Manage the Weed Seed Bank—Minimize “Deposits” and Maximize “Withdrawals” One of the most important—yet often neglected—weed management strategies is to reduce the number of weed seeds present in Weed Seedbanks The weed seedbank in ƒi in the current period, then becomes equal to those surviving seeds that do not germinate in the seedbank at t−1, sbi,t−1, and the contribution of seeds Agricultural soils contain thousands of weed seeds per square foot. The density of weed seeds in the weed seedbank is influenced by past farming practices and will vary from field to field.
Manage the Weed Seed Bank—Minimize “Deposits” and Maximize “Withdrawals”
One of the most important—yet often neglected—weed management strategies is to reduce the number of weed seeds present in the field, and thereby limit potential weed populations during crop production. This is accomplished by managing the weed seed bank.
What is the Weed Seed Bank, and Why is it Important to Organic Farmers?
The weed seed bank is the reserve of viable weed seeds present on the soil surface and scattered throughout the soil profile. It consists of both new weed seeds recently shed, and older seeds that have persisted in the soil from previous years. In practice, the soil’s weed seed bank also includes the tubers, bulbs, rhizomes, and other vegetative structures through which some of our most serious perennial weeds propagate themselves. In the following discussion, the term weed seed bank is defined as the sum of viable weed seeds and vegetative propagules that are present in the soil and thus contribute to weed pressure in future crops. Agricultural soils can contain thousands of weed seeds and a dozen or more vegetative weed propagules per square foot.
The weed seed bank serves as a physical history of the past successes and failures of cropping systems, and knowledge of its content (size and species composition) can help producers both anticipate and ameliorate potential impacts of crop–weed competition on crop yield and quality. Eliminating “deposits” to the weed seed bank—also called seed rain—is the best approach to ease future weed management. Over a five-year period in Nebraska, broadleaf and grass weed seed banks were reduced to 5 percent of their original density when weeds were not allowed to produce seeds. However, in the sixth year, weeds were not controlled and the seed bank density increased to 90 percent of the original level (Burnside et al., 1986).
Weed seed banks are particularly critical in organic farming systems, which rely on cultivation as a primary means of weed control. Because a cultivation pass generally kills a fixed proportion of weed seedlings present, a high initial population will result in a high density of weeds surviving cultivation—escapes—and competing with the crop. Initial weed population is directly related to the density of seeds in the seed bank (Brainard et al., 2008; Teasdale et al., 2004); thus, effective cultivation-based weed control requires either a low seed bank density (Forcella et al., 1993) or multiple cultivation passes to achieve adequate weed control. In addition, dense weed stands (for example, a “sod” of smooth crabgrass or other grass weed seedlings) can interfere with the efficacy of cultivation implements in severing or uprooting weeds (Mohler, 2001b).
Cultivation efficacy—weed kill—can vary considerably based on equipment, soil conditions, weed growth stage, and operator experience. Eighty percent mortality would be considered quite respectable, a level of weed control far less than that achieved with most herbicides. Therefore, without the “big hammer” of selective herbicides to remove heavy weed populations from standing crops, effective measures to reduce weed seed banks become vital for the organic farmer.
Inputs (“Deposits”) and Losses (“Withdrawals)
Organic growers aim to manage their weed seed banks in the opposite fashion from a long term savings account: minimize “deposits,” and maximize “withdrawals” (Forcella, 2003). Weed seed bank deposits include:
- The annual weed seed return (or seed “rain”) from reproductively mature weeds in the field or in field margins
- Production of new rhizomes, tubers, and other vegetative reproductive structures by perennial weeds
- Weed seeds brought into the field through inputs and farm operations, such as manure, mulch hay, irrigation water, farm machinery, and custom operators
- Weed seeds introduced by natural forces beyond the farmer’s control, such as wind, floodwaters, and migrating birds
Whereas the first two kinds of deposits have the greatest influence on future population levels of existing weed species, the latter two can introduce new weed species to the farm—somewhat analogous to opening a new kind of bank account with a small initial deposit and a sky-high interest rate. Even two or three viable seeds or propagules of a highly aggressive new weed species can spell trouble in years to come. Thus organic farmers strive both to prevent heavy deposits through propagation of existing weeds, and to prevent establishment of new weed species by excluding their seed and promptly eradicating new invaders. This topic is discussed further in Keeping New Weedy Invaders Out of the Field.
Weed seed bank withdrawals include:
- Seed germination
- Fatal germination, in which the seed or propagule sprouts but fails to reach the soil surface due to excessive depth or death from allelochemicals (natural phytotoxic substances released by plants), microbial pathogens, insects, or other organisms in the soil
- Consumption of weed seeds by ground beetles, crickets, earthworms, slugs, field mice, birds, and other organisms (=weed seed predation)
- Loss of viability or decay of seeds over time
The first type of withdrawal—germination leading to emergence—is, of course, how weeds begin to compete with and harm crops each season. It is also the foremost mechanism for debiting the seed bank, an effective strategy if emerged seedlings are easily killed by subsequent cultivation or flaming (the stale seedbed technique, for example). Even in species with relatively long-lived seeds such as pigweeds, velvetleaf, and morning glory, the vast majority of weed emergence from a given season’s seed rain takes place within two years after the seeds are shed (Egley and Williams, 1990). Thus, timely germination (when emerging weeds can be readily killed) can go far toward minimizing net deposits into the seed bank from recent weed seed shed. Knowing when to promote or deter weed seed germination, and how to do so for the major weeds present, are important skills in seed bank management.
Weed Seed Bank Dynamics
Weed seeds can reach the soil surface and become part of the soil seed bank through several avenues. The main source of weed seeds in the seed bank is from local matured weeds that set seed. Agricultural weeds can also enter a field on animals, wind, and water, as well as on machinery during activities like cultivation and harvesting (explored further in Keeping New Weedy Invaders Out of the Field).
Weed seeds can have numerous fates after they are dispersed into a field (Fig. 1). Some seeds germinate, emerge, grow, and produce more seeds; others germinate and die, decay in the soil, or fall to predation. The seeds and other propagules of most weeds have evolved mechanisms that render a portion (a large majority in some species) of propagules dormant (alive but not able to germinate) or conditionally dormant (will not germinate unless they receive specific stimuli such as light) for varying periods of time after they are shed. This helps the weed survive in a periodically disturbed, inhospitable, and unpredictable environment. Weed seeds can change from a state of dormancy to nondormancy, in which they can then germinate over a wide range of environmental conditions. Because dormant weed seeds can create future weed problems, weed scientists think of dormancy as a dispersal mechanism through time.
Figure 1. Fate of weed seeds. Inputs to the seed bank are shown with black arrows and losses with white arrows. Figure Credit: Fabian Menalled, MSU Extension, Montana State University.
Maintaining excellent weed control for several consecutive seasons can eliminate a large majority of the weed seed bank, but a small percentage of viable, highly dormant seeds persist, which can be difficult to eliminate (Egley, 1986). Researchers are seeking more effective means to flush out these dormant seeds through multiple stimuli (Egley, 1986).
