Final Review Flashcards
What is a weed?
A weed is a plant that interferes with the growth of desirable plants, is persistent and negatively affects human activity .
Why is weed science a broad discipline?
Weeds create problems in many or most environments. It’s the study of the interference of these plants in human activity. It’s not just agronomic crops anymore, it’s in managed and natural ecosystems.
Negative impacts of weeds
- Decrease quality/quantity of crops
- increase production costs
- human health and allergies
- decrease land value
- reduce water quality
- recreation value
- livestock poisoning
- decrease biodiversity
name a significant wetland weed
Purple loosestrife
significant terrestrial weed
horseweed
name one significant parasitic weed
dodder
what are several growth traits that are common to weeds?
- propagules can germinate in many enviroments
- discontinuous germination
- rapid growth
- ability to compete interspecifically
- well adapted to disturbance
- co adapted with crops
- -herbicide resistance
- -growth habits (prostrate)
- -crop mimics
several beneficial traits of weeds?
- erosion control
- medicinal purposes
- insect and disease traps
- pollution control
- food
How are weeds classified?
life cycle
habitat
morphology
botanically
->must consider the biology of the weed, its neighbors, the constraints of the environment, and budget
what are several reproductive traits that are common to weeds?
seed produced as long as conditions permit
seed dormancy
self-pollinating
short and long distance dispersal mechanisms
cross-pollination by unspecialized pollinators or wind
Why is weed management so complex?
synonymous with vegetation management
a manipulation of plants against the natural evolutionary pressures, and to promote the growth of a desired species
what are the requirements of a Weed Management Plan?
- **Ghiboiie! **
- Goals
- History
- ID
- Biology
- Options
- Integration
- Implementation
- Evaluation
prevention versus control versus eradication
prevention is the most effective, economical and ecologically sound approach to controlling weeds. Control is working to manage populations thart are already established. Eradication is very hard and requires getting rid of all parts of a plant and its propagules.
What is EDRR?
Early Detection and Rapid Response
controlling populations while the populations are localized and small
costs are typically less
more likely to eradicate weed infestations and prevent them from growing to unmanageable sizes.
What are some specific prevention techniques?
- Planting certified seed
- keeping equipment clean and screen water
- spot treat small infestations
- use mulches and other barriers
- make inspections regularly
What are some laws to prevent the spread of weeds?
- Federal noxious weed act (1975)
- Plant Protection Act (200)
- State Noxious Weed Laws
- Oregon Noxious Weed Law
- A (not yet here)
- B (Problem, biocontrol)
- T (targeted, priority)
- Oregon Seed Law
- Prohibited versus restricted
How is weed control defined?
the suppression of a weed to the point where its impact is minimized
What are the four major breeding systems of weeds?
Outcrossing, Self-pollination, clonal, mixed mating
Define Outcrossing
gametes that form zygote are genetically dissimilar
- leads to more diversity within a population
- less diversity among populations
Define self-pollination
gametes that form the zygote are genetically very similar
genetic diversity within a population is low
diversity among populations is high
define vegetative propagation
offspring are genetically identical
genetic diversty is hard to predict
define mixed mating systems
most common method in weeds
common combos:
-falcutative selfing in an outcrossing species
predominately selfing with low level of outcrossing
outcrossing with clonal capacity
facultative apomoxis
define apomixis
asexual form of reproduction where a seed is formed without fertilization
-dandelion, blackberry, kentucky bluegrass
Advantages of seed reproduction:
+ = genetic diversty
seeds are a complete package - preserved in time and against harsh environments, easily dispersed, many are produced
disadvantages of seed production
less competitive than those that reproduce vegtatively
producing seeds uses resources that could be used for plant competition
five ways seeds are dispersed
wind, water, animals, mechanically (explosion), machinery
management of seed dispersing seeds
must get the plants before they set seed
remove entire plant from field (seeds could be viable)
apply control methods before seeds are viable
management of veg. reproductive weeds
good candidates for phloem mobile herbicides (downward movement)
good candidate for biological controls
no good for hand mowing, pulling, grazing due to underground structures, could increase
Characteristics of annual plants
reproduce quickly
often weeds in annual crops
adapted well to disturbance
highly differentiated populations
characteristics of biennial plants
Season 1: rosette
Season 2: stem, flowers, seeds, dies
less adapted to disturbance
characteristics of perennials
short lived (3-5 years)
long lived (5+)
less among population diversity compared to other life forms, don’t tolerate disturbance well
Define soil seedbank
all of the viable seed in the soil or mixed into soil debris
contains seeds produced over many years
buffer against enviromental pressures
what are the major events in the seedbank pathway?
