lecture 05

lecture 05 - 1/26/11 Weed Seed Characteris;cs Weed...

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Unformatted text preview: 1/26/11 Weed Seed Characteris;cs Weed seeds are usually small in comparison to crop seeds.  ­ single weed plant commonly produces from100’s to 100,000’s of seed per year.  ­  infest cropland soils in the amount of millions per acre. Weed Seed Persistence and Dormancy BTNY 304 The number produced per plant is apparently related to the total weight of the seeds produced: the smaller the seed, the more seeds produced per plant. Weed seeds can be found throughout a deep profile of the soil  ­ majority found within the top 6 inches.  ­ In undisturbed soil, mainly in the top 2 inches The length of ;me that a seed is viable varies species and environmental condi;ons. However, many weed seeds can remain viable for 40 years or longer when buried 2 inches or deeper in the soil where the oxygen supply is reduced. Weed Seed Characteris;cs Large numbers produced Produce seeds under adverse condi;ons Seeds exhibit periods of dormancy Buried seeds remain viable for years Seeds survive adversity Seeds may be difficult to detect in or removed from crop seeds •  Seeds adapt to aid in their dispersal •  •  •  •  •  •  Weeds are persistent – able to produce numerous, long ­las;ng and easily transported seeds Annuals – seeds essen;al for survival Perennials – seeds and vegeta;ve reproduc;ve parts able to persist and spread Seed Banks The accumulated deposit of seeds on or in the soil is called the seed bank. Seed Bank survival is affected by many seed ­related and environmental factors 1 1/26/11 Seed Produc;on and Soil Longevity Weed Seeds/Plant Pigweed up to 200,000 Jimsonweed up to 25,000 Velvetleaf up to 20,000 Morningglory up to 6,000 Foxtails up to 1,000 Alive 40 years 80 years 30 years 7 years 10 years Seeds Found per Acre (under corn produc;on) Weed Shaaercane Pigweed Velvetleaf Seeds/Acre 100,000,000 2, 000,000,000 16,000,000 Giant Foxtail Seed Produc;on Germina;on of Weed Seeds Buried in the Soil for 38 Years Germinates in late May late July late August Seeds per Head 1,000 400 100 Germina;on % 100 85 < 10 Weed Jimsonweed Moth Mullein Velvetleaf Evening Primrose Lambsquarters Green Foxtail % Germina;on 91 48 38 17 7 1 Why Weeds Persist Why Weeds Persist What happens over ;me to weed seed in the soil?  ­  Some germinate,  ­  some stay dormant for a variety of reasons,  ­  some rot,  ­  some are eaten by bugs, worms, criaers, etc. As more seeds are produced,  ­  the food supply increases for the soil organisms,  ­  more pathogens are produced,  ­  more animals are produced,  ­  more seeds stay dormant,  ­  and more seeds germinate Over a number of years, the level of weed seeds in the soil becomes constant This is par;cularly true of weeds that produce more than 1,000 seeds per plant Weed seed popula;ons per acre  ­ high and stable in the soil. The soil microenvironment can adjust for available food supply. 2 1/26/11 Weed Seed Predators Weed Seed Predators Earthworms (dew worms) (Regnier et al.) Weed Seed Preda-on •  18 % of velvetleaf and 22% of giant foxtail seed consumed/day (Hartzler 2003) •  99.8 % of current years seed rain (barnyardgrass, redroot pigweed, and common lambsquarters) was eliminated under a canopy of alfalfa by field mice (Ghersa et al. unpublished) Why Weeds Persist Even With Herbicide Use Take a field that had 100,000 foxtail seeds per acre. If all germinated and produced 1,000 seeds each, what % weed control would be needed to maintain original level of 100,000 seeds per acre? 100,000 x 1,000 = 100,000,000 seeds If 99% weed control achieved, there would be 1,000,000 seeds remaining, or an increase in the seed supply If 99.9% weed control achieved, there would be 100,000 seeds remaining, or the original number. If 99.99% weed control achieved, there would be 10,000 seeds remaining, or a decrease in the seed supply. Dormancy In the above example, a weed control level of 99.9% would consistently be needed just to maintain the weed supply that was in the field from the beginning. Maintaining 100% weed control will  ­  decrease the seed supply,  ­  but will not eliminate it unless it is achieved con;nuously for the number of years that the seeds stay viable in the soil. •  Condi-on of the seed that impedes its germina-on even though all environmental condi-ons seem favorable 3 1/26/11 Why does dormancy maPer? •  Dormancy affects –  The -ming and dura-on of germina-on –  The number of seedlings that emerge Weed Seed Dormancy Dormancy – a temporary condi;on in which live seed fail to germinate even when appropriate levels of moisture, air, temperature, and light are available  ­ An important condi;on to weed persistence  ­ Protects seed from adverse environmental condi;ons Weed Seed Dormancy Two types of seed dormancy Primary dormancy – undergo while s;ll aaached to parent plant Secondary dormancy – occurs aler seeds are dispersed in the seed bank and is induced over ;me by environmental condi;ons  ­ Burial in the soil  ­ Exposure to poor germina;on condi;ons Mechanism of seed dormancy  ­ Hard seed coats that restrict air and water Requires mechanical or microbial rupture of the seed coat to germinate  ­ Chemical inhibitors that must be leached out of the seed coat  ­ Red light exposure to buried seeds (phytochrome regulated) Canopy shading can affect  ­ Alerripening requirements Quiescence – not a form of dormancy A failure of a living seed to germinate because some factor is missing  ­  Oxygen  ­  Water  ­  Light Dormant seeds will eventually germinate Alerripening – the loss of dormancy Occurs over ;me in response to favorable environmental condi;ons by a process called stra;fica;on Seed dormancy and alerripening ensure that seeds will remain viable in the seed bank for a long ;me and will germinate under appropriate condi;ons 4 ...
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