Weed species also differ in the seasonal timing of their germination and emergence. Germination of many species is governed by growing degree–days (GDD)—the summation of the number of degrees that each day’s average temperature exceeds a base temperature. This concept is founded on the assumption that, below the base temperature, the organisms (in this case seeds) are quiescent, and that as “thermal time” accumulates above this temperature, their development proceeds. In addition, some newly shed weed seeds must first undergo a period of unfavorably cold or hot conditions before they can germinate in response to favorable temperatures. This initial, or primary, dormancy delays emergence until near the beginning of the next growing season—late spring for warm-season weeds (dormancy broken by cold period over winter), and fall for winter annual weeds (dormancy broken by hot period in summer)—when emerging weeds have the greatest likelihood of completing their life cycles and setting the next generation of seed.
The Iowa State University Cooperative Extension Service has evaluated seed germination response of common weeds of field corn in relation to GDD calculated on a base temperature of 48°F beginning in early spring, and categorized the weeds into germination groups (cited in Davis, 2004). For example, winter annuals like field horsetail and shepherd’s purse germinate before any GDD accumulate in the spring; giant ragweed and common lambsquarters require fewer than 150 GDD and therefore emerge several weeks before corn planting; redroot pigweed, giant foxtail, and velvetleaf germinate at 150–300 GDD, close to corn planting time; whereas large crabgrass and fall panicum require over 350 GDD and usually emerge after the corn is up. A few species, such as giant ragweed, emerge only during a short (8 weeks). Knowing when the most abundant species in a particular field are likely to emerge can allow the farmer to adjust planting dates and cultivation schedules to the crop’s advantage.
Several factors other than mean daily soil temperature have a major impact on the timing of weed germination and emergence in the field. Adequate soil moisture is critical for germination, and good seed–soil contact is also important in facilitating the moisture uptake that is required to initiate the process. Thus more weeds may emerge from a firmed soil surface, such as occurs under planter press wheels, than from a loose, crumbly, or fluffy soil surface (Gallandt et al., 1999). For example, densities of common chickweed and common purslane in seeder tracks—in the crop rows—were roughly double those over the rest of the field, whereas annual grass weeds and yellow nutsedge did not show this pattern. (Caldwell and Mohler, 2001).
In addition, many weed seeds are also stimulated to germinate by light (even the very brief flash occasioned by daytime soil disturbance), fluctuations in temperature and moisture, or increases in oxygen or nitrate nitrogen (N) levels in the soil. Tillage, which exposes seeds to these stimuli, is therefore a critical determinant of seed germination. The timing of N fertilizer applications can also influence the number of weeds germinating. For example, many weed species can be stimulated by large increases in soluble N after incorporation of a legume cover crop, or inhibited by delayed applications of N fertilizer.
Shallow soil disturbance during periods of peak potential germination can be an effective tactic for debiting (drawing down) the weed seed bank (Egley, 1986). This phenomenon is exploited when timely cultivated fallow is used to reduce the weed seed bank, and in the establishment of a stale seedbed prior to planting. These tactics encourage the conditionally dormant portion of the seed bank to germinate so that the crop can be sown into a reduced initial weed population.
Weed seeds disperse both horizontally and vertically in the soil profile. While the horizontal distribution of weed seeds in the seed bank generally follow the direction of crop rows, type of tillage is the main factor determining the vertical distribution of weed seeds within the soil profile. In plowed fields, the majority of weed seeds are buried four to six inches below the surface (Cousens and Moss, 1990). Under reduced tillage systems such as chisel plowing, approximately 80 to 90 percent of the weed seeds are distributed in the top four inches. In no-till fields, the majority of weed seeds remain at or near the soil surface. Clements et al. (1996) have shown that soil texture may influence weed seed distribution in the soil profile under these different tillage systems (Fig. 2).
Figure 2. Vertical distribution of weed seeds in a loamy sand soil (top) and a silty loam soil (bottom). Figure credit: adapted from Clements et al. (1996) by Fabian Menalled, MSU Extension, Montana State University.
Understanding the impact of management practices on the vertical distribution of seeds is important because it can help us predict weed emergence patterns. For example, in most soils small-seeded weeds such as kochia, Canada thistle, and common lambsquarters germinate at very shallow depths (less than ½ inch). Large seeded weeds such as common sunflower have more seed reserves and may germinate from greater depths.
Thus, one strategy for managing the weed seed bank, especially for smaller-seeded weeds, is to maintain seeds at or near the soil surface. It is here that seeds experience the greatest exposure to environmental cues that will encourage germination—the most effective means of debiting the seed bank—as well as greater exposure to seed predators (see Encouraging Weed Seed Predation and Decay). Studies have confirmed that some weed seeds, including velvetleaf, morning glory, and pigweed, germinate in larger numbers in untilled than in tilled soil during the first year after seed shed (Egley and Williams, 1990). It may be tempting to use inversion tillage to place seeds below the depth from which they can emerge. This may be an effective strategy for species with short-lived seeds (see below), but it may simply protect longer-lived seeds from mortality factors like seed feeding animals and decomposer fungi, only to be returned to the soil surface by the next deep plowing event.
Factors Affecting Weed Seed Longevity
The number of viable seeds remaining from a given year’s weed seed return declines over time as a result of germination (successful or fatal), predation, and decay. The percentage remaining declines in an approximately exponential manner, similar to the decay curve for a radioactive chemical element—the time for the number to decline by 50% is roughly the same, regardless of the initial num. The half-life of weed seeds varies widely among weed species; for example, hairy galinsoga and some annual grass weeds, such as foxtail species, last only one to a few years, whereas some curly dock and common lambsquarters seed can last over 50 years.
The actual seed longevity in the soil depends on an interaction of many factors, including intrinsic dormancy of the seed population, depth of seed burial, frequency of disturbance, environmental conditions (light, moisture, temperature), and biological processes such as predation, allelopathy, and microbial attack (Davis et al., 2005; Liebman et al., 2001). Understanding how management practices or soil conditions can modify the residence time of viable seeds can help producers minimize future weed problems. For example, seeds of 20 weed species that were mixed into the top 6 inches of soil persisted longer in untilled soil than in soil tilled four times annually (Mohler, 2001a), which likely reflects greater germination losses in the disturbed treatment. On the other hand, a single tillage can enhance the longevity of recently-shed weed seeds, because buried seeds are usually more persistent compared to those left at the surface where they are exposed to predators, certain pathogens, and wide fluctuations of temperature and moisture. However, soilborne pathogens may also contribute to attrition of buried seeds, even in large-seeded species like velvetleaf (Davis and Renner, 2008).
Although seed longevity of agricultural weeds is a cause for notoriety, and a proportion of the population may remain viable for several years or decades, most of the seeds of many weed species will either germinate or die shortly after being dispersed from the parent plant. The seeds of many grasses are particularly short lived. For example, in a field study conducted near Bozeman, MT, wild oat seeds were incorporated into the top four inches of a wheat–fallow field, and approximately 80 percent of them died during the first winter (Harbuck, 2007). It is important to note, however, that postdispersal survival varies widely among weed species.
