Discuss the role of a weed seed bank

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The role of weed seed bank, its function in crop production.

Title: The role of weed seed bank, its function in crop production.

Keywords: weeds, seed bank, crop production

Introduction

Any plant that is objectionable or interferes with the activities or welfare of man is defined as a

weed. Weeds are competitive in nature, persistent and pernicious; hence they have a vital function

in crop production. Weeds are undesirable and are considered to be pests just as insects and disease

organisms are considered to be pests. They compete with field crops for materials essential for

growth and production. In a weed infested field, no matter how much fertilizer or water is applied to

a crop weeds will always get a disproportionate share of the resources (Herren 2011, p.344).

There is an old adage which says “One year is seeding-seven year is weeding”. The importance of

the adage has increased with advent of herbicide resistance in weeds. It is very likely that herbicide-

resistant weeds will produce seeds that will germinate and produce plants that are also herbicide

resistant (Shrestha, 2004). For those farmers who do not rely on chemical weed control, herbicide

resistant weeds is not a problem to them.

However, it is equally important for them to understand weed seed banks because it is the main

source of weeds in agricultural fields. According to (Zimdahl 2013, p.113) said “Most weeds start

their life cycle from a single seed in soil, if these weeds escape control strategies employed by the

farmer; they grow and produce thousands of seeds.” An example is Amaranthus fricolor (pigweed)

which produces 117 000 seeds per weed.

The weed seed bank is the reserve of viable weed seeds present on the soil surface and scattered

throughout the soil profile (Mahesh and Robert 2007, p.38). 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 (Douglas et al.,

2008).

In the following discussion, the term weed seed bank is defined as the sum of viable weed seeds and

vegetative propagates 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

propagates per square foot. Hence the understanding of what happens to seeds in the seed bank can

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be an important component of overall weed control (Robert and Shirtliffe, 2009).

This article will seek to discuss one of the most important yet often neglected weed management

strategies of reduce the number of weed seeds present in the field, and thereby limit potential weed

population during crop production. This is accomplished by managing the weed seed bank.

Dynamics of Weed Seed Bank

Seed banks are generally composed of numerous species belonging to three groups. The first group

includes dominant species accounting for 70 to 90 % of the total seed bank (Callagner 2013, p.57).

These species represent most of the weed problems in a cropping system. Second group of species

comprise of 10 to 20 % of the seed bank, including those adapted to the geographic area but not to

current production practices. The final group accounts for a small percentage of the total seed and

includes recalcitrant seeds from previous seed banks of the previous crop (Rao 2010, p.37).

Weed seeds can have numerous fates after they are dispersed into a field (figure. 1). Some seeds

germinate, emerge, grow, and produce more seeds; others germinate and die (fatal germination),

decay in the soil, or fall to predation by insects, birds or mammals. The seeds and other propagules

of most weeds have evolved mechanisms that render a portion of propagules dormant (alive but not

able to germinate) or conditionally dormant for varying periods of time after they are shed (Baskin

2003, p.47).

Figure 1| Fate of weed seeds. Inputs to the seed bank are shown with black arrows and losses with

white arrows. Figure Sourced from: (Fabian Menalled, 2001) MSU Extension, Montana State

University.

The cycle shown above in (figure. 1) helps the weed to survive in a periodically disturbed,

inhospitable, and unpredictable environment. Weed seeds can change from a state of dormancy to

non-dormancy, 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.

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In agronomic situations, we are most interested in the majority of seeds that germinate during the

first few years after shed. “Understanding the short term dynamics of these seeds and their resultant

weed population will aid in predicting potential for crop yield loss and control costs.” (Blackshaw

2008). Management practices have major impacts on these processes and represent opportunities for

regulating seed bank characteristics in crop production systems.

Harmful effects of weeds to crop production

Weeds have serious impacts on agriculture production. It is estimated that in general 5 % loss in

agricultural production in most developed countries and 25 % in least developed countries. Yield

losses due to weeds vary with crops. Every crop is exposed to severe competition from weeds. Most

of these weeds are self-sown and they provide competition caused by their faster rate of growth in

the initial stages of crop growth (Naylor, 2008). These losses caused by weeds in some of the

important crops are shown in table 1.

Table 1 | Effects of weeds to crop yield losses (Naylor, 2008)

Crop Reduction Due to Weed %

Rice (Oryza sativa) 41.6

Wheat (Triticum sativum) 16.0

Maize (Zea mays) 39.8

Cotton (Pennisetum glaucum) 72.5

Soybean (Glycine max L.) 30.5

*In some crops, the yields are reduced by more than 50 % due to weed infestation.

Weeds compete with crops for water, nutrients, light and space and this will reduce the crop yield.

An estimate shows that weeds can deprive the crops 47 % N, 42 % P, 50 % K, 39 % Ca and 24 %

Mg of their nutrient uptake (Young and Pierce, 2013).

Weeds also act as alternate hosts that harbour pests, insects and diseases and other micro-organisms,

this will have an indirect negative impact to crop production.

