Direct seeded rice: research strategies and opportunities for water and weed management

Abstract

Anticipated water crisis, traditional rice cultivation having standing water for most of the growth stages and
mounting labour shortage necessitates the search for alternative water management methods to increase the
water productivity in rice cultivation. The major benefit of direct seeded rice (DSR) is its low- input demand.
DSR with non- puddled and non- flooded conditions has the potential to maximize the water productivity under
deficit soil moisture conditions. The major constraint for DSR is water and weed management for sustaining the
yield. Exposure of rice plants to water deficit stress leads to the nutrient deficiency and panicle sterility, which
ultimately leads to reduction in yield. One of the prominent reasons for yield penalty under DSR is weed
infestation, which accounts for enormous losses in economic terms and sometimes crop failures. High weed
density not only compete with the rice plant but often provides a shelter for growth of various harmful insect,
pest and pathogens, which adversely affect the rice production. Compared to manual weeding, weed control by
herbicides are considered to be more efficient and economical in wet direct seeded rice. Hence, the identification
and selection of cultivars based on their competitive ability with weeds coupled with drought tolerance is of
paramount importance. In order to achieve long term sustainable and economic weed control in DSR, an
integration of different weed management strategies involving cultural, mechanical, biological and chemical
methods are very much essential. In this review, we discuss the experiences, potential benefits and major
challenges associated with DSR, and suggest sustainable management practices for direct seeded rice cultivation.

Full text

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ABSTRACT
Anticipated water crisis, traditional rice cultivation having standing water for most of the growth stages and
mounting labour shortage necessitates the search for alternative water management methods to increase the
water productivity in rice cultivation. The major benefit of direct seeded rice (DSR) is its low- input demand.
DSR with non- puddled and non- flooded conditions has the potential to maximize the water productivity under
deficit soil moisture conditions. The major constraint for DSR is water and weed management for sustaining the
yield. Exposure of rice plants to water deficit stress leads to the nutrient deficiency and panicle sterility, which
ultimately leads to reduction in yield. One of the prominent reasons for yield penalty under DSR is weed
infestation, which accounts for enormous losses in economic terms and sometimes crop failures. High weed
density not only compete with the rice plant but often provides a shelter for growth of various harmful insect,
pest and pathogens, which adversely affect the rice production. Compared to manual weeding, weed control by
herbicides are considered to be more efficient and economical in wet direct seeded rice. Hence, the identification
and selection of cultivars based on their competitive ability with weeds coupled with drought tolerance is of
paramount importance. In order to achieve long term sustainable and economic weed control in DSR, an
integration of different weed management strategies involving cultural, mechanical, biological and chemical
methods are very much essential. In this review, we discuss the experiences, potential benefits and major
challenges associated with DSR, and suggest sustainable management practices for direct seeded rice cultivation.
Key words: Direct seeded rice, weed control, water management, breeding approach
Rice is the vital staple food of Asia, where ~92% of
the global rice is produced and consumed. It is the
source for ~35-80% of total calorie intake of Asian
population (IRRI 1997). Nearly 133 Mha out of the
total of 156 Mha of global rice growing area is in Asia
producing ~540 million tonnes (Mt) out of the total global
production of 660 Mt of rice. India is the 2nd largest
producer of rice next to China, where it is grown in an
area of 45 Mha annually with a production of 90 Mt
and accounts for~45% of food grain production in the
country (Singh et al. 2013). Worldwide, rice demand is
increasing @ 6.0% due to change in the dietary habit
of majority of the population of western and central
Africa (Carriger and Vallee 2007). The most common
methods of rice crop establishment are direct sowing
(dry direct seeding and wet direct seeding) and
transplanting (Kumar et al. 2015 a, b; Chatterjee et al.