Evaluating the Weed Seed Bank
One way to estimate a field’s weed seed bank is to wait and see what weeds emerge during the first season. However, knowing something about seed bank content before the season starts can help the farmer prevent severe weed problems before they develop. Davis (2004) recommended the following simple procedure for scouting the weed seed bank:
A little effort in understanding your weed seedbank [sic] can give you valuable information about what weeds to expect in a given growing season, weed density, and when most weed germination will take place. To get a weed preview, you can germinate weeds indoors as you’re waiting to plant. For summer annual weeds, such as velvetleaf, foxtail, lambsquarters, and pigweed, March–April is a good time to sample weed seedbanks [sic] in the North Central region. Using a soil probe or a garden trowel, take 20 samples to a 2” depth in a ‘W’ pattern from the field you’re interested in. Place the soil in a pie dish, put in a warm place (> 65 º F) and keep moist. Within one to two weeks, you should have an idea of what weeds will be emerging in your field as the soil warms.
~ Davis, 2004
For a more representative sampling, collect sufficient soil samples to fill several pie dishes, or a seedling flat. The larger the sample, the more closely the observed weed emergence will reflect field populations.
Keep in mind that this method is not likely to reveal all the species present in a field. However, in combination with field observations on seasonal patterns of weed emergence, greenhouse weed emergence tests can help anticipate when control tactics are likely to be needed in the coming season, and to begin developing a seed bank management strategy.
Some Weed Seed Bank Management Practices
Use these strategies to minimize annual inputs (deposits) to the weed seed bank:
- Kill weeds before they set seed—before flowering to be safe, because some weeds (such as hairy galinsoga) can mature seeds from flowers that are pollinated before the weeds are pulled or severed . If in doubt, attempt to thresh the seeds from the fruits or flowers of flowering weeds; dough-consistency and firm seeds can be considered mature and should be removed from the field if possible.
- Control creeping perennial weeds before they can form new rhizomes, tubers, or other propagules.
- Keep crops ahead of the weeds—small weeds overshadowed by a good crop canopy may have less than 1% of the seed forming capacity of vigorous individuals growing in full sun.
- Walk fields to remove large weed escapes before they flower. Getting the largest 10% of individuals can reduce seed production by 90% or better.
- Mow field margins to minimize seed set by weed species that have the potential to invade fields. (Balance this with the potential role of field margins as beneficial insect habitat).
- Mow or graze fields promptly after harvest to interrupt weed seed production.
- Utilize good sanitation practices to prevent introduction of new weed species into the field, and remove new invaders before they propagate.
Another measure that can help contain seed bank populations is to increase the diversity of crop rotations. Although data on the effects of crop rotations on weed seed banks in organic systems have not been consistent, there is some evidence suggesting that more diverse rotations, especially those that include one or more years in red clover, alfalfa, or other perennial sod crops, can help reduce seed inputs from velvetleaf and other annual weeds, and promote seed bank declines through seed predation and decay (Davis et al., 2005; Teasdale et al., 2004; Westerman et al., 2005).
Use these strategies to maximize losses (withdrawals) from the weed seed bank:
- Till or cultivate to stimulate weed seed germination at a time when the seedlings can be easily knocked out by additional cultivation or flaming (stale seedbed), or will be freeze-killed before they can reproduce. Rolling after tillage can further enhance germination by improving seed–soil contact.
- If practical, time this tillage or cultivation to take place when seeds of the major weeds present are least dormant, and/or during the season of the weeds’ peak emergence, in order to maximize the seed bank withdrawal.
- Time crop planting to facilitate destruction of flushes of weed seedling emergence. For example, if the major weeds in a given field are known to reach their peak emergence in mid May, delay corn planting until end of May to allow time to remove this flush prior to planting.
- Maintain habitat for weed seed predators—vegetation or mulch cover—in at least part of the field for as much of the year as practical.
- Reduce or avoid tillage during critical times for weed seed predator activity. If a significant weed seed rain has occurred, leave weed seeds at the surface for a period of time before tilling to maximize weed seed predation.
Because soil microorganisms can play a role in weed seed decay, maintaining a high level of soil biological activity through good organic soil management might be expected to shorten the half-life of weed seed banks. In addition, incorporation of a succulent legume or other cover crop may either stimulate weed seed germination by enhancing soil nitrate N levels, or promote weed seed or seedling decay as a result of the “feeding frenzy” of soil microorganisms on the green manure residues. However, the potential of these practices as weed seed bank management tools requires verification through further research.
While it is sometimes advantageous to cause weed seeds to germinate, it is important at other times to keep them quiescent long enough for the crop to get well established. Several practices can help reduce the number of weeds emerging in the crop.
- Cultivate at night or with light shields over the cultivation implement to minimize the light stimulus to weed seeds.
- Leave a loose soil surface after planting or cultivation to reduce seed–soil contact for near-surface weed seeds, thereby deterring germination. If practical, cover newly seeded rows with loose soil to reduce within-row weed emergence.
- Minimize soil disturbance at or near the time of planting. Do major tillage in fall or very early spring several weeks before planting. Use flame or very shallow cultivation to prepare the seedbed.
- Avoid practices that result in early pulses of nitrogen that may stimulate weed emergence. Use split N fertilizer applications and slow releasing forms of N, such as compost and legume–grass cover crop mixtures) to make N availability patterns over the season match N needs of the crop rather than the weeds.
- Avoid planting crops in fields with heavy populations of weeds with similar life cycles. For example, fields dominated by late emerging summer annual weeds might best be planted in early crops like peas.
- Time crop planting to take place well before the most abundant weed species in the field are expected to emerge.
- Time crop planting to take place after the expected major weed seedling flushes, and remove the latter by shallow cultivation or flame weeding.
- Invert the soil to a depth from which weed seeds cannot emerge (most effective for weeds with small, short-lived seeds).
Incorporated green manures or surface residues of cover crops can reduce the establishment of small-seeded weeds through allelopathy and/or physical hindrance. Thus, these practices can provide a measure of selective weed control for transplanted or large-seeded crops, which are tolerant to the stresses imposed by cover crop residues. This selectivity does not apply to small-seeded, direct sown vegetables like carrots and salad greens, which are at least as sensitive to these cover crop effects as small-seeded weeds.
Challenge of Weed Seed Bank Diversity
Remember that none of these strategies can be expected to eliminate the weed seed bank, and also that you may need to change seed bank management strategy as the seed bank itself changes. The reason the weed seed bank is so difficult to manage is because it contains not only many seeds, but many different kinds of seeds, with typically 20 to 50 different weed species in a single field. In other words, the grower may have to deal with 20 to 50 different plant survival strategies! Thus, there will almost always be some weeds that tolerate, or even thrive on, whatever combination of seed bank management strategies the farmer adopts.
For example, some but not all weed species have light-responsive seeds, and dark cultivation reduces emergence only in the light responders. Similarly, careful nitrogen (N) management can reduce problems with nitrate responders but have no effect on nonresponders and could even favor a weed that is well adapted to low levels of soluble N. The best approach to weed seed bank management is to design your strategy around the four or five most serious weeds present, then monitor changes in the weed flora over time, noting what new weed species emerge as the original target weed species decline. Then change your seed bank management strategy accordingly. Plan on making such adjustments every few years, and if possible, keep a sense of curiosity and humor about the weeds!
