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Table of contents
Executive Summary
...


Introduction
...
Types of weeds
...
1
...
7
2
...
Perennial weeds
...
3
...
10
3
...
11
3
...
Soil moisture competition
...
2
...
13
3
...
Light
...
4
...
15
4
...
15
4
...
Cultural control
...
1
...
Crop rotation
...
1
...
Proper spacing
...
1
...
Timely planting
...
1
...
Improving soil fertility
...
1
...
Vigorous and viable seed
...
2
...
18
4
...
1
...
19
4
...
2
...
19
4
...
Biological control
...
4
...
20
4
...
1
...
20
4
...
2
...
21
4
...
3
...
22
5
...
23
5
...
23
5
...
1
...
25
5
...
2
...
25
1

5
...
3
...
26
5
...
27
7
...
31

LIST OF TABLES
Table 1: Annual weeds types, characteristics and their control methods……………………
...
…10
Table 3: Weed with higher densities in conventional tillage fields……………………………
...
28

2

Executive Summary
Weed management is the most challenging component of maize agronomy
...
However, there are instances
when weeds become a problem
...
This FAR Focus draws on
information from five years of investment into weed research
...
Effective weed control in maize requires attention to detail
...


Successful weed control is important for achieving maximum yield in maize grain and silage crops
...
The critical time for weed control in maize is
between crop emergence and canopy closure
...
When maize is grown on new cropping ground, broadleaf
weeds are the most prevalent, but after several years of continuous maize cropping, grass weeds
become the dominant weed type
...

Uncontrolled weeds also contribute to the weed seed bank
...
The weed seed bank in maize fields is often large
and weed germination can start before the maize is planted
...


3

1
...
It may be native
to an area or introduced from other parts of the country or world
...
In general weeds are able to grow and
survive under conditions, to persist and reproduce almost everywhere
...
Wild and rank, producing dense thickets
or vines that crowd out or cover up more desirable species
...
Abundant existing in dense
population, prolific seed producers spontaneous establishing from seed that may have been
dominant for many years or from parts of roots or rhizomes
...
Worldwide maize
production is hampered up to 40% by competition from weeds which are the most important pest
group of this crop (Nathal et al
...


In southern Africa there have been reports of a doubling in labour required for hand hoe weeding
of maize and cotton grown under planting basins (Haggblade and Tembo, 2003) as well as
increases in weeding intensity in minimum tillage compared to conventional mouldboard plough
tillage (Baudron et al
...
Research done in the region
indicated that minimum tillage was associated with high weed density scores (Muliokela et al
...
Giller et al
...
Generally
weeds reduce crop yields by competing for light, nutrients, water and carbon dioxide as well as
interfering with harvesting and increasing the cost involved in crop production
...
Weeds not
only cause severe crop losses but also compete with farmers and their families to spend a
considerable amount of their time on weeding
...
Weeds are a serious threat to
4

primary production and biodiversity
...
The costs of weeds to the natural environment are
also high, with weed invasion being ranked second only to habitat loss in causing biodiversity decline
...
An
efficient weed control program can only be developed after the weed has been properly identified
...
The most effective management of weeds is usually
achieved through collaboration and co-operation, in partnerships between the community, land owners,
agriculture, industry and the various levels of government, using a combination of methods in conjunction
with a thorough follow-up campaign (Gorsi, et al
...


Weed management is an important component of plant protection improving the production
potential of crops
...
Weed management is done through the
mechanical, cultural and chemical means
...
Use of herbicides is an important method in the modern
concept of much in use
...
New hand-tools and implements have also been designed to assist in
wed-management programme
...
) is the most important grain crop in South Africa (SA) and is produced
throughout the country under diverse environments
...
However, grain yields obtained by most smallholder irrigation
farmers, are far below potential with an average of less than 3 t ha-1 being common (Bembridge,
1996; Averbeke et al
...
, 2004; Fanadzo, 2007)
...
8 t ha-1 (Fanadzo, 2007)
...
(1998) and
Fanadzo (2007)
...
Provided
nutrients and moisture are not limiting, successful cultivation of maize depends largely on the
efficacy of weed control
...
Poor weed control decreases water and nitrogen use
efficiency, the two most important inputs to achieving high yields under irrigation (Thomson et
al
...


