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123
The Korean Society of Crop Science
J. Crop Sci. Biotech. 2015 (June) 18 (2) : 133 ~ 145
REVIEW ARTICLE
DOI No. 10.1007/s12892-014-0085-2
Cover Crops as a Means of Ecological Weed Management in
Agroecosystems
Fikre Lemessa*,Mulatu Wakjira
Department of Horticulture and Plant Sciences, College of Agriculture and Veterinary Medicine, Jimma University, P. O. Box 307,
Jimma, Ethiopia
Received: August 8, 2014 / Revised: December 2, 2014 / Accepted: January 27, 2015
ⒸKorean Society of Crop Science and Springer 2015
Abstract
Weeds cause an estimated crop yield loss of about 43% world-wide. The heavy use of herbicides in weed management has several
environmental and health risks. Therefore, systems-oriented approaches to weed management that make better use of alternative
weed management tactics need to be developed. One of these approaches is the use of plants with strong weed-suppressing abilities
as a component of integrated crop management. Cover crops are well-suited in such a holistic approach as they provide many other
agroecosystem services besides suppressing weeds. However, compared to the use of herbicides, the use of cover crops as a weed
management tool needs careful follow-up throughout the intended period to maximize the agroecosystem services they provide, min-
imize the disservices they exert, and optimize their selectivity between weed and crop suppression. Although there are many review
papers that address the contribution of cover crops for managing weed problems in agroecosystems, few systematically address the
difficulties that are encountered in fitting in the cover crops in the different cropping systems and the methods how to overcome
these difficulties. Therefore, this paper is to examine how to maximize weed suppressive effects of cover crops and minimize the
negative side effects of introducing cover crops into annual and perennial cropping systems and how to optimize selectivity between
weeds and crops. We suggest further research regarding the selection of cover crops that are compatible with annual and perennial
cropping systems under different climatic conditions and identify the traits responsible for adaptation in various cropping systems
and environments.
Key words: Cover crops, Ecosystems disservices, Ecosystem services, Ecological weed management
Weeds are important biotic constraints in agroecosystems
(Chauhan and Gill 2014) that interfere with crop plants and
cause an estimated crop yield loss of about 43% world-wide
(Oerke 2006). To overcome the negative effects of weeds on
crop production, a number of weed management methods
have been employed since the beginning of agriculture.
Nowadays, heavy reliance on synthetic herbicides is consid-
ered objectionable due to the potential negative impacts of
herbicide compounds on food safety, non-target organisms,
beneficial species, public health, and the environment (Ali et
al. 2014; Chauhan and Gill 2014; Isik et al. 2009; Kropff and
Walter 2000; Liebman 2001). There is also the development
of herbicide resistance in weeds as a result of continuous
exposure to heavy doses of chemical herbicides (Booth et al.
2003; Moss 2003). Furthermore, herbicides are not allowed
in organic agriculture and are minimized in integrated agri-
cultural systems. Hence, minimizing or even avoiding the use
of synthetic herbicides has gained interest in weed manage-
ment research (Barberi and Kropff 2002; Samedani et al.
2014; Ulla et al. 2014).
To solve the weed problem in agroecosystems, it is not
enough to merely replace synthetic herbicides by other direct
control measures due to their negative consequences
(Kruidhof et al. 2008). For instance, mechanical weed control
methods depend heavily on fossil fuel. As a result, they sig-
nificantly impact the environment and are often out of reach
Introduction
Fikre Lemessa ( )
e-mail: fikre.lemessa@ju.edu.et
from resource-poor farmers of developing countries. Besides,
mechanical control measures like cultivation and hoeing can
damage soil structure and the root systems of shallow-rooted
crop plants, and their efficacy is strongly dependent on
weather conditions (Kruidhof et al. 2008). Hand weeding is
often used, but it is labor-intensive and costly (Kruidhof et al.
2008). Another alternative is biological control that has
gained attention since the 1970s (Charudattan and Dinoor
2000). However, the classical approach is less suitable for
controlling weeds in agroecosystems (Bastiaans et al. 2008)
whereas field efficacy of bioherbicides is significantly lower
as compared to synthetic herbicides (Hallett 2005).
Therefore, systems-oriented approaches to management of
weeds that make better use of alternative weed management
tactics need to be developed (Barberi 2002; Kruidhof et al.
2009; Liebman and Davis 2000; Mirsky et al. 2013).
One of such approaches is the use of plants with strong
weed-suppressing ability as a component of integrated crop
management (Khan et al. 2002; Liebman and Davis 2000;
Teasdale 2013). Cover crops are well-suited in such holistic
approach as they provide many other agroecosystem services
besides suppressing weeds (Samedani et al. 2014; Ulla et al.
2014). The major agroecosystem services provided by cover
crops include improving soil organic matter content and
hence the aggregate stability and structure of the soil (Liu et
al. 2005; Nakhone and Tabatabai 2008; Sarrantonio and
Gallandt 2003) reducing runoff and soil erosion (Mirsky et
al. 2013; Zuazo and Pleguezuelo 2008) improving soil
hydraulic properties (Bodner et al. 2008; Carof et al. 2007)
minimizing dependency on artificial fertilizer by fixing
atmospheric nitrogen by legumes (Hartwig and Ammon
2002), recycling of unused nitrogen (Hartwig and Ammon
2002; Kruidhof et al. 2008), and reducing nitrate leaching
(Rinnofner et al. 2008; Thorup-Kristensen 2001; Vidal and
Lopez 2005), improving soil biological activity (Schutter and
Dick 2002), and controlling pathogens (Conklin et al. 2002;
Manici et al. 2004), nematodes (DuPont et al. 2009; Jibrin et
al. 2014), and insect pests (Peachey et al. 2002).