This article is part of a series on Twelve Steps Toward Ecological Weed Management in Organic Vegetables. For more on managing the weed seed bank, see:
References and Citations
- Brainard, D. C., R. R. Bellinder, R. R. Hahn, and D. A. Shah. 2008. Crop rotation, cover crop and weed management effects on weed seedbanks and yields in snap bean, sweet corn and cabbage. Weed Science 56: 434–441. (Available online at: http://dx.doi.org/10.1614/WS-07-107.1) (verified 23 March 2010).
- Burnside, O. C., R. G. Wilson, G. A. Wicks, F. W. Roeth, and R. S. Moomaw. 1986. Weed seed decline and buildup under various corn management systems in Nebraska. Agronomy Journal 78: 451–454. (Available online at: https://www.agronomy.org/publications/aj/abstracts/78/3/AJ0780030451) (verified 4 April 2011).
- Caldwell, B., and C. L. Mohler. 2001. Stale seedbed practices for vegetable production. HortScience 36: 703–705.
- Clements, D. R., D. L. Benoit, and C. J. Swanton. 1996. Tillage effects on weed seed return and seedbank composition. Weed Science 44: 314–322. (Available online at: http://www.jstor.org/stable/4045684) (verified 23 March 2010).
- Cousens, R., and S. R. Moss. 1990. A model of the effects of cultivations on the vertical distribution of weed seeds within the soil. Weed Research 30: 61–70. (Available online at: http://dx.doi.org/ 10.1111/j.1365-3180.1990.tb01688.x ) (verified 23 March 2010).
- Davis, A. S. 2004. Managing weed seedbanks throughout the growing season [Online]. New Agriculture Network Vol. 1 No. 2.
- Davis, A. S., J. Cardina, F. Forcella, G. A. Johnson, G. Khttp://eorganic.info/node/2806/editegode, J. L. Lindquist, E. C. Lusheri, K. A. Renner, C. L. Sprague, and M. M. Williams. 2005. Environmental factors affecting seed persistence of annual weeds across the US corn belt. Weed Science 53: 860–868. (Available online at: http://dx.doi.org/10.1614/WS-05-064R1.1) (verified 23 March 2010).
- Davis, A. S., and K. A. Renner. 2006. Influence of seed depth and pathogens on fatal germination of velvetleaf (Abutilon theophrasti) and giant foxtail (Setaria faberi). Weed Science 55: 30–35. (Available online at: http://dx.doi.org/10.1614/W-06-099.1) (verified 23 March 2010).
- Davis, A. S., K. A. Renner, and K. L. Gross. 2005. Weed seedbank and community shifts in a long-term cropping systems experiment. Weed Science 53: 296–306. (Available online at: http://dx.doi.org/10.1614/WS-04-182) (verified 23 March 2010).
- Egley, G. H. 1996. Stimulation of weed seed germination in soil. Reviews of Weed Science 2: 67–89.
- Egley, G. H., and R. D. Williams. 1990. Decline of weed seeds and seedling emergence over five years as affected by soil disturbance. Weed Science 38: 504–510. (Available online at: http://www.jstor.org/stable/4045064) (verified 23 March 2010).
- Forcella, F. 2003. Debiting the seedbank: Priorities and predictions. p. 151–162. In R. M. Bekker et al. (ed.) Seedbanks: Determination, dynamics and management. Aspects of Applied Biology 69. Association of Applied Biologists, Wellesbourne, UK.
- Forcella, F., K. Eradat-Oskoui, and S. W. Wagner. 1993. Application of weed seedbank ecology to low-input crop management. Ecological Applications 3: 74–83. (Available online at: http://www.jstor.org/stable/1941793) (verified 23 March 2010).
- Gallandt, E. R., M. Liebman, and D. R. Huggins. 1999. Improving soil quality: Implications for weed management. p. 95–121. In D. D. Buhler (ed.) Expanding the context of weed management. Food Products Press, New York.
- Harbuck, K. Z. 2007. Weed seedbank dynamics and composition of Northern Great Plains cropping systems. MS Thesis. Montana State University, Bozeman, MT.
- Liebman, M., C. L. Mohler, and C. P. Staver. 2001. Ecological management of agricultural weeds. Cambridge University Press, New York.
- Menalled, F. 2008. Weed seedbank dynamics and integrated management of agricultural weeds. Montana State University Extension MontGuide MT200808AG. (Available online at: http://www.msuextension.org/publications/AgandNaturalResources/MT200808AG.pdf) (verified 11 March 2010).
- Mohler, C. L. 2001a. Weed life history: identifying vulnerabilities. p. 40–98. In M. Liebman et al. Ecological management of agricultural weeds. Cambridge University Press, New York.
- Mohler, C. L. 2001b. Mechanical management of weeds. p. 139–209. In M. Liebman et al. Ecological management of agricultural weeds. Cambridge University Press, New York.
- Teasdale, J. R., R. W. Magnum, J. Radhakrishnan, and M. A. Cavigelli. 2004. Weed seedbank dynamics in three organic farming crop rotations. Agronomy Journal 96: 1429–1435. (Available online at https://www.agronomy.org/publications/aj/articles/96/5/1429?highlight=JmFydGljbGVfdm9sdW1lPTk2JnE9KGF1dGhvcjolMjJUZWFzZGFsZSUyMikmcT0oam91cm5hbDphaikmbGVuPTEwJnN0YXJ0PTEmc3RlbT1mYWxzZSZzb3J0PQ%3D%3D ) (verified 4 April 2011).
- Westerman, P. R., M. Liebman, F. D. Menalled, A. H. Heggenstaller, R. G. Hartzler, and P. M. Dixon. 2005. Are many little hammers effective? Velvetleaf (Abutilon theophrasti) poplution dynamics in two- and four-year crop rotation systems. Weed Science 53: 382–392. (Available online at: http://dx.doi.org/10.1614/WS-04-130R) (verified 23 March 2010).
Published August 20, 2013
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The weed seedbank in ƒi in the current period, then becomes equal to those surviving seeds that do not germinate in the seedbank at t−1, sbi,t−1, and the contribution of seeds dispersed in the landscape in the current period, as shown in Eqs (5) and (6):(5)sb1,i,t=sb1,i,t−1×1−sm1×1−gr1+∑j=1nD1,j,i,tfjfi(6)sb2,i,t=sb2,i,t−1×1−sm2×1−gr2+∑j=1nD2,j,i,tfjfi
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The Future of Agricultural Landscapes, Part III
David A. Bohan , . Michael J.O. Pocock , in Advances in Ecological Research , 2021
22.214.171.124 Seedbank counts
France: The weed seedbank was assessed using the methods described in Heard et al. (2003) . Seedbank abundance was estimated by taking five soil cores (1.5 L in total between 0 and 20 cm depth) at the 4 and 32 m sampling points along the transects. The seedbank was estimated by germination of soil samples in a greenhouse under controlled conditions (18/15 °C day/night temperature regime with 12:12 h light:dark cycle). Counting and species identification of the germinated seeds in the samples were done up to 18 weeks after sample preparation. Weed seeds germinating from the soil samples collected in 2017 and 2018 were identified to species and summed to a total count of seeds per sampled field.
