Weed Life Cycles

1
Weed Life Cycles

• Most weeds in annual cropping systems are
  annuals, they complete a life cycle by
  germinating from a seed and produce seed in one
  year.
• Summer annuals - germinate in spring and produce
  seed in late summer and fall.
• Winter annuals - germinate in the fall and
  produce seed in late spring or early summer.

2
Weed Life Cycles

• A few weeds are biennials, they complete a life
  cycle in less than two years. They must start
  from seed every two years.
• In the first year the plants form a rosette of
  leaves. This is the over-wintering stage.
• The next spring the plants produce a stem with
  flowers and seeds (Bolting).
• Many pasture weeds are biennials (e.g. bull
  thistle,wild carrot).

3
Weed Life Cycles

• Some weeds are perennials, they can often live
  for more than two years and can reproduce from
  seed or via vegetative structures.
• Common vegetative structures include
• Rhizomes - an underground creeping stem.
• Tuber - a swollen underground storage stem.
• Rootstock - an over-wintering root.
• Stolon - a creeping stem that roots at the nodes.

4
Perennial Weeds - the challenge

• Very difficult to control
• Repeated treatment is necessary
• whatever it is
• Survivors
• regrowth
• Carbohydrate depletion

5
Seasonal Carbohydrate Levels In Hemp Dogbane Root
Crowns
Dry Weight
Early bud
Mow, Till
Herbicide
Herbicide
Mow, Till
Early bud
Mid flower
Mid flower
Control window
Control window
Month Sampled
6
Cultivation to Eradicate C.Thistle, ID.
8
9
11
6
Days to eradicate
6
7
18
9
4
Tillage interval (Days)
Plow 6 deep when 5 tall, then duckfoot cult.
5 deep Indicates total no. of tillage events
needed to eradicate
Seely, C.I. 1952 Univ. Idaho Exp. Stat. Bull. 288
7
The Seed Cycle
Mature plants
Seed rain
Emergence
Other sources (wind, machinery, manure, etc.)
Seedbank
Decay and predation
Other losses (Machinery, erosion, etc.)
8
Annuals - the dilemma

• Prominent in disturbed soil
• Colonizers (voted most likely to succeed)
• Seedbanks (long-term memory)

9
Annual Weeds

• Weed seed dormancy is a key factor in weed
  management.
• Dormancy means that you must manage weeds, not
  eliminate them.

10
Annual Weeds

• Seed viability in soil
• Weed-seed banks contain multiple weed species.
• Weed species differ in their longevity (broadleaf
  gt grass).
• Weed seed buried deeper in the soil stays viable
  longer.
• Majority of seeds are lost within 2-3 years.
• Recovery of seed bank requires only 1 year of
  poor weed control.

11
Impact of seedbank on weed management

• Level of inputs required to control weeds
  directly related to seed bank size
• Seed production by weeds needs be considered in
  management decisions
• What is an acceptable level of seed production?

12
Weed Management is Affected by Weed Density

• Assume a weed management tactic controls 99 of
  the weeds present (not bad).
• If initial weed density was 5 plants/ft2 then
  only 0.05 plants/ft2 would survive.
• If initial weed density was 1,200 plants/ft2 then
  12 plants/ft2 would survive.
• Why didnt my herbicide work!!!!

13
Annual Weeds

• Seed size influences depth of emergence. This
  can influence
• Effectiveness of mechanical weed control.
• Effectiveness of soil-applied herbicides.
• Time of weed emergence.

14
Weed Management- the basics

• No free lunch concept
• Weed management options
• labor
• herbicides
• tillage
• cultural practices

15
Question?Can Weed Science Knowledge and
Producer Wisdom Reduce the Risk in Weed
Management?
16
Producers Perception of Risk

• Desire minimal variability of crop yield and net
  returns across the field and over growing
  seasons.
• Herbicides often meet these desires.

17
Producers Perception of Risk

• Producers focus for weed management is often
  within a 1 to 2 year time frame.
• Producers have time and labor management
  limitations.
• Therefore IWM is perceived as risky and difficult
  to implement.

18
Weed Science Perception of Risk

• Biological time constraints relevant to weed
  management
• Crop planting date
• Weed emergence periods
• Rate of weed growth
• Critical period of weed control

19
Crop Planting Date

• Sets the stage for future weed/crop interactions
• Rate of weed/crop emergence and growth and
  development will influence the outcome.