2016; Kumar et al. 2016e). Presently, in direct seeded
rice (DSR) is gaining momentum due to labour shortage
during peak season of transplanting and availability of
water for short periods (Kumar et al. 2016c,d; Kumar
et al. 2015a, b; Singh et al. 2017;
Review article Oryza Vol. 53 No.4, 2016 (354-365)
Received :29 November 2016 Accepted :02 February 2017 Published :03 February 2017
Direct seeded rice: research strategies and opportunities for water and
weed management
R Kumar1*, N Kumawat2, S Kumar3, Ravikant Kumar1, M Kumar4, RP Sah5, U Kumar5 and
A Kumar5
1ICAR-Research Complex for Eastern Region, Patna-800 014, Bihar, India
2AICRP on Maize-Zonal Agricultural Research Station, Jhabua-457 661, MP, India
3ICAR-Research Complex for NEH Region, Manipur Centre, Lamphelpat, Imphal-795 004, Manipur, India
4ICAR-RC for NEH Region, Nagaland Centre, Jharnapani-797 106, Nagaland, India
5ICAR-National Rice Research Institute, Cuttack-753 006, Odisha, India
*Corresponding author e-mail: rakeshbhu08@gmail.com

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Prakash et al. 2014). Direct seeding involves sowing
of pre-germinated seeds in wet (saturated) puddled/
dry soils. In recent years there is a serious concern
about the availability of water for rice production due
to sharp decrease in water table (Hugar et al. 2009). It
has been reported that ~2 M ha of fully irrigated and
13 M ha of partially irrigated lands in Asia during wet
season experience physical water scarcity and 22 M
ha of irrigated lands in the dry season would face
economic water scarcity by 2025 (Ali et al. 2014).
These facts lead to shift from transplanting to DSR in
many Asian countries including India.
According to Lafitte et al. (2002), concept of
DSR comprises of use of rice varieties, which are
nutrient-responsive and well adapted to aerobic soils
with yield potential of 70-80% of high-input flooded rice.
The irrigation scheduling in DSR done through surface
irrigation aims at keeping the soil wet, but not flooded
or saturated. In practice, irrigation is applied to bring
the soil water content up to the field capacity, once the
lower threshold limit has been reached. For most of
the upland crops, the threshold limit of irrigation are
usually when the soil water content reached halfway
between field capacity and wilting point (Doorenbos
and Pruitt 1984). The greatest problem with DSR
system is yield sustainability issue as frequent yield
declines even failures have been reported from different
parts of world (George et al. 2002).In the current
scenario of shrinking agricultural land, acute shortage
of irrigation water and decreased availability of labour,
the adoption of improved rice varieties having higher
water productivity and modern agronomic technology
are the only way out to meet the global rice demand
(Leeper 2010).
Constraints in DSR
In DSR system, dry rice seeds are sown with or without
tillage and irrigation is applied periodically to maintain
soil moisture at field capacity. So, this method enhances
water productivity and conserve considerable amount
of irrigation water (Kumar et al. 2016c) and method of
planting also reduces the total labour requirement by
11-66% compared to puddled transplanted rice (PTR),
depending on season, location, and type of DSR. It
allows faster and easier planting and often DSR rice
matures ~7-10 days earlier than the PTR, facilitating
timely planting of the succeeding rabi crop (Singh et
al. 2006). Additionally, DSR improves the soil health,
emits less methane and often ensure higher profit in
area with assured irrigation supply (Kumar et al.
2016c). Weeds are the one of the major cause of yield
loss in DSR as well as in low-land transplanted
condition (Roy et al. 2011). Yield loss of ~17-24% takes
place if weeds were allowed to compete till 4 week
after seeding (Chauhan and Johnson 2011). Weed
management practices under DSR system may vary
depending upon the socio-economic conditions of
growers and several other factors associated with them
(Mandal et al. 2011a, b; Chatterjee et al. 2016).
Traditionally, hand-weeding was the most common
method to control weed, and presently weeding through
hand hoeing is not economical as it takes time and
requires more man days (Chatterjee et al. 2016).
Surface mulching through crops residue may selectively
provide weed suppression through their physical
presence on soil surface and can be a part of integrated
water management (IWM) program.