Seedbank diversity in Catalonia
Izquierdo et al. (2009) examined how the spatial distribution of weed seedbank diversity was affected by weed control. They examined the changing spatial distribution of weed seeds in an 8-ha winter wheat field (Triticum aestivum) field in western Catalonia from 2001 to 2003. The field was regularly treated with herbicides to control grass and broadleaf species, except only grass herbicides were administered in 2002 and 2003. 16-cm 2 soil cores were taken at 10-m intervals on a 150 × 150 m grid in the wheat field in January of each year. Seeds were allowed to germinate in a greenhouse, identified, and the density per square meter estimated for each species at 254 sample points (2 points were skipped). The distribution of weed seed diversity within the 2.25-ha area was mapped for each year. Izquierdo et al. found that the spatial distributions of Shannon diversity and evenness became increasingly patchy over time. Both grass and broadleaf weed patches moved and varied in size from year to year. In general patches of broadleaf weeds decreased in response to herbicide application, but the absence of a grass herbicide application in the first year enabled grass patches to expand contributing to increased patchiness. Izquierdo et al.’s Table 1 gives the density and SE of seeds (#/m 2 ) for 30 weed species. Despite the year-to-year variation in diversity and spatial distribution, the 3 years’ mixed-species TPLs do not differ significantly and are best described by a single line ( Fig. 6.8 ; Appendix 6.K).
Fig. 6.8 . There is no difference between years in the mixed-species TPL (NQ = 254, NB = 68) of a community of 30 weed species in the seedbank of a field in Catalonia. Each point in this graph is a different weed species recovered in sampling the seedbank by soil core.
Data from Table 1 in Izquierdo et al. (2009) .
Weedy rice (Oryza spp.)
Understanding factors influencing weedy rice seed decay in the soil can have important implications for management targeting weed seedbank of weedy rice. However, information on factors affecting the decay of weedy rice in DSR systems is meager. Soil moisture has been probably best documented for its influence on weedy rice seed decay. For example, winter flooding in Italy between rice crops reduced the viability of weedy rice seeds on the soil surface by >95% compared to a reduction by 26%–76% when the field is left dry ( Fogliatto et al., 2010 ). The study concluded that the reduction in viability was partly due to seed decay of nongerminated seeds under low-temperature conditions and flooding. However, another study conducted in Korea by Baek and Chung (2012) also observed that winter flooding reduced the germination of weedy rice but the effect was not as dramatic as reported by Fogliatto et al. (2010) . They observed that >60% weedy rice seeds could overwinter under flooded conditions, whereas in dry conditions, about 90% weedy rice seeds could overwinter.
Ecological weed management in Sub-Saharan Africa: Prospects and implications on other agroecosystem services
Paolo Bàrberi , in Advances in Agronomy , 2019
1.2.3 Reduced weed seedbank size
The major part of weeds in agricultural land reproduce and survive as seeds, thus the soil weed seedbank represents the main source of future weed infestations. Depletion of the weed seedbank can be obtained by increasing seed losses and/or reducing seed inputs. Losses can occur through seed predation, seed decay, and increased germination ( Gallandt, 2006 ).
Weed seed predation, especially after seeds have been shed on soil, may be an important determinant of seedbank losses ( Davis et al., 2013 ; Westerman et al., 2011 ). Insects and small rodents are the main contributors to weed seed predation, thus manipulation of agricultural habitats as to attract them (e.g., no-till, delayed stubble cultivation, introduction of uncultivated strips within fields or as field margins) is expected to increase the number of weed seeds predated ( Landis et al., 2005 ). Carabid beetles are among the most important consumers of weed seeds. It should be kept in mind that seed consumption by carabids is influenced by several factors, including weed species, seed physiological state, insects gender, activity-density level, and seed burial depth ( Kulkarni et al., 2015, 2016 ).
Weed seed decay is a mechanism so far poorly understood and consequently poorly exploited. It refers to the creation of soil conditions that are conducive to increased seed mortality through, e.g., fungal attack. Recently, some interesting results have been obtained by Gómez et al. (2014) , who nevertheless pointed out that are differences in weed species susceptibility to decay, indicating the need to develop species- and cropping system-specific management solutions.
Increased weed seed germination results in an output to the seedbank. This can be achieved, e.g., by the application of the false- and stale-seedbed techniques, i.e., the anticipated soil seedbed preparation which allows stimulation of germination and emergence of weed seedlings that are subsequently destroyed before the actual crop seeding or crop emergence takes place ( Cloutier et al., 2007 ). In the false seedbed technique seedling destruction usually occurs by harrowing or similar mechanical tools whereas in the case of the stale seedbed technique it occurs by chemical herbicides or by thermal methods (flame weeding or soil steaming), to avoid any further soil disturbance. Weed seed losses can also occur when seed germination is not followed by seedling emergence, usually because the seed is placed too deep down the soil and has not enough reserves in its endosperm to sustain seedling growth until it reaches the soil surface and becomes autotroph. This phenomenon is referred to as “fatal germination” ( Fenner and Thompson, 2005 ).
Weed seedbank replenishment can also be avoided by preventing production and shedding of new seeds. This can be obtained as an outcome of increased competition or as an effect of a well planned crop rotation ( Légère et al., 2011 ). However, it is also important to prevent seed shedding from late emerging weeds that, although usually unable to diminish crop yield in the same growing season, may create potential weed problems in subsequent crops or growing seasons through their seed inputs. Similarly, it is important to avoid weed seed shedding (e.g., by stubble cultivation or mowing) in the period between two crop growing cycles, an important issue that many farmers tend to disregard.
Integrated Weed Management in Organic Farming
Charles N. Merfield , in Organic Farming , 2019
5.4 Weed Seed Rain and Seedbank
As discussed in Section 5.2.4 , the core of weed management rests on minimizing the weed seed rain and therefore minimizing the weed seedbank. This is why managing the weed seed rain is the most important part of integrated organic weed management and must therefore be the first priority.
To illustrate the direct relationship between the weed seedbank and in-crop weeds, a study by Rahman et al. (1996) studying the number of emerged weeds vs. the number of viable weed seeds in the soil found a clear, almost one-to-one relationship ( Fig. 5.9 ). Clearly, the larger the weed seedbank, the larger the population of in-crop weeds.
Figure 5.9 . Number of weed seedlings emerged versus the number of viable seeds found in each of 48 individual soil samples. Note log scales both axes.
Adapted from Rahman et al. (1996) .
In terms of the linkage between weed seed rain and in-crop weed populations, Gallandt et al. (2010) found that by preventing weed seed rain they could reduce subsequent years’ weed seedbanks compared with other autumn treatments between 45% and 93% and weed seedling densities by 23% to 90%. In Western Australia preventing the seed rain of annual ryegrass ( Lolium rigidum) reduced in-crop ryegrass emergence by 90% in 4 years ( Walsh et al., 2013 ). Another perspective is illustrated by a study by Rahman et al. (1998) whereby they tilled soil monthly for 4 years, achieving an exponential decline in the weed seed bank, represented by four weed species, both monocotyledons and dicotyledons ( Fig. 5.10 ).
Figure 5.10 . Decline in seed numbers of four weed species at the Ruakura Research Centre site following monthly tillage over a period of 4 years. Note y-axis is a logarithmic scale.
Adapted from Rahman et al. (1998) .
