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Soil & Tillage Research 111 (2011) 87–98
Review
Rice direct seeding: Experiences, challenges and opportunities
M. Farooq a,b,*, Kadambot H.M. Siddique b, H. Rehman c, T. Aziz a, Dong-Jin Lee d, A. Wahid e
a Department of Agronomy, University of Agriculture, Faisalabad 38040, Pakistanb The UWA Institute of Agriculture, The University of Western Australia, Crawley, WA 6009, Australiac Department of Crop Physiology, University of Agriculture, Faisalabad 38040, Pakistand Department of Crop Science and Biotechnology, Dankook University, Chungnam, Republic of Koreae Department of Botany, University of Agriculture, Faisalabad 38040, Pakistan
A R T I C L E I N F O
Article history:
Received 24 May 2010
Received in revised form 12 October 2010
Accepted 13 October 2010
Keywords:
Direct-seeded rice
Resource conservation
Rice production system
Seed priming
Water-saving
Weeds
A B S T R A C T
Rice is one of the most important food crops in the world, and staple for more than half of the global
population. Looming water crisis, water-intensive nature of rice cultivation and escalating labour costs
drive the search for alternative management methods to increase water productivity in rice cultivation.
Direct seeded rice (DSR) has received much attention because of its low-input demand. It involves
sowing pre-germinated seed into a puddled soil surface (wet seeding), standing water (water seeding) or
dry seeding into a prepared seedbed (dry seeding). In Europe, Australia and the United States, DSR is
highly mechanised. The development of early-maturing varieties and improved nutrient management
techniques along with increased availability of chemical weed control methods has encouraged many
farmers in the Philippines, Malaysia, Thailand and India to switch from transplanted to DSR culture. This
shift should substantially reduce crop water requirements, soil organic-matter turnover, nutrient
relations, carbon sequestering, weed biota and greenhouse-gas emissions. Still, weed infestation can
cause large yield losses in DSR. In addition, recent incidences of blast disease, crop lodging, impaired
kernel quality and stagnant yields across the years are major challenges in this regard. In this review, we
discuss the experiences, potential advantages and problems associated with DSR, and suggest likely
future patterns of changes in rice cultivation.
� 2010 Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
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Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
2. Experiences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
2.1. Role of direct seeding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
2.2. Seed priming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
2.3. Yield benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
2.4. Resource conservation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
2.5. Varietal development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
2.6. Weed management. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
3. Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
3.1. Weeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
3.2. Diseases and insect pests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
3.3. Panicle sterility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
3.4. Stagnant yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
3.5. Varietal development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
3.6. Nutrient dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
3.7. Lodging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
4. Opportunities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
4.1. Management options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
4.1.1. Integrated weed management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
* Corresponding author at: Department of Agronomy, University of Agriculture, Faisalabad 38040, Pakistan. Tel.: +92 41 9201 098; fax: +92 41 9200 605.
E-mail addresses: [email protected], [email protected] (M. Farooq).
0167-1987/$ – see front matter � 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.still.2010.10.008
M. Farooq et al. / Soil & Tillage Research 111 (2011) 87–9888
4.1.2. Nutrient management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
4.1.3. Water use and water use efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
4.1.4. Lodging. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
4.1.5. Physiological approaches. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
4.1.6. Greenhouse gas (GHG) emission. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
4.2. Genetic and biotechnological approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
[(Fig._1)TD$FIG]
Low High
Wage rate
High
Low
Water availability
TPR WS/TPR
DS/TPR DS
Fig. 1. Factors affecting the choice of rice establishment methods.
TPR = transplanting, WS = wet seeding, DS = dry seeding (adapted from Pandey
and Velasco, 2002).
1. Introduction
Direct seeding of rice refers to the process of establishing a ricecrop from seeds sown in the field rather than by transplantingseedlings from the nursery. There are three principal methods ofdirect seeding of rice (DSR): dry seeding (sowing dry seeds into drysoil), wet seeding (sowing pre-germinated seeds on wet puddledsoils) and water seeding (seeds sown into standing water; Table 1).Dry seeding has been the principal method of rice establishmentsince the 1950s in developing countries (Pandey and Velasco,2005).