Seed production (loss to predation and other losses)
condition of seed (dormant or non-dormant)
germination
emergence
establishment
define seed rain
seeds input into the seed bank due to plants that are producing the seed within the site
Primary dormancy
seed that doesn’t germinate under favorable conditions
define after-ripening
when a seed goes from non-dormant from dormant after exposure to favorable conditions after the seed has left the plant
what conditions facilitate loss of dormancy?
largely a function of temperture but also to a lesser degree, moisture
Describe the possible fates of a seed from dispersal to germination
What is the equation for predicting seeds in the next season?
N(t+1)=Nt + Nt(G*S*Y) - Nt* G-Nt*D, where G is germination rate, S is survival rate, Y is Yield, D is geath
how to manage the soil seedbank?
induction of long term dormancy
stimulate germination and then manage the seedlings (stale or false seedbed)
reduce the inputs into the soil seedbank (requires weed-free conditions)
what factors will influence weed seed emergence?
Cool season weed species will emerge early in the spring or fall
warm season weed species will emerge late spring or early summer
soil moisture
soil temperature
soil disturbance
How do you manage weeds in terms of emergence?
apply mulches before they emerge
herbicides are better applied post emergence and timed with maximum emergence
timing of emergence of the crop and the weed is influential on competition
what are two positive plant interactions?
Mutualism (+,+) and Commensalism (+, 0)
What is commensalism?
epiphytic plants benefit from living on other plants but take no nutrients. Gain access to light, moisture and air
What are three negative plant interactions?
Competition (-,-)
Amenalism (-,0)
Parasitism (-,+)
What are the hosts of dodder? Where is it found?
alfalfa, clover, tomato. it’s found everywhere crops are cultivated
What are the hosts of broomrape?
clover, lettuce, sunflower. Mediterranean and western europe
what are the hosts of witchweed and where is it found?
corn, sorghum, grasses. West Africa and S. Carolina.
Why are parasitic weeds so difficult to control chemically?
chemical has to be very highly selective due to close relationship between weed and host
often the seeds are underground
if it’s attached then it’s too late to save the host plant
name two strategies from dealing with parasitic weeds
false hosts: hosts that attract and induce germination.
plant non-hosts if an infestation is predicted
herbcidal seed treatments
methyl bromide
define allelopathy
chemical interaction between plants in which one is negatively affected. Different than competition because one is not outcompeting the other for limited resources
where do broomrape and witchweed attach to the plant?
the roots! broomrape attaches to broadleaf, witchweed to grasses (remember that because witch grass is a thing)
Which species have been shown to have allelopathic effects?
Brassicaceae
define competition
interaction between plants that is mutually detrimental
what is the critical period of weed control?
the time in which weeds must be controlled to prevent yield losses
generally before weeds reach a height of 4-8”
depends on the competitiveness of the desired species
generally between 2 and 10 weeks
how do you manage weeds when considering competiton?
manipulate the environment to favor resource capture by desired species
limit resource capture by weeds
what are factors that influence competition?
weed species
biomass
duration
why haven’t action thresholds been developed for weeds?
discontinuous germination in many species
thresholds for a suite of species very difficult
number might be reached by timing for management strategy is wrong
sites of competition?
light: leaves!
water and nutrients: roots:
What is WUE?
measure of a plants photosynthetic production rate relative to the rate at which it transpires water to the atmopshere. Not always related to competition
what is luxary consumption?
weeds taking up more nutrietnts than they need
how do nutrients move through the soil>
N moves with mass flow, P and K are less mobile and move through diffusion.
What are the factors that influence water competition?
plant species, root volume and density, WUE, Water availablilty
What is the relationship in terms of WUE between crops and their associated weeds?
the WUE between crops and their associated weeds are usually similar.
what is the definition of an herbicide?
a chemical used to kill, control, or supress a plant, or interrupt their normal growth process. It’s the most widely used tool in weed management
What was the first herbicide and when did it appear?