Most smallholder farmers are aware of the detrimental effects of weeds, but do not have the time
or the means to control them, especially where tractor mechanization has resulted in an increased
area of cultivated land (Steyn, 1988)
...
Shortage of
labour means that smallholder farmers invariably weed a large proportion of the crop late, after
the crop has already suffered significant yield damage (Chivinge, 1990; Mashingaidze, 2004)
...
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
...
Weeds compete
with crops for water soil, nutrients, light, and space, and thus reduce the crop yields
...


6

2
...
1
...
g
...
The seeds they produce will
germinate even faster than the maize crop and are able to survive by producing a lot of seeds that
will germinate next season
...
They will definitely interfere in
the growth of the crop during the critical period of the first three weeks
...
Striga spp are parasitic weeds that attack mainly cereals like
maize and other crops like sorghum, millet and upland rice
...
Yield losses attributed to Striga in
maize can go up to 100% at farm level
...
Produce numerous tiny seeds (50,000 - 500,000
seeds per plant)
...
Once shed, the seeds can stay viable in the soil for up to 20 years
...

Once germinated, the weed establishes parasitic attachments with the root of the host and starts
deriving all nutrients from the host
...
Underside of
leaf is often hairy
...

A narrow-leafed
prostrate summer
annual, Leaf blades
coarse to touch, often
with sparse, stiff
hairs
...


Summer annuals are
best controlled in
spring, and winter
annuals are best
controlled in fall
Control these species
by preventing their
germination with
preemergence
7

Barnyardgrass(billion
dollargra)

Monocot weed

Cheatgras (rye
bromegrass)

Monocot weed

Magnoliopsida

Dicot weed

Magnoliophyta

Dicot weed

than most annual
herbicides, or treating
Monocots
...

A very wide-bladed
Control may involve
semiprostrate summer both chemical and
annual, Stems may be cultural practices
...

application of a pre
Seedhead has
emergence and post
multiple short
emergence
...
Seeds are
coarse with short
burrs
A narrow-leafed,
Grazing, combined
mostly erect winter
with multiple
annual
...

open with multiple
semi-erect branches
carrying 1-4 seed
clusters
Usually characterized Many control
by broad leaves with
methods which have
net-like veins
...

These are seed bearing,
flowering plants
...

These are the most
common plants along
the roadside
...


2
...
Perennial weeds
These are weeds that are always in a maize garden all time every year
...
g
...
Mechanical weeding only cuts off the tops but the bottom continue
consuming the nutrients and water meant for the maize plurope and centants
...

Table 2: Perennial weeds types, description and their control method
NAME

TYPE OF WEED

DESCRIPTION

CONTROL
METHOD

Chufa sedge(Cyperus
esculentus)

Monocot weed

Perennial, is a crop of
the sedge family
widespread across
much of the world
...


(Wild garlic)Allium
canadense

Monocot weed

Cool-season
perennial, the narrow
grass-like leaves
originate near the
base of the stem,
which is topped by
doom like cluster
...

An acceptable and
practical approach to
reduction of an
infestation is a
program of spring
cropping, where
possible cultivation
should extend from
through spring
cultivation, breaking
dominance in hard
offset
...
Plants need to be
sprayed more than
once and for more
than one season
...


Mexican
poppy(Argemone
Mexicana)

Dicot weed

Cover cropping
appears to be a
successful,
sustainable strategy
for weed
management
...

cover on the new
mexicana
...

9

(Santa
Maria)Parthenium
hysterophorus

Dicot weed

Invades disturbed
land, it infests
pastures and
farmland, causing
often disastrous loss
of yield
...