Dominant weed flora in DSR
Echinochloa colona and E. crusgalli are most serious
weeds affecting significantly in all methods of rice
establishment (Mandal et al. 2011a, b). Other weeds
of major concern in rice includes, Ammannia baccifera,
Cyperus iria, Cyperus difformis, Eclipta alba,
Fimbristylis miliacea, Ischaemum rugosum,
Leptochloa chinensis, Monochoria vaginalis,
Paspalum distichum and Spaenoclea zeylanica. E.
colona is predominantly observed in dry seeded rice
as it requires less moisture than E. crusgalli. Cyperus
rotundus and Cynodon dactylon are other major
weeds posing problems in upland conditions, particularly
in poorly managed fields. Enormous amount of
variations occur in dominance and abundance of weed
species with change in crop establishment and weed
control methods (Singh et al. 2005). In recent era,
weedy rice is emerging as a major problem in DSR.
Javier et al. (2005) has observed a shift in weed flora
by the change in crop establishment method. Yaduraju
and Mishra (2005) has reported that direct seeding also
favours sedges such as Cyperus difformis, Cyperus
iria, Cyperus rotundus and Fimbristylis miliacea.
Several weed management strategies was exploited in
India depending upon weed flora as well as critical
period of crop-weed competition (Prasad et al. 2013).
Weed management strategies includes physical,
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chemical and biological methods
Weed management system
Various cultural methods are available depending on
location and availability of resources. Some of the
methods are discussed as below:
Sanitation
Rice seeds infested with noxious weed seeds have a
chance to introduce the problematic weed species to a
new field and increase the seed numbers in the soil
weed seed bank. Other than clean crop seed, farm
machinery and tools used for tillage, sowing, harvesting
or threshing operations should also be cleaned before
moving it from one field to another. Movement of seeds
or weeds of propagules should be avoided to some
extent by cleaning of bunds and irrigation canals.
Land leveling
A well prepared land helps in minimizing the weed
densities by providing a weed free seed bed at the time
of sowing. Proper land leveling also ensures uniform
plant stand in the field. Rickman (2002) reported that
laser land levelling reduces the weed population by up
to 40% and the labor requirement for weeding by 75%
(16 man-days ha-1).
Stale seedbed method
Stale seedbed is a technique in which weed seeds are
forced to germinate and then they are killed by either
through use of non-selective herbicide (paraquat,
glyphosate, or glufosinate) or by shallow tillage before
sowing of direct seeded rice. This technique offers a
great potential for suppressing weeds and is feasible
under zero till (ZT)-DSR as there is ~ 45-60 days of
fallow period between wheat harvests and sowing of
rice. Stale seedbed reduced weed population by 50%
compared with treatments in which this was not used
(Singh et al. 2007). Additionally, this technique is equally
effective in reducing weed seed bank.
Crop rotation and cropping system
In age old practices of traditional farming, crop rotations
comprised of crops with different life cycles were a
key component of weed management. Manipulation in
date of planting and harvesting of different crops in
rotation may provide opportunities for farmers to
prevent either weed plant establishment or seed
production. Weedy rice, which has become a major
threat in rice tract can be controlled to the some extent
through crop rotation with other crops such as soybean,
mungbean, cotton, maize etc., which allow using other
herbicides and cultural practices that cannot be used in
rice (Singh et al. 2013). Rotating rice with mungbean
was found to be very much effective for weedy rice
control because volunteer rice seedlings failed to
survive in mungbean (Watanabe et al. 1998).
Intercropping of rice with sunhemp ( Crotalaria juncea
L.), cowpea [Vigna unguiculata (L.) Walpers],
soybean [Glycine max (L.) Merr.], and prostrate
sesbania (Sesbania rostrata Brem.) along with one
inter-cultivation at 15 days after emergence (DAE) and
one hand-weeding at 40 DAE, was found to be much
effective in managing weeds (Angadi and Umapathy
1997) in rice.
Seed priming
In seed priming, seeds are allowed to be hydrated
partially to that point where germination-related
metabolic activities occur, but seeds do not reach the
irreversible point of radicle emergence (Bradford,
1986). Seed priming had beneficial effects on inert seed
as it enhanced the germination and seedling emergence
ability (Anwar et al. 2012b). It can improve the traits
closely associated with weed competitiveness of rice
i.e. growth rate, early crop biomass and early vigour.
