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Weed Control Plant Science 280 Mr Gomes
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Seed Production A live source of life! A mature fertilized ovule (embryo) that has stored energy reserves A small plant packaged for shipment…just needs the right environment to go Prevention of seed production, especially in annuals, is one of the biggest keys to reducing weed pressure
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Seed Size/Abundance Seed size is generally extremely small, most weeds can produce a tremendous amount Shepherd’s purse can have as many as 5 million seeds/pound!! Weed seed has a unique ability to survive in the soil based on archaic genetic traits Estimates vary, but numbers vary anywhere from 120 million to as high as 1.4 billion seeds/acre
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Seeds Produced by Common Weeds
Common Name Seeds/Plant Seeds/Pound Barnyardgrass , ,286 Common cocklebur ,270 Curly Dock , ,286 Field Dodder , ,806 Lambsquarters , ,570 Shepherd’s Purse 38, ,729,166
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How can they be Dispersed??
Mechanical Wind Water Human Aided Animal Aided Machinery Mimicry Other
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Mechanical Dispersal Think the seeds ability to move itself
Cocklebur’s ability to attach to socks and fur PUNCTURE VINE!!! Bye bye bike tires Weeds use these methods in order to move across greater territory
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Wind Dispersal Most seed is small and light…high mobility
Can move great distances with the slightest breeze Dandelion and Tumbleweed are great examples
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Water Dispersal This is a bigger one than most people realize!
Where does most of our surface water come from in CA?? A study found that of 82 species in irrigation water, 27 endured linger than 12 months and some lasted up to 60 months
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Human Aided Dispersal Human inability to properly screen out seeds from irrigation or mechanical means Imported in for livestock feed for years and years. Much of our “weeds” are non natives from Europe and Asia Ex. Penny Newman and other grain processors use the screenings from grain crops to sell as cattle feeds. This is not cooked, thus weed seeds survive
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Animal Aided Dispersal
Hay fed to cows in Ca is a big one Screenings as a cheap feedstuff In Ca, most feed won’t stay in the rumen longer than 24 hrs, viability remains easily Liquid and dry manure for fertilizer spreads those viable seeds After dry is spread, what do most farmers do right after??? DISC and Plow, essentially inviting germination Attachment to animals and displacement
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Machinery Dispersal Moving equipment without proper cleanout, like combines Conventional tillage Less of an issue in Ca in 2018, but for years a farmers grain drill Dodder, wild oats, bindweed, etc. can easily be missed if not professionally conditioned Big problem outside Central Valley
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Mimicry Very common where weeds have evolved to mimic seed size, shape, or color of the crop they infest This can also be the mimicry of plant morphology or growth habit. Very difficult to distinguish visually, thus they are not removed. A great example of this is barnyardgrass in closely related wild and domestic long grain rice
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Other Literally any other method that helps propagate any unwanted plant. Downy Brome first entered Ca in packing material for glassware from Europe Nursery Stock and Ornamentals. Ex. Morning Glory in corn!! Highway Construction where “fill dirt” is brought in
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Soooooo… Consequences?!?!
Challenges with control make eradication nearly impossible Beauracracy is an ever challenging thing with chemistry development in CA Rapid infestation US Bureau of Land Management estimates that alien plants are expanding at a rate of 14% each year Quick recovery in most cases Weeds generally recover quicker after disaster than native species and definitely crops. Most recent, CA fires where cattle graze Ecological impact Increased weed pressure generally means increased erosion from shallow root zone.
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Seed Germination-Dormancy
Two important aspects of understanding weed biology Germination is the act of going from seed to plant This is the actual process of development from embryo to plant Dormancy is a state of suspended animation – alive but not actively growing Describes the state at which the seed waits in the soil profile for the ideal environmental conditions Unique trait to more land race weed genetics
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Germination Most seed will germinate when proper environmental conditions exist, however not all do. Soil disturbance may or may not initiate germination Are generally driven by; Temperature Moisture Soil disturbance
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Germination - Light At least half of the annual weeds in crops have seed that requires light for germination, especially small seeded annuals Light only can permeate into the top few mm’s of soil Chickweed, purslane, johnsongrass, lambsquarters, etc require light Wild onion and jimsonweed prefer darkness
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Germination Triggers Immature Embryo
Some plants, like smartweed and bullrush, shed immature embryo’s that are unable to immediately germinate. This prevents germinating at the wrong time Impermeable Seed Coat Many weeds like pigweed, wild mustard, and shepherd’s purse have seed impermeable to water, oxygen, or both This is referred to as “hard seed” They require tillage, microbes, and certain soil acids to drive the germination process
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Triggers Cont… Oxygen Temperature
Much of the reason most weed seed germinates near the surface is driven by Oxygen. Higher oxygen concentrations generally starts the process Counter to that, compaction reduces this and can usually inhibit germination. This is why tillage can start a varying degree of weed species Minimum and Maximum temps help dictate timing Individual species dictate this Wild oats can germinate in 35 degree temps while pigweed requires it substantially warmer
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Dormancy Conversely, dormancy is driven by a higher number of circumstances, they are; Soil temperature Soil water content Light Surface drying and wetting Percentage oxygen or carbon dioxide in soil air
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Dormancy cont… Soil Temperature Soil water content
Over the course of hundreds of years, weed species have evolved to ensure seed germinates at the right time, a constant soil temp, cold or hot, drives this If this was not the case, climate differences would allow for easier control In cases like barnyardgrass, the seed goes dormant until enough moisture ripens the seed However, some seed requires higher oxygen levels during emergence, and will wait until the soil profile dries
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Surface drying and wetting
Dormancy cont… Light Surface drying and wetting Successional Species Soil disturbances mix with increased light, telling the seed that resources are more readily available Reduction of competition also Seed will wait for these triggers Often times this is characteristic of disturbed sites where higher resources are available This correlates perfectly to cultivated fields Drying and wetting breaks the exterior coating of the seed over time, which brings the seed out of dormancy and starts germination
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Classes of Dormancy Innate Dormancy
Can be dependent on receipt of specific environmental stimuli. i.e. seed ripening requirement Induced Dormancy When seed goes dormant because of exposure to specific environmental conditions such as dryness, Hi CO2 concentrations, high temps, etc i.e. seed buried to deep stays dormant until it gets tilled up Enforced Dormancy Also depends on environment, but does not persist when conditions change
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Consequences of this!!! “1 years seeding equals 7 years weeding”
Weeds are designed by nature to persist!! They’re ability to germinate immediately OR sit dormant for periods of time make proper planning crucial Seed can hang out in the profile for decades in some cases
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Burial Study 1 yr 71 species 91% of Jimsonweed 6 yrs 68 species
Burial Period Germinated Germination of weed seed after 38 years 1 yr species 6 yrs 68 species 10 yrs 68 species 20 yrs 57 species 30 yrs 44 species 38 yrs 36 species!! 91% of Jimsonweed 48% of mullein 38% of velvetleaf 7% of lambsquarters 1% of foxtail 1% of curly dock That means after 38 years of being buried and dormant, you would still have 3,242 Curly Dock Seeds/acre and 6,485 Lambsquarters seeds viable enough to germinate!!!!
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Take Care of the Problem!!!
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