In the traditional transplanting system (TPR), puddling creates ahard pan below the plough-zone and reduces soil permeability. Itleads to high losses of water through puddling, surface evaporationand percolation. Water resources, both surface and underground,are shrinking and water has become a limiting factor in riceproduction (Farooq et al., 2009a). Huge water inputs, labour costsand labour requirements for TPR have reduced profit margins(Pandey and Velasco, 1999). In recent years, there has been a shiftfrom TPR to DSR cultivation in several countries of Southeast Asia(Pandey and Velasco, 2002). This shift was principally broughtabout by the expensive labour component for transplanting due toan acute farm labour shortage, which also delayed rice sowing(Chan and Nor, 1993). Low wages and adequate water favourtransplanting, whereas high wages and low water availability suitDSR (Fig. 1; Pandey and Velasco, 2005). TPR has high labourdemands for uprooting nursery seedlings, puddling fields andtransplanting seedlings into fields.
The adoption of a direct-seeded method for lowland rice culturewould significantly decrease costs of rice production (Flinn andMandac, 1986). To date, no specific varieties have been developedfor this purpose. Existing varieties used for TPR do not appear to bewell-adapted for seedling growth in an initially oxygen-depletedmicroenvironment. As a result, farmers often resort to the costlypractice of increasing the seeding rate for DSR by 2–3 times. Newvarieties suitable for DSR must be able to emerge and grow from anon-flooded soil.
DSR is a major opportunity to change production practices toattain optimal plant density and high water productivity in water-scarce areas. Traditionally, rice is grown by transplanting one-month-old seedlings into puddled and continuously flooded soil.The advantages of the traditional system include increased
Table 1Classification of direct-seeded rice systems.
Direct seeding system aSeed condition bSeedbed condition and
environment
Dry direct-seeded Dry Dry soil, mostly aerobic
Wet direct-seeded Pre-germinated Puddled soil, may be aerobic or
anaerobic
Water seeding Dry or pre-germinated Standing water, mostly anaerob
a Source: Thakur et al. (2004).b Source: Balasubramanian and Hill (2002).
nutrient availability (e.g. iron, zinc, phosphorus) and weedsuppression (Surendra et al., 2001). With respect to yield, bothdirect seeding (viz. wet, dry or water seeding) and transplantinghad similar results (Kukal and Aggarwal, 2002).
In Southeast Asia, DSR is more often adopted in the dry seasonthan in the wet season probably due to better water control; butdry-season rice accounts for less than one-quarter of riceproduction in this region. At present, 23% of rice is direct-seededglobally (Rao et al., 2007). In the United States, Australia andEurope, rice is planted into either a dry-seeded or water-seededsystem (Gianessi et al., 2002; Ntanos, 2001; Pratley et al., 2004). InAustralia, for instance, most rice is aerially sown in water (Pratleyet al., 2004), while in Africa, broadcasting and dibbling are commonseeding practices for rice sowing (Ampong-Nyarko, 1996). Direct-seeding in saturated soil has been widely adopted in southernBrazil, Chile, Venezuela, Cuba, some Caribbean countries, and incertain areas of Colombia (Fischer and Antigua, 1996). In Asia, dryseeding is extensively practiced in rainfed lowlands, uplands, andflood-prone areas, while wet seeding remains a common practicein irrigated areas (Azmi et al., 2005; de Dios et al., 2005; Kim et al.,2001; Luat, 2000).
bSeeding pattern bWhere practiced
Broadcasting; drilling
or sowing in rows
Mostly in rainfed areas and some in
irrigated areas with precise water control
Various Mostly in irrigated areas with good drainage
ic Broadcasting on standing
water
In irrigated areas with good land levelling
and in areas with red rice problem
M. Farooq et al. / Soil & Tillage Research 111 (2011) 87–98 89
DSR has been practiced for some time, but has not gainedpopularity; even though many research studies suggest its benefitsover TPR (Bhushan et al., 2007; Farooq et al., 2008; Jehangir et al.,2005; Singh et al., 2005a). This review sums up the most recentexperiences, potential advantages, associated problems and likelypatterns of changes in DSR.