Various priming techniques employed to improve speed
and synchrony of seed germination are pre-soaking,
hardening, hormonal priming, hydro priming, halo
priming, osmo-conditioning, and ascorbate priming.
Brown manuring through Sesbania co-culture
Brown manuring is a practice, which involves, co-
cultivation of rice crops along with green manure crops
(sesbania), thereafter, 25-30 days of growth, it is killed
by application of 2, 4-D ester @ 0.50 kg ha-1. This
technique is efficiently able to manage weed population
by nearly half without any adverse effect on yield of
rice (Singh et al. 2007). Other than weed suppressions,
several additional benefits of sesbania co-culture are
atmospheric nitrogen fixation and facilitation of crop
emergence in areas, where soil crust formation is a
problem. The best time for sowing sesbania to get
maximum weed suppression is on the day of rice sowing
(Singh et al. 2007).
Research strategies for direct seeded rice Kumar et al

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Soil solarisation
Soil solarisation is a method of heating the soil's surface
by using transparent low-density polyethylene (LDPE
film) sheets placed on the soil's surface to trap solar
radiation. Soil solarisation using plastic mulch increases
soil temperature at 5 cm depth by 10- 15°C and at 10
cm depth by 10-12° C. Other than weed control, soil
solarization also improves the soil structure, availability
of essential plant nutrients and control of soil borne
pathogens such as nematode, Fusarium, Rhizoctonia
etc. Khan et al. (2003) reported that covering soil prior
to planting with 100 µ thickness (400 gauge) LPDE
sheets for 30 days was effective in reducing density of
grassy and broad leaved weeds.
Mechanical method of weed control
Mechanical method of weed control involves use of
implements that either destroys weeds or make
environment less favorable for weed seed germination
and weed survival. These methods include hand-pulling,
hoeing, mowing, ploughing, disking, cultivating and
digging. Mechanical weeding is most commonly
practiced on row seeded rice since, inter row cultivation
with either hand tools/animal traction equipment reduces
time in weeding and minimizes crop damage. However,
weeds within row are difficult to remove by this method.
The sufficient soil moisture is another critical factor to
achieve satisfactory results by using these weeders.
Although mechanical weeding using hand pushed
weeders (e.g., cono-weeder) is tedious and time
consuming, but still it is very much common for many
small and marginal farmers of Asia and Africa. Sarma
and Gogoi (1996) reported that in rainfed upland rice,
manually operated peg type dryland weeder and a twin
wheel hoe were effective in controlling weed when
used twice at 20 and 30 day of emergence (DOE).
The use of mechanical weeders is feasible only where
rice is planted in rows. Weedy rice is generally taller
than cultivated rice and chopping must be done before
seed setting takes place. In many countries, weedy rice,
panicles are cut with the help of a machete or a special
knife attached to a stick (Singh et al. 2013).
Chemical method of weed control
Chemical method of weed control should not be
considered as a replacement for other weed control
methods, however, should be integrated with them
(Mishra et al. 2016;Kumar et al. 2016 a, b; Chatterjee
et al. 2016). Hill et al. (2001) reported that the success
of herbicidal method of weed control is closely linked
to water management to provide suitable condition for
achieving specificity in weed control and minimizing
the risk of phytotoxicity to rice seedlings. Judicious
selection of herbicide, correct time of application, proper
dose and right method of application are the important
criteria for achieving higher weed control efficiency
and crop yield. Jacob et al. (2014) reported that the
major advantage with herbicidal control of weeds in
DSR is the reduction in cost of cultivation. De Datta
(1981) opined that despite of some adverse
environmental effects, herbicides are considered to be
the most effective, practical and economical means of
weed management in DSR. Chemical method of weed
control is becoming more popular day by day and is
the best alternative to hand weeding as hand weeding
needs high labour involvement (190 man days ha-1), is
tedious; time consuming and impractical under adverse
weather conditions (Begum et al. 2011).
Sunil and Shankaralingappa (2014) reported
that application of pyrazosulfuron @ 25 g a.i./ha alone
was unable to control heavily infested weeds and it
failed to control goose grass (Eleusine indica (L.)