While this level of tillage in real-world farming is clearly excessive and would be highly damaging to soil, it, along with the other examples, clearly illustrates the importance of minimizing weed seed rain and the ability to reduce an existing weed bank. Depleting the seedbank also underlies the false and stale seedbed techniques, as these deplete the emergable weed seedbank (see Section 10.8.2.1 ).
In terms of replenishing the weed seed bank, the potential is almost limitless. Many weeds can produce tens, hundreds of thousands, even millions of seeds ( Robbins et al., 1953; Salisbury, 1961; Gwynne and Murray, 1985 ), though seed production per plant is typically much lower due to competition. As an example a weed population of 1000 weed m 2 (one weed per 10 cm 2 ) and 1000 seeds per weed, represents ten billion seeds per hectare or one seed per square millimeter! While this shows the potential to rapidly refill the weed seed bank, it should not be concluded that the effects will persist for many years, rather the above research shows that it can be quickly reduced again.
Biology and Ecology of Weeds and the Impact of Triazine Herbicides
Homer M. LeBaron , Gustav Müller , in The Triazine Herbicides , 2008
Herbicides and Weed Biology
The use of triazine herbicides resulted in the control of many weed species with one application. Research showed that repeated control of weeds resulted in reductions in the weed seedbank in soil after several years. In a 6-year study in Colorado, Schweizer and Zimdahl (1984) found the number of seeds in the seedbank decreased by approximately 70% after 3 years of annual atrazine application plus interrow cultivation. Atrazine use was ceased in some plots after the first 3 years, and weeds were controlled with one or two cultivations. After 3 years of cultivation only, the weed seedbank was approximately 25 times greater than those where atrazine use and cultivation were continued. A similar study was conducted at five locations in Nebraska ( Burnside et al., 1986) . Broadleaf and grass seed density in the soil declined by 95% after a 5-year weed-free period. During the sixth year, herbicide use was ceased, and seed density increased to 90% of the original level at two of the five locations. These studies demonstrate that weed management has a great impact on the weed seedbank, resulting in a rapid decline in the seedbank when seed introductions are minimized or prevented. However, a small number of seeds of most weed species remain viable for long periods in the soil, and when weed management practices are not entirely effective, these seeds can germinate, mature, and produce enough seed to replenish the seedbank ( Buhler et al., 1998) .
Norris (1992) proposed that with proper use of herbicide and weed management technology, we can eliminate weeds from an area by preventing weeds from producing seed. He further stated that the economic threshold, defined as the pest population at which control action should be initiated in order to prevent the population from increasing to or exceeding the economic injury level, should not be adopted in weed management as it has been in entomology for insect management. Weed management must recognize long-term weed population dynamics, including the nature of the seedbank. He recommended that weed management, especially for serious problem weeds, should adopt a ‘no-seed’ threshold. This threshold implies that weeds should not be permitted to set seed. He cited several cases where this has worked in California on high-value crops where the same growers are in control of the land for many years. Norris (1999 , 2000 ) further stated that a ‘no seed’ threshold can only be successful when weed management technologies are integrated, including the use of hand labor for controlling low-weed populations that have not succumbed to other management tools.
Jones and Medd (2000) proposed that a longer-term management approach is needed to manage weed seedbanks and to determine the optimal level of intervention required for a specific weed situation. Managing seedbanks is complex because of the difficulty in preventing seed production and introduction, as well as the persistence of certain seeds in the seedbank and the high seed production potential of many weed species ( Buhler et al., 1998) . Weed seedbanks are an ever-present component of agricultural land, and resources directed to understanding, interpreting, and predicting seed germination potential can improve agricultural production. Management systems can be devised that minimize the impact of the resultant weeds.
Cousens and Mortimer (1995) confirmed that fields receiving herbicides annually for more than 20 years may be reinfested with damaging weed flora if left unsprayed, often within one or a few years.
Weed populations are never constant, but are in a dynamic state of flux due to changes in climate, environmental conditions, tillage, husbandry methods, use of herbicides, and other means of control. Weeds that were at one time of minor importance, but not controlled by certain broad-spectrum herbicides, have increased to become major problems. Reduction in tillage has sometimes led to the increased occurrence of perennial weeds and annual grasses, particularly of those species that readily establish near the soil surface and have relatively short periods of dormancy. Many perennials have increased in importance under minimal cultivation (e.g., field bindweed and Canada thistle). The occurrence of herbicide-resistant weed biotypes is also a phenomenon of increasing concern. Some research results show that large changes in the seedbank can impact weed control efficacy. Winkle et al. (1981) and Buhler et al. (1992) found large increases in weed densities reduced weed control with herbicides and mechanical practices.
Webster and Coble (1997) reported on weed shifts in major crops of the Southeastern states over a 22-year period (1974–1995) when herbicides were the major means of weed control. Sicklepod and bermudagrass had become the most troublesome weeds. The largest decreases in weed pressure were found with Johnsongrass, crabgrasses, and common cocklebur. Morningglories and nutsedges remained relatively constant. The weeds of greatest importance in soybean, peanut, and cotton are the pigweeds.
Webster and Coble (1997) listed several factors that may play an important role in the future weed species composition of cropland: (1) Herbicide-resistant weeds represent a change in the weed spectrum in some of the management systems, with almost every state having at least one reported herbicide-resistant weed. (2) Cropping systems that use fewer tillage operations may allow weeds that are unable to survive frequent disturbances (e.g., biennials and simple perennials) to invade and become problem weeds in fields. (3) A reduction of triazine herbicides used in corn and cotton weed management systems may allow previously controlled broadleaf weeds to become major weeds again. (4) The widespread use of herbicide-tolerant crops may have a further significant impact on the weed species composition.
Changes in weed species and populations also cause changes in plant diseases and insect pests since certain weeds serve as their hosts ( Bendixen et al., 1981 ; Manuel et al., 1982 ; Weidemann and TeBeese, 1990 ; Norris and Kogan, 2000) . Herbicide-resistant weed biotypes are present in our weed populations, although often at very low frequencies, even when herbicides are not used. Weed species have acquired built-in genetic adaptability to survive most control methods used against them. For example, dandelions usually develop a vertical growth habit when growing wild, but when growing in a frequently mowed lawn, more prostrate or flat-growing biotypes evolve. We should continually add to our weed control technology and keep tools available in order to address the adaptability of weeds to different control methods. For further information on the biological characteristics of weeds, including growth strategies, mimicry with crops, plasticity of weed growth, photosynthetic pathways, weed seed reservoir, and vegetative reproduction see Cousens and Mortimer (1995) and Buhler et al. (1998) .
Weeds Resistant to Nontriazine Classes of Herbicides
Homer M. LeBaron , Eugene R. Hill , in The Triazine Herbicides , 2008
Use of Modeling in Managing Herbicide Resistance
The rate of evolution of resistant weeds is based on several factors, including characteristics of the weed and herbicide, gene frequency, size and viability of the soil seedbank, weed fitness, herbicide potency, frequency and rate of application, and persistence in soil. Various attempts have been made to use modeling to determine the relative importance of these factors and to predict the probability of resistance, as well as to evaluate how to avoid, delay, or solve the problem ( Gressel and Segel, 1990 ).