Dinoseb, 1932
When was the last time herbicide development saw a new chemistry?
1990
How many herbicides existed in 2002?
140
When did 2,4-D appear? When did glyphosate appear?
1940’s, 1970
What are several properties of an ideal herbicide?
applied in small amounts, many methods of application, easy to apply, effective in many environments, highly selective, low soil mobility, low volatlity, minimal site disturbance, rapidly biodegradable, profitable, low animal toxicity, low toxicity of adjuvants, low toxcity of biproducts
What are the advantages to chemical weed management?
- reduces labor needs
- control weeds when other methods can’t
- pre-emergence applications
- contol perennial and parastic weeds
- can be used when tillage or manual control can’t
- often effective in a single application
what are the disadvantages to chemical weed control
- applicators must be current on their licenses, etc.
- herbicides harmful to humans, enviromnment
- crop injury can occur
- volatiles can injure non-targets
- persistance longer than anticipated, long hal life will impede crop rotation
- resistance development and weed shifting
what is a preplant herbicide?
on that is applied to the soil surface before seeding or transplanting
what is a pre-emergence herbicide?
- the herbicide is applied prior to emergence of either the crop or the weed, not necessarily both.
- usually refers to after planting and before crop emergence
- in established crops, prior to weed management
what is a post emergence herbicide?
one that is applied after the crop or a specific weed has emerged
can be further divided into early, late and directed treatments
Who are the two organizations that decide which chemicals can be used in organic systems?
NOP and OMRI,
what is cultural control?
manipulation of cropping practices to suppress weed growth while promoting development of the plant.
it is not mechanical, biological or chemical
Identify the cultural control methods that we discussed in class (7)
crop rotation
cultivar choice
seeding rate
time of bed prep
plant spacing
multiple cropping
sanitation
what is crop rotation? Why is it effective?
Crop rotation is seasonally changing the crop that is grown in a particular site. It keeps any one weed species from becoming dominant, as well as diversifying the system (as weeds are similar to crops). It also reduces the chance of resistance. Changes in the crop also changes the methods and timings of seedbed prep, tillage, herbicides. This will influence which weeds are present.
what is cultivar choice and how is it effective in reducing weeds?
varying cultivars may be more competitive than others in terms of weeds. There is not much data in terms of this. However, choosing cultivars that have characteristics that would give an advantage to the plant could help.
- early emergence
- rapid root growth
- increased root size
- rapid canopy closure
- greater seedling vigor
- increased leaf expansion
- increased plant height
- profuse tillering or branching
What is the advantage for crops to have a larger seed size?
They can emerge from position in the soil that are 1 to 2” deeper than weeds.
What is seeding rate and how is it effective in reducing weed populations?
Increased seeding rate usually increases crop competitiveness. At greater densities, the crop can cover the ground quicker and shade the weeds. crop roots are established more evenly to compete for nutients
What is bed prep timing and how does it help control weeds?
stale seedbed is created by tilling the soil early and encouraging the weeds to germinate. weeds are then killed without disturbing the soil, such as with herbicides or flamers or mowing. Crop is then seeded or planted with minimal disturbance.
What is plant spacing and how does it help to reduce weed populations?
Planting pattern is changed without altering the seeding rate. Rows can be put closer together to make the canopy close sooner and shade out weeds.
decreaseing the row spacing can influence which weeds will be present.
What is multiple cropping, relay, or double cropping?
It’s the practice of planting multiple types of crops in the same plots. Weeds have less time to grow without competition, and the second (or third ) crop fills the niche that would likely be filled by one of the weed species.
What is sanitation and how does it help in reducing weed populations?
Santitation methods include using certified seed, keeping edges free of weeds, removing fallen leaves and branches and reducing litter accumulation.
What is mechancal weed management?
physical removal of the weeds by hand pulling, flaming, burning or preventing them from growing through mulching or solarization.
What are some of the advantages to mechanical weed control?
- herbicide use may be reduced or eliminated.
- It is an effective method of use on herbicide resitant weeds.
- It can be highly selective, especially in the case of hand weeding.
what are the disadvantages to mechanical weed control
- cultivation reduces SOM
- may increase erosion and compaction
- highly dependent on timing, soil moisture and weather
- less flexibility compared to other mgmt techniques
- within row weeds are hard to reach
- slow or ineffiecient
- can’t be practiced on large acreages.