Cover crops interfere with the development of weed popu-
lations through two important mechanisms (Kruidhof et al.
2008, 2009). First, introduction of cover crops into the agroe-
cosystem prevents growth and development of weeds
through niche pre-emption (Amoghein et al. 2013; Kruidhof
et al. 2008; Liebman and Staver 2001). In this case, cover
crops occupy the space and utilize the resources that would
otherwise be available to weeds. Second, incorporated or soil
surface-placed cover crop residues can inhibit or retard ger-
mination and establishment of weeds (Kruidhof et al. 2009;
Ohno et al. 2000). Cover crop residues used as surface
mulches, suppresses or retards weed germination, emergence
and growth due to both allelopathic and physical effects
(Liebman and Davis 2000). Incorporated residues of allelo-
pathic cover crops can also inhibit or retard germination,
emergence, and growth of weeds. For this purpose, cover
crops that have a high level of allelochemicals seem to be
well-suited (Kruidhof et al. 2009).
Besides physical- and allelopathy-mediated weed suppres-
sion, there are many other mechanisms through which cover
crop residues affect weed germination and establishment
(Kruidhof et al. 2009). Nutrients released from the cover crop
residues, mainly nitrogen, can stimulate weed seed germina-
tion (Teasdale and Pillai 2005), whereas temporary immobi-
lization of nitrogen as a result of slow decomposition rate of
high C/N cover crop residue decomposition can inhibit ger-
mination (Liebman and Mohler 2001). Soil surface-placed
residues can result in less fluctuation in soil temperature and
physically reduce penetration of light, both of which have
been demonstrated to inhibit germination (Liebman and
Mohler 2001). Moisture may be better conserved when soil is
amended with cover crop residues (Teasdale and Mohler
1993). Furthermore, soil amendment with fresh residue mate-
rial may in some cases either stimulate (Manici et al. 2004)
or suppress (Matthiessen and Kirkegaard 2006) the multipli-
cation and activity of soil microbes which have an impact on
weed seed bank.
Despite these multiple agroecosystem services provided
by the cover crops, the use of cover crops as a weed manage-
ment tool needs careful follow up throughout the intended
period unlike the direct use of herbicides. Hence, careful
management of cover crops is required to optimize the agroe-
cosystem services they provide and minimize the disservices
resulting from introducing cover crops in a cropping system.
Although there are many review papers that address the con-
tribution of cover crops for managing weed problems in
agroecosystems, there are few papers that systematically
address the difficulties that are encountered in fitting in the
cover crops in the cropping system and the methods how to
overcome these difficulties in annual and perennial cropping
systems. Therefore, the main aim of this paper is to examine
how to maximize weed suppressive effects and minimize the
negative side effects of introducing cover crops into the crop-
ping system and how to optimize selectivity between weeds
and crops. This paper is therefore structured in two major
parts: (1) cover crops and their agroecosystem services and
disservices, and (2) suitability, selectivity, and management
of cover crops. Finally, future research directions will be
indicated in respect to successful introduction of cover crops
as a component of ecological weed management in annual
and perennial production systems under different environ-
ments.
Cover crops and their agroecosystm
services and disservices
What are cover crops?: Cover crops are defined as plants
grown primarily to cover the soil in order to protect it from
soil erosion and nutrient losses between periods of regular
crop production, and between trees and vines in orchards and
vineyards (Brady and Weil 1999; Sarrantonio and Gallandt
2003; Sullivan 2003). Cover crops provide a wide array of
Cover Crops as a Means of Weed Management
124
JCSB 2015 (June)18 (2) : 133 ~ 145
125
beneficial agroecosystem services (Teasdale 2003, 2013).
Though the species can vary, cover crops can be integrated
into both temperate and tropical climates and in annual and
perennial production systems (Bowman et al. 1998; Liebman
and Davis 2000; Teasdale 1998).
Cover crops are introduced into the cropping system in
two major ways (Kalinova 2010). First, growing the cover
crops during the off-season and killing before planting the
main crop, a common practice in annual cropping systems.
Secondly, growing the cover crops at the time of the main
cropping for all or part of the growing season as living
mulches which best fit with the perennial cropping system
(Hartwig and Ammon 2002). In this regard, cover crops that
adequately suppress weeds for long duration while compati-
ble with the the perennial production systems with no or min-
imal deletirious effects on the main crop are highly needed.
In all these syetems, cover crops provide layers of vegetation
and plant residues that suppress weeds through competition
for soil nutrients, water, and light, creating environmental
conditions unfavorable for weed germination and establishe-
ment and exerting allelopathic effects that inhibit grmination,
emergence, and growth of weeds (Locke et al. 2002;
Schipanski et al. 2014; Teasdale 1998; Teasdale et al. 1991).
Agroecosystem services and disservices of cover crops:
Agroecosystem services are complex entities composed of
several interacting elements spatially and temporally in a
way that the effects of individual elements cannot be easily
separated from the system (Barberi and Mazzoncini 2001).
The agricultural ecosystems are highly disturbed systems
redesigned to produce food, feed, fiber, and industrial raw
materials and are known to receive and provide agroecosys-
tem services (Heal and Small 2002). The Millennium
Ecosystem Assessment (MA 2005) defined ecosystem serv-
ices as the benefits people obtain from ecosystems and classi-
fied them into provisioning, regulating, cultural, and support-
ing services (MA 2005). Cover crops are known to provide
diverse services to the agroecosystem (Teasdale 2003) (Fig.
