Weed Science - Exam 2

another exam, another sob session

What are pesticides?

Chemicals that deter, incapacitate, or kill a pest

FIFRA (Federal Insecticide, fungicide, and rodenticide act)

• Developed in 1947 by USDA

• Stated that pesticide must meet proper standards

• Pesticides intended for interstate shipment must be registered with US secretary of Agriculture

The EPA

• created Dec 2 1970

• Enforce environmental legislation, including the pesticide registration improvement act, FIFRA, and the clean water act

What does the label mean to the manufacturer?

• permit to sell

• How they pay for research and development and earn profits

• no money made until the product is registered

What does the label meant to the government?

• method to control distribution, storage, sale, use, and disposal

Trade Name

• Proprietary name of pesticide

• usually trademarked by company

• flashy name to attract customers

Common Name

• simplified version of chemical name

• used worldwide, doesn’t vary by region

Chemical Name

• IUPAC name that gives the proper chemical notation to completely describe the structure of a compound

General Use (unclassified)

• will not cause adverse effects on the environment if used as directed

• Safe for use by public without special training or a permit

Restricted Use

• Most possess proper license

• For use only by certified applicator or under their supervision

• Most states have a specific license, some reciprocity

Caution

• Slightly toxic if consumed, inhaled, or absorbed through skin

Warning

• Moderately toxic if consumed, inhaled, or absorbed through skin

Danger

• Very poisonous or irritant (to skin or eyes); use extreme care

Certified commercial applicator

• using restricted or general use

• employed by a business and receiving compensation

Certified noncommercial application

• Using restricted or general use

• pesticides only used on land owned or rented by the applicator or their employer

Certified public applicator

• using restricted or general use

• pesticide use by anyone employed by the state of missouri or any governmental energy

Active Ingredient

• chemical that will control the plants

• clearly identified on label

Inert/Inactive ingredient

• not a pesticide, but improves the activity of the ai

• shown as % of content

• compounds to make the herbicide soluble or miscible in water

EPA Registration number

• signifies that epa approves the product

• license to sell

Establishment Number

• where the product was made

1A License

Agricultural plant pest control

3 License

Ornamental and turf pest control

6 License

Right of way pest control

Food and Drug act of 1906

• first law addressing pesticides

• arsenic poisoning in uk due to use in protecting apples from pests led to support for this law

Insecticide Act of 1910

• targeted unethical people from selling chemicals that didnt do what they were stated or resulted in injury to people

• responsibility moved from government to manufacturers

Federal food, drug, and cosmetic act (FFDCA) of 1938

• set standards that a product had to contain a list of active ingredients and that these ingredients are in the amount listed

Delaney cancer clause of 1958

• “no substance known to cause cancer in humans or animals shall be deliberately added to or found as a contaminant in food”

• “any amount detected”

FIFRA amendment in 1972

• extended federal regulation to all pesticides

• regulates pesticide sales in US and between US and other countries

• Shifted responsibility of regulating pesticides from USDA to the EPA

Food Quality Protection Act (FQPA) of 1996

• abolished the Delaney clause

• set new standards for reasonable certainty regarding safety of pesticide

• introduced concept of risk cup

• mandated reregistration of pesticides every 15yrs

Risk Cup

• each use of a pesticide contributes a specific amount of exposure to humans

• risk cup is per active ingredient

• all uses are combined and added in a cup

• When the cup is full, no additional uses of the pesticide are permitted

Process for pesticide registration

• Discovery (computer assisted)

• Testing trials (first screening, greenhouse testing, field trials)

• Toxicology work

• Petitioning the EPA

Early Use Permit

• only can be issued by the us epa

• company must apply for it, can be sought if registrant has already provided epa with all registration material and is waiting for approval

• effective for 1yr, epa can renew or extend

Section 18

• allows unregistered use of herbicide for a limited time; herbicide already has federal registration for some crop

