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Weed ecology and management. Martha Rosemeyer Ecological Agriculture November 4, 2003. Sustainable Agriculture. Three attributes: Economically viable, environmentally sound, socially just An integrated system including: natural resources: land/soil, crops, animals, climate

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weed ecology and management

Weed ecology and management

Martha Rosemeyer

Ecological Agriculture

November 4, 2003

sustainable agriculture
Sustainable Agriculture
  • Three attributes: Economically viable, environmentally sound, socially just
  • An integrated system including:
    • natural resources: land/soil, crops, animals, climate
    • socio-economic resources: capital, labor and management
that are organized to satisfy the following goals
That are organized to satisfy the following goals:
  • Provide food and fiber
  • Enhance environmental quality and natural resource base
  • Make the most efficient use of non-renewable and on-farm resources
  • Sustain the economic viability of farm operations
  • Enhance the quality of life for farmers, their families, communities and society as a whole
  • Definition
  • Weed ecology and succession
  • Weed problems and benefits
  • Weed management: systems modifications vs. input subsitution

Labor 40-70% labor in traditional systems or herbicides to remove weeds

what is driving the system why are weeds so important
What is driving the system? Why are weeds so important?
  • Disturbance and Recovery via Succession
  • Succession is the process by which one community gives way to another
    • Here referring to plants
  • Primary vs. secondary succession
two approaches to agriculture
Two approaches to agriculture
  • Don’t let succession proceed
    • Invest much energy in the form of human, animal or fossil fuel
    • Herbicides are used to prevent succession from occurring
  • Try to mimic successional stages of the natural ecosystem, the “analogue model”
    • plant a corresponding successional stage, filling that niche. If the niche is filled a “weed” can’t compete
analogue approach
Analogue Approach
  • Commonly used in tropics where forest is climax
  • Disturbance can be introduced at any stage to bring back to beginning stage of succession
  • Defined as “practices that intentionally retain trees on land used for crop production or grazing”
  • Over space or time (rotations that use fallows with trees in it)
  • fallow:
  • Trees can be spatially arranged for different effects
roles of trees
Roles of trees

Good foundation for developing the

“emergent properties”

of more complex ecosystems”

Other roles:

Gliessman p259

  • microsites for beneficials
  • reduction of wind
optimizing the positive impacts
Optimizing the positive impacts
  • Potential negative effects:
  • shading, allelopathic influence
  • microclimate effects that favor disease
  • branches etc. can fall on harvestable portion
  • Can be mitigated by planting plan

Fallow= field not planted to a crop

what is a weed a plant out of place
What is a weed? “A plant out of place”
  • A weed is any plant, native or non-native, that interferes with crop production by doing more harm than good and encroaches where it is not wanted
  • A successful weed has closely adapted to the life-cycle of the crop and farming practices
    • Most of our crops and farming practices came via Europe with European weeds
  • Learn to work with succession instead of preventing it
when working against succession weed problems competition
When working against successionWeed problems: competition
  • Nutrients
  • Light
  • Water
  • Results in lower yields and poor crop quality
interference with harvesting operations
Interference with harvesting operations
  • Weed roots such as bindweed wrap around sub-soil blades used for undercutting root crops during harvest
  • European, extensive problem, difficult to eradicate, root system to 20’, seeds viable for 50 years
  • Reproduction by seed or rhizomes

