Weed Science Exam 2

What does herbicide behavior affect?

success or failure of weed control

presence or absence of crop injury

persistence of the herbicide (length of control; potential for carryover)

environmental impact

Soil impact on POST herbicides

Soil will have no effect on efficacy of herbicides with foliar activity only. Example would be glyphosate.

Soil can, however, have an effect on efficacy of POST herbicides which also have soil activity. Example would be atrazine.

How can one determine if the soil impacts activity of a POST herbicide?

You test soil and foliar contact, foliar contact only, and soil contact only, which pot has the best control is the one that is more important

How does soil behavior affect the rain-free requirement?

The herbicides that require more foliar contact need a longer rain free period than herbicides that are more soil active

What does environmental fate of pesticides mean?

Where it goes, how it affects the environment, how long it hangs around

Environmental fate - degradation processes

Photochemical decomposition

microbial decomposition

chemical decomposition

Environmental fate - transport processes

volatilization

runoff and erosion

absorbed by clay and organic matter

leaching

plant uptake

herbicide adsorption definition

Binding of herbicide to soil colloids (clay and organic matter fraction of soil)

Determines herbicide availability to plants

Also impacts leaching, volatilization, and microbial degradation

factors affecting herbicide adsorption

1. Organic (humic) matter content

2. Clay content

3. Soil moisture

4. Chemical properties of herbicide

5. Soil pH (affects some herbicides, not others)

How organic (humid) matter affects hericide adsorption

greater OM = greater adsorption

Negative surfaces (ionic binding) and organophillic surfaces (non-ionic binding)

Humic matter definition

Humic matter is the highly degraded organic fraction of soil; humic matter fraction is what binds most of the herbicide

Humic matter and organic matter correlation

Humic matter and organic matter contents highly correlated; absolute values differ

As OM increases HM increases

No practical way for grower to convert HM to OM

cation exchange capacity

The capacity of the soil to hold on to these cations is called the cation exchange capacity (CEC). These cations are held by the negatively charged clay and organic matter particles in the soil through electrostatic forces (negative soil particles attract the positive cations). The cations on the CEC of the soil particles are easily exchangeable with other cations and as a result, they are plant available. Thus, the CEC of a soil represents the total amount of exchangeable cations that the soil can adsorb.

How clay content affects herbicide adsorption

more clay = more adsorption

clay is negatively charged, ionic binding with positively charged herbicides

What’s the difference between 2:1 and 1:1 clays?

1:1 clays are highly weathered, non expanding, have 1 Al to Si, more of these in NC, LESS BINDING

2:1 clays are expanding, 2 Si for 1 Al, have more surface area and more negatively charged sites meaning they can absorb more herbicide

2:1 clays adsorb more than 1:1 clays

coarse soil texture on herbicide label =

sand, loamy sand, sandy loam

medium soil texture on herbicide label =

loam, silt loam, silt, sandy clay loam, sandy clay

fine soil texture on herbicide label =

silty clay loam, silty clay, clay loam, clay

Why does clay and organic matter affect application rates and limitations on use?

Affects how much is available to the plant, leaching, and potential injury to crop, efficacy affects

Soil moistures affect on herbicide adsorption

residual herbicides more tightly bound to drier soils due to less competition with water for binding sites under dry conditions

longer persistance in dry soils, better weed control in moist soils

partition coefficient, Kd definition

the ratio of herbicide bound to soil particles compared to the amount left in the soil solution

Kd equation

amount absorbed by soil / amount in soil solution

Koc equation

Kd / organic carbon content

sorption coefficient Koc definition

measures the tendency for pesticide adsorption by soil, adjusted for organic carbon content of soil

values for most herbicide can be found online

what do the Koc values mean

low Koc - not thightly bound to soil, most of the herbicide is in soil solution and available for uptake, leaching, volatilization, microbial degradation

high Koc - Tightly bound to soil. Most of the herbicide not available for plant uptake, leaching, volatilization, microbial degradation