• urgent, non routine situation, must be an emergency condition

• Authorized for up to 1yr and then must submit another request

• Ex- growers identify a weed that registered pesticides cannot control

Section 24c

• special local need

• state can register a new pesticide if there is a special local need

• pesticide must have established residue tolerance

• no expiration date unless established by state

Herbicide

synthetic or natural compound designed to control unwanted vegetation

Advantages of chemical weed control

• biological selectivity

• herbicides control weeds where other techniques may fail

• permitted removal of weeds where previously not possible

• herbicides reduce dependence on tillage

• herbicide use is more effective, efficient, and consistent

• controls annuals, biennials, and perrenials

Disadvantages of herbicides

• off target injury

• spray tank contamination

• movement into the environment

• persistence

• disposal of containers and residues

• crop injury following application

Broadcast Application

• treat the entire area

Types of broadcast applications

• ground applicators

• aerial applications

• herbigation

Banding

• refers to a specific area that is treated, not the entire field

• ex- spraying pesticide underneath the trees in an orchard and not in the rows since they aren’t a concern

Directed Spray

• limited area that you are treating, similar to banding, but designed for the applicaiton without the pesticide touching the crop

• precise application of herbicide in targeted area

Spot Spray

• intended for post emergence

• herbicide targets only specific weeds in patches or isolated areas

Invasive Species

• non indigenous species or strains that become established in natural plant communities and wild areas and replace native vegetation

• likely to cause economic or environmental harm or harm to human health

Noxious Species

• a weed designated by state or national authorities as a plant that is injurious to agricultural and/or horticultural crops and/or humans and livestock

Novel Weapons Hypothesis

• production of chemicals/allelochemicals is more successful in the new environment than in the native environment

• species in native environment have adapted to the plant

• in the new environment, species have not adapted to the invasive plant and are more susceptible to the allelochemicals

Enemy release hypothesis

-upon introduction to the new area, healthiest plants are introduced and natural predators are left behind

Better competitiors hypothesis

• in habitats, native plants occupy the space and resources

• for invasives to take over, they must be more successful at competing for the resources than native species

Class Theory

• most invasives start at low densities over a broader area and reach densities that are too high to eradicate by the time we notice them

When submitted to the epa, what three criteria must be satisfied?

1. Pesticide composition and activity must be validated

2. Proposed label conforms to EPA standards

3. no adverse affects on non target organisms AND the environment

Integrated weed management

• practice that employs multiple techniques to control weeds

• purpose to keep weeds off balance and prevent selection for tolerant or resistant to any one technique

• uses physical, biological, chemical, cultural

Soil Uptake of Herbicides

• seed imbibition of water

• roots

• hypocotyl and coleoptile

Foliar uptake

• leaves and stems

• leaf cuticle is waxy and repels water

• uptake into plants via leaves is a challenge

Lipophilic

• lipid loving

• not water soluble

Lipophobic

• fears lipids

• water soluble

Hydrophilic

• water loving

• water soluble

Hydrophobic

• fears water

• lipid soluble

Casparian Strip

• around the endodermis

• waxy, repels water

Cuticle

• outermost layer of leaf, waxy and repels water

• surfactants play a major role to improve herbicide uptake in leaves

• if the volume of the droplet of herbicide can be spread out, more sites for places of contact for the herbicide to be driven into plants

• major problem for herbicides is going from hydrophobic surface to hydrophilic medium

Hairs on leaf surface

• can prevent some droplets from reaching the cuticle

Apoplast

• transportation through the non living parts of the plant

• includes the xylem, cell wall, and intercellular space

• does not require energy (passive)

• moves around the outside of cells

Symplast

• transportation through the living parts of the plant

• includes the plasma membrane to the inside of the cell

• requires energy (active)

• most important pathway when taking things up from the foliage

Driving force for uptake of herbicides FROM THE ROOTS

• evapotranspiration of water

Driving force for uptake of herbicides THROUGH THE LEAVES

• Photosynthesis

• It takes energy to move things through plants

Mechanism of action

• the specific way the herbicide interferes with a plant

• one step

Mode of action

• sequence of events following the initial application of an herbicide until the effect is observed

• start to finish

Pathways herbicides target

• photosynthesis

• pigments

• lipid synthesis

• hormone synthesis

• cell division

• amino acids

Chloroplast

• most important target organelle for herbicides

• photosynthesis takes place in the inner membrane of the chloroplast (in the granum)

Membrane

• important to maintain cell organelles integrity

• composed of lipids

• located around cell membrane, vacuole, nucleus, chloroplast

Chemical Herbicide Processes

• can change the parent molecule structure, not always, depends

• includes adsorption, absorption, and desorption

Adsorption

• does NOT change the parent molecule structure

• herbicide ions stick to the surface of soil particles

Kd

• soil sorption coefficient

• ratio of herbicide bound to the soil vs free in soil solution

• higher number means more adsorbed to soil than free in solution

Absorption

• passage of an herbicide through the surface of a particle

• absorbed INTO the particle, not the surface

Desorption

• the release of absorbed or adsorbed herbicides

• makes them available for uptake from the soil

• rain is important for desorption

Why is rain important for desorption?