Gaertn.). The herbicide mixtures (both tank and
proprietary mixture) broaden spectrum of weed control
in a single application (Damalas 2005). A narrow leaved
effective herbicide in combination with a herbicide that
kill broad leaf weeds would be effective in controlling
both types of weed. Similarly, a grass effective herbicide
in combination with herbicide that control both broad
leaf weeds and sedges will provide a wider spectrum
of weed control (Mukherjee 2006). Aurora and De
Datta (1992) reported herbicides used in combination
reduced the usage rate as compared to single herbicide
use. Chauhan and Yadav (2013) opined that in future,
combination of two or more herbicides may become
an effective and integrated approach to control
complex weed flora in DSR. Application of 10%
common salt (NaCl) were found to be effective weed
management in upland direct seeded jhum rice
especially broad leaved weeds in acidic soils of
Nagaland (Chatterjee et al. 2016).
Biological method of weed control
Biological method of weed control offers environmental
friendly approach that supplements conventional
method. It involves deliberate use of insects,
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nematodes, bacteria, fungi or other bio-agent that
reduce weed populations. Different herbivorous bio-
agents like fish, tadpoles, shrimps, ducks and pigs were
utilized for control of weeds in irrigated lowland rice in
a few countries (Smith 1992). However, these bio-
agents could not be utilized in DSR because, there is
no standing water. Recently micro-herbicides are being
evaluated and validated to reduce the herbicide
dependency. Collego, a powder formulation of
Colletotrichum gloeosporioides (Penz.) Sacc. f. sp.
aeschynomene, controls northern joint vetch weeds
(Aeschynomene virginica (L.) B.S.P.) in rice (Smith
1992). Other useful fungi identified for bio-control of
barnyard grass are Exserohilum monocerus and
Cocholiobolus lunatus (Khadir et al. 2008),
Alternaria alternata for control of barnyard grass
(Jyothi et al. 2013). Setosphaeria sp cf rostrata for
the control of Leptochloa chinensis (Thi et al. 1999).
In future, attention must be paid for extensive research
in order to develop the broad spectrum micro-herbicide
formulation for effective control of weeds in DSR.
Water management in transplanted rice Vs
DSR cultivation
As compared to wheat and maize, the water
productivity (Wp) of transplanted rice is very low, which
ranges from 0.33 g kg-1 to 0.20 g kg-1 (Kumar et al.
2016c). Average water requirement to produce 1 kg of
rice is ~2350 liters (L), which varies in the range of
1700-3000 L. The Wp of rice crop depends on a
number of factors like water availability (rains +
irrigation), soil type (texture, organic matter content,
hydraulic conductivity, percolation rate etc.), and climate
(temperature, sunshine hours, humidity, wind velocity
etc.). Gupta et al. (2002) reported that there was an
increase in total water required by rice crop from 1566
mm in a sandy clay loam soil texture to 2262 mm in
sandy loam soil texture. Beside other factors, this
increase in water requirement was mainly due to an
increase in percolation loss from 57% in clay loam to
66.9% in sandy loam soil. Due to indiscriminate use of
surface or ground water by industrial, domestic and
agricultural sectors, availability of water is getting
scarce day by day and it is predicted that by 2025, only
50-55% of the total world water will be available for
agriculture as against 66-68% in 1993 (Sivannapan
2009). Saving of water has demanded attention of
irrigation scientists worldwide and hence efforts are
underway to develop water saving technologies in rice
such as alternate wetting and drying (AWD), keeping
soil saturated continuously, irrigation based on soil
moisture tensions varying from 0 to 40 kPa at root zone
depth or irrigation at an interval of 1-5 days after
disappearance of ponding water (Bouman and Tuong
2001; Kumar et al. 2016c, 2017, Pal et al. 2013).
During the growth period of rice plant, some
phases are highly sensitive to moisture stress, and
moisture stress at these phases' results in yield loss.