- great operator skill required.
- crops can be damaged
*
what are the advantages or disadvantages of hoeing or hand pulling?
very selective
used in high value crops
cna be expensive
labor intensive - used in developing countries
time consuming
what is flaming? advantages and disadvantages?
burning the tops of weeds. destroys cell membranes. weeds must be small. can get some selectivity by rate and speed.
immediate results, doesn’t disturb soil.
doesn’t work on monocots, but can work well in monocot cropping systems (kill weeds but crop revives)
what is open field burning? what are the advantages and disadvantages?
burning cover vegetation. used in ditches
+ prepares soil for crop production, produces immediate results. doesn’t disturb soil, no chemical residue. some ecosystems well adapted to fire.
- can catch dry vegetation, smoke, selective, no residual control, many burn temps not high enough to kill seeds, banned in many areas
what is tillage? what are the advatages and disadvantages?
tillage controls weeds by uprooting or burying seeds or both. Primary tillage is for breaking up soil. secondary is to pull or cut up weeds, or cultivation after crop has been sown. can be within and between rows
needs dry, level surface, warm temperatues for the dessication of weed seeds
tillage on wet soils destroys the soil structure
- weeds and crops are phenologically or morphologically similar. tillage is indiscriminate. vegetative weeds can be spread. A lot of art involved.
what is soil steaming and how does it help reduce weeds?
hot steam is injected into the ground up to 6”. common in vineyards
there is less risk compared to fire. can control perennials
$$$, high consuption of fossil fuels
band steaming can reduce costs
What is mulching and what are the advantages and disadvantages?
- controls weeds by excluding light, any substance can be used
- cheap and easy to work with
- black plastic and paper are often used. plant material such as straw, sawdust, woodchips.
- Perennial weeds are not generally controlled well .
- can be costly
- thin spots and tears, decay, need replacement
- often limits management techniques
how does solarization work and what are the advantages and disadvantages?
heat from the sun is captured under clear plastic
to work you must have soil moisture at field capacity, smooth seed bed, 4-6 weeks, high sunlight and low wind, a solid tarp
weed species differ in sensitivity, weeds with a hard seed coat are unaffected, rhizomes are not controlled
what are the three conditions that are necessary for an herbicide to be effective:
1) contact the plant surface
2) stay in contact with the plant surface long enough to be absorbed into the plant
3) reach a target site in sufficient concentrations to disrupt physiological processes
what is the relationship between water solublity and movement across the cuticle.
any plant structure that imbibes water is capable of absorbing herbicides (not dormant seeds)
what is KOW?
KOW is a measure of how easily a compound moves through a lipophilic region. higher = easier
describe the path of root absorption
herbicides are absorbed through mass flow(water), diffusion or root interception.
passes though the casparian strip (waxy layer, less developed than cuticle) and can travel between cell walls
what are the factors that influence root absorption?
conditions that favor movement of the herbidcide into soil solution also increase root absorption
a soil applied herbicide must have properties that result in the majority of herbicide being bought to soil colloids and only a small amount in the soil solution
don’t want leaching. availability similar to nutrients and available binding sites
What are the environmental fates of an herbicide after it is applied to the soil?
leaching, runoff, volatization, photodegradation, chemical breakdown (hydrolysis), degradation by microbes, adsorption by soil, absoption by plants
Which fates are considered “transfer and transport”?
adsorption, runoff, leaching, volatility
Which fates are considered Decomposition?
plant uptake - absorption
degradation by microbes
photodecomposition
transformation by chemical methods
diagram the fates of soil applied herbicides
what are the consequences of misuse of herbicides?
crop injury, weed control failures, environmental pollution
what are the benefits of soil applied herbicides?
preplant and preemergence control early season weeds
provide residual control
generally more convenient
controls some weeds that aren’t controlled in any other way
what are some risks of soil applied herbicides?
application is weather dependant
some weeds emerge even before PPI and PRE herbicides can be applied
residual herbicides are associated with a greater likliehood of developing resistance
often a need for weed control later in the season
what is the chemical barrier>
the top two inches of soil in which soil applied herbicides will come into contact with weed seeds and seedlings.
when should soil applied herbicides be applied?
They should be applied before weed emergence and should be based on the history of weeds in the field.