2. Experiences
Direct seeding of rice was the major method of standestablishment for about six decades. It was replaced withtransplanting during the 1970s in most parts of the world (Pandeyand Velasco, 2005). As a result of water scarcity and labour issues,farmers are again considering direct-seeding systems in riceproduction. Yield benefits, resource conservation, varietal devel-opment and weed management of DSR are summarised below.
2.1. Role of direct seeding
Direct-seeding methods have several advantages over trans-planting (Singh et al., 2005a). In addition to higher economicreturns, DSR crops are faster and easier to plant, less labourintensive and consume less water (Bhushan et al., 2007; Jehangiret al., 2005), are conducive to mechanisation (Khade et al., 1993),generally flower earlier leading to shorter crop duration (Farooqet al., 2006a,b; Santhi et al., 1998) and mature 7–10 days earlierand have less methane emissions (Balasubramanian and Hill,2002; Pandey and Velasco, 1999) than TPR. Typically, DSR isestablished earlier than TPR without growth delays fromtransplant injury; which hastens physiological maturity andreduces vulnerability to late-season drought (Tuong et al., 2000).
Dry-seeding on flat land or raised beds with successivesaturated soil conditions reduces the amount of water neededfor land preparation and thus overall water demand (Bouman andTuong, 2001). Direct seeding also offers the option to resolveedaphic conflicts (between rice and the subsequent non-rice crop)and enhance sustainability of both the rice–wheat cropping systemand succeeding winter crops, particularly early sown wheat(Farooq et al., 2008; Ladha et al., 2003; Singh et al., 2005a,b).
Yield in DSR is often lower than TPR principally owing to poorcrop stand and high weed infestation (Singh et al., 2005a).Moreover, cost for weed control is usually higher than TPR. Highweed infestation is a major constraint for broader adoption of DSR(Rao et al., 2007). Likewise, micronutrient deficiencies such as Znand Fe, due to imbalanced N fertilisation and high infiltration ratesin DSR, are of major concern (Gao et al., 2006; Saleque and Kirk,1995). Nonetheless, owing to water and labour shortages (Pandeyand Velasco, 1999) and soil degradation under intensive TPR (Sinhaet al., 1998), farmers are inclining to adopt DSR. The area under DSRis increasing as it is more productive and profitable to compensatethe production costs.
2.2. Seed priming
After shifting from transplanting to direct seeding, crop standestablishment becomes a critical factor affecting subsequentgrowth, development and yield in rice. The rate, uniformity andpercentage of seedling emergence are critical determinants of cropestablishment and yield (Farooq et al., 2006a,b). Successful cropestablishment requires high quality seeds, which emerge promptlyunder a wide range of field conditions (McDonald, 1998; Phill,1995). Early emergence of a vigorous crop stand provides betterroot anchorage and improves nutrient absorptive capacity(Watanabe, 1997).
In seed priming, a pre-sowing hydration technique, seeds arepartially hydrated such that germination processes begin, but
radical emergence does not occur (Farooq et al., 2009b). Thistechnique allows some metabolic processes to occur withoutactual germination (Basra et al., 2005). Seed priming techniquesare a promising solution to poor stand establishment in DSR(Farooq et al., 2006a). Seed priming techniques, such as hydro-priming (Farooq et al., 2006c); on-farm priming (Harris et al.,2002); osmohardening (Farooq et al., 2006a,b,d); hardening(Farooq et al., 2004); and priming with growth promoters likegrowth regulators and vitamins have been successfully employedin rice to hasten and synchronise emergence, achieve uniformstands, and improve yield and quality (Basra et al., 2006; Farooqet al., 2006a,b).
