Impact of Weed Management on Weed Populations

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Impact of Weed Management on Weed Populations
Crop Production
Waterhemp is but a current example

• Jeffrey L. Gunsolus Roger L. Becker
• Extension Weed Science
• University of Minnesota

Numerous content contributions by Bob Hartzler at
Iowa State University
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Weed Species Shifts

• A change in composition is
• a change in density
• a change among or within species (biotypes)
• Most changes in composition due to increase in
  density rather than introduction of new species

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Composition of Infestations

• 15 to 30 weedy species in most fields
• A few species dominate infestation
• Illinois study
• 25 species in seed bank
• 4 species accounted for 85 of infestation

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Factors Influencing Weed Shifts

• Management
• Herbicides
• Tillage
• Crop rotation
• Row spacing
• Fertility

• Biological characteristics
• Herbicide tolerance
• Emergence pattern
• Propagule dissemination
• Diversity within population

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Roundup x Velvetleaf

• Naturally infested fields
• Roundup Ready soybeans, 30 rows
• Roundup Ultra applied to 3-4, 5-7 or 10-12
  velvetleaf.
• Plants tagged prior to application and monitored
  throughout growing season

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Selection
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Variation in tolerance within a species
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Hypothetical Development of a Resistant Weed
Population with Repeated Herbicide Applications
0 applications
0.0001
0
10
20
30
40
50
60
70
Resistant Weeds
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Barriers to Managing Weed Species Shifts

• A dollar today is worth more than a dollar
  tomorrow
• An attitude of planned obsolescence
• A Borg mind set Resistance is futile

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My Response to these Barriers

• Does the capital currently exist in the
  agrichemical industry to replace existing
  herbicide programs?
• Weed seed bank and dormancy are the cropping
  systems memory
• We dont know how enough about gene flow and its
  impact on weed species shifts

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Gene Flow

• Maximum gene flow potential exists for
  cross-pollinated plants with traits that are
  nuclear (pollen transfer), homozygous, and
  dominant in expression.
• Limits to corn gene flow
• Pollen movement (less than 660 feet is the
  standard)
• Timing of pollination

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Waterhemp - Taxonomy

• Two species
• Amaranthus rudis Common
• Amaranthus tuberculatus Tall
• Frequent hybridization
• Recommended that only one speciesbe recognized
  (A. tuberculatus)

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Waterhemp Identification

• Key characteristics
• Lack of stem hairs
• Narrow leaves

Waterhemp Redroot
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Dioecious Plant
Female flowers Male flowers
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Weedy Characteristics

• Emergence patterns
• Prolific seed producer
• Seed persistence
• Diversity

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Emergence patterns of giant foxtail and waterhemp
Giant foxtail
Waterhemp
5/17 5/24 6/4 6/10
6/19 6/27 7/5 7/16
8/15 9/30
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Weed Cast 2.0

• Uses daily rainfall and maximum and minimum air
  temperatures to forecast annual weed emergence
  and growth
• Potential teaching and scouting tool
• www.morris.ars.usda.gov

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April 1st Planting Date
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Impact of Prolonged Emergence

• Significant emergence after
• preemergence herbicides degrade
• postemergence herbicides are applied
• How competitive are late-emergers?

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Emergence Timing Studies

• Roundup Ultra applied at V2, V4 and V6
• Waterhemp emerging within 5 days of application
  tagged
• Data height, dry wt and seed production
• Five locations, two years

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Plant survival and height
Average of four locations
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Dry weight and seed production
Average of four locations
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Genetic Potential of WaterhempMean vs
Exceptional Site
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Seed Persistence
of 1994 seed remaining
Buhler and Hartzler. Iowa State University.
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Herbicide Resistant Waterhemp

• ALS Inhibitors
• Cross resistance to all classes
• Triazine
• Multiple resistance

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Waterhemp and Roundup

• Several problem fields identified in Iowa,
  Missouri and Illinois
• Univ. of Missouri study
• 3 populations treated with 1.5 pt Roundup at 2
  height
• Dead Injured No obs. effect
• Pop1 100 - -
• Pop2 86 6 8
• Pop3 73 12 15

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Roundup and Waterhemp

• Relatively high initial tolerance
• Variability in tolerance among biotypes
• Strong environmental influence on Roundup
  activity
• 1 2 Inconsistent performance

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Population Dynamics Model

• Based on seedbank and date of emergence studies
• Assumptions
• 30 inch rows continuous soybean
• 50 annual loss in seedbank
• Plants emerging after post application unaffected
  by herbicide (RR system)
• Does not consider early-season competition

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Population Dynamics Model

• Control levels
• Excellent 100 kill of emerged plants
• Good 95 kill 75 reduction in fitness
• Fail 50 kill 30 reduction in fitness

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Waterhemp Population Dynamics(V6 application
100 control)
Seeds/sq m
Plants/sq m
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Waterhemp Population Dynamics(95 control in
Year 2 and 5)
Seeds/sq m
Plants/sq m
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Risk Efficiency for Soybean Treatments at Waseca
in 1998-2000
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Risk Efficiency for Soybean Treatments at
Lamberton in 1998-2000
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Risk Efficiency for Corn Treatments at Waseca in
1998-2000
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Risk Efficiency for Corn Treatments at Lamberton
in 1998-2000
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Summary

• Most weed shifts involve an increase in density
  rather than introduction of new species
• Rate of shifts increase in management systems
  with little diversity in weed management
• Factors other than herbicides influence shifts
• Catching problems early is easiest means of
  managing weed shifts

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Future Shifts in Herbicide Use Patterns

• Will significant changes in herbicide use
  patterns result in more weed species changes?
• If these weed spectrum changes come to pass, will
  they be due to herbicide resistance or simply a
  shift to species not effectively controlled?
• For the grower, is there a difference?