Richter et al. (2002) have reviewed the use of models to evaluate the dynamics of herbicide resistance and to develop suitable anti-resistance strategies. Herbicide resistance is impacted by a high initial frequency of resistance alleles in a population, out-breeding, dominance of inheritance, a short persistence of the seed bank in the soil, and the lack of a fitness penalty for resistant versus susceptible biotypes of a weed species, along with agronomic factors having a positive influence on weed development. The occurrence of herbicide-resistant weeds in a field usually means the loss of an effective control measure. This is particularly serious if resistance develops in species for which there are few if any effective alternatives. As a rapid increase in the development of herbicides with new modes of action is not likely, and since economic and environmental conditions often will not support cultural control measures or alternative cropping systems, it is important to manage resistance wisely in order to avoid further loss of herbicides.
Using a model to maximize strategies for herbicide-resistant blackgrass, Cavan et al. (2000) gave estimates on the effectiveness of various strategy options. Based on research with a long-term model for control of blackgrass and annual bluegrass, Munier-Jolain et al. (2002) concluded that threshold-based weed management strategies can be more cost-effective than spraying every year and may enable important reductions in herbicide use. However, the highest long-term profitability was obtained for the lowest weed level threshold tested.
Müller-Schärer et al. (2000) reviewed the progress made during 1994–1999 by 25 institutions within 16 European countries on biological weed control. These efforts were aimed at control of major weed species, including common lambsquarters, common groundsel, and species of pigweed, broomrape and bindweed in major crops, including corn and sugar beet. No practical control has yet been reached for any of the five target weeds, however, the authors concluded that potential solutions have been identified.
Weed and soil management: A balancing act☆
Trevor Kenneth James , Charles Norman Merfield , in Reference Module in Earth Systems and Environmental Sciences , 2021
Weed management interacts with soils in multiple ways. The primary aim of weed management is to limit weed populations below the levels where they cause economic losses, both in current and future crops. Weed management therefore deliberately aims to reduce plant biodiversity, both in the amount and range of non-crop species, which produces a cascade of negative ecological effects, including reduced soil health. Where weed management achieves a very large reduction in weed populations bare soil can result which is at risk of erosion and other damage. Tillage (e.g. mouldboard ploughing/inversion tillage) is an integral part of weed management in many farming systems, particularly cropping systems (e.g., cereals and vegetables), and tillage’s negative impacts on soil are well documented, including in this encyclopedia. In response to the harmful effects of tillage, especially inversion tillage, on soil health, reduced/minimum tillage and no-tillage systems were developed and have been widely adopted. Changing tillage systems can have large impacts on both weed flora as well as soil health.
The soil is where the weed seedbank is situated, and particularly for therophyte weeds, managing the weed seedbank is critical for effective weed management, especially under non-chemical and integrated weed management systems ( Fig. 1 ). Soil health and function also have a large impact on the weed seedbank, as there are many organisms in soil, from microorganisms through invertebrates to vertebrates that predate on seeds, so soil management and soil health can affect how much of the weed seedbank is lost to predation. In comparison, mechanical techniques to reduce the weed seedbank, such as fallowing are highly damaging to soil due to both repeated tillage and absence of living plants, resulting in compaction, loss of structure, reduction in soil biological diversity, etc.
Fig. 1 . Weed seed banks can contain millions of seeds giving rise to large numbers of weeds, Digitaria sanguinalis (L.) Scop. uncontrolled in maize (Zea mays L.).
Integrated weed management (IWM) is the approach promoted by weed scientists as the most effective long-term and damage limiting approach to weed management. It systematically integrates physical, chemical, biological and ecological approaches using the technique that achieves optimal weed control with the fewest negative outcomes for the wider environment for any given issue. However, physical and chemical techniques are often harmful to soil, and greater use of biological and ecological techniques is required. Additionally, over reliance on chemical control can lead to herbicide resistant weeds. One such technique is cover cropping, which has many permutations and can achieve good weed management while at the same time avoiding the damage and consequences of physical and chemical techniques and even improving soil health at the same time.
System level farm tools for weed management, for example, diversified rotations, can have positive effects on soil health, e.g. the inclusion of a pasture phase in a cropping rotation, improves all aspects of soil quality and health, and reduce the soil seedbank and therefore the need to use harmful agrichemicals.
While weed management has multiple effects on soil, many negative, effective weed management is critical in farming, particularly cropping. If not controlled, weed populations can rapidly reach exceptionally high levels to the point that crop yield is dramatically reduced, even to the level of complete crop loss. Further, for any given crop or pasture there will always be many different species of weeds present and any given weed species can infest a very wide range of crops and pastures, which means weeds are the ubiquitous pests of crops and pasture. This contrasts with invertebrate pests and diseases (e.g. fungi and bacteria) of plants which are typically highly host specific, i.e., any given pest can attack only a narrow range of host species, and vice versa any given crop species is only attacked by a small range of pests and diseases. Weeds can also host crop plant pests and diseases. Effective weed management is therefore vital for successful agriculture and horticulture.
Weed management therefore interacts with soil in many different ways, and while often having negative effects, there are many techniques available to help ameliorate negative effects and increase positive effects. Weed management is therefore a balancing act, between achieving sufficient weed control for good crop yield and quality, while minimizing negative impacts on soil and the rest of the farm environment.
Weeds of farm crops
Some weed species are able to produce thousands of seeds per plant. Examples of prolific seed producers include corn poppies and mayweed species. The seed reservoir (weed seed bank) in some soils can be as high as 40 000/m 2 . Not all the seed produced in one year will germinate the next year; the percentage emergence may only be around 2–6% of the weed seedbank. Many species have some sort of seed dormancy mechanism that has to be broken before they will germinate. Once dormancy has been broken environmental conditions must also be correct for germination; this accounts for some of the variation in weed populations between years. Losses of seed and seed viability are taking place all the time. Depth of burial in the soil, number and type of cultivations, soil type and weed species affect the rate of decline. The seed viability of some species such as fumitory, charlock, black bindweed, wild oats and corn poppy declines very slowly compared with the rapid decline of some grasses such as barren brome.
Preventive Weed Management in Direct-Seeded Rice
Adusumilli N. Rao , . David E. Johnson , in Advances in Agronomy , 2017
2.5.2 Stimulating Fatal Germination With Crop and Cover Crop Rotations
Rotational crops (including cover crops) often entail the application of practices which stimulate germination (e.g., tillage and irrigation) as well as those that kill emerged seedlings (e.g., cultivation and herbicides). Rotational crops may result in the germination and death of rice weeds in a manner analogous to the stale seedbed approach described earlier.
As with a stale seedbed, the success of rotational crops in reducing the rice weed seedbank depends on an appropriate stimuli being applied at the right time (when seeds are relatively nondormant), as well as on the use of effective postemergence termination methods. Likewise, the efficacy of rotational crops in promoting fatal germination is likely to be the greatest for weeds with limited dormancy and in rotational crops for which weed management is relatively easy and inexpensive. Indeed, if rice weed seeds are stimulated to germinate in rotational crops and are not effectively terminated, they may exacerbate weed problems through reproduction.
In the rice–wheat rotation of India, the inclusion of mungbean during the fallow period between wheat harvest and rice planting resulted in 84% and 40% reduction in the population of D. aegyptium in the subsequent rice crop under ZT and CT systems, respectively ( Fig. 2 ). This was because of the greater emergence of this weed species following irrigation during the mungbean cropping, followed by effective termination using nonselective herbicides (in ZT) and shallow tillage (in CT).