2
...
Characteristics of weeds
Weeds are also like other plants but have special characteristics that tend to put them in the
category of unwanted plant
...
g
...
It has been observed that among 61
perennial weeds, the average seed-production capacity was 26,500 per plant
...
They can
germinate under adverse soil-moisture conditions, have short period of plant growth, generally
grow faster rate and produce seed earlier than most of the crops growing in association
...
g
...
Weed seeds have a tremendous capacity to disperse from one place to another
through wind, water and animals including man
...
Successful
cultivation of maize depends largely on the efficacy of weed control
...
Annual yield losses occur as a result of weed infestations
in cultivated crops
...
The loss occurs as a result of weed competition for nutrients, water and light
...
Certain seeds,
such as those of the thorn apple (Datura), may be poisonous when consumed by animals or human
...
Effects of weeds in maize production
Maize is very sensitive to weed competition during the critical period between the V3 (leaf stages
based on the number of leaves i
...
collar of 3rd leaf visible 16-23 days) on the corn plant and the
V8 stages (collar of 8th leaf visible 31-38 days)
...
Maize needs a
period between the V3 and V8 stages when few weeds are present
...
In
the later part of the cycle, weeds are important mainly if water or nutrient stress is a problem, or if
a very aggressive weed overtops the maize and shades it, or if the weed has some allelopathic
effect
...
Some
weed species are more damage than others
...


Two types of approaches are utilized in most competition studies between weeds and maize
(Rajcan and Swanton, 2001)
...
Hall et al
...
Grain yield in maize can be increased by increasing the
number of hoeing’s, even though differences are not always significant (Bezerra et al
...

Maximum leaf area of maize was noted in those treatments where weeds were controlled the
experiment was done by Khan et al
...
If there were no subsequent weeding, respective yields would be approximately 100%,
75%, 25%, and 8% of potential
...
Weed species differ in their response to management practices
because they have different life cycles, nutrient requirements and modes of reproduction (Martin
& Pol, 2007)
...
1
...
Water stress during vegetative
dry matter (DM) accumulation can limit the height, vegetative biomass (Denmead and Shaw, 1960
and Stewart et al
...
In general, plant
species are more vulnerable to moisture stress during reproductive rather than during early
vegetative stages of development
...
e
...
Stomata closing will affect the rate of leaf photosynthesis, which may
influence the grain yield
...
e
...
, 1997)
...
e
...
e
...
Maize grown together with weeds may have a less developed root system compared to
maize grown under weed-free conditions
...
Maize grow together
with weeds may have a less developed root system compared to maize grow under weed free
conditions, thus the more limiting factor in uptake during reproductive DM accumulation may be
less developed root system, rather than water availability per se
...


Water is the most common limitation to maize production in the tropics
...
The main effect of drought
in the vegetative period is to reduce leaf growth, so the crop intercepts less sunlight
...
Grain yield can be seriously affected if drought occurs during this period
...
In general,
maize needs at least 500-700 mm of well- distributed rainfall during the growing season
...
The occurrence of moisture stress usually varies greatly from year to
year
...
If drought is a common problem, it will
reduce yield by more than 20% in one year out of four
...
2
...
After silking, less assimilates are supplied to the roots as the kernels become the
major sink for photo assimilates
...
, 1984, Anderson et al, 1985, Moll et al, 1987 and Rajcan and Tollenaar, 1999)
...
Therefore, an adequate N supply during the period of N uptake by maize is
essential in order to achieve optimum yields
...


This would imply more depletion of soil N under weedy conditions
...
e
...
However, to the best of our knowledge, there
is no literature available to support this speculation
...
The yield of maize was reduced by weeds more under limited than under luxury
N supply (Staniforth, 1957, Nieto and Staniforth, 1961 and Tollenaar et al, 1997)
...
(1997) under N limiting conditions maize yields were 47% lower
under weedy than weed-free conditions
...
Similar results were found by (Nieto and
Staniforth 1961)
...
A multi-disciplinary approach, involving scientists from other disciplines such as soil
science, plant physiology/mineral nutrition, crop physiology and crop modeling will be necessary
in order to develop a holistic approach to understand the competition process for nutrients
...
3
...
In a mixed crop
– weed community, mutual shading of leaves causes reduction of available photosynthetic photon
flux density (PPFD), which results in reduction of photosynthetic rates
...
most of the weed flora in a maize
canopy at silking and thereafter, however is below 1m (Tollenaar et al
...
Thus, direct
competition for incident PPFD by weeds in maize canopy is relatively weak
...
,
(1994b) reported that 13% of incident PPFD in maize canopy was intercepted by weeds under high
weed pressure
...