• water is a polar molecule and competes with herbicide for adsorption to the soil

• this frees the herbicide from the soil and allows for weeds to take herbicide up

Cation Exchange Capacity

• total amount of cations a soil can retain

• increases with increase in clay or organic matter

• adsorption of herbicides increases as CEC increases

Photodegradation

• effect of radiation on internal chemical bonds

• uv light is strong enough to break herbicide bonds

Physical Soil Processes

• CANNOT change the parent molecule structure

• includes leaching and volatility

Leaching

• movement of herbicide with water

• less likely to occur with high koc

• influenced by soil texture, permeability, water volume, adsorption, absorption, solubility of herbicide in water, and soil pH

Volatility

• when the herbicide goes from a solid or liquid state to a gas

• do not undergo changes to the parent molecule, just physical location

• can result in loss of herbicide

• more likely in warm, moist soils

Koc

• the kd divided by the weight fraction of the organic carbon in the soil

• higher value = less mobile herbicides

• lower value = more mobile herbicides

Microbial Herbicide Processes

• ALWAYS changes the parent molecule structure

• the SINGLE MOST IMPORTANT means of herbicide degradation

• result of some active microbes that break down herbicides

• happens because herbicides are a carbon source for microorganisms

Photosynthesis Equation

Light + CO2 + H2O → C6H12O6 + O2

Z Scheme Steps

1. Transfer light energy absorbed by pigments

2. Chlorophyll molecule at P680 becomes excited

3. Electron transfer from p680 to Chlorophyll p700

4. Chlorophyll molecule at P700 becomes excited

5. NADPH is made

What happens when we block photosynthesis with herbicides?

• Excited chlorophyll cannot pass the energy on, so it gives it to Oxygen in the thylakoid membrane

• The Oxygen becomes an oxygen radical

Oxygen Radicals

• Created when Chlorophyll cannot pass energy on due to photosynthesis block

• Oxygen radical is unstable and wants to give its energy away

• Oxygen reacts with lipids in the membrane and breaks carbon bonds, creating holes int the membrane

• This leads to the granum leaking into the chloroplast. The oxygen radicals are still present, and tear holes in the chloroplast

• Eventually, they tear holes in the cell wall, leading to the cell dying

QB - Quinone

• Protein that wants to react and accept an electron from QA in Photosystem II

• Herbicide binds to QB and prevents the electrons from passing through photosynthesis

What herbicide families are photosynthetic inhibitors that bind to QB?

• Triazines

• Ureas

• Uracils

Result of photosynthetic inhibitor herbicides

• loss of cell function, including chlorophyl death

• Interveinal chlorosis of leaves, which eventually turns into necrosis (browning) of the leaf

• Eventually, leads to plant death

Mode of action for Triazines, Ureas, and Uracils

• Uptake and translocation to chloroplast (thylakoid membrane)

• Inhibition of electron transfer

• Light energy to chlorophyll generates reactive oxygen species

• Oxygen radicals initiate autocatalytic lipid peroxidation

• cells die

• plant dies

Preplant

• Prior to crop planting

• surface or incorporated

Pre emergence

• after planting but prior to crop and weed emergence

• allows the crop to be established

• crop can be emerged or not, but the weeds are NOT emerged

Post emergence

• post emergence of the weeds, can be small or big

Burndown

• type of post emergence application

• early in the season

• getting rid of all the existing vegetation before the crop

Brownout

• herbicide is applied during the season and kills plants where it was sprayed

• other vegetation is fine and still green

• looks bad in public eye

Composition of an herbicide

• active ingredient

• inactive/inert ingredient

• additives

Additives

• materials designed to help herbicide:

  • not react with water

  • penetrate the cuticle

  • not move off target

  • spread evenly over the waxy leaf

  • form an emulsion

  • not freeze

• add desirable traits to herbicide

Triazine Representative herbicide

Atrazine

Ureas representative herbicide

Diuron

Uracil representative herbicide

terbacil