On-farm priming in DSR has increased the rate of germinationand emergence (by 1–3 days), resulting in more uniform andvigorous seedling growth (Harris et al., 2002). Priming with 0.05%imidacloprid (N-[1-[(6-Chloro-3-pyridyl)methyl]-4,5-dihydroimi-dazol-2-yl] nitramide) resulted in the highest seedling densityduring the early vegetative phase of semi-dry rice, with comparableresults from NaCl 0.5%, KCl 2% and the bio-fertiliser Azospirillum 2%(Mohanasarida and Mathew, 2005a). However, priming withimidacloprid resulted in more plant height, root weight, dry matterproduction and root length; while the Azospirillum treatment hadthe highest shoot:root ratio during early vegetative growth and themaximum tillers (Mohanasarida and Mathew, 2005a). Furthermore,seed priming with imidacloprid increased yield by 2.1 t ha�1
compared with the control (non-primed), which was attributed tohigher panicle numbers and more filled grains per panicle(Mohanasarida and Mathew, 2005b). Seed priming also reducedthe need for high seeding rates, but was detrimental for seedlingestablishment when soil was at or near saturation (Du and Tuong,2002). Likewise, soaking seeds of three upland rice varieties in waterfor 24 h, followed by air drying, improved stand establishment by23–43%, compared with non-primed seeds, and grain yield by 11–24% over three consecutive years (Singh and Chatterjee, 1981).
Priming rice seeds for 12 and 24 h improved crop establishmentand subsequent growth (larger leaf area, taller plants, higher rootand shoot dry weights measured 4 weeks after sowing) in Ghana(WARDA, 2002). Primed plants also had significantly more tillers,panicles and grains per panicle than non-primed plants (WARDA,2002). Farmers’ opinions on seed priming tend to agree with thosereported by Harris et al. (1999) – that primed crops emerge fasterand more completely, produce more vigorous seedlings, flower andmature earlier, and yield better than non-primed crops.
Osmohardening with KCl or CaCl2 improved germination andemergence, allometry, kernel yield, and grain quality in direct-seeded medium grain and fine grain aromatic rice compared withtraditional farmer seed-soaking, which resulted in poor and erraticemergence of seedlings followed by poor crop performance(Farooq et al., 2006a,b). Faster and uniform seedling emergencefrom primed seeds was attributed to improved a-amylase activityand increased levels of soluble sugars in these seeds, suggestingthat physiological changes produced by osmohardening enhancestarch hydrolysis, making more sugars available for embryogrowth, vigorous seedling production and improved growth,kernel yield and quality attributes at maturity (Farooq et al.,2006a,b). In direct-seeded fine grain rice, however, osmohardeningwith CaCl2 had the best kernel yield (2.96 t ha�1 vs. 2.11 t ha�1,untreated control), straw yield (10.13 t ha�1 vs. 9.35 t ha�1) andharvest index (22.61% vs. 18.91%; Table 2). Higher yield wasattributed to more tillers, 1000 kernel weight and kernel yieldwhile quality was attributed to improved kernel protein and kernelwater absorption ratio (Farooq et al., 2006b). In direct-seededmedium grain rice, osmohardening with KCl had the best kernelyield (3.23 t ha�1 vs. 2.71 t ha�1, untreated control), straw yield(9.03 t ha�1 vs. 8.12 t ha�1) and harvest index (26.34% vs. 24.02%),followed by osmohardening with CaCl2, hardening and ascorbic
Table 2Effect of seed priming on kernel yield and yield traits of direct seeded (DSR) and transplanted (TPR) fine grain rice.
