
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Typical N contents for cover crop types

• Before flowering
• Annual legumes 3.5-4
• Perennial legumes 2.5-3
• Grasses, brassicas 2-3
• When flowering
• Annual legumes 3-3.5
• Perennial legumes 2-2.5
• Grasses, brassicas 1.5-2.5

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To estimate how much of the N will be available
to the crop that season

• If conventionally tilled
• divide total N by 2
• If left on surface
• divide by 2 in warmer climates
• divide by 4 in cooler climates.

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Goal improve soil physical properties

• Generally improve soil physical properties by
• increased SOM
• increased microbial activity and production of
  extracellular glues that enhance aggregate
  stability
• increased water infiltration due to increased
  porosity - both macro and micropores
• It may be beneficial to include a non-legume to
  provide organic material that breaks down more
  slowly than a legume alone
• Some deep-rooted species can help to break
  through compacted layers in the soil and improve
  drainage.
• E.g. Sorghum-sudan grass
• In a recent study cover crop use in moderately
  saline soils impaired soil physical properties

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Goal Weed suppression

• Achieved by either
• outcompeting weeds by rapid canopy development
  and more vigorous growth
• production of allelopathic compounds
• provision of dense mulch
• Examples
• good options include triticales, sorghum/sudan
  and other cereals, brassicas such as rapeseed and
  oilseed radish and high biomass or allelopathic
  legumes. Also a well-balanced mix can also work
  providing the canopy closes quickly
• cereal rye is also effective due to combination
  of a dense canopy and allelopathy, but can become
  a problematic weed itself in small grain systems.
  
• For effective mulches use species/mixes that
  produce lots of slowly decomposing biomass (have
  a relatively high CN ratio)

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Cereal Rye in vineyards
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What to avoid...

• Some legumes such as vetch produce hard seed
  that remains viable in the soil for years, and
  can become a weed if allowed to go to seed prior
  to incorporation.
• Just as crop rotation is important it may be
  desirable to avoid using the same cover crop
  every year, particularly a single species.
• Otherwise populations of weeds most competitive
  with that species will build up, as well as pests
  and disease organisms that also attack that
  species.

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Goal Provide mulch to conserve soil moisture

• As for weed suppressive mulches look for a
  combination of high above ground biomass and
  moderate or high CN ratio residues.
• Most legume residues with their high nitrogen
  content will decompose too rapidly to be
  effective.

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Goal Scavenge nutrients prevent leaching

• To maximize nutrient scavenging the cover crop
  should have an extensive root system that
  develops quickly after planting.
• Non-legumes such as small grains, cereal rye,
  triticale, rapeseed, annual rye grass oilseed
  radish and mustards work well, but some legumes
  are also suitable.

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Goal Prevent soil erosion

• Here the key is to choose a species that rapidly
  covers the soil surface.
• Many of the species that are good nutrient
  scavengers also provide excellent ground cover.
• But... while annual rye grass is a good nutrient
  scavenger, it has fine leaves and is slow to
  cover the soil surface, and will not be good at
  reducing surface runoff. (However extensive root
  system will help stabilize soil)

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Importance of ground cover for reducing erosion
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Goal Protect water quality.

• This is achieved by selecting species that both
  prevent soil erosion and scavenge nutrients
  during periods of high rainfall.
• It is also important to avoid turning in high N
  cover crops when the soils will be vulnerable to
  leaching

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Goal reduce disease and pest severity

• Effects of cover crops depend on the species used
  and timing of incorporation and planting of the
  subsequent crop.
• Some species such as cereal rye, triticale,
  forage rapeseeds, mustards and oil seed radish
  are known to suppress certain plant parasitic
  nematodes and soil borne diseases, whereas most
  legumes are highly susceptible and can increase
  nematode populations.
• Need to have information on pests and diseases
  prevalent in an area to identify which cover
  crops should be avoided, or only used in mixtures.

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Nematode suppressive cover crops
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Goal Provide habitat for beneficial insects and
spiders

• From work in orchards and vineyards it is clear
  that cover crops provide habitat for beneficials,
  due to
• vegetative cover
• by providing food sources such as extrafloral
  nectaries (e.g. vetch) or flowers
• This aspect of cover crop ecology has not been
  well studied for annual systems, and there may be
  more potential than has been recognized.
• The key in annual systems is to make sure that
  when the cover crop is turned in there are
  alternative habitats for the beneficials to move
  to
• Field margin vegetation, strips of undisturbed
  cover crops or insectary plantings

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How to choose a cover crop

• Once you have decided it is desirable to include
  a cover crop in a given production system there
  are 3 basic steps to follow
• Identify what you want the cover crop to do.
• Identify the planting windows where the cover
  crop can fit in your crop rotation and what the
  climatic and soil conditions are at that time.
• Once these two questions are answered, then you
  can select the best species or mix to be used.

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Step 1. Identify what you want the cover crop to
do?

• Address the most important factors limiting the
  productivity and sustainability of your
  production system.
• Possible goals include
• Improve nutrient availability and provide
  nitrogen
• Improve soil physical properties
• Reduce erosion, leaching and protect water
  quality
• Suppress weeds, pests and diseases
• provide beneficial habitat
• provide surface mulch

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Step 2 Identify where the cover crop fits in
your crop rotation

• Examine cash crop rotations to identify windows
  where cover crops can fit
• Most cover crops are planted in the fall to
  provide cover over the winter months
• In summer when temperatures are high, fast
  growing species such as sorghum/sudan, cowpeas,
  and buckwheat and other can provide a good
  biomass return in a short growth period.
• It is critical to minimize conflicts in timing of
  field operations for cover crops and cash crops.
  
• Once the window is identified then the species
  selection will depend on the climatic and soil
  conditions during the window. e.g.- frost
  patterns, soil and air temperatures for
  germination and growth, soil pH.

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Step 3 Select species/mix to meet the goals and
requirements from steps 1 2.

• the final step is to match the potential
  candidates identified in step 1 with the required
  characteristics identified in step 2.
• Remember - it is as important to consider the
  characteristics you dont want as well as those
  you are looking for.
• It is rare that the perfect cover crop will
  exist and trade-offs will need to be made between
  different goals.
• A final consideration will be the cost and
  availability of seed, and the number and types of
  field operations required for the different
  options to make a sound economic assessment of
  the alternatives.