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COVER CROPS IN SUSTAINABLE FARMING SYSTEMS

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Title: COVER CROPS IN SUSTAINABLE FARMING SYSTEMS


1
COVER CROPS IN SUSTAINABLE FARMING SYSTEMS
DR. CAROL SHENNAN THE CENTER FOR AGROECOLOGY AND
SUSTAINABLE FOOD SYSTEMS UNIVERSITY OF
CALIFORNIA, SANTA CRUZ
2
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.

3
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

4
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.

5
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

6
Lablab beans
cowpeas
Summer cover crops - tropical species
Sorghum (C4 plant)
Pigeon peas
7
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.

8
Effect of CN ratio on size and timing of peak
soil nitrate levels over 3 years - UC Davis field

(yr 1, yr2, yr 3)
9
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.

10
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11
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.

12
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.
13
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

14
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.

15
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

16
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)

17
Cereal Rye in vineyards
18
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.

19
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.

20
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.

21
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22
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)

23
Importance of ground cover for reducing erosion
24
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

25
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.

26
Nematode suppressive cover crops
27
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

28
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.

29
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

30
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.

31
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.
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