Table 2 | Weeds as alternate hosts for crop pests and pathogens.

Crop Pest Alternative host

Zea mays (maize) Downy mildew Sacharum spontaneum (Kanz grass)

Triticum sativum (wheat) Black rust Agropyron repens (Couch grass)

Pennisetum glaucum (pearl millet) Ergot Cenchrus ciliaris (Foxtail grass)

Oryza sativa (rice) Stem borer Echinolochloa (Barnygrass)

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Some weeds release into the soil inhibitors of poisonous substances that may be harmful to the crop

plants (the effect of allelopathy).

Weeds reduce the quality of marketable agricultural produce. Contamination of weed seeds of

Datura and Argemone is harmful to human health and weed seeds present in the produce cause

odour. Price dockage for cotton flint at the market is much common due to contamination by Bidens

pilosa (black jack) seeds (Inderjit 2004, p.38).

Weeds not only reduce yield but also interfere with agricultural operations for maximum crop

production. Weeds make mechanical sowing a difficult process and render harvesting difficult,

leading to increased expenditure on labour, equipment and chemicals for their removal.

Beneficial effects of weeds to crop production

On contrary, weeds can offer some beneficial properties, particularly when occurring at low

densities. These aspects should be used in the farming system. Some of the potential benefits of

weeds are;

They help in conservation of soil moisture and prevent erosion. A ground cover of weeds will

reduce the amount of bare soil exposed helping to conserve nutrients particularly nitrogen which

could be leached away, especially in sandy soils (Barker, 2011).

Food and shelter can be provided for natural enemies of pests and even alternative food sources for

crop pests. The actual presence of weed cover may be a factor in increasing effectiveness of

biological control of pests and reducing pest damage (Clark, 2008).

Managing the seed weed bank “Limiting the weed seed bank”

The higher fecundity of agricultural weeds coupled with their persistence in the seed bank indicates

that management strategies should not only focus on minimizing weed density and yield reductions,

they should also include approaches to minimize weed seed banks (Gulden et al., 2004). Preventing

weeds from setting seeds may not benefit the current crop but will pay-off in the long term. Any

seed that is produced will only add to the seed bank and contribute to future weed populations

which impede crop production (Derksen et al., 1993).

The study of weed seed bank composition and dynamics is a relatively new interesting concept in

weed science. Still, several management practices can be implemented to deplete the seed bank

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reservoir. Prevention is the most efficient approach to reduce weed seed banks it does not allow

weeds to set seed in the field. Care should be taken to avoid bringing new weed seeds into a field

through irrigation, equipment or animals (White et al., 2007).

This can be achieved by screening irrigation water, washing equipment before bringing it into the

field, keeping canal banks and irrigation systems weed free and keeping grazing animals in

quarantine before moving them from a weedy field to a clean one. Bidens pilosa (Black jack) and

Acanthospermum hispidum (Upright starbur) seeds have projected hooks that attach to animal fur,

this allows weed seeds dispersal to clean fields (Thompson et al., 2003).

Left| mature Bidens pilosa (black jack) seeds with projected hooks Right| mature Acanthospermum hispidum (Upright starbur) seed

Figure 2 | Seed morphology for Bidens pilosa (black jack) and Acanthospermum hispidum (upright

starbur) with projected hooks that facilitate their dispersal by clinging on animal fur.

The other approach for managing seed bank is by reduction, reduction not only minimizes future

weed problems, and it also reduces the speed at which weed patches expand across crop fields.

Increasing crop interference by increasing seeding rate and filling empty niches with cover crops

helps minimize weed seed inputs into the seed bank. Other approaches include mowing weeds prior

to seed production and controlling weeds with herbicides or cultivation (Monaco et al., 2002).

The annual broadleaf weed Xanthium strumarium (common cocklebur) has high prolific seed

production of 7000 seeds per plant in the absence of crop competition and 1000 seeds per plant

while growing in association with Glycine max (soybean) (Moyer, 2000). According to (Rao 2000,

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p.11) said “Seed production of Abutilon theophrasti (velvetleaf has reduced up to 82 % by

competition with soybean. Herbicide doses that do not kill weed plants may help to reduce seed

production. In research done in Nebrasaka, Sublethal doses herbicide reduced seed production of

several weed species as much as 90 % (Monaco et al., 2002).

High seed longevity (table. 1) of weed seeds is a special characteristic that can be used as a tool for

seed bank management. Burying weed seeds by tilling increases the longevity of the seeds in the

seed bank, while leaving weed seeds on the soil surface expose them to predation by birds, insects

and pathogens, reducing their abundance in the seed bank. If seeds in the shallow zone are

stimulated to germinate, for example by pre-irrigation, the emerged seedlings can be controlled and

prevented from producing seeds.

Table 3 | Time at which seed longevity decline in soil. Adapted (Major 2004, p.77).