1). The services provided by introducing cover crops into the
agroecosystems in relation to each of the above classes of
services will be briefly discussed hereunder.
Provisioning services are the products obtained from the
cover crops including food for humans, forage for animal
feed, production of biofuel, source of seed and propagation
material, conservation of genetic resources and sources of
biochemicals and natural medicines (de Groot et al. 2002;
MA 2005; Sandhu et al. 2010).
Regulating services include the benefits obtained from the
regulation of ecosystem processes as a result of introducing
cover crops into the cropping system (Sandhu et al. 2010).
The major regulating services provided by the cover crops to
the agroecosystem can be grouped into biotic and abiotic
services. The biotic ecosystem services provided by cover
crops include biological management of insect pests
(Peachey et al. 2002), soil pathogens (Bailey and Lazarovits
2003; Conklin et al. 2002; Manici et al. 2004), parasitic
nematodes (DuPont et al. 2009; Wang et al. 2006) and weeds
(Hatcher and Melander 2003; Kremer and Li. 2003; Kruidhof
et al. 2008; Liebman and Davis 2000), increased biological
activity, and pollination (MA 2005; Sandhu et al. 2010).
Cover crops reduce damage on crops due to diseases and
insect pests by various mechanisms (Sarrantonio and
Gallandt 2003). For instance, introduction of cover crops into
the cropping systems directly affect pathogens through
changing the soil physical and chemical properties and by
releasing exudates and decomposition products. Cover crops
also contribute to suppression of disease and insects by serv-
ing as hosts for competitors, parasites, and predators by
changing above and below ground environmental factors
such as moisture levels and aeration and by influencing the
entire health of succeeding or concurrent crops. In addition,
cover crops mechanically hinder the movement of insects or
can confuse insect visual or olfactory clues. Furthermore,
cover crops inhibit or retard the germination, emergence,
growth, and development of weeds (Ohno et al. 2000;
Weston 1996) through competition for limited resources such
as light, water, and nutrients, and allelopathic (Kruidhof et al.
2009) and physical effects (DenHollander et al. 2007). Cover
crops also play in improvement of soil biological diversity
and activity (Schutter and Dick 2002).
The abiotic services provided by cover crops to the agroe-
cosystem include erosion control, reducing water pollution,
soil moisture conservation, improving soil structure, and car-
bon sequestration (MA 2005; Power 2010; Sandhu et al.
2010). Cover crops play key roles in improving soil physical,
chemical, and biological properties (Nakhone and Tabatabi
2008; Sarrantonio and Gallandt 2003). Evidently, cover crops
improve soil physical properties, aggregate stability, and soil
structure (Liu et al. 2005; Power 2010), and soil hydraulic
properties (Bodner et al. 2008; Carof et al. 2007; Power
2010); and reduce runoff and soil erosion (Meyer et al. 1999;
Zuazo and Pleguezuelo 2008). Cover crops also reduce water
runoff and soil erosion and ultimately improve soil produc-
tivity (Hartwig and Ammon 2002). The vegetative and
residue ground cover provided by cover crops reduces the
detachment of soil particles and minimizes soil erosion.
Continuous coverage of the ground with cover crops greatly
reduces surface water runoff and consequently, the loss of
nutrients and pesticides (Hall et al. 1984). For instance, water
runoff, soil loss, and pesticide loss were reduced by 95 to >
99% when corn was planted into birds foot trefoil or crown-
vetch living mulch on a land of 14% slope (Hall et al. 1984).
The reduction in the rate of runoff also consequently improve
moisture infiltration rate (Hartwig and Ammon 2002).
Supporting services are the services that are required for
the production and availability of the other ecosystem goods
and services and include fixing atmospheric nitrogen by
legumes (Hartwig and Ammon 2002), reducing nitrate leach-
ing (Rinnofner et al. 2008; Thorup-Kristensen 2001; Vidal
and Lopez 2005), recycling unused nitrogen (Hartwig and
Ammon 2002), adding organic matter to the soil, maintaining
soil fertility, and providing habitat for different organisms
(MA 2005; Sandhu et al. 2010). Legume cover crops fix
atmospheric nitrogen (Hartwig and Ammon 2002) and the
nitrogen released from the breakdown of stem, leaf, and root
residues of legume cover crops can be used by the preceding
crop (Ebelhar et al. 1984). However, the total nitrogen contri-
bution of a cover crop depends on the amount of dry matter
produced by the cover crop (Holderbaum et al. 1990). The
added organic matter and the breakdown products of cover
crop residues bond soil particles into aggregates that
improves soil structure resulting in improved soil tilth, water
holding capacity and aeration, and consequently productivity
(Danso et al. 1991; Hartwig and Ammon 2002). Cover crops
also take-up the excess nitrogen in the soil and hence reduce
nitrate leaching and minimize the environmental problems
associated with excess nitrogen from ecosystems (Hartwig
and Ammon, 2002).
Cultural services are the nonmaterial benefits humans
obtain from the inclusion of cover crops into the agroecosys-
tem and include aesthetic values, recreational and eco-
tourism, educational and knowledge systems and spiritual
and religious values (MA 2005; Sandhu et al. 2010).
Agroecosystem disservices can be described as the nega-
tive impacts of introducing cover crops into the cropping sys-
tem on the associated agroecosystem. The disservices can be
grouped into economic, biological and operational aspects
(Mallory et al. 1998; Sarrantonio and Gallandt 2003; Stivers-
Young and Tucker 1999) (Fig. 1). These aspects will be dis-
cussed in detail in the following paragraphs.