Relationship between Koc and water solubility

inverse, as Koc increases water solubility decreases

What type of herbicides does pH not affect

non-ionizable herbicides - have no charge regardless of soil pH, no effect of soil pH on adsorption

cationic herbicides - always postively charged, very tightly bound to colloids, soil pH has no effect

What type of herbicides does pH affect

basic herbicides - act like a weak base, Charge on molecule is pH dependent, Neutral or positively charged depending upon pH

Positively charged at lower pH; neutral at higher pH

Greater adsorption under low soil pH

better weed control at higher pH

Bronsted-Lowry definition of acids and bases

Acid: donates a proton (H+)

Base: accepts a proton (H+)

Charge of basic herbicides

neutral or positively charged, depending upon pH

How pH affects basic herbicides

Low soil pH - HB+ positive herbicide, greater adsorption, less available for plant uptake (less weed control), less available for leaching, microbrial degredation, volatilization

High soil pH - HB neutral herbicide, better weed control, increased leaching, volatilization, microbrial degradation

pKa definition

the pH at which half of the herbicide molecules are neutral and half are charged

pKa relative to pH impact on herbicides

when soil pH is above pKa herbicides not attracted to soil and more available to plant, better weed control

when soil pH is below pKa herbicides positively charged and attracted to soil, less available for plant, worse weed control

what would the charge be of an acidic hericide at pH above the pKa?

negative

what would the charge be of a basic herbicide at pH above the pKa?

neutral

what would the charge be of an acidic herbicide below the pKa?

neutral

what would the charge be on a basic herbicide below the pKa?

postive

what would the charge be of an acidic herbicide when pH equals the pKa?

50% neutral 50% negative

what would the charge be of a basic herbicide when pH equals the pKa?

50% neutral 50% positive

soil pH affect on persistence

for basic herbicides, greater persistence at lower pH

How pH affects acidic herbicides

Charge on molecule is neutral at low pH, negatively charged at high pH

soil pH has little to no effect on adsorption

Why does soil pH have little to no effect on adsorption of acidic herbicides?

Because there’s not a strong attraction between the soil colloids (negative) and the molecule because its either neutral or negatively charged

Soil pH effect on water solubility

for some herbicides soil pH can affect water solubility of the herbicide

Ex: water solubility of sulfentrazone (acidic herbicide) increases as pH increases

herbicide leaching definition

downward movement of herbicide through soil profile by water

importance of leaching

1. Provides for activation of PRE herbicides (leaching into weed seed germination zone)

2. Excessive leaching may reduce weed control (diluted concentration in weed seed germination zone less than lethal)

3. Can explain crop selectivity, or lack of selectivity (for root absorbed herbicides if the roots are out of the leaching zone crop should be fine)

4. Can contribute to ground water contamination (can cause environmental, human health, drinking water, and irrigation water issues)

Factors affecting herbicide leaching

1. herbicide chemical properties

2. soil characteristics

3. volume of water flow

How the herbicide chemical characteristics affect leaching

1. water solubility - as water solubility increases leaching increases

2. degree of adsorption - tighter adsorption = less leaching

PLP - pesticide leaching potential

determined by water solubility and extent of adsorption

low water solubility + high adsorption (Koc) = low PLP

high water solubility + low Koc = high PLP

How soil characteristics affect leaching

texture and organic matter affect adsorption and water infiltration

greater infiltration in coarser textures

SLP - soil leaching potential

determined by soil texture and organic matter

GWCP - Ground water contamination potential

Determined by combining the pesticide leaching potential (PLP) and soil leaching potential (SLP) (think of the table)

with PLP and SLP both low GWCP is low

with PLP and SLP both high GWCP is high

with one low and one high it can be moderate

How does volume of water flow affect leaching

increased water flow (irrigation, rainfall) means increased leaching

herbicide runoff definition

Movement in surface water leaving site; herbicide dissolved or suspended in water

Herbicide attached to soil carried by runoff water

Herbicide can enter rivers and reservoirs

Factors affecting amount of herbicide transported by runoff

Herbicide application rate

Time of first rainfall, intensity, amount - high intensity rainfall leads to less infiltration = more runoff