Weed Name Maximum longevity (years)

Stellaria media L. (Common chickweed) > 20

Chenopodium album L. (Common lambsquarters) > 20

Polygonum aviculare L. (Prostrate knotweed) 10-15

Descurainia sophia L. (Flixweed) 16

Avena fatua L. (Wild oat) 4-6

The following strategies should be used to minimize annual inputs (deposits) to the weed seed bank.

Weeds should be killed before they set seed or before flowering to be safe, because some weeds

such as Galinsoga quadriradiata (hairy galinsoga) can mature seeds from flowers that are

pollinated before the weeds are pulled or severed. If in doubt, attempts should be made 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 (Ross and Lembi, 2009).

Creeping perennial weeds such as Rumex crispus (curly dock) should be controlled before they can

form new rhizomes, tubers, or other propagules. Crops are to be kept ahead of the weeds; small

weeds overshadowed by a good crop canopy such as Helianthus annuus L. (sunflower) may have

less than 1 % of the seed forming capacity of vigorous individuals receiving photosynthetic active

radiation (Buhler et al., 2008).

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Walking in fields to remove large weed escapes before they flower will help in managing the seed

bank, getting the largest 10 % of individuals can reduce seed production by at least 90 %

(Thompson et al., 2003). Field margins should be mowed to minimize seed set by weed species that

have the potential to invade fields like Lantana camara (tick berry). More so, farmers should mow

or graze fields promptly after harvesting to interrupt weed seed production.

Another measure that can help contain seed bank populations is to increase the diversity of crop

rotations. More diverse rotations, especially those that include one or more years in Trifolium

pratense (red clover), Medicago sativa (alfalfa), or other perennial sod crops, can help reduce seed

inputs from Abutilon theophrasti (velvetleaf) and other annual weeds, and promote seed bank

declines through seed predation and decay (Ross and Lembi, 2009).

These following strategies can be used to maximize losses (withdrawals) from the weed seed bank,

timing of crop planting 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-November, delay

Zea mays (corn) planting until end of November to allow time to remove this flush prior to planting

(Buhler et al., 2008).

Habitat for weed seed predators should be maintained, 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. According to (Liebman

2001, p.125) said “Incorporated green manures or surface residues of cover crops can reduce the

establishment of small-seeded weeds through allelopathy or physical hindrance.” 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.

Farmers have to reduce or avoid tillage during critical times for weed seed predator activity. If a

significant weed seed rain has occurred, weed seeds should be left at the surface for a period of time

before tilling to maximize weed seed predation (Ascard, 1994).

In summary, “an ounce of prevention is worth a pound of care”, weeds should be prevented from

producing seeds. Doing so will limit future weed populations and addition of herbicide resistant

weed seeds to the seed bank, treating the cause rather than just the symptoms of weed infestations.

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Challenge of Weed Seed Bank Diversity

None of these management strategies can be expected to eliminate all the weeds in a seed bank,

There is a need to change seed bank management strategy as the seed bank itself changes. The

reason why 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

(Robert and Shirtliffe, 2009). 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 weed species have light-responsive seeds Cerastium fontanum (Starweed), and

dark cultivation reduces emergence only in the light responders. Similarly, careful nitrogen

management can reduce problems with nitrate responders but have no effect on non-responders and

could even favour a weed that is well adapted to low levels of soluble nitrogen. 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 humour about the weeds (Major, 2004).

In brief, biodiversity in weedy populations result from taxonomic diversity, as well as diversity in

those traits that affect the survival, mortality and reproduction of individual weeds. Biodiversity

also arise as a result of differential survival mechanisms of individual weed species. Successful

weed management requires a thorough knowledge of weed biodiversity (Rao, 2010).

Conclusion and Recommendations

Weed seed banks are an important component of the weed life cycle. There are many fates and

processes that occur in the weed seed bank, many of which are not very well understood. The sheer

difficulty of monitoring a process that occurs mostly underground has deterred weed scientists from

gaining a full understanding of the weed seed bank. Never the less, current knowledge about weed

seed banks has shown some potential management options.

Reducing inputs to the seed bank is an important component of seed bank management, while other

strategies like using a no-till cropping system can be used to directly affect germination, persistence

and mortality of weed seeds. Managing weed seed banks should be an important component of

integrated weed management, but more often than not, seed bank management is not being

exploited to its fullest potential.

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Since the seed bank is the real weed problem for most crop producing farmers, they should bear in

mind that agricultural management practices can promote weed seed germination, mortality or

persistence. In order to reduce weed seed banks it is important to promote germination and

especially prevent new seed production on the fields. Reductions in the seed bank should allow

reductions in weed control cost in the long run. Goals for long term weed seed bank management

should not be forgotten which are; to promote “withdrawals” (seed germination and decay) and

prevent “deposits” (reduce new seed production).

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Baskin C.C. and J.M. Baskin. (2003) Seeds: Ecology, Biogeography, and Evolution of Dormancy

and Germination. San Diego: Academic Press. pp 666.

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Discuss the role of a weed seed bank