Economic issues include requirement of high establish-
ment and management costs and they are regarded as the
major economic issues that hinder the inclusion of cover
crops into a cropping system. For instance, additional estab-
lishment cost is required to buy cover crop seed and addition-
al labor and time is required for planting and management of
the cover crops (Mallory et al. 1998; Sarrantonio and
Gallandt 2003). Cost of cover crop establishment is species
dependent. Evidently, the cost of cover crop establishment is
10 times higher for legumes than grass cover crops (Labarta
et al. 2002; Roberts and Swinton 1996). This might be due to
larger seed size of legume cover crops that necessitates
requirement of more seed weight, mechanisms of seed dis-
persal that increase harvesting expense and seed cost, and
Cover Crops as a Means of Weed Management
126
Fig. 1. Services and dis-services provided by cover crops to the agroecosystem. Sizes of arrows depict the degree of contribution.
JCSB 2015 (June)18 (2) : 133 ~ 145
127
weak emergence that necessitates extra investment in tillage,
irrigation, and fertilization practices and high seeding rates
(Snapp et al. 2005). In addition, special equipment could be
required such as no-tillage seeders, transplanting equipments,
and mowers to handle the large quantity of cover crop
residues in no-till cropping systems (Lu et al. 2000).
Additional management is also required for suppressing or
killing and incorporation of the cover crops (Snapp et al.
2005). Furthermore, it is difficult to kill and incorporate very
vigorous cover crops and this will need unexpected addition-
al costs (Snapp et al. 2005). Delayed planting of the main
crop, and competition or substitution by the cover crops as a
result of introducing cover crops into the cropping system
could lead to yield loss and this is considered as the major
opportunity cost forgone from the main crop (Snapp et al.
2005).
Biological issues include becoming habitat for pests and
diseases and a potential weed, competing with crops, deplet-
ing soil moisture, leading to considerable nitrogen losses
through leaching or volatilization, and exerting allelopathic
effects on the crop. Cover crops harbor insects, diseases and
nematodes that affect the main crop. Insect pests and
pathogens could use the living cover crops growing during the
off-season as an alternate host and attack the main crop the
following season (Lu et al. 2000). Living cover crops could
also interfere with the main crop as unmanaged cover crops
competitive for the scarce resources required for growth and
hence act as weeds (Locke et al. 2002; Lu et al. 2000). This is
because those cover crops that competitively suppress weed
emergence and growth also suppress the main crop (Alford
2003; Reddy and Koger 2004; Teasdale et al. 2007). Living
cover crops sometimes deplete soil moisture reserves during
early spring and this could be a severe problem in arid and
semi-arid areas where soil moisture is a limiting factor for
crop production (Munawar et al. 1990; Teasdale 2003). Cover
crops also compete with crop plants for pollination (Zhang et
al. 2007). On the other hand, living cover crops can be affect-
ed by the same chemical and physical factors that contribute
to weed control. Crop species may also be hampered by the
allelochemicals released from the cover crops. If cover crops
are not rotated, there is a possibility of buildup of weed,
pathogen, and pest populations and allelochemicals in the
agroecosystem which have a deleterious effect on the main
crop and the cover crop (Kalinova 2010).
Cover crop residues interfere with establishment and
growth of the main crop by physically hindering seed sow-
ing, by their cooling effect on the soil, by the allelochemicals
released from their decomposition, by reduced availability of
nutrients, or by stimulating the incidence of seedling diseases
(Davis and Liebman 2003; Gallagher et al. 2003; Westgate et
al. 2005). Unless nitrogen is assimilated, there could be a
considerable probability of N-losses through nitrate leaching
and N-volatilization from residues rich in nitrogen
(Rosecrance et al. 2000). Other bottlenecks include the slow
nitrogen release rate from non-legume cover crops and the
difficulty to accurately estimate mineralization of cover crop
residue and this create difficulty in synchronization of crop
demand with nutrient release from the residue (Snapp and
Fortuna 2003; Snapp et al. 2005).
Operational issues include requirement of continuous
management and interference with field operations
(Sarrantonio and Gallandt 2003). Inclusion of the cover crops
in the cropping system requires high and continuous manage-
ment to maximize the agroecosystem services and to mini-
mize the disservices and this could overshadow its benefits
(Teasdale et al. 2007). Cover crops and their residues inter-
fere with planting and harvesting time and operations
(Giesler et al. 1993). For instance, the high quantity of cover
crop residues create a mechanical barrier that hinder plowing,
cultivation, or no-tillage planting operations and maintain
spring temperatures too cold (Grisso et al. 1985; Teasdale
2003). In conventional and integrated farming systems where
herbicides are used, cover crops act as physical barriers that
hinder uniform spraying and cover crop residues bind
sprayed herbicides consequently resulting in reduced activity
and efficacy of herbicides. Some cover crops are difficult to
control from producing seed and preventing them from estab-
lishing and as a result they could act as a weed (Mutch and
Snapp 2003). Furthermore, cover crops that produce hard-
coated seeds or other adaptation mechanisms may enter and
stay in the weed seed bank and continuously germinate over
several years (Benech-Arnold et al. 2000).