20
Annual Weeds

• Weed emergence and growth is often rapid.
• This gives weeds a competitive edge over some
  crops.
• This is why existing weeds must be controlled at
  the time of crop planting.
• The primary goal of weed management is to shift
  the competitive edge towards the crop.

21
Annual Weeds

• Weed seed germination is influenced by
• Loss of seed dormancy.
• Soil temperature.
• Soil moisture.
• Varies from year to year, however, the order of
  weed species emergence is fairly predictable.

22
Weed Emergence Periods

• Most important with non-residual
  postemergence herbicides
• Order and duration of weed emergence will
  influence time(s) of postemergence application

23
Weed Emergence Patterns. 1997.
Post Herbicide
Planting
Gi.foxtail W. cupgrass F. panicum Velvetleaf G
. ragweed Kochia Morningglory Waterhemp
What are the costs of these weeds?
1 2 3 4 1 2 3 4 1
2 3 4 1 2 3 4
1 2
April May June July Aug
Sandell, Hartzler and Buhler. Iowa State
University.
24
Weed Science Perception of Risk

• Weed emergence periods
• Diversity of weed species and emergence patterns
  will influence the time of herbicide application
• Weed emergence sequences often make two-pass weed
  control practices necessary

25
Weed seedling emergence as a function of tillage
date Ames, IA
Average seedling
counts / ft2 Mean
Tillage Date Colq Gift Pesw Rrpw Vele Yr 1 Yr 2
Avg
May- week 1 1.26 158.66 0.69 7.24 2.22 45.20 13.89
29.55 May- week 2 0.49 134.82 0.16 17.39 1.06 39
.10 14.98 27.04 May- week 3 0.48 122.04 0.06 6.46
0.78 27.15 19.51 23.33 May- week 4
0.49 69.05 0.18 4.11 0.79 8.20 20.25 14.23 June-
week 1 0.24 83.75 0.19 13.93 0.89 23.55 11.44 17.5
0 June- week 2 2.13 64.36 0.06 2.09 0.73 14.45 10.
44 12.45 Y 0.85 105.43 0.23 8.54 1.08
_
26
Weed Science Perception of Risk

• Rate of weed growth is dependent upon
  environmental conditions
• A typical foxtail growth rate in Minnesota is
• 4 weeks to reach 4 inches
• 5 weeks to reach 5 inches
• 6 weeks to reach 8 inches

27
Rate of Weed Growth

• Do not let slow early-season weed growth lull
  you into a false sense of security
• The critical period of weed control illustrates
  the consequences of untimely weed control

28
Weed Science Perception of Risk

• Critical period of weed control
• Early season competition - No interference until
• 2-5 WAE in corn
• 4-6 WAE in soybean
• Late emerging weeds - No interference if weeds
  emerge 4-5 WAE

29
(No Transcript)
30
Influence of critical periods on window of
opportunities for one-pass postemergence weed
control.
Max
Late emerging weeds
Yield loss
Early season competition
A
A
0
B
0
B
0
C
1 2 3 4 5
6 7 8 9
Application Timing (Weeks after emergence)
Window of opportunity to prevent yield losses A.
4 weeks -(low weed populations) B. 1 week -
(mod. to high populations) C. No opportunity -
(mod. to high pops.)
Slide courtesy of Bob Hartzler Iowa State
University
31
Weed Science Perception of Risk

• A key communication point between weed science
  and the producer is how to align time and labor
  management issues with site-specific biological
  time constraints
• This is where wisdom is important

32
Challenges to the Adoption of IWM as a Risk
Management Strategy

• Short-term economics often predominates over
  biology
• Often difficult to explain the connection of
  biological interactions to time and labor
  management decisions

33
Limitations of Weed Science Wisdom to
Site-Specific Management

• Time cost to gather and process weed biology
  information
• Does it out weigh the value of the information?
• Weather variability will always limit our ability
  to carry out IWM strategies
• Look for patterns of consistent IWM practices
  within a defined region

34
How Does the Weed Spectrum Change?

• Development of herbicide resistant weed biotypes
• problem continues to expand across Midwest
• Shifts resulting from repeated use of a
  particular herbicide family
• shifts to species more difficult to control
• Introduction of new, previously obscure species
• species not prevalent in previous years
• Shifts in response to tillage, cropping changes
• no-till weed problems

35
Selection
36
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
37
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, whats the difference?