Field bindweed, (Convulvulus

arvensis), Morningglory Family

build up of soil seedbank vs crop seed bank for storing crops
Build up of soil seedbank vs crop seed bank (for storing crops)
  • Weed seeds present in soil
  • Seedbank can increase rapidly in one season with fertilization and irrigation and ineffective control
  • One weed plant can produce thousands of seeds
  • Affected by management
redroot pigweed amaranthus retroflexus amaranthaceae fam
Redroot Pigweed Amaranthus retroflexus Amaranthaceae Fam.
  • Common throughout the West
  • One plant can produce thousands of seed
  • Relative’s seed eaten by Andean populations (Grain amaranth, A. hypochondiacus)
  • North American indigenous crop (A. blitum)
hairy galisoga galinsoga ciliata asteraceae fam
Hairy Galisoga, Galinsoga ciliataAsteraceae Fam.
  • One of the most difficult-to-control weeds in vegetable production on fertile soil, esp. in NE US
  • Continually produces seeds throughout growing season
  • Can produced 7500 seeds/plant
allelopathic affect on crop plant
Allelopathic affect on crop plant
  • Black walnut and tomatoes
  • Lamb’s quarters roots secret oxalic acid
  • Velvetleaf, quackgrass, Canada thistle, giant foxtail, black mustard and yellow nutsedge
  • Mechanism: root secretion, decomposition of residues, effects microbial symbionts
weeds can harbor diseases and pathogenic fungi esp crop relatives
Weeds can harbor diseases and pathogenic fungi, esp. crop relatives
  • Classic case of wheat rust disease on wheat with alternative host European barberry
  • 1970-1990 caused $100 million annually
  • Barberry eradication project saved farmers $30 million per year
can be directly parasitic
Can be directly parasitic
  • Dodder (Cuscuta sp.) Convolulaceae (Morningglory) Family
  • Major problem in West US with alfalfa, clover, potatoes, safflower
  • First germinates root then when finds host becomes parasite
striga is serious problem of corn and sorghum in africa
Striga is serious problem of corn and sorghum in Africa
  • Striga in corn, Witchweed, Scrophulariaceae Fam.
benefits of weeds
Benefits of weeds
  • Enhance soil structure and water penetration
  • Improve soil tilth
  • Capture nutrients that would otherwise be lost
  • Provide habitat for beneficial insects
indicate soil characteristics and suitability for crops
Indicate soil characteristics and suitability for crops
  • Creeping buttercup, Ranunculus repens, Ranunculaceae Fam.
  • Introduced from Europe as ornamental
  • Reproduces from seed or rhizomes
  • Toxic to cattle
  • Could indicate moist soil conditions
may be edible good weed bad weed concept
May be edible: good weed/bad weed concept
  • Common purselane Portulaca oleracea Portulacaceae Fam.
  • Introduced from Europe
  • Seeds can remain dormant for years
  • Edible fresh or cooked, esp. used in Mexican cooking
know your weed biology critical to control life habit classification
Know your weed biology: Critical to controlLife habit classification
  • Annual: A plant that completes its life cycle in one year
  • Summer annual
    • Purslane, galinsoga, pigweed, lambsquarters, pineappleweed
winter annuals common chickweed black mustard annual bluegrass
Winter annuals: common chickweed, black mustard, annual bluegrass

Brassica nigra Stellaria media

Brassicaceae Caryophullaceae

A plant that completes its life cycle in two years above ground

Examples are wild carrot (Daucus spp.), bull thistle (Circium vulgare) and poison hemlock (Conium maculatum)



Wild carrot is a host of leafhopper-vectored

aster yellows phytoplasma

  • A plant that lives a number of years, often producing seed once each year
  • Simple perrenials reproduce by seed
  • Examples are dandelion ((Taraxacum officinale), curly dock (Rumex crispus) and plantain (Plantago sp.)

Broadleaf plantain, (Plantago


creeping perennials
Creeping perennials
  • Reproduce by seed and asexually through rhizomes, stolons (a horizontal above ground stem that roots at the nodes =runners) tubers and rootstalk
  • Examples are Johnson grass, Bermuda grass, Nutsedge (Cyperus sp.), Field bindweed
management goal balance with crop not complete eradication
Management Goal: Balance with crop, not complete eradication
  • Retain diverse community
  • Economic threshold concept
    • Do not start to modify agricultural practices unless it reaches a point where it pays to deal with it in long and short term
  • Can include ecological concepts as well
    • If ecological benefit then may want to leave
weed control systems approach vs input substitution
Weed control: Systems approach vs. Input substitution
  • Systems approach: Direct intervention is last resort!
  • First modify:
    • soil conditions
    • change crop rotation
    • cultivation and sowing practices
    • increase competitiveness of crop
    • introduction of animals as grazers
farming practices affect weed community composition
Farming practices affect weed community composition
  • No-till increases percentage of perennial weeds (annuals shaded out by residue)
  • Decreased reliance on grass “leys” (2-3 yr rotation into grass (European) increases perennial weeds
    • grass shades and cutting reduces thistle
  • Organic production may have worse problems with perennials
  • Annuals increased by crop nutrition
Prevention because of system effect
    • Crop rotation
    • Tillage
    • Fallow
    • Mulches
      • black plastic, organic, living
  • Direct effects
    • Pesticides
    • Flame and soil sterilization
    • Chemicals- vinegar
systems approach vs input substitution
Systems approach vs input substitution
  • Systems approach modifies system so that weeds are not a problem
  • Prevention of weed problems
  • Alternate summer and winter crops (where possible)
  • Rotate crops- different crops favor different weed., pest and disease populations

Crop Rotation effects on weeds

Warn season weeds

Cool season weeds

From: Harwood

weed suppressive cover crops
Weed-suppressive cover crops
  • Sudan grass, buckwheat, sesbania (hot summer areas)
  • Perennial rygrass (Lolium perenne) (PNW)- dense growth and allelochemicals suppress weeds (at organic farm)
  • Optimize planting time to ensure uniform cover
  • Pre-plant tillage to allow weed seeds to sprout-- removing part of weed seed bank
    • may till several times depending on weed pressure
  • One of main weed control methods of organic farmers
  • Can also use moldboard plow to bury the weed seeds
  • For control of perennial weeds like
    • Johnson grass, crab grass and Bermuda grass
  • Tillage can actually increase infestation
  • Black plastic mulch in strawberry (also warms soil)
  • Organic mulches- straw, sawdust, tree leaves, secondary succession vegetation
  • Living mulches
    • intercrop with a