Soil texture (infiltration rate) and slope

Chemical properties of herbicide - Water solubility and Extent of adsorption (PLP)

Best management practices for reducing herbicide runoff

No-till (reduced soil erosion)

Soil tilth, residue cover - gives a barrier between the soil surface (water infiltration)

Vegetative buffer strips (trap runoff soil)

Containment ponds around nurseries - traps it rather than allowing it to runoff

Timing of application in relation to anticipated rainfall event

herbicide volatilization definition

Change from a solid or liquid phase to a gaseous phase, with subsequent dissipation into the atmosphere

Factors affecting extent of herbicide loss by volatilization

1. vapor pressure of herbicide

2. soil moisture content

3. soil temperature

4. adsorption

5. application method

How vapor pressure of herbicide affects volatilization

as vapor pressure increases volatilization increases

the primary factor affecting volatilization losses

How soil moisture content affects volatilization

as soil moisture increases volatilization increases

due to higher adsorption in dry soils

How soil temperature affects volatilization

as soil temperature increases volatilization increases

due to vapor pressure increasing with temperature

Adsorption affect on volatilization

as adsorption increases volatilization decreases

Application method (PPI vs PRE) affect on volatilization

mechanical incorporation decreases volatilization losses of soil applied herbicides due to trapping it in the soil

importance of volatilization

excessive volatilization losses can adversely affect weed control

volatilization can cause injury to off-target vegetation through vapor drift - example: clomazone turning trees white

Degradation processes of microbial degradation

Aerobic microorganisms (require oxygen)

Rate of herbicide degradation related to population of microorganisms in soil

What are microbial populations and activity affected by

Soil temp

Soil moisture

Soil aeration

How does soil temp affect microbial degradation

80-90 degrees F best, cool soil slows microbial degradation

How does soil moisture affect microbial degradation

50 to 100% field capacity is best, dry soil slows degradation

How does soil aeration affect microbial degradation

poor aeration and flooding slows aerobic degradation because they need oxygen

How is half life and degradation related

As half life decreases degradation increases

Accelerated microbial degradation

a significant increase in the rate of degradation of a pesticide in soil following a previous application of the same pesticide or one of similar chemical structure.

microbes adapt to feeding on that pesticide leading to more of the microbe in the soil and increased degradation

thiocarbamate herbicides (butylate, EPTC, vernolate) particularly subject to this

Chemical degradation of herbicides

Non-biological, chemical reactions

Affected by temperature; more rapid at higher temperatures

May be affected by soil pH

hydrolysis or reductive dealkylation two examples

Photodegration or photolysis definition

breakdown of herbicide by sunlight (UV)

some herbicides very susceptible others not - DNA’s group of herbicides very susceptible

herbicide persistence definition

Length of time a herbicide remains phytotoxic in soil

Typically expressed as half-life (t1/2), or the time it takes for 50% of the herbicide to breakdown to an inactive form

Half-lives can vary considerably, depending upon soil and environmental conditions

important of persistence

Length of weed control

Toxicity to following crop (carryover)

Possibly illegal residues in next crop

Impacts environmental fate

factors affecting herbicide persistence

1. soil properties - organic matter, texture

2. climatic condition - moisture, temp

3. herbicide properties - ionic/ nonionic, acidic/basic

absorption definition

Entry of herbicide into the plant; often called “uptake”

translocation definition

movement of herbicide from site of entry to other locations within plant

what are translocation sites of entrys

foliar tissue or roots

metabolism definition

Changes in chemical structure of herbicide molecule within the plant

Often referred to as degradation

Most metabolic changes lead to reduced phytotoxicity (make herbicide less toxic to plant)

In a few cases, initial steps of metabolism increase phytotoxicity

fate of postemergence herbicides applied to plants

1. volatilize off leaf surface before absorption

2. washed off leaf surface before absorption

3. photodegraded before absorption

4. remain on leaf surface not to be absorbed (cuticle is the main barrier)