The study of growth and biomass accumulation in has shown than biomass production is dependent
to leaf area index and the amount of photosynthetic active radiation received during growth period
...
Asseng et al, 2004
...
Successful modeling of plant growth are
very much related to complete description of leaf area index, light extinction coefficient and light
use efficiency
...
Photosynthetic active radiation Reception by plants and its
consumption in biomass production shows the fundamental processes which control agricultural
plants (Purcell et al, 2002)
...
(Stewart et al, 2003)
...
One of the most important factor
is planting arrangement and plant density which the amount of absorbed light and light extinction
coefficient in plant community can be greatly changed through changing these two factors
...

Through density increase, the size of maize and the weight of grains of one maize decreases
...


3
...
Harmful effects
Weeds have serious impacts on agricultural production
...
Plants can influence each other, by allelopathy
which is usually harmful (Boonitee and Ritdhit, 1984); sometimes the effect is beneficial (Newman
and Andrews, 1973)
...
Allelopathic substances are most commonly found in plant extracts and in plant
residues of soil, some were found in live plant exudates and as volatile gases liberated from leaves
and rhizome (Keeley, 1987)
...
Yield losses due to weeds vary with the crops
...
Alternate hosts of some of the pest
and diseases
...
Control of weeds in maize production
Integrated weed management (IWM) is a broad term covering many methods that can be combined
and applied in various ways to the crop to constitute an IWM strategy
...
Substantial reductions in herbicide input can
be achieved in maize through using IWM
...
Inter-row cultivation is an important element in an
IWM strategy for maize either by supplementing band-spraying or by controlling weeds that have
survived previous control actions
...


15

Whenever herbicides are applied, this should preferably be done as band-application to limit the
area sprayed
...
It is recommended that cover crops be used and crop rotations be diversified
...
Careful and
regular monitoring of weed infestations helps in choosing the right solutions and may identify
herbicide-resistant species at an early stage when applying IWM
...
Weed control refers to actions used
to achieve the immediate elimination of an existing weed population, usually through the use of
herbicides and tillage
...
Thus, while weed control reacts to problems after they
occur, weed management emphasizes understanding the causes of weed problems with the goal
of preventing weeds from becoming problematic in the first place
...


The IWM involves a combination of cultural, mechanical, biological, genetic, and chemical
methods for an effective and economical weed control that reduces weed interference with the crop
while maintaining acceptable crop yields
...
However, if various components of IWM are implemented in a systematic
manner, significant advances in weed control technology can be achieved
...
The IWM
involves a combination of cultural, mechanical, biological genetic, and chemical methods for an
16

effective and economical weed control that reduces weed interference with the crop while
maintaining acceptable crop yields
...
However, if various components of IWM are implemented in a systematic manner,
significant advances in weed control technology can be achieved
...
Selection of a well-adapted crop variety or hybrid with good early-season vigor and appropriate
disease and pest resistance
...
Appropriate planting patterns/spacing and optimal plant density, improved timing, placement,
and amount of nutrient application
...
Appropriate crop rotation, tillage practices, and cover crops
...
1
...
, 2000; Akobundu, 1993)
...
, 2002) and
suppressing weed growth and competitiveness (Zimdahl, 1999; Murphy et al
...
, 1967; Wiese et al
...
Increased rates of crop growth as a
result of adequate fertilization and precise placement of fertilizer in the vicinity of crop roots
ensures that the crop has a ‘starting position’ advantage over weeds in the capture of resources
including incoming radiation (Mahler et al
...
, 1995; Tanner, 1984)
...
, 2002; Mesbar and Miller, 1999)
...
1
...
Crop rotation
Rotate maize with leguminous crops (such as cowpea, groundnuts, pigeon pea, soya beans, kidney
beans, lablab, mucuna and bambara nuts) and with other trap crops like sunflower
...
However, this cannot
completely eradicate striga because seeds can stay viable in the soil for a long time
...
Crop rotation is considered as a “panacea” as for controlling several insect
pests, diseases and weeds under crop field ecosystems so for maintaining soil health and sustained
crop production
...
1
...
Proper spacing
If crops are planted at recommended spacing, the plants cover the ground quickly reducing the
need for weeding
...
In areas of less reliable rainfall, weedfree conditions should be maintained until flowering to minimize the risk of moisture stress at this
critical stage
...
1
...
Timely planting
Maize planted at the right time has vigorous growth and could be well established before the
growth of weeds
...

4
...
3
...