Several workers reported that effect of water stress
on rice yield was more severe (50% yield loss), when
drought occurred during the reproductive phase,
whereas water stress at the vegetative phase resulted
in less yield loss (34%). The main reason for yield
decrease was delayed anthesis and reduced number
of spikelets/panicle with reduced number of filled grains
(Kumar et al. 2017; Datta et al. 1975). Mendoza (2010)
also reported that when rice field was kept flooded during
vegetative phase and under AWD condition in
reproductive phase of growing period, it resulted in
maximum yield and water productivity.
In Aerobic Rice System (ARS), soils are kept
aerobic almost throughout crop growing season. Like
upland crops such as maize, wheat and sugarcane, ARS
aims at growing rice without puddling and flooding under
non-saturated soil conditions. To optimise the water
economy is the main motto behind adopting ARS, which
is reported to give a water saving of 73% in land
preparation and 56% during crop growth (Castaneda
et al. 2003). Yadav et al. (2011) reported 30-50%
irrigation water saving in DSR irrigated at 20 kPa
compared with PTR irrigated at 20 kPa due to reduced
seepage and runoff losses. Yields of PTR and DSR
with daily irrigation and 20 kPa irrigation threshold were
similar. Further, tiller density, leaf area index and growth
rate was better in DSR than PTR with daily and 20
kPa irrigation scheduling. No effect on crop yield was
noticed up to soil moisture suction of 160±20 cm, while
increasing soil matric suction to 200 and 240±20 cm
decreased rice grain yield non-significantly by 0-7%
and 2-15%, respectively, over different years compared
to recommended practice of 2-days interval for
scheduling irrigation. Irrigation at 160±20 cm soil matric
suction saved 30-35% irrigation water as compared to
that used with 2-day interval irrigation (Kukal et al.
Kumar et al
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2005). One more method of scheduling irrigation is
based on IW/CPE ratio. Irrigation at IW/CPE ratio of
1.2 recorded higher crop growth and yield with no
moisture stress, minimal proline accumulation and
sterility coefficient (Maheswari et al. 2007). In Japan,
under aerobic rice condition total amount of water
supplied (irrigation plus rainfall) was 800-1300 mm when
irrigation was scheduled based on tensiometer reading
between 15 and 30 kPa at 20-cm depth and average
yield under aerobic conditions was similar to or even
higher than that achieved with flooded conditions (7.9 -
9.4 t /ha for aerobic versus 8.2 t/ha for flooded),
whereas average water productivity in aerobic
conditions was 0.8-1.0 kg grain/m3 water (Kumar et
al. 2016c).
The effect of manipulating irrigation schedule
on grain yield, water use efficiency and irrigation
efficiency of rice was investigated by Nwadukwe and
Chaude (1998) and reported significantly higher grain
yield and water use efficiency with irrigation schedule,
which maintained the soil moisture regime at saturation
rather than at submergence or field capacity.
Experiments were conducted in Wagner pots by
Anbumozhi et al. (1998) to evaluate the effect of
different ponding depths (0, 3, 6, 9, 12, 15 and 18 cm)
on rice growth and yield and results revealed that
ponding depth of 9 cm was found to give higher plant
height, grain yield and water productivity. Water saving
irrigation has profound effect on yield as well as various
phenological and biochemical components of rice plant.
Yang et al. (2007) proposed limiting values of soil water
potential as irrigation indices. These indices were related
to specific growth stages, so that wetting and drying
could meet the growth and development of rice. He
compared the conventional irrigation where drainage
was in mid-season to flood at other times, and observed
that the water-saving irrigation increased grain yield
by 7.4 to 11.3%, reduced irrigation water by 24.5 to
29.2%, and increased water productivity by 43.1 to
50.3%.
Breeding approaches for Direct-Seeded Rice
Conventional puddled transplanted rice is facing water
and labour scarcity hence DSR can be a possible answer
because of high potential for saving water, reduced
labour needs, and acclimatizing to climatic risks. Few
varieties have been developed for aerobic condition like
CR Dhan 200, CR Dhan 203, CR Dhan 205, CR Dhan
206, CR Dhan 207, CR Dhan 209 that has considerable
yield potential. Still further improvement is required to
develop variety that possesses traits specifically needed
to produce high yield under dry direct-seeded conditions
that may be prone to drought and low fertility. The major
yield and productivity contributing traits (genes/ QTLs)
than can be introgessed in line through molecular
breeding for DSR is being discussed.