Suitablity,selectivity, and management
of cover crops
Suitability of cover crops: Introduction of cover crops in a
cropping system begins with selection of suitable cover crops
for the preceding crop or the crop under which the cover crop
is sown and the cropping system whether it is annual or
perennial. The selection of suitable cover crop species
depends on the environment (pedo-climatic conditions), the
cropping system including the current crop rotation schemes,
and the sets of services required from farmers’ perspectives
(Scholberg et al. 2010; Zibilske and Makus 2009). These will
be discussed hereafter grouped into three including general
suitability that includes the species requirements, manage-
ment, and farmers’ preferences, suitability in terms of envi-
ronment and suitability in annual and perennial cropping sys-
tems.
General suitability of cover crops: Cover crops that pro-
vide multiple agroecosystem services are generally preferred
by the farmers. Though the perfect cover crop does not exist
(Scholberg et al. 2010), the major features of cover crops that
provide wide array of agroecosystem services and that are
required to satisfy the ecological principles and needs of
farmers include (1) easy to establish and fast growing to pro-
vide rapid ground cover (Sullivan 2003), (2) ability to grow
in mixed stands and having a complementary growth cycle,
canopy traits, and root functionality with the main crop
(Muhamman and Gungula 2006; Scholberg et al. 2010), (3)
production of broad leaves which can rapidly cover the soil
and reduce raindrop impact and protect soil from erosion
(Muhamman and Gungula 2006), (4) ability to fix atmos-
pheric N, catch excess N, or recycle N (Varhallen et al.
2003), (5) extensive or deep root system that helps in explor-
ing and utilizing water and nutrients from deeper layers and
in stabilizing the soil and maintaining soil structure and
hence reduce soil erosion (Varhallen et al. 2003), (6) succu-
lent foliage production that facilitates rapid increase in bio-
logical activity and rapid decomposition and quick release of
nutrients and allelochemicals under conditions in which rapid
nutrients are required by the crop and when fast inhibition of
weeds is required (Varhallen et al. 2003), (7) ability to sup-
press insect pests, diseases, and weeds (Muhamman and
Gungula 2006; Sarrantonio and Gallandt 2003), and (8) lack
of undesirable traits such as unfavorable residue properties
(e.g. excessively high C : N ratio, coarse, recalcitrant
residues that hamper seed bed preparation, and allelopathic
effects on the main crop), competition for essential growth
resources with the main crop, weediness and excessive
regrowth, and ability to harbor pests and diseases (Scholberg
et al. 2010).
Regarding weed suppression, there are important charac-
teristics to consider before selecting cover crops for inclusion
in a given cropping system. For instance, the ideal living
cover crop for weed suppression should have various charac-
teristics (Teasdale 2003). High initial growth rates for height
and leaf area development and hence rapid establishment and
growth with better competitive ability so as to develop a
close dense canopy faster than weeds and the ability to cover
the ground completely is one important quality of the cover
crops to be considered (Akanvou et al. 2001; Hartwig and
Ammon 2002). Selectivity between suppression of weeds
and the associated crops is another factor to be considered to
minimize the impact of cover crops on the main crop. In
addition, cover crops that produce high amount of biomass
provide high amounts of weed suppressive residue
(Hutchinson and McGiffen 2000; Teasdale 2003).
Furthermore, cover crops that produce allelochemicals can
play key roles in suppressing weeds.
On the other hand, there are some important aspects of
cover crop residues to consider for proper suppression of
weeds. The extent and the time course of allelochemical
release in the soil and the associated cover crop residue-
mediated weed suppression depends on the quantity and
quality of the residue, the residue management method
including the level of tissue disruption and residue place-
ment, and on the allied environmental conditions including
the soil physical, chemical, and biological characteristics
(Kruidhof et al. 2009; Liebman and Mohler 2001). Weed
suppression potential of cover crop residue depends on quan-
tity of the residue used. Accordingly, cover crop allelopathy-
mediated weed suppression is more pronounced if the high
amount of residue is incorporated or placed on the soil sur-
face as mulch because higher quantity of allelochemicals can
be released (Batlang and Shushu 2007). Quality of the
residue also plays a key role as the inhibitory effect depends
on the quantity of the allelochemical produced and this is
affected by the rate of release which is related to the residue
quality. For instance, cover crop residues that have high C :
Nratio decompose slowly and provide weed suppression for
extend period of time (Kruidhof et al. 2009; Mohler and
Teasdale 1993). Cover crop residues that produce adequate
quantity of allelochemicals with higher inhibitory effect can
also play role in weed suppression.
On the other hand, the more the tissue disruption the faster
the rate of release of allelochemicals either directly (Morra
and Kirkegaard 2002) or indirectly by altering the decompo-
sition rate (Ambus and Jensen, 1997; Angers and Recous
1997). Cover crop placement method also affects the degree
of weed suppression by cover crop residues. In this regard,
cover crops retained on the soil surface decompose more
slowly and may result in slower release rate of allelochemi-
cals than residues incorporated in the soil (Dou et al. 1995;
Kruidhof et al. 2009). Furthermore, the soil physical, chemi-
cal, and biological characteristics also affect the rate of
release and the activity of allelochemicals in the soil.
Suitability in terms of environment: The climate (day
length, temperature, radiation and rainfall), edaphic (soil)
characteristics and environmental factors, under which the
cover crops are to be grown influence the development and
growth duration of cover crops and determine the chosen
species of cover crops (Scholberg et al. 2010). Ideal cover
crops that tolerate the adverse environmental conditions such
as drought, flooding, heat, cold, water logging, low pH, nutri-
ent limitations, and shading conditions are preferred
(Anderson et al. 2001; Scholberg et al. 2010).