Fig. 2 . Effects of crop rotation (by including mungbean) and tillage in the rice–wheat rotation system of India on the cumulative emergence of Dactyloctenium aegyptium during fallow/mungbean period and during the subsequent direct-seeded rice crop (Kumar et al., unpublished data).
In temperate cropping systems, cover crops have been evaluated for their potential to promote the fatal germination of weed seeds. For example, Mirsky et al. (2010) reported declines in weed seedbanks by encouraging fatal germination associated with soil disturbance in cover crop treatments. Cover crops stimulated weed seed germination and the germinated weeds were either suppressed by the cover crop or controlled by subsequent tillage and preempted weed seed rain. The stimulative effect of certain cover crops has been proven particularly helpful in the management of parasitic weeds. In upland DSR fields in East Africa, green manure ( Crotalaria ochroleuca, M. invisa, and Cassia obtusifolia) exhibited a potential to induce the suicidal germination of S. asiatica ( Kayeke et al., 2007 ). The cover crops in this case served as a false-host by stimulating the germination of Striga without providing conditions necessary for survival.
Agricultural soils contain thousands of weed seeds per square foot. The density of weed seeds in the weed seedbank is influenced by past farming practices and will vary from field to field (Table 1, Renner, 1999) and even between areas within fields. In intensively cropped fields in the north central corn belt, the weed seedbank ranged from 56 – 14,864 seeds per square foot (Forcella et al., 1992).
Table 1. How many weed seeds are in the weed seedbank?
Number of seeds per square foot
W.K. Kellogg Biological Station—Long term Ecological Research Site, Hickory Corners, Michigan
Composition of Weed Seedbanks
Seedbanks are made up of numerous weed species although only a few species will comprise 70 to 90 percent of the total seedbank. Common lambsquarters (Chenopodium album) is the dominant weed seed in many field soils in the north central region of the United States, including Michigan. Common lambsquarters dominated the weed seedbank in three of five cropping systems at the Long Term Ecological Research (LTER) site at the Kellogg Biological Station in Michigan (Figure 1).
Weed Seed Distribution in Soil
The location of seeds in the weed seedbank is influenced by the tillage system. More weed seeds will remain near the soil surface when tillage is reduced or no-till farming is practiced (Figure 2). These changes in the distribution of the weed seeds in the weed seedbank will influence weed emergence and the resulting weed population in farm fields.
Sources of Weed Seed
Weed seeds can reach the soil and become pat of the seedbank through several avenues. The main source of weed seed in the seedbank is from, weeds that matured in the field and set seed. Annual weeds produce large numbers of seeds (Table 2, Renner, 1999). Weed seed can also enter the seedbank by wind, water, animals, birds, and human activity. Some weed seeds (such as dandelion) are wind-dispered. Weed seeds can reach a field site following flooding of drainage ditches or adjacent rivers. Wildlife and livestock can spread weed seed, either directly or by spreading of manure. Farming operations can add weed seed to the seedbank by moving soil that contains weed seeds on farm equipment or moving weed seeds to other fields when harvesting crops.
Table 2. Typical Michigan weed seed production
Number of seeds per plant
Weed density (per 33 feet of crop row)
Weed Seed Fate
A weed seed can have numerous fates once it is dispersed in a field (Figure 3, Renner, 1999). Some weed seeds will decay in the soil. Other seeds will decay in the soil. Other seeds will not decay but will no longer have the ability to germinate (the seeds are not viable). Some weed seeds will germinate and die, while other weed seeds will germinate and emerge. Some weed seed will be predated by various predators including birds, rodents, crickets, carabid (ground) beetles, and ants. Seed predation occurs mainly on or near the soil surface. Many weed seeds will remain dormant in the soil and not germinate regardless of environmental conditions. However, dormancy is not permanent and seeds of many weed species change from a state of dormancy to non-dormancy. This is called dormancy cycling (Figure 4, Renner, 1999). Seed dormancy is a survival mechanism and it is a major barrier to weed control in agroecosystems. Only a fraction of the weed seeds (less than 10 percent of most weed species) germinate each year. Therefore dormant seeds perpetuate the weed seedbank and weed populations in farm fields.
Weed Seed Persistence
Under agricultural conditions the average time that a weed seed will persist in soil and still be capable of germinating (remain visible) is less than five years. Some weed species will have more persistent seed than others. Velvetleaf (Abutilon theophrasti) and clovers (Trifolium sp.) have persistent seedbanks. Tillage also influences seed longevity in soil since weed seeds usually remain viable longer if they are buried. Seed on or near the soil surface is exposed to predators and seed decay which reduces seed persistence.
Managing the Weed Seedbank
The best way to manage the weed seedbank is to not allow weeds to set seed in the field. Over a six-year period in Colorado, common lambsquarters and redroot pigweed seeds were reduced to 6 and 1 percent, respectively, of the original seedbank in a continuous corn rotation where herbicides were applied and the fields cultivated (Schweizer and Zimdahl, 1984). In a Nebraska study, the broadleaf and grass weed seed density in soil declined by 95 percent over a five-year period. However in the sixth year weeds were not controlled and the weed seedbank increased to within 90 percent of the original level at two of five locations (Burnside et al., 1986). These studies illustrate two important points in weed seedbank management. First, there is a rapid decline in the weed seedbank when weeds are not allowed to set seed. Secondly, the few weed seeds remaining in the weed seedbank are capable of infesting the farm fields and returning the number of weed seeds in the seedbank to high levels. Therefore weeds must be managed every year to reduce the weed seedbank.
Other farming practices can influence the weed seedbank. Burying weed seed by tilling the soil increases longevity of weed seeds in the seedbank. Leaving weed seeds on the soil surface exposes weed seeds to predation which will reduce the number of weed seeds in the seedbank. Leaving weed seeds on or near the soil surface may increase the number of weed seeds that decay after being infected by fungi or other microorganisms. Livestock manure that is stored has fewer viable weed seeds compared to fresh manure. Cleaning tillage and harvest equipment can reduce the movement of weed seed from field to field.
Understanding weed seedbank dynamics is the first step in managing the weed seedbank. Reducing the number of weed seeds in the weed seedbank will improve our management of weeds in agroecosystems.
Burnside, O. C., R. G. Wilson, G. A. Wicks, F. W. Roeth, and R. S. moomaw. 1986. Weed seed decline and buildup under various corn management systems across Nebraska. Agronomy Journal. 78:451-454.
Forcella, F., R. G. Wilson, K. A. Harvey, D. A. Alm, D. D. Buhler, and J. Cardina. 1992. Weed seedbanks of the U.S. corn belt: magnitude, variation, emergence, and application. Weed Science. 40:636-644.
Renner, K. A. 1999. Weed ecology and management. Pages 51-68 in Michigan Field Crop Pest Ecology and Management. M. Cavigelli, ed. Michigan State University Extension Bulletin E-2704.
Schweizer, E. E. and R. L. Zimdahl. 1984. Weed seed decline in irrigated soil after six years of continuous corn and herbicides. Weed Science. 32:76-83.