Anaerobic germination and tolerance of early
submergence
The untimely extended rains immediately after sowing
during monsoon season adversely affect establishment
of DSR and causes mortality of young seedlings due to
submergence (Ismail et al. 2009). Therefore, ability
for anaerobic germination (AG) and flash floods (early
submergence) will help in weed suppression too. The
QTLs responsible for a significant percentage of
variation in submergence tolerance (Sub1) and
anaerobic germination tolerance (qAG-9-2 or AG1) in
rice have been previously identified. Sub1, accounts
for about 69% of the variation (Xu and Mackill 1996),
while qAG-9-2 for anaerobic germination tolerance
accounts for ~ 33% of the variation (Angaji et al. 2010).
Initially, IR64- Sub1+AG1 and IR64-AG1 has not shown
any difference with original line of IR64, in terms of
yield and agronomic traits evaluated under normal
conditions. Later changing genetics background of
parents like in Ciherang-Sub1+ AG1 showed
significantly higher yield compared to Ciherang.
Early vigor
Early seedling vigor is the essential desirable trait for
DSR to dominate and smoother the weed growth. Rapid
germination, rapid shoot and root growth, and long
mesocotyls and coleoptiles are important seedling vigor-
related traits (Cui et al. 2002). Some important QTLs
for seedling vigour traits have been identified recently
by Anandan et al. (2016). They reported that marker
alleles on chromosome 2 were associated with shoot
dry weight on 28 DAS. Further, they also identified
QTL for leaf length on 14 DAS on chromosome 1
(RM13); SSR genomic region RM230 and RM125
mapped on chromosome 8 and 7 were QTLs controlling
leaf width variation on 14 DAS; the marker RM230
allele associated with root length on 14 DAS. The root
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dry weight QTL was detected on chromosome 5 (by
linked marker allele RM249) on 14 DAS coincided with
QTL of root length on 28 DAS. For root dry weight at
28 DAS, marker RM250 on chromosome 2 is
responsible. Zhang et al. (2005) also reported RM230
allele marker on chromosome 8 as contributing to root
length. Karla et al. (2016) identified one major effect
QTL, qFW1, on chromosome 1 and contributed to about
35% of the phenotypic variation for seedling fresh
weight. Further, region on the distal end of the long
arm of chromosome 1, which is about 4 Mb away from
the sd1 gene (position 38,381,339), harbors a gene or
genes that contribute to seedling vigor in both Indica
and Japonica rice. Improvement in the early vigor was
identified in varieties like 'Sabita' (a known early vigor
genotype), Varshadhan, Vandana, AC4387 and Pyari
which can be used for identifying rice genotypes
acquiescent to direct seeded.
Crop competitiveness against weeds
Weeds are one of major constraints in DSR.
Differences in cultivar for weed competitiveness exist
in many crops, including rice (Haefele et al. 2004).
This crop weed competitiveness can minimized with
the ability of crop plants to a) weed tolerance and b)
weed-suppressive ability (Zhao et al. 2006). The
competitiveness in plants is judged by growth and
development in a particular time and the allelopathy
reponse. Ebana et al. (2001) mapped QTLs using RFLP
markers controlling allelopathy (F2 population using PI
312777 (Cross of Taichung 65x2 and TN-1), a highly
allopathic accession). Seven allelopathy QTLs on
chromosomes 1, 3, 5, 6, 7, 11, and 12 were identified.
The major QTL is on chromosome 6 explained 16 % of
the total phenotypic variation. But, multiple QTLs
considered jointly explained 5 important QTLs explained
36.6 % of phenotypic variation. A genotypes IAC165,
an allelopathic Japonica upland rice cultivar from
Brazil, and CO39, a weakly allelopathic Indica cultivar
from India were used to develop RILs to map
allelopathy. Four main effect QTLs on chromosomes
2, 3, and 8 explained 35% of the phenotypic variation
observed in population (Jensen et al. 2001). Lee et al.