cover crop

i direct intervention pesticide what is a pesticide
I. Direct intervention: pesticideWhat is a pesticide?
  • An umbrella term used to describe any chemical that controls or kills a pest
  • Classified by the type of pest they kill
    • insecticide
    • fungicide
    • bactericide
    • herbicide- kills plants
    • rodenticide- kills rodents
    • nematicide- kills nematodes
    • acaricide- kills mites
Contact herbicides
    • Paraquat: persistent, high acute mam. Toxicity, slightly to high toxic aquatic
    • Triazine herbicides: Atrazine- persistent, low acute mam. toxicity, carcinogen,

endo. disruptor, ground water

  • Systemic herbicides
    • glyphosate (OP)- not persistant, low acute mam. tox., slightly toxic aquatic, hormones
    • (2,4-D (auxin); 2,4,5-T) - not persistent, moderate acute mam. tox.,

poss. carcinogen; sus. endo. disruptor,

potential ground water contaminant

not only is there stimulation of cancer but new story endocrine disruptors
Not only is there stimulation of cancer but new story: Endocrine disruptors

Theo Colborne, 1987. Our Stolen Future.

  • PhD UW, observations of birds of Great Lakes
  • Led to endocrine disruptor hypothesis
  • Data still emerging
  • National EPA still not released list of suspected chemicals, though Illinois EPA, Keith, Colborn and Benbrook list and Canada
function of endocrine disruptors
Function of endocrine disruptors
  • They can act like a natural hormone and bind to a receptor. This causes a similar response by the cell, known as an agonist response.
  • They can bind to a receptor and prevent a normal response, known as an antagonistic response.
  • A substance can interfere with the way natural hormones and receptors are synthesized or controlled.
source of agricultural endocrine disruptors
Source of Agricultural Endocrine Disruptors
  • Agricultural runoff /Atmospheric transport
    • Organochlorine Pesticides (found in insecticides, many now phased out)
      • DDT, dieldrin, lindane
    • Carbamate insecticides
      • **Aldicarb (PAN-pesticide database)
  • Agricultural runoff
    • Pesticides currently in use
      • **Atrazine, trifluralin, permethrin
atrazine triazine herbicide
Atrazine (triazine herbicide)
  • #1 herbicide in corn in the MidWest
  • Now banned in EU because of human health and environmental concerns
  • Just re-approved in US
  • Discrediting of US scientists by Syngenta (e.g. Hayes), where Atrazine is 1/4 of total revenues
  • Proven endocrine disruptor-- in frogs changes testes to ovaries (Lumenstyk,2003 Chronicle of Higher Educ 50:A26)
Atrazine detected

in groundwater

in Wisconsin

other direct interventions used by organic farmers
Other “direct” interventionsUsed by organic farmers
  • Flaming
  • Solarization
  • Biological control
  • Vinegar (salt not recommended)

Impt to have

small seeds

Can use propane

gas tank on small scale

solarization with clear or black plastic
Solarization with clear or black plastic
  • Useful to kill annual weed seeds
  • Can kill certain perennials, e.g. creeping buttercup in the PNW
  • Can be combined with

cabbage or cole crop

decomposition for


biocontrol of weeds using insects or plant pathogens a pathogen causes a disease
Biocontrol of weeds using insects or plant pathogens (a pathogen causes a disease)
  • One of major natural occurrences- Only 25% of weed seeds do germinate- thought to be due to insects and pathogens in the soil
  • Used only in specific circumstances-

ie not widely used,

unlike diseases of insects

Carabid beetle

  • 5% (household white vinegar)-10% concentrations of organic vinegar had 85-100% kill rate
  • Canada thistle- one of most susceptible with 5% concentration with 100% kill rate
  • Spot spraying in corn cost $20-30 per acre
    • ARS researchers Radhakrishnan, Teasdale and Coffman
  • Not tested for effects on soil that cannot be replace by lime
  • Salt not recommended
the systems approach
The systems approach
  • How to avoid the use of direct intervention methods?
    • Organic farmers growing annuals use long (4-7 yr) rotations and a combination of other methods
  • How can we mimic the natural ecosystem to avoid the effects of weeds?
    • How do does letting succession take place avoid problems with weeds?
  • Weed photos identification and management: www.ipm.ucdavis.edu
  • Whitson et al. 2000. Weeds of the West (9th Edn).Western Society of Weed Science.
  • Taylor, R. 1990. Northwest Weeds. Mountain Press, Missoula
  • Uva, R., J. Neal, J. DiTomaso. 1997. Weeds of the Northeast. Comstock Press.
  • Beeby, John. 1997. Test Your Soil with Plants. Ecology Action Self-teaching Mini-Series Booklet #29
  • Would you consider the lack of weeds an “emergent property” of an agroecosystem? Explain.
  • Ch 17 Question #1, 2 (spend some time on this),
  • Upper division Ch 17 # 4, 6