Weeds – Why Control Them and How?

Hort 301

November 26, 2007

What is a weed?

• A plant that forms populations that are able to enter habitats cultivated, markedly disturbed or occupied by man, and potentially depress or displace the resident plant populations which are deliberately cultivated or are of ecological and/or aesthetic interest

Navis, 1991

Weeds• Well adapted to environmental

disturbances

• Thrive under conditions generated by agriculture field practice

• Produce large numbers of seeds– Pigweed 100,000 seed/plant

– Lambsquarters 70,000 seed/plant

– Barnyardgrass 7,000 seed/plant

– Purslane 50,000 seed/plant

Why Control Weeds?• Lower yields and reduced

income• Less efficient land use• Decreased production efficiency• Higher costs of insect and

disease control• Poorer quality products• Water management problems• Lower human efficiency

Cost of Weeds

• Common on all 485 million acres of US cropland– Reduce yields by 12% annually ( ~ $36 billion loss) –

$4.1 billion loss in return– w/o herbicides loss > $20 billion– $4 billion spent on herbicides– $3 billion on other methods

• Other concerns– Invasive species, waterways, pastures, range lands,

lawns, gardens, golf courses

Types of Weed Control

• Prevention– Stopping a weed from contaminating a given area

• Control– Limit weed infestation by using some removal

technique

• Eradication– Complete elimination of all live plant parts and

seeds from an area

Methods Used to Manage Weeds

• Weed identification (taxonomic/picture keys) and life-cycle knowledge– Annuals, biennials, perennials

• Cultural and Mechanical Control• Biological • Chemical

– Herbicides• Genetic – GMO’s• Crops• weeds

Mulching – Physical and/or chemical

Plastic Mulch

Cover crop mulch – physical and allelopathic

•What is the easiest and most consistent way to manage weeds?

Herbicides• Herbicide action

- Nonselective - kills all plants - Selective - kills some plants

- Soil applied – no translocation or translocated - Foliar applied - contact or systemic

Genetic Approaches

• GMO’s– Herbicide resistant crops

• Roundup Ready - corn, soybeans, canola, cotton, sugar beets, wheat

• Liberty Link - corn, canola, sugar beet

• Poast - corn

• STS - soybeans

• BXN – cotton

• Clearfield – corn, wheat

• No horticulture crops released yet – – lettuce, mint, tomato, potato

Roundup Ready Corn

GMO Crops

• Corn, Soybeans, Cotton, Canola

• 10 million farmers in 22 countries planted over 100 million hectares with GMO crops in 2006

• 80% of GMO crops planted have the a herbicide resistance gene

• US – 95% of soybeans, 90% cotton, 60% corn acres are Roundup Ready

What are the values of Herbicides?

• Practical – make weed control much easier• Scientific

– Great tools to study physiological processes• Photosynthesis• Amino Acid biosynthesis• Pigment formation• Hormone action – auxins• Lipid biosynthesis• Cell division• Cell wall synthesis• Uptake, translocation, metabolism

What is Herbicide Mode of Action?

The sequence of events from herbicide absorption into the plant until the plant dies

Herbicide Mode of Action

• Herbicide action is related to:– Adequate contact with the weed– Absorbed by the weed– Movement within the plant to site of action

with no deactivation– Toxic levels reach the site of action– Plant injury and death

Regions of Herbicide Absorption

Foliar Deposition

Dissipation

runoffwashoffvolatilizationphotolysis

Abiotic sequestrationamorphous depositioncrystallization

Cuticularpenetration

Apoplastic penetration - cell wall- xylem

Symplastic penetration- parenchyma- phloem

Herbicides applied to plants and fate

Root uptake

Site of Action

For the herbicide to work…

The herbicide must be delivered to the site of action at a concentration that is sufficient to cause death to the pest.