4
...
4
...
It is always
advisable to use treated seed bought from a recognized stockist
...
This way, low vigor seeds could diminish
percentage of emerged seedlings and seedling emergence speed, initial growth, leaf area, and dry
mass accumulation (Schuch et al
...

4
...
Mechanical weed control
Tillage is the most common method of mechanical weed control and it can be divided into two
categories: Pre plant tillage and row crop cultivation
...
To control weeds during the season, crops
may be planted in wide rows for mechanical control
...

18

4
...
1
...
Field cultivators and discs are
commonly used by growers, and they are highly effective for controlling weed seedlings if used
properly
...
When growing a competitive crop, this head start may
allow for reduced herbicide use
...
2006)
...
2
...
Row-crop cultivation
Row crop cultivation is a good complement to chemical weed control
...
Two or more passes with the cultivator may be necessary, but cultivation can allow
for reduced herbicide rates, especially when the herbicide is banded
...
Weeds should be shorter than 4 inches and the crop tall enough (greater than 6 inches) to
avoid being buried under soil or crop residue
...
This single later cultivation may suffice if early weeds have been controlled with
herbicides
...
Highresidue row crop cultivators can be used to help manage small annual weeds in reduced-tillage
systems
...


4
...
Biological control
The biological control approach makes use of the weed’s naturally occurring enemies to help
reduce the weed’s impact on agriculture and the environment
...
Weeds are reunited with their natural enemies
and achieve sustainable weed control
...
For example, a commercial bio-herbicide Colego, a fungal herbicide,
19

has been used to control northern jointvetch (Aeschynomene americana L
...
Considerable host-specificity testing is
mandatory as per many government rules and regulations prior to the release of biological control
agents to ensure that they will not pose a threat to non- target species, such as native and
agricultural plants
...
Developing a biological
control project requires a substantial investment, sometimes costing millions of dollars
...


4
...
Chemical Weed Control
Application of herbicides is the most important method of weed control in corn
...
Herbicides can be applied at different time intervals, such as
before the crop is planted (pre-plant), after the crop is planted but before emergence (pre
emergence), and after crop emergence (post emergence)
...
Many corn
growers use more than one herbicide applications that may provide a season-long weed control
...
4
...
Pre-plant Herbicides
For control of winter annuals and early-spring annual weeds, herbicides applied on emerged weeds
are known as “burn down herbicide treatment
...
Many farmers include residual herbicides with early-spring burn down
treatments
...
The magnitude of this reduction will depend
on the time period and weather encountered between applications and planting and the herbicide
rate
...


If applications are to be made a few weeks earlier than normal, the product rates should be
evaluated carefully in order to maximize the contribution of the residual weed control after crop
emergence
...
With
this application method, the herbicide is applied to the soil surface and mechanically incorporated
into the top 5–8 cm of soil with tillage
...


In addition, it also reduces the chance of herbicide loss through volatilization
...
Buttle observed that soil
incorporation led to a significant reduction in the total metolachlor loss in runoff water relative to
application as pre emergence
...

4
...
2
...
Soil-applied pre emergence herbicides may either be
broadcast on the field or be applied in bands over the planted crop rows
...
If there is no rainfall or source of irrigation,
mechanical incorporation by a rotary hoe can move some of the herbicide into the weed
germination zone
...
If weeds are
emerged at the time of application, pre emergence herbicide can be tank-mixed with foliar active
herbicides to expand weed control spectrum
...
Several pre-emergence herbicides have been registered for weed control in corn
...
Several residual preemergence herbicides can be applied after corn emergence
...
g
...
Metolachlor, alachlor, and

21

dimethenamid are acid amide herbicides, also known as chloroacetamide herbicides
...
] Scop
...
] Beauv
...
Wright])
...

4
...
3
...
They
usually have foliar activity on emerged weeds with a good crop safety if applied as directed on the
label
...
Foliar-applied
post-emergence herbicides do have a requirement for rainfall after application
...
For example, time
until herbicides are rainfast for 2, 4-D is 1 h, glyphosate 1–4 h depending on glyphosate
formulation, and glufosinate 4 h
...
Wide-scale adoption of glyphosate-resistant corn has resulted in heavy reliance on
glyphosate for weed control for many years in Midwestern United States
...
In addition, more than
90 % of the soybean hectares are planted with glyphosate-resistant cultivars, placing extreme
selection pressure for glyphosate resistance in weeds
...
The hydroxyphenylpyruvate dioxygenase
(HPPD)-inhibiting herbicides, such as mesotrione, tembotrione, topramezone, and isoxaflutole,
are important herbicides for control of broadleaf weeds in grain and seed corn
...