Anderson et al. (2001) demonstrated that some cover
crops have specific tolerance towards drought (e.g. Cajanus
cajan,Canavalia ensiformis,Mactroptilium atropurpureum),
waterlogging (e.g. Sesbania sesban,Aeschynomene spp.), or
shaded conditions (e.g. Calopogonium muconoides,
Centrosema pubescens,Desmodium ovalifolium). Cover crop
type and growth duration influence water requirement of
cover crops (Scholberg et al. 2010). Leguminous cover crops
are known to be poorly adapted to either extremely acidic or
alkaline and poorly drained soils (Cherr et al. 2006). Some
cover crops are able to tolerate high aluminium levels in acid
soils such as Vigna unguiculata,Lablab purpureus,Arachis
pintoi,Stylosanthes guianensis,and Centrosema spp. where-
as many cover crops also tolerate soils with poor fertility
(Anderson et al. 2001). Many of the tropical legume cover
crops including the genera of Neonotonia,Canavalia,
Mucuna,Vigna,Lablab,Centrosema,and Desmodium are
short-day and care is required when taking seed from one lat-
itude to another as flowering and seed production are influ-
enced by day length.
Based on their environmental requirement, cover crops
can be grouped into tropical/warm and temperate/cool cover
crops (Anderson et al. 2001; Kiff et al. 1996). Tropical cover
Cover Crops as a Means of Weed Management
128
crops may thrive under hot temperature conditions (> 35°C)
but do not tolerate freezing temperature regimes (< -2°C).
These groups of cover crops grow throughout the year under
tropical climates and easily grown during the summer season
in the sub-tropics (Anderson et al. 2001; Scholberg et al.
2010). Some of the major leguminous tropical cover crop
species include the genus Canavalia (e.g. Jack bean
Canavalia ensiformis), Crotalaria (e.g. sunn hemp Crotalaria
juncea), and Mucuna (e.g. velvet bean Mucuna pruriens)
(Anderson et al. 2001; Scholberg et al. 2010). Temperature,
precipitation, soil type, and pH requirements of major tropi-
cal cover crops has been summarized by Baligar and Fageria
(2007) and the various tropical and temperate legume and
non-legume cover crops have synthesized by Cherr et al.
(2006).
Cover crops with specific traits are required for adaptation
to a certain environment. For instance, cover crops with low
specific leaf area (SLA) could reduce soil moisture depletion
while maintaining soil covering and carbon and nitrogen fix-
ation benefit and this trait is greatly required in arid and
semi-arid regions where water is limiting (Wilke and Snapp
2008). Leaf pubescence is another important response trait
that plays key roles in reflectance of photosynthetically
active radiation and hence moderating leaf temperature and
reducing evapotranspiration and contributing to water reten-
tion in dry, warm environments (Wilke and Snapp 2008).
Temperate cover crops may survive a freezing temperature
of up to -10°C but their growth may be altered under hot con-
ditions (> 25 to 30°C). These cover crops are known to grow
well under temperate conditions whereas during the winter
season in sub-tropical conditions or under tropical highlands
(Cherr et al. 2006; Scholberg et al. 2010). Freezing / cold tol-
erance that determines winter survival is the most important
response trait that determines the field survival of cover
crops under temperate climates (Brandsaeter and Netland
1999; Brandsaeter et al. 2008; Wilke and Snapp 2008).
Furthermore, biomass production of the hairy cover crops
evaluated under Norway condition strongly correlated to
winter survival (Brandsaeter et al. 2008).
Though cold tolerance is an important trait in cold climat-
ic regions, this could be achieved at the expense of rapid
growth which is a valuable trait of cover crops (Ehleringer
and Bjorkman 1978; Wilke and Snapp 2008). Cover crops
that are well-adapted to the warmer temperate regions have
the capability to (1) produce a uniform stand of readily estab-
lished plants in autumn before the onset of cold weather, (2)
survive freezing weather during the winter season, and (rap-
idly grow during cool conditions before planting the main
crop in spring (Teasdale et al. 2007). Among the cover crops,
rye (small-grain cover crop) and hairy vetch (winter annual
legume cover crop) which are vigorous and well-adapted to
the warm temperate regions provide competitive and provide
ground cover leaving relatively few weeds at the time of
planting a spring crop (Teasdale et al. 2007). In northern
temperate regions like northern Europe and Canada, where
the climate is characterized by freezing winters and relatively
cool and short summers, the commonly used cover crops sys-
tems include undersowing clover or clover-grass mix as a
green manure (in organic farms) or grass as a catch crop
(conventional and organic farms) in cereals, and annual green
manure cover crops in rotation with cash crops (Teasdale et
al. 2007).
Suitability in terms of cropping system: Cover crops can
be introduced into plantations such as orchards and vineyards
or in rotation with annual crops. Cover crop choice could be
influenced by the cropping system, annual or perennial, in
which the cover crop is introduced. Farmers prefer to intro-
duce cover in a way that the cover crops occupy an underuti-
lized special or temporal niche because introduction of cover
crops into an established cropping system can pose logistical
problems to the farmers (Sarrantonio and Gallandt 2003).
Therefore, before selecting a cover crop (mixture) a thorough
analysis of the existing cropping and management systems at
afield level is required. The analysis should include crop
rotation schemes, main crop duration, intercrop / fallow peri-
od, and systems of tillage coupled with main crop pest and
disease potential risk assessment (Scholberg et al. 2010).
Besides selection of compatible species and varieties, the
development of a successful main crop/cover crop associa-
tion depends on the implementation of suitable management
practices (Scholberg et al. 2010). These include appropriate
sowing dates, patterns of planting and spacing, mowing / cut-
ting to minimize competition, killing methods when no
longer needed, management and mitigation of negative
effects of mulches such as fire risk, and above-, and below-
ground competition.
