What is Weed Science?
The discipline that investigates the
biology and ecology of weeds and
how best to manage these plant
species for the betterment of
mankind.
Weed Science is Multi-Disciplinary
Includes other fields, such as:
plant systematics engineering
plant ecology toxicology
plant anatomy soil chemistry
plant physiology soil fertility
chemistry soil physics
biochemistry entomology
molecular biology plant pathology
genetics
What is a Weed?
Simplistic definitions:
• A plant out of place
• A plant growing where
it is not wanted
What is a Weed?
Simplistic definitions:
• A plant out of place
• A plant growing where
it is not wanted
Characteristics of Weedy Plants
1. Able to colonize disturbed environments; rapid population
establishment
2. High reproductive capacity (sometimes multiple methods)
3. Short time to reproduction
4. Germination and reproduction over a range of environmental
conditions
5. Seed dormancy; discontinuous germination, seed longevity
6. Presence of vegetative reproductive structures
7. Adaptations for spread
8. Ability to compete well with crops (see comments in notes)
What is a Weed?
• Weed Science Society of
America definition:
any plant that is objectionable
or interferes with the activities
or welfare of man.
What is a Weed?
• Plants that are competitive,
persistent, and pernicious,
and are undesirable because
they interfere with human
activities
How Do Weeds Reduce Yield?
I. Competition
• Finite amount of resources.
• Competition occurs when combined resource
demands of crop and weeds exceed available
supply.
• Resources used by weeds are unavailable
to crop.
How Do Weeds Reduce Yield?
I. Competition
• Competition occurs when combined resource demands of
crop and weeds exceed available supply.
• Resources used by weeds are unavailable to crop.
• A few weeds do not cause measurable yield loss.
• As weed density increases, amount of resources used by
weeds increase.
• Reach a point where resources become limiting to crop;
crop is adversely affected.
Utilizing crop competition
as a weed management tool
Any practice that leads to more a uniform stand of
healthy crop plants that grow off rapidly (get ahead
of weeds) will aid weed suppression.
• Narrow rows
• Denser crop stands
• More equidistant crop spacing
• Good fertility, pH, nematode control, seedling
disease control
• Variety selection
Effect of soybean row spacing on weed resurgence.
Yelverton and Coble, 1991.
All plots received early POST application to control all weeds.
Percent photosynthetically active radiation (PAR)
intercepted by soybean. Yelverton and Coble, 1991.
All plots received early POST application to control all weeds.
How Do Weeds Reduce Yield?
II. Allelopathy
• Release of an organic chemical by one
plant that adversely affects the growth
and development of surrounding plants
• Root exudates or leaf leachates from
living weeds
• Release of volatile compounds
• Compounds released from dead weeds as they decay in the soil
Allelopathy may be useful
in weed management systems
1. Allelochemicals in cover crops
2. Allelopathic potential of crops may be
genetically controlled. Breed/select
for cultivars with greater allelopathic
potential.
Extent of interference depends upon:
1. Crop species
2. Weed species
3. Length of time crop exposed to
interference
Comparison of Actual and Percent Yield Loss and Economic Loss for
Cotton, Corn, Peanut, and Soybean under Similar Weed Pressure
Weeds: 2 cocklebur, 2 signalgrass, 3 lambsquarters, 1 pigweed
Crop Actual loss Percent loss
Corn
150 bu/acre @ $2.85/bu
9.7 bu/acre 6.5%
Soybean
35 bu/acre @ $6.25/bu
7.0 bu/acre 20.1%
Cotton
750 lb L/acre @ $0.62/lb L
247 lb L/acre 32.9%
Peanut
3500 lb/acre @ $355/ton
1503 lb/acre 42.9%
Example of effect of weed species on season-long weed
interference, NC data:
Crop Weed Weed density % yield loss
Peanuts Broadleaf 1 per 16 ft of row 7
signalgrass 1 per 2 ft of row 28
Peanuts Fall 1 per 16 ft of row 25
panicum 1 per 2 ft of row 65
Crops do not need to be weed free all season
to avoid yield losses
• If good weed control is obtained early in the
season, crops can tolerate weeds emerging at
some point later in the season.
Hypothetical yield of crop as affected
by length of early season weed control
(Critical weed-free requirement)
Yield loss No yield loss
If the crop is kept weed
free for more than 6 weeks
after crop emergence, crop
yield will not be reduced
Crops do not need to be weed free all season
to avoid yield losses
• If good weed control is obtained early in the
season, crops can tolerate weeds emerging at
some point later in the season.
• Crops can tolerate weeds for a short period early in
the season, assuming they are effectively controlled
thereafter.
Hypothetical yield of crop as affected by
length of early season weed interference
(critical time of removal)
No yield loss Yield loss
If you allow the weeds
to compete with the
crop for no more than
3 weeks, crop yield
will not be reduced.