Atrazine is the base for the weed control program in corn in the USA
...


22

However, a long-term and continuous use of atrazine resulted in accumulation of atrazine andits
breakdown products in the environment, groundwater, and aquatic systems
...


Therefore, the use of atrazine for crop production has been banned in several European countries,
including France, Germany, Italy, and Sweden
...
More research is required to explore the potential of reducing
atrazine-use rates while maintaining effective weed control in corn and environmental quality
...
Because most corn
growers have a number of broadleaf and grass weed species in their fields, tank-mixing atrazine
with other herbicides—such as mesotrione, isoxaflutole, or acetochlor—might be desirable to
broaden the weed control spectrum
...
Effect of tillage methods on weed population
One of the principle reasons why farmers plough the soil is to control weeds
...
Weed control without tillage is more complicated and requires more
knowledge
...
This can lead to a rejection
of the technology (Thiefelder et al
...
The diversity of species within weed communities is of
agronomic significance because of it is indicative of the response of weed species to soil
management
...
,1993)
...
1 Conservation Agriculture
Weed infestations are claimed to decrease with time under CA resulting in a weed community that
is more manageable when recommended CA practices are followed
...
It is therefore
23

important to review literature on the effects of individual CA principles before weed dynamics
under CA are studied
...
The
mulching effect on weed suppression was more pronounced in the ripping and planting basin
options
...


This is in contrast to findings by Mashingaidze et al
...
At Domboshawa, however, while similar processes
may have promoted weed seed germination, mulch most likely smothered the germinated seedlings
across the treatments, such that at quantification, only surviving individuals were accounted for
...
Flash floods were
experienced during the months of December and January in 2011–2014, leading to excessive water
logging, particularly under the basin option
...
For
example, regardless of fertility treatments, a Shannon–Wiener diversity index of 2
...
8 for basins where no mulch was applied, although the same
herbaceous annual, G
...
The same trends were observed for the
ripping option
...
05) weed diversity (mean 2
...

Crop rotation is considered as a “panacea” as for controlling several insect pests, diseases and
weeds under crop field ecosystems so for maintaining soil health and sustained crop production
...
Any material that blocks light will suppress or prevent growth of weeds
...
,1994)
...
Organic mulches break down over time, and
original thickness can typically reduce by 60 percent after one years
...
, 1989; Creamer,
1995)
...
1
...
Crop residue mulching effects on weeds
Among the benefits reported to be associated with retention of crop residue on the soil surface in
CA is weed suppression which can lead to improvement in weed management in CA (FAO,
2010; ZCATF, 2009)
...
Crop residue mulches have been reported to reduce weed density
(Bilalis et al
...
, 2007; Chauhan and Johnson, 2008) and weed biomass
(Gill et al
...
, 2003)
...
, 2007)
...

The smothering effect of residues the number of weeds present in the field
...
However, the residues also make hand
weeding more difficult
...
,2007)

5
...
2
...
The seeds in the seed bank were previously shed by standing vegetation or
dispersed into the area from other regions
...
For weed species that reproduce from seed, the weed seed bank is viewed as
the driver of annual weed infestations in the field
...
The size and weed diversity of the seed bank under
arable fields is believed to be a reflection of past and current farming practices (Buhler et al
...
However, an increase in the size of the seed bank under no-till has been observed
in other research (Dorado et al
...
The increase in weed seed bank
has been attributed to protection of weed seed by crop residue and less movement of seed through
25

soil profile resulting in less dormancy breaking mechanism in soils with fewer disturbances
(Vencill et al
...
In contrast, Barberi and Lo Cascio (2001) observed no differences in seed
bank size between no-till and mouldboard plough
...
In terms of weed composition, weed diversity has been
reported to increase (Dorado et al
...
The small-seeded
Portula oleracea L
...
, 1999)
...

