Section 1 Basic Impacts of Weeds - Crop and Soil … 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,

Section 1

Basic Impacts of Weeds

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?

What is a Weed?

Simplistic definitions:

• A plant out of place

• A plant growing where

it is not wanted

What is a Weed?

People define weeds.

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

Weeds are undesirable

because they cause

some type of loss

or problem

Losses Caused by Weeds

I. Direct losses

II. Indirect losses


I. Direct losses in crops

A. Yield loss

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.

Weeds compete with crops for:

1. Light

2. Water

3. Nutrients

4. Essential gases????


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.

Yield loss as affected by weed density.

Weed density/unit area

Weed density/unit area

Crop competition with weeds

can be an important

component of a weed

management program.

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.


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

Effect of water extracts of johnsongrass rhizomes on

soybean growth. Lolas, 1979.

Allelopathy may be useful

in weed management systems

1. Allelochemicals in cover crops

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.

Weed Interference:

collective effects of competition

and allelopathy

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 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 can tolerate weeds for a short period early

in the season, assuming they are effectively

controlled thereafter.




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



A. Yield loss

B. Reduced Harvesting

Efficiency

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



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


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



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



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.



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


B. Sanitation concerns, such as

cover and breeding sites for

rodents and snakes


C. Safety issues

Example: obstruction of

traffic signs


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


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


H. Reduced property value

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

Questions?

Documents

Section 1 Basic Impacts of Weeds - Crop and Soil … 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,