Crops do not need to be weed free all season
to avoid yield losses
• Crops can tolerate weeds for a short period early
in the season, assuming they are effectively
controlled thereafter.
• If good weed control is obtained early in the
season, crops can tolerate weeds emerging at
some point later in the season.
There is a critical period of interference, or a time
when weeds must be controlled. When is it?
Hypothetical example of critical period
of weed control.
Criticalperiod
Critical weed-free requirement
Critical time of removal
If the crop is kept weed free for
more than 6 weeks after crop
emergence, crop yield will not
be reduced
If you allow the weeds to
compete with the crop for
no more than 3 weeks, crop
yield will not be reduced
Weeds may reduce
harvesting efficiency
1. Delayed start of harvest
2. Slower harvest speed
3. Increased harvesting losses
4. Wear and tear on equipment
Effect of Palmer amaranth on cotton stripper harvesting efficiency.
Smith et al., 2000.
Machine Time to Total % total
Weed Lint time to remove harvest trash due
density yield strip stoppage time to weeds
(no./A) (lb/A) (min/A) (min/A) (min/A) (%)
0 446 32 0 32 0
260 455 33 29 62 13
525 429 35 51 86 11
1320 348 36 74 110 15
Losses Caused by Weeds
I. Direct losses in crops
A. Yield loss
B. Reduced Harvesting
Efficiency
C. Reduced Quality of
Harvested Crop
Weeds may reduce
quality of harvested crop
1. Foreign matter dockage
2. Off-flavors in products made from crop
3. Crop rejected due to toxic weeds
4. Increased moisture content
5. Loss of quality due to delayed harvest
Losses Caused by Weeds
II. Indirect losses in crops
A. Increased Crop Production Costs
1. Cost of herbicides & application
costs (equipment and labor)
2. Other control costs, such as
cultivation & additional land prep
3. Management time
Losses Caused by Weeds
II. Indirect losses in crops
B. Damage to crop
1. Herbicide damage to target crop
2. Herbicide damage to other crops
due to drift or carryover
3. Mechanical damage
4. Moisture loss from cultivation
5. Enhance other pest problems
Losses Caused by Weeds
II. Indirect losses in crops
C. Limited rotational choices
1. Weeds can determine which crops
can and cannot be planted due to
necessary management strategies.
2. Herbicide program necessary to
control weeds in one crop may
dictate crops which can and cannot
be planted next year.
Losses Caused by Weeds
II. Indirect losses in crops
D. Weeds may be alternate hosts for
other pests
Horsenettle -- TMV
Johnsongrass -- MCDV
Sicklepod -- SCN
III. Other types of losses by weeds
A. Health hazards to humans,
livestock
1. Poisons
a. Dermal
b. Internal
2. Allergins
III. Other types of losses by weeds
B. Sanitation concerns, such as
cover and breeding sites for
rodents and snakes
III. Other types of losses by weeds
D. Problems in water use
1. Impede flow in drainage and
irrigation canals
2. Utilize water intended for irrigation
3. Impede river navigation
4. Interfere with recreational uses of
water and aesthetic value of water
5. Fish kills
III. Other types of losses by weeds
E. Aesthetics
Lawns, ornamentals,
golf courses
F. Utility of recreational turf
Other Types of Losses
Caused by Weeds
G. Maintenance of non-crop areas
roadsides, utility rights-of-way,
electrical substations, farmsteads
Noxious Weeds
• Federal Noxious Weed Act of 1974
• Defined a noxious weed as “any living stage of a parasitic
or other plant of a kind which is of foreign origin, is new to
or not widely prevalent in the U.S., and can directly or
indirectly injure crops, other useful plants, livestock, poultry
or other interests of agriculture, including irrigations,
navigation, fish and wildlife resources, or the public health.”
• Act gave Sec. of Ag. authority to designated plants as
noxious weeds and to enforce associated regulations.
• Federal noxious weed list presented in your notes.
Invasive Weeds
• An invasive weed is one which is introduced,
by human activities, into an area where it did
not previously occur naturally, becomes
capable of establishing a breeding population
in the new location without further human
intervention, and spreads rapidly throughout
the new area.
Invasive Weeds• Spread rapidly, dominate the habitat
• Cause adverse environmental impacts
– Destroy wildlife habitat
– Displace threatened or endangered species
– Reduce plant or animal diversity
– Disrupt waterfowl flight patterns, nesting sites
– Interfere with agricultural production
– Natural disasters (such as fires)
Invasive Weeds• Spread rapidly, dominate the habitat
• Cause adverse environmental impacts
• Economic impacts
– Reduce crop, pasture, forestry production
– Reduce property values
– Reduce recreation value of land (or water)
– Interfere with irrigation
– Treatment costs
Examples of Invasive Weeds
Johnsongrass
Japanese stiltgrass
Kudzu
Multiflora rose
Hydrilla
Water hyacinth
Tropical spiderwort