This is because soil inversion is the primary cause of vertical seed movement in agricultural lands
(Benvenuti, 2007)
...
, 2007)
...
, 1995; Barberi and Lo Cascio, 2001; Cardina et al
...
, 2006b; Vasileiadis et al, 2007)
...
1
...
Composition and diversity of weed species
Changing tillage regimes changes the disturbance frequency of the farm field, which results in a
shift in weed species (Pollard and Cussans, 1981; Buhler et al
...
, 1994;
Swanton et al
...
, 2015)
...
, 1981; Tuesca et al
...
As NT can favor certain granivore species over others,
the associated shift in preferred seed consumption may contribute to altered seedbank composition
(Brust and House, 1988)
...
,
1993), although this depends heavily on the crop rotation (Derksen et al
...
While there is
consensus that the weed species composition will shift in response to changes in tillage, whether
the diversity of the weed community increases is less clear
...
Compared to tilled soils, higher weed

26

species diversity has been observed in NT seedbanks (Cardina et al
...
,2006)
emerged weed communities (Derksen et al
...
, 2001) or both (Murphy et al
...
Studies that report no increase in diversity with NT all found either crop rotation or weather
had a larger effect on weed species diversity
...
, 1997; Legere et al
...
,
2011; Boscutti et al
...


Figure 1: Theoretical effect of tillage on number of emerged weed seedlings (expressed as a

percentage of highest number of emerged weeds observed) over time assuming zero seed rain,
adapted from Anderson (2005)
...
2 Conventional tillage
Under conventional plough tillage fresh weed seeds are buried at depths from which successful
emergence is low for most weed species (Forcella et al
...
This is because for a viable seed,
light, temperature and moisture are the main drivers of the germination process (Grundy, 2003)
27

and these become less favourable for germination with increase in soil depth
...
, 1998)
...
A consequence of this is that the ploughing operation brings to the
surface soil layer previously buried weed seed
...
, 2007; Chauhan and Johnson, 2010)
...
Conventional plough tillage
has been associated with summer dicot weed species (Derksen et al
...
and Digitaria sanguinalis L
...
(Vencill et al
...
Tillage reduces the mechanical strength of
soil and this enables more seedlings to emerge (Mohler and Galford, 1997)
...


Comparing zero and conventional tillage systems in Manitoba was conducted
...
For each zero tillage field surveyed, a conventional tillage field was selected nearby
that had a similar crop rotation history
...
As well, only fields seeded with knife, spoon or disc type openers were
used; those on which cultivator shovels were used at seeding time did not qualify
...

Fields qualified under the conventional tillage category if there was a minimum of two tillage
operations before seeding (in the spring or previous fall)
...
The centre part of each
field was sampled, as well as the border, to look for weeds that might have been encroaching from
the field's edge
...
However, other annual weeds had higher densities in conventional tillage
systems
...
Annual weeds
that had lower densities in zero tillage fields included redroot pigweed, green foxtail and kochia
...


Differences were also observed in weed populations in field borders and the main body of both
types of fields
...
This is an important
observation, since it has been suggested that weeds such as dandelion and Canada thistle (with
seeds that are spread by wind) might be more abundant in the borders of zero tillage fields - because
their seeds are trapped by stubble and litter as they blow in
...
It should be noted that the consistent increase of perennial weeds such as dandelion, perennial
sow-thistle, Canada thistle and quackgrass in zero tillage fields has been and still is a concern
...
,1994)
Weed

Zero
Tillage
Density (#/m2)

Conventional Tillage
Density (#/m2)

Green foxtail

22
...
1

Volunteer barley

0
...
6

Wild mustard

15
...
6

Redroot pigweed

1
...
2

Volunteer flax

17
...
7

Night
catchfly

flowering 1
...
0

Kochia

0
...
3

Lamb's-quarters

4
...
5

29

6
...
The dynamic
nature of weeds species distribution and infestation demands intensive research hence integrated
weed management in this regard is an indispensable approach to regulate weed population below
that causes economic loss to maize productivity
...
No weed management technology used alone is
sustainable since weeds will adapt to any single tactic used repeatedly for many years
...
Although the IWM systems are regarded as the best approach to dealing
with weeds in the small holder sector, there seems to be a large gap between theory and practices
...
Developing more effective supplementary weed control options,
such as low-disturbance mechanical weed or weed seed destruction, may enhance the feasibility
of organic zero tillage and other conservation tillage systems
...
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