In annual intensive vegetable rotations, annual cover crops
are most often used because full establishment of the rhi-
zomatous and stoloniferous perennial cover crops may take
several months to even a year (Abdu-Baki et al. 2002; Rouse
and Mullahey 1997). In perennial cropping systems, cover
crops could be introduced in an established orchard or vine-
yard with varying level of shade. Cover crop species that are
adapted to low initial light regimes are required for over-
sowing cover crops into established main crops that create
shady condition (Scholberg et al. 2010). Wilke and Snapp
(2008) indicated that cover crops with high SLA may be bet-
ter adapted to under shady low light conditions. However,
these cover crops utilize high water, are more productive
with higher leaf turnover rates, and may be less resilient to
variations in environment and decomposition process (Wilke
and Snapp 2008).
In perennial cropping systems, cover crops that adapt to low
light conditions and that can be easily managed to minimize
their competition with the main crop during its critical growth
stage are required. Grasses or legumes planted in the alleyways
between rows in orchards and vineyards are the commonly
used living cover crops in perennial cropping systems
(Kalinova 2010). For example, hairy vetch is the most promis-
ing cover crop for orchard weed management (Fujii 2003).
JCSB 2015 (June)18 (2) : 133 ~ 145
129
Selectivity of cover crops
The cover crops that are competitive enough and able to
suppress weeds can also have the potential to suppress the
main crop (Alford 2003; Reddy and Koger 2004; Teasdale et
al. 2007). Teasdale (2003) suggested the following practices
to optimize selectivity of cover crops: (1) growing cover
crops that are low-growing and that competes primarily for
light, (2) adjusting the sowing time of cover crop so that its
peak time of growth doesn’t coincide with the critical period
at which competition greatly impacts crop yield, (3) enhanc-
ing the relative competitiveness of the crop relative to the
cover crop by increasing crop population density, (4) supple-
mental application of water and nutrients mainly N to com-
pensate for the quantity used by the cover crop, and (5)
reducing the competitiveness of the living cover crop mainly
at the critical time of crop growth by suppressing either
mechanically or chemically.
Allelopathic cover crops might also hamper the emer-
gence and growth of the main crop. Hence, it is necessary to
use cover crops that have allelopathic effect on crops in the
way they will not cause significant impact on the crop. To
make use of allelopathy to suppress weeds but not crops,
there are three possible ways (Liebman and Davis 2000).
First, seeding a sensitive crop several weeks after residue
incorporation as the toxicity of plant resides can substantially
decline through decomposition, usually more than two weeks
after residue incorporation (Dabney et al. 1996; Kruidhof et
al. 2009). Secondly, clearing allelopathic residues from
bands where cover crops sown by ridge-tillage equipments to
reduce the allelochemical load (Exner et al. 1996). Third,
transplanting seedlings of small-seeded crops instead of
direct sowing as they are more sensitive to allelochemicals
compared to large-seeded crops (Putnam and DeFrank 1983).
Plant species having smaller seeds are more inhibited by
allelochemicals compared to the large-seeded weed species.
The main reasons for the differential suppression of smaller
seeded species by allelochemicals could be due to the charac-
teristics of seeds and seedlings that are eco-physiological in
nature (Liebman and Davis 2000). Small-seeded species have
greater amounts of absorptive surface area through which alle-
lochemicals may enter as they have greater amounts of root
length per unit of root mass. In addition, the amount of stored
reserve in the small-seeded species is lower to enable seedlings
to tolerate or detoxify allelochemical stress agents. Large-seed-
ed species that can emerge from deeper in the soil profile may
be exposed to lower allelochemical toxin concentrations than
smaller seeded species that tend to germinate from higher in
the profile if allelochemicals are concentrated near residues
placed at the soil surface (Liebman and Davis 2000).
Management of cover crops
Once suitable cover crops are selected, proper manage-
ment of cover crops is crucial to maximize the benefits
obtained from cover crops and minimize their negative
impacts on the main crop and optimize their selective sup-
pression on the weeds (Teasdale 2003). The management
should consider pre-sowing care, appropriate time and
method of sowing cover crops, post-sowing care, and residue
management to enhance weed suppressive ability. These
issues will be discussed thoroughly hereunder.
Pre-sowing care: Cover crops require essential nutrients for
their growth and development and to successfully provide the
aspired agroecosystem services (Sullivan 2003). Soil fertility
management prior to sowing cover crops is therefore
required based on soil analysis. It is also important to know
the pest history of the field in order to avoid sowing cover
crops that can be damaged by similar pest or disease
(Kalinova 2010; Sullivan 2003). To avoid the herbicide
carry-over and the associated injury, it is important to check
the herbicide applied the previous season. Before sowing,
pre-plant tillage is needed to control weeds, disrupt insect
and disease life cycles and improve cover crop establishment.
However, no-till sowing is recommended to avoid erosion
and stimulation of weed seed germination and emergence
(Teasdale 2003). During sowing, good seed to soil contact is
needed for proper germination and emergence.
Appropriate time and method of sowing cover crops: The
time of introduction of cover crops determines the mecha-
nism of weed suppression (as a living cover crop or as cover
crop residue), weed suppressive ability, and the effect on the
main crop. To minimize the level of damage on the main
crop, the best strategy is growing the cover crops when the
main crop is not in the field (Kruidhof et al. 2008). However,
this strategy is not applicable in perennial cropping systems.
