1 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 structur es 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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1
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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The role of weed seed bank, its function in crop production.
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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The role of weed seed bank, its function in crop production.
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.