Treatments No. of tillers (m�2) 1000 kernel weight
(g)
Straw yield (t ha�1) Kernel yield
(t ha�1)
Harvest index (%)
aDSR bTPR DSR TPR DSR TPR DSR TPR DSR TPR
Control 517.3c 548.0e 14.67cd 15.33 09.35c 10.03e 2.11d 2.85e 18.91d 22.27d
Traditional soaking 526.3c 606.0d 14.33cd 15.00 09.23c 10.33de 2.01e 3.06e 17.88e 22.85c
Hydropriming 608.3b 666.0bc 15.33bcd 15.00 09.65b 10.84c 2.71b 3.17d 21.92a 22.62c
Osmohardening (KCl) 625.3b 669.3b 15.67abc 15.67 10.01a 10.84c 2.76b 3.57b 21.61a 24.77a
Osmohardening (CaCl2) 684.7a 707.0a 17.00a 17.00 10.13a 11.40a 2.96a 3.75a 22.61a 24.57a
Vitamin priming 608.3b 672.3b 14.00d 16.00 09.87ab 10.91bc 2.63c 3.41c 21.04c 23.81b
Hardening 640.3b 670.7b 16.33ab 16.00 10.00a 11.34ab 2.75b 3.58b 21.56b 22.48cd
Means not sharing the same letters in a column differ significantly at P<0.05.a Source: Farooq et al. (2006b).b Source: Farooq et al. (2007).
M. Farooq et al. / Soil & Tillage Research 111 (2011) 87–9890
acid priming (Table 3; Farooq et al., 2006a). Likewise, seed primingtreatments improved kernel quality in fine grain (Farooq et al.,2006b) and medium grain (Farooq et al., 2006a) DSR.
During on-farm evaluations of different seed priming techni-ques in DSR near saturated soil conditions, osmohardening withCaCl2 improved stand establishment, crop growth and kernelquality, and increased plant height, number of kernels per panicle,straw and kernel yield, and harvest index. Improved kernel qualitywas attributed to reduced numbers of sterile spikelets, abortiveand chalky kernels, and increased numbers of normal kernels andkernel length. Moreover, osmohardening with CaCl2 improved P,Ca and K uptake, closely followed by osmohardening with KCl.Thus, osmohardening with CaCl2 or KCl can improve crop stands,growth, yield and quality in DSR culture in farmers’ fields (Rehmanet al., 2010). Despite that, more results from research related toDSR are needed to disseminate to farmers.
2.3. Yield benefits
DSR is both cost- and labour-saving, although grain yield in DSR iscomparatively less than TPR (Tables 2 and 3; Farooq et al., 2006a,b,2007, 2009c; Naklang et al., 1996). Some reports claim similar oreven higher yields of DSR with good management practices (Table4). For instance, substantially higher grain yield was recorded in DSR(3 t ha�1) than TPR (2 t ha�1), which was attributed to increasedpanicle number, higher 1000 kernel weight and lower sterilitypercentage (Dingkuhn et al., 1991; Sarkar et al., 2003). Among semi-dwarf rice cultivars (IR-56, IR-58, IR-64 and IR-29732-143-3-2-1),IR-58 had superior yield when seeds were directly broadcastedrather than nursery transplanted (Dingkuhn et al., 1991). The DSR inmoistened soil produced taller plants, more dry matter, lowerchlorophyll contents and specific leaf weights, and more paniclesand sterile spikelets than transplanted rice (Sarkar et al., 2003).
Table 3Effect of seed priming on kernel yield and yield traits of direct seeded (DSR) and trans
Treatments No. of tillers (m�2) 1000 kernel weight
(g)
aDSR bTPR DSR TPR
Control 623.3b 664b 16.33b 17.33
Traditional soaking 675.7bcd 673b 15.33b 17.67
Hydropriming 737.7a 669b 16.67b 17.33
Osmohardening (KCl) 728.7ab 778a 19.00a 18.00
Osmohardening (CaCl2) 656.0cd 758a 16.33b 17.00
Vitamin priming 705.7abc 741a 16.67b 16.67
Hardening 713.3abc 761a 17.00ab 17.00
Means not sharing the same letters in a column differ significantly at P<0.05.a Source: Farooq et al. (2006a).b Source: Farooq et al. (2009c).