Depth of sowing is also important though it depends on the
seed size. During sowing cover crops in established perennial
cropping systems, care is required to minimize the damage
on the root system of the main crop.
Post-sowing care: Weed growth suppressing ability of a
cover crop species is proportional to the time to closure and
the amount of canopy produced (Liebman and Davis 2000;
Sarrantonio and Gallandt 2003). Management factors that
lead to high initial growth rates for height and leaf area
development and hence rapid establishment and growth so as
to develop a close dense canopy are important to cover the
ground completely with dense vegetation (Akanvou et al.
2001; Hartwig and Ammon 2002). Therefore, how soon a
complete ground cover is formed by cover crop depends on
the management intensity during the first three months after
sowing (Bradshaw and Lanini 1995). In annual cropping sys-
tems, effective killing or suppression of the cover crop is
needed before emergence of the main crop to avoid competi-
tion as yield of the subsequent crop can be adversely affected
if cover crops are not adequately killed (Kalinova 2010;
White and Worsham 1990). The commonly used cover crop
suppression methods include tillage, mowing, use of herbi-
cides, and selection of species that winterkilled or have a
short life cycle (Sullivan 2003). However, some cover crops
are difficult to kill and may be more than one herbicide appli-
cation is required (Griffin and Dabney 1990) or may be nec-
Cover Crops as a Means of Weed Management
130
essary to combine varying herbicides (White and Worsham
1990). It is important to rotate used cover crops to minimize
build-up of weeds, pathogens, and pest populations, and alle-
lochemicals in the agroecosystem (Kalinova 2010).
Residue management: Manipulation of the relative timing
of placement of cover crop residues relative to seed sowing
time needs to be manipulated to reduce the toxic effects
exerted on the emerging crop seedlings (Kalinova 2010). The
effects of surface mulches in influencing weed seed germina-
tion, emergence, and early growth of seedlings generally
depend on the type (quality), quantity and structure of mulch
(Teasdale and Mohler 2000). Therefore, one important aspect
of cover crop residue is the quality that determines the C/N
ratio and the decomposability and the composition of allelo-
chemicals. Another important aspect is the quantity of cover
crop residue as high amounts are required for optimal weed
suppression-whereas lower amounts stimulate weed emer-
gence by either being insufficient to inhibit weeds-or provide
more uniform moisture conditions by retarding evaporation
compared to the bare soil and release of nitrogen (legumes)
that stimulate germination of selected weed species (Mohler
and Teasdale 1993; Teasdale 2003; Teasdale and Mohler
2000).
Creating cover crop residue mulch with multiple layers of
densely packed material is important to enhance the weed
suppressing ability of mulching materials (Teasdale 2003).
Hence, management practices that create the maximum
mulch area and solid volume or, conversely that minimize
empty mulch volume maximize weed suppression. On the
other hand, the degree of tissue disruption and residue place-
ment method determines the rate decomposition of cover
crop residues and rate of release of allelochemicals (Kruidhof
et al. 2009). Evidently, the more tissue is disrupted by chop-
ping, the faster is rate of decomposition. Regarding place-
ment method, residues incorporated into the soil decompose
more rapidly than residues placed on the soil surface and
results in a faster rate of release of allelochemicals (Dou et
al. 1995; Kruidhof et al. 2009).
Conclusions
Cover crops are well-suited in systems-oriented approach-
es to weed management as they provide many other agroe-
cosystem services besides suppressing weeds. Living cover
crops suppress the development of weed populations through
niche pre-emption and cover crop residues suppress or retard
weed emergence and growth due to both allelopathic and
physical effects. Compared to the straightforward use of her-
bicides, the use of cover crops as a weed management tool
needs careful follow up throughout the intended period to
maximize the agroecosystem services they provide and mini-
mize the disservices and optimize the selective suppression
of weeds compared to the main crop. In this paper, we exam-
ined the ways in which to maximize weed suppressive effects
and minimize the negative side effects of introducing cover
crops into the cropping system and how to optimize selectivi-
ty between weeds and crops.
Cover crop-mediated ecological management of weeds
chiefly depends upon cover crop species and management.
Hence, introduction of cover crops in a cropping systems
begins with selection of suitable cover crops for the preceding
crop or the crop under which the cover crop is sown and the
cropping system whether it is annual or perennial. The selection
of suitable cover crop species depends on the environment,
cropping system, and the farmers’ preferences. However, cover
crops that provide multiple agroecosystem services are general-
ly preferred ecologically and by the farmers.
Regarding weed suppression, the important characteristics
to consider before selecting cover crops for inclusion in a
given cropping system include high initial growth rates for
height and leaf area development and with better competitive
ability and hence rapid establishment and growth so as to
develop a close dense canopy, and the ability to cover the
ground completely with dense vegetation, selectivity between
suppression of weeds and the associated crops, production of
allelochemicals and high amount of biomass, and the amount
of C : N ratio and the degree of residue pre-treatment during
residue management. Once suitable cover crops are selected,
proper management of cover crops is crucial to maximize the
benefits obtained from cover crops and minimize their nega-
tive impacts on the main crop under annual and perennial
cropping systems. The management should consider pre-
sowing care, appropriate time and method of sowing, post-
sowing care, methods of enhancing the selectivity between
the weed and the crop, and residue management to enhance
weed suppressive ability. Previous research mainly concen-
trates on the use of cover crops in annual and perennial crop-
ping systems. We suggest further research regarding selec-
tion of cover crops that are compatible with annual and
perennial cropping systems under different climatic condi-
tions and identify the traits responsible for adaptation in vari-
ous cropping systems and environments.
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