5. penetrate cuticle, remain tied up in cuticle (does not reach living part of plant, does not kill plant)

6. pass through cuticle, enter apoplast or symplast (now in the living part of plant)

What affects how much herbicide will volatize off the leaf surface before it can be absorbed?

vapor pressure of herbicide

environmental conditions - mainly temp (as temp increases volatility increases, as humidity increases volatility decreases)

how rapidly the herbicide is absorbed into the plant

What affects how much herbicide is washed off leaf surface before absorption?

speed of absorption

time of rainfall after application

chemical properties of the herbicide especially water solubility (as water solubility increases wash off increases)

rain-free requirement or rain fastness definition

A rain-free requirement is the length of time without rainfall (or irrigation) one needs following POST herbicide application so that reduced control does not happen, can be found on the label

What affects photodegradation of herbicides off the leaf surface?

chemistry of the herbicide (if its susceptible to photodegradation)

whether in soil or water or on plant

speed of absorption into plants

light intensity

as photodegration increases efficacy decreases

How do herbicides with very short half lives still work?

Rapidly absorbed and active at low rates

How do Weed Scientists Study Absorption, Translocation, and Metabolism of Herbicides?

Primary method is through use of radiolabeled herbicide. Radiolabel gives us a way to “see” the herbicide.

1. autoradiograph - xray film, phosphorescence imaging

2. quantify radioactivity through grinding tissues and combusting it

3. determine metabolites by chromatography

apoplast definition

Non-living, continuous network of cell walls, intercellular spaces, and xylem tissue that functions as the conduit for water and mineral nutrients from roots to shoot

apoplastic translocation

Translocation in the apoplast

Apoplastically mobile herbicides follow same pathway and move in same direction as the net movement of water in plant

Transpiration is the driving force

Where is an apoplastically mobile herbicide going to be found in the plant some time after application?

Follows the net movement of water/transpiration

All leaves are transpiring, largest leaves the most, more at the tips and extends back, some interveinal movement

So should be found at the tips of all leaves with the most on the largest leaves and some on the stem

A plant with injury at older growth, largest leaves with veins visible is what kind of translocation

apoplastic translocation

Where will an apoplastic mobile herbicide move when spotted on a single leaf?

Will stay on that leaf expanding to the tip because water is coming into the leaf not going out of it

symplast definition

Continuous network of living cells, intercellular protoplasmic connections (plasmodesmata), and phloem tissue that transports assimilates (food) from site of production (leaves) to site of use (root and shoot meristems, developing fruit, seed, storage organs)

symplastic translocation

Translocation in the symplast

Symplastically mobile herbicides follow a “source to sink” direction of movement; source is site of assimilate production (or storage in case of young perennials) to sites of assimilate use (new growth)

Follows the net movement of photoassimilates

Bidirectional movement - acropetal (upward movement) and basipetal (downward movement)

net direction of movement depends on location of areas of greatest assimilate supply and demand

symplastically mobile herbicides almost always applied postemergence

Where is a symplastically mobile herbicide going to be found in the plant some time after application?

Follows net movement of the assimilates so most goes to the new growth - ends of roots and newest shoots, some will be found in all other leaves because they are also still growing with more found in younger leaves

When there is injury at new growth or the growing point what type of translocation is that

symplastic

When is the best time to apply symplastically mobile herbicides?

The “boot-to-head” stage of growth, when the plant is large enough that net assimilate movement is going towards the rhizomes to get the herbicide in there and get better control

Really any time its putting energy into the rhizomes (before a frost etc.) but you need to give it time to translocate (so like 7 days before tillage)

susceptibility definition

Positive response to the presence of a given herbicide, resulting in some growth process being altered.

A response is observed.

Susceptible plant is killed or significantly injured by the herbicide.

tolerance definition

Lack of response by the plant to a given herbicide

Tolerant plant is not affected or only slightly injured by the herbicide

selectivity definition

Differences in response of plant species to a given herbicide

selective herbicide definition

kill or severely injure some species while having little effect on other species; i.e., kill weeds but do not hurt crop