• Volatalized or washed off

• Remain on outer surface

• Remain associated with the cuticle

• Enter and move apoplastically

• Enter and move symplastically

Foliar ApplicationThe five fates of retained

herbicide

Leaf Surface and Cuticle

Very hydrophilic

cell wall

Very hydrophilic

cell wall

Hydrophilic pectin

strands

Hydrophilic pectin

strands

Lipophilic cutin matrixLipophilic

cutin matrix

Very lipophilic wax layer

Very lipophilic wax layer

Herbicides must cross ALL of these chemically varied layers to reach the

interior of the plant

Wettability - Smooth• Smooth wax layer -

devoid of crystals• Relatively easy to

wet

PigweedPigweed

• Heavily crystalline wax layer

• VERY difficult to wet

Wettability - Crystalline

LambsquartersLambsquarters

• Root uptake– No cuticular barrier

– Must cross the Casparian strip

• Shoot uptake– Very little cuticular barrier in seedlings

Soil Application and Absorption

Translocation routes….• Following root uptake

– Xylem

– Then phloem

• Following shoot/foliar uptake– Not translocated - contact

– Localized transport • Primarily xylem

– Systemic transport• Primarily phloem

For the herbicide to work…

The herbicide must be delivered to the site of action at a concentration that is sufficient to cause death to the pest.

• All biochemical targets of herbicides are inside the cell membrane

• All herbicides MUST cross at least one membrane to reach their target

Crossing the membrane

Categories of Herbicide Mechanisms of Action

• Photosynthesis inhibitors • Pigment inhibitors• Cell membrane disrupters • Amino acid inhibitors • Lipid biosynthesis inhibitors• Cell growth inhibitors – cell division • Cellulose inhibitors – cell walls • Growth regulators – abnormal hormone action• Auxin transport – disrupt auxin transport• Miscellaneous - Asulox, Prograss, DSMA, MSMA

All underlined herbicide groups ultimately result in the same method of plant death

Photosystem II Inhibitors as an example

Plant Response and Mechanism of Action

Symptoms on the plant develop slowly over several days

Leaves develop chlorosis (yellowing) followed by necrosis (tissue death)

Chlorosis due to chlorophyll destruction (photooxidation)

Necrosis due to membrane disruption from lipid peroxidation

Inhibit electron transport in Photosystem II

Atrazine injury on soybean from root uptake

Photosynthesis overview

6 CO2 + 6 H2O C6H12O6 + 6 O2

Light

• overall this is a highly unfavorable reaction• it must be carried out in many, small steps

Site of Inhibition in PS II by Photosynthesis Inhibiting Herbicides

Results in a blockage of electron transport to PQ

What Happens With all this Unused Energy ?

High energy free radicals result in cellmembrane disruption

• Other herbicide groups that result in the production of free radicals as part of their biochemical mechanism of action

Herbicides that inhibit PSIDiquat & Paraquat

Examples of Postemergence Foliar Applied Contact Herbicides

Membrane Disrupters - Paraquat

PPO Inhibitors• Diphenylethers - acifluorfen,

fomasafen, lactofen, oxyfluorfen,

• Oxidiazoles - oxidiazon, fluthiacet

• N-phenylheterocycles - carfentrazone, flumiclorac, sulfentrazone, flumioxazin, fluthiacet

Typical cupped leaf injury from diphenylether herbicide application to soybean

Pigment Inhibitors - Symptomology

• Gradual whitening (bleaching) of susceptible plants - loss of pigments

• Cessation of growth, and

• Tissue necrosis followed by death.

Glufosinate Activity

• Non-selective - Contact-like active

• Foliar applied at 1 to 1.5 lb/A

• Controls many annual and some perennial weeds

• Plant necrosis occurs in 1 - 5 days

• No activity by root uptake

• Efficacy is dependent on Uptake and Transport (Grasses > Broadleaves)

Free Radicals Lead to Lipid Peroxidation and Membrane Disruption

Take Home Points

• What is a weed?

• Why worry about them?

• Methods of management

• Herbicides most common

• How do herbicides work?

Take Home Points

Fatal Effect of Many Herbicides on plants

1. Many result in production of toxic free radicals

2. Destroy of plant cell membranes

3. Cells lose integrity

4. Cells become leaky

5. Plant dies