Weed Ecology and Management

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Weed Ecology and Management
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Characteristics of Weed Species

• Good colonizers of cultivated land and disturbed
  habitats
• High seed production, often over a long period of
  time
• Good seed dispersal
• Longevity of seeds and dormancy. Seed Bank
  reservoir of seeds in soil, available to
  germinate over long period of time (many years)
• Persistence in perennial crops
• Tolerance to environmental conditions
• Good interspecific competitors

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Weed Competition with Plants Depends on

• Weed density
• Timing
• Environment
• Plant physiology

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Weed Competition with Plants Depends on

• Weed density Crop yield is related to weed
  density if weed numbers are low, maybe no effect
  on yield.
• Timing
• Environment
• Plant physiology

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Relationship between Crop Yield and Weed Numbers
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Relationship between Crop Yield and Weed Numbers
Not known for many crop/weed combinations
Future predictions relative to density in seed
bank
Depends on species or mixture
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Weed Competition with Plants Depends on

• Weed density
• Timing
• Environment
• Plant physiology

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Timing Period of crop growth relative to weed
growth

• Early-season weeds may be serious
• Critical period for weed control recognized in
  most crops
• Weeds germinating when crop is near harvest may
  not be important
• Planting date can be manipulated to lessen weed
  impact

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Weed Emergence over Time
Critical period for weed control in crop
Important to match up these two !
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Weed Competition with Plants Depends on

• Weed density
• Timing
• Environment Does soil type, temp., rainfall,
  etc., favor crop or weed?
• Plant physiology C4 weeds vs C3 plants

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Nutsedge
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Bermudagrass
What is a weed ?
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Nightshade
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Lambsquarter
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Pigweed
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Hairy indigo
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Black Medic common winter weed
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Stages in Dynamics of Weed Population in a Site
Are there weaknesses in weed life cycle that
provide opportunities for management ?
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Weed Management Pepper with Weeds - Immokalee
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Weed Management

• Favoring crop growth over weeds
• Need depends on weed density
• Many different management practices

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Weed Management

• Herbicides
• Mechanical cultivation
• Hand weeding
• Other weed management practices

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Herbicides

• Largest pesticide use in US agriculture (85 if
  ag pesticides, 55 of US pesticides US EPA,
  1995)
• highly effective, selective
• - - - and high energy costs, pollution
• Selective vs nonselective herbicides

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Herbicide Application Relative to Crop Growth
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Selective Herbicides

• For specific crop and weed combinations (see
  extension recommendations)
• Some may be highly specific (plant growth
  regulators, etc.)
• Grass vs Broadleaf is common distinction
• Timing is critical (pre-, post-, etc.)
• E.g., Atrazine used in corn

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Selective Herbicides Cautions

• Drift to nontarget plants
• Roots of woody plants, etc.
• Carryover (old residue from previous herbicide
  may affect crops)
• Water, leaching (e.g., atrazine in wells)

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Non-selective Herbicides

• General cleanup (along roads, etc.)
• Desiccant crop defoliation kill crop residues
  for easy mechanical harvest (cotton, potatoes)
• Kill crop residues and weeds in no-till and
  min-till systems

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Roundup -- Glyphosate

• Most widely used herbicide
• GMO genes for Roundup tolerance moved into crop
  plants
• Roundup-Ready cultivars of crops (soybean,
  cotton, corn, etc.)

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Roundup
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Fumigation with Methyl Bromide1. Make plant beds
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Chisels Used to Inject Methyl Bromide Toxic to
Living Organisms
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Inject Fumigant and Cover Bed with Plastic
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Finished Plant Bed with Cover to Seal in Methyl
Bromide Gas
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Non-Selective Herbicides Cautions

• Non-target effects
• Resistance/tolerance in weeds, selection for
  Roundup tolerant weeds
• GMO crops as weeds
• Narrowing of crop genetic base if few cultivars
  are planted
• Water pollution less than with previous
  herbicides
• Air pollution -- Methyl bromide affects ozone
  layer

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Weed Management

• Herbicides
• Mechanical cultivation (energy costs)
• Hand weeding (labor costs)
• Other weed management practices

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Weed Management Practices that improve crop
competition

• Narrow row spacing
• Increased seeding rate
• Transplanting vs seeding
• Optimum planting date
• Competitive crop cultivar

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Weed Management Practices that reduce weed
densities

• Soil Solarization
• Mulching
• Cover Crops
• Rotation Crops
• Intercropping
• Biological Control
• Prescribed Fire

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Solarization

• Cover soil with clear plastic
• Maintain for 6 wk
• Heats to high soil temp (50C122F)
• Kills weeds and other soil pests

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Solarized bed
Solarization on flat surface
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Solarization Plastic Application
Clear plastic
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Clear vs Gray Plastic Mulch in Nutsedge Field
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Nutsedge Problems on Plastic Bed
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Failure Due to Prolonged Cloud Cover

• Cloudy weather in October, 2001.
• High nutsedge pressure.

Mulch removed after solarization
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Plastic not sealed into soil
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Beds after Solarization vs Untreated (hairy
indigo)
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Weed Management Practices that reduce weed
densities

• Soil Solarization
• Mulching
• Cover Crops
• Rotation Crops
• Intercropping
• Biological Control
• Insects -- maybe
• Geese, sheep --- often
• Prescribed Fire

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Weed growth in an agricultural crop good or bad
for managing other plant pests (insects,
diseases, nematodes)?

• Weeds may serve as reservoirs for natural
  enemies of insect pests, provide barriers to pest
  movement, etc.
• - - Weeds may serve as alternate hosts or food
  sources for crop pests, increasing threat to crop
  (e.g., weeds in fruit orchards as reservoirs of
  plant viruses, weeds as hosts of root diseases,
  nematodes, foliar fungal diseases, etc.)

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Weed colonization differs from intercropping, in
which plant species in the mixture are chosen or
controlled
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References

• Text, pp. 109-110, 139-147, 180-187.
• Altieri, 1987. Ch. 14.
• Carroll et al. 1990. Ch. 8.
• Radosevich, S., J. Holt, and C. Ghersa. 1997.
  Weed Ecology. John Wiley, New York.