(2005) identified nine QTLs controlling allelopathic
effects of rice on E. crusgalli on chromosomes 1, 2, 3,
4, 5, 8, 9 and 12. Of these, QTLs on chromosomes 1
and 5 were the most allelopathic and explained 36.5%
of total phenotypic variation. An ideal plant type with
an ability to compete against weeds would have early
seedling vigor, high specific leaf area in vegetative
growth, high chlorophyll producing ability compatible
with high yield and weed competitiveness. The lines
RU9701151 (cross of PI 338046 and Katy)
(Moldenhauer et al. 1999), PI 312777 (Cross of
Taichung 65x2 and TN-1) were identified for weed
competitiveness in rice.
Modified panicle architecture
Direct seeding in dry season can face dry spell even in
complete irrigated conditions, which adversely affecting
spikelet fertility. Increase in sink size is a priority trait
to increase yield potential in rice (Dingkuhn et al. 2015
), which can be possible by increase the number of
spikletes/panicle (Peng et al. 1999). Several genes/
QTLs related to panicle architecture and number of
spikelets/panicle like, Gn1a, OsSPS1, and SPIKE were
identified. Rebolledo et al. (2016) reported 25 new loci
that involved in the genotypic variation for number of
spikelets/panicle. SNPs with high association
probabilities were closer as previously identified
candidate genes, e.g., Osjag in q-1 and OsCKX2 in q-
3. They also reported that the genes such as MOC1,
LAX1/2, OsCKX2, SP1, DEP1/2/3, and IPA1/WFP
were found to modify panicle architecture (Qiao et al.
2011), likewise for panicle density (q-22, gene GSK22)
or floral organ identity (q-25, gene NL1) and two genes
with a function on tillering ability (q-23, OsHRZ2 gene)
or carbon and nitrogen content (q-6, gene OSAAT7).
These genes may be suggested as candidates gene to
improve the number of spikelets per panicle in rice
through molecular breeding
Modified root system
A clear-cut difference in rooting pattern of direct-
seeded or transplanted rice plants can be seen. Sandhu
et al. (2015) developed BC2F4 mapping populations
(crosses of Aus276, a drought-tolerant variety, with
MTU1010 and IR64, high-yielding indica) to identify
traits and QTLs for development of dry direct-seeded
rainfed rice varieties. They identified 26 QTLs in
Aus276/3 x IR64 associated with 23 traits and 20 QTLs
associated with 13 traits in Aus276/3 x MTU1010
populations. The QTLs like qGY6.1, qGY10.1, qGY1.1,
and qEVV9.1 were found to be effective for both
populations under a wide range of conditions. The co-
Kumar et al
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361
r r
location of QTLs for grain yield with root hair length
on chromosome 9 and early vegetative vigour on
chromosome 1 were identified. Whereas, QTLs for root
hair density and phosphorus uptake were co-located in
this study on chromosome 5. QTLs previously reported
for root length (Sandhu et al. 2013), root thickness,
and root number (Li et al. 2005) were located near
qEVV9.1 and qGY9.1. Introgression of these QTLs
may increase nutrient uptake in rice to improve rice
yield under dry direct-seeded conditions.
Present day rice cultivation is under threat by
anticipated water scarcity and mounting labour crisis,
so direct seeded rice (DSR) offers an attractive
alternative. A successful DSR demands breeding of
special rice varieties and developing appropriate
management practices. Under DSR, optimization of
irrigation scheduling and occurrence of diverse range
of weeds is a serious problem. Identification of optimum
water management practices may help in maximizing
water productivity under DSR system. Weeds are
dynamic in nature and a shift in their abundance and
dominance in DSR is a major challenge for the
researchers. Herbicide is the smartest and most
economic tool to fight against the weeds. But recurrent
use of one herbicide for a long time may result in
development of herbicide resistant weed biotypes. The
use of any single strategy cannot provide effective,
season-long and sustainable weed control as weeds
vary in their dormancy and growth habit. To achieve
effective, viable and long term sustainable weed control,
integrated weed management practices are to be wisely
utilized in such a compatible way as to reduce weed
population below the economic threshold levels without
deteriorating environment quality.
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