Farmer and researcher trials in the Indo-Gangetic Plain reportedirrigation water savings of 12–60% for DSR on beds, with similar orlower yields for transplanted compared with puddle-floodedtransplanted rice (Gupta et al., 2003), and usually slightly loweryields with DSR in flat fields (Balasubramanian et al., 2003; Guptaet al., 2003). A study evaluating the effect of different seedingtechniques, cultivars, seed rates and soil types on basmati ricefound 44% and 30% higher grain yield in direct-drilled compactedand puddled plots, respectively than un-compacted/un-puddledplots (Yadav et al., 2007). On-farm studies in India revealedcomparable rice yields in some DSR and TPR systems when weedcontrol was adequate. Nonetheless, a yield reduction by �20% wasobserved in DSR compared with TPR on 36 farms (Johnson et al.,2003). Likewise, when weeds were controlled, average yieldsunder DSR and TPR were similar in the Philippines (Tabbal et al.,2002). Similarly, Ali et al. (2006) observed that during both wet anddry seasons, DSR yielded the same as TPR, and dry seeding had ahigher benefit:cost ratio.
While comparing productivity and economics of variousplanting techniques in rice-based cropping systems in the Indo-Gangetic Plain, Gangwar et al. (2008) recorded higher yield, rootdry matter, net cost:benefit ratio and infiltration rate for a DSR-based cropping system using hybrid rice than TPR. Higher values ofbulk density, soil organic carbon, available P and K were recordedunder mechanical transplanting. Similarly, higher total nutrientuptake was recorded in a rice–wheat sequence under mechanicaltransplanting than manually transplanted rice under puddleconditions. Gupta et al. (2003) reported 10% higher yields inDSR than flooded TPR. In a two-year field experiment in the Indo-Gangetic Plain evaluating various establishment systems, riceyields under conventional puddled transplanting and direct-seeding on puddled or non-puddled (no-tillage) flat bed systemswere the same (Bhushan et al., 2007).
planted (TPR) medium grain rice.
Straw yield (t ha�1) Kernel yield (t ha�1) Harvest index (%)
DSR TPR DSR TPR DSR TPR
8.12c 9.34e 2.71d 3.51e 24.02f 27.31c
8.02c 9.38e 2.61de 3.87 d 24.55e 29.20a
8.40c 9.62cd 2.78d 3.94d 24.86d 29.05a
9.03a 10.27a 3.23a 4.28a 26.34a 29.41a
8.93b 9.95abc 3.11b 4.07bc 25.83b 29.02a
8.87b 9.84bc 3.01c 4.02cd 25.33c 29.00a
0.896b 9.99ab 3.03c 4.01ab 25.27c 28.64ab
Table 4Comparison of grain yield in direct seeded and transplanted rice (t ha�1).
Direct seeded rice Transplanted rice Reference
5.90 4.40 Ho and Romli (1998)
5.38 5.32 Ko and Kang (2000)
2.27 1.83 Sharma and Ghosh (2000)
4.63 3.88 Oyediran and Heinrichs (2001)
3.18 2.31 Hayashi et al. (2007)
4.14 4.79 Cabangon et al. (2002)
2.93 3.95 Farooq et al. (2006a, 2009c)
2.56 3.34 Farooq et al. (2006b, 2007)
4.60 4.14 Ali et al. (2006)
5.31 5.28 Ali et al. (2006)
7.30 7.30 Bhushan et al. (2007)
7.20 6.60 Bhushan et al. (2007)
3.70 3.69 Sarkar et al. (2003)
3.00 3.63 Sarkar et al. (2003)
3.15 2.99 Sarkar et al. (2003)
M. Farooq et al. / Soil & Tillage Research 111 (2011) 87–98 91
2.4. Resource conservation
Rice farming is ongoing but subject to rapid change. The DSR is aresource conservation technology as it uses less