What is CEC? Which soils have higher CEC? What is CEC a function of?
a measure of the adsortive sites present in the soil. CEC is based primarily on the clay and OM of a soil. CEC is a function of the surface areas of a colloid and its charge.
Which soils tend to have high CEC? What happens to herbicides as CEC increases?
Clay soils with high OM have high CEC. As CEC increases, more herbicide is bound to soil colloids and less is available in the soil solution. This is why recommeded rates are based on soil type.
What is the most important factor regarding soil applied herbicide availability to plants?
Adsorption. Herbicide acitivity is a function of herbicide that is nod adsorbed and remains in the soil solution .
What is the most important factor in herbicide adsorption?
Amount of OM in the soil. A 1% increase in soil OM results in adsorption of 5-10%
How does pH affect adsorption?
in low pH soils, herbicides can be released several months later. pHs about 7 will maintain a neutral charge, where less herbicide is adsorbed and made available to plants.
Define point source
storage and handling facilities of herbicides, improperly discarded containers, or rinsed equipment in loading and handling areas.
They are discrete, identifiable locations that discharge high concentrations of herbicides.
What is a non-point source of groundwater contamination?
occurs when a mobile herbicude is applied in areas with a shallow water table.
This is reduced by choosing appropriate herbicides or alternative control strategies
What is herbicide resistance?
heritable response of a biotype to survive an herbicide treatment to which the wiltype is susceptible
what is herbicide tolerance?
tolerance is when the wild type was never susceptible to the herbicide. this is what selective herbicides are based on.
What are S-biotypes? What are R-biotypes?
Resistant and susceptible biotypes
what is cross-resistance?
cross resistance is resistance to other herbicides with the same site of action. Generally, weeds are resistant to others in the same group.
What is multi-resistance?
Multi-resistance is less common that cross-resistance. it’s resistance to two or more chemically unrelated herbicides with different sites of action.
How quickly can resistance occur?
4-20 years. It will occur more quickly if the mutation is present at a high rate or the herbicide related to the mutation is sprayed every year.
what is the likliehood that resistance will evolve in a field?
it’s a function of genetic and biological characteristcs of the weeds
properties of the herbicide
management practices
it’s about the number of resistant ind. that survive, and the number of offspring that they generate
why would a population of weeds have more resistant individuals compared to another population?
high mutation rate occurs naturally in the weed species or the field has been sprayed previously by the same herbicide or one with the same site of action.
What is the result in terms of resistance of of applying an herbicide in which the weeds are highly susceptible?
A high amount of selection pressure will kill off a large number of the susceptible individuals. The resistant individuals will increase quickly.
Which kinds of plants are more likely to evolve resistance?
Annuals that…
reproduces multiple times a year or
produce a lot of seed
What kinds of herbicides will promote resistance?
those that are persistent. greater selection pressure will result in herbicides that control multiple flushes
Why is resistance more likely to occur with the use of herbicides that only occur at a single target site?
less mutuation needs to occur for the resistance to happen.
how does cross resistance occur?
it occurs when the mutation happens in overlapping binding pockets in an enzyme
What is an altered target site?
modification of the herbicide binding site that prevents the herbicide from binding
what is enhanced metabolism?
complex resistance that may result in resistance to unrelated herbicides.
could occur when is able to deal with functional groups on herbicide molecules
what is reduced translocation / exclusion?
resistance based on the fact that herbicide does not reach the site of action due to
- not being taken into the plant
- not moved within the plant
- may be sequestered
name three herbicide resistant weeds in OR
prickly lettuce, Russian Thistle, common groundsel
what are the symptoms of resistance in the field?
- one weed is not controlled where others that should be controlled are
- irregular shaped patches of uncontrolled weed
- records show a previous failure
- no herbicide symptoms on target weed
rule out problems with the application
check records for repeated use of herbicides with the same site of action
what are three keys to preventing resistance?
knowledge of herbicides
knowledge of biology of the weeds
willingness to make changes
what do you do if you suspect resistance?
do not respray with the same herbicide
do not use a higher rate
document resistance and contact extension, chem dealer, salesperson
collect seeds
Ways to prevent resistance:
- diversify crops planted in terms of seasons, herbicides, and weeds
- scout! before and after herbidice applications
- use non-chemical management techniques
- plant certified seed
- clean equipment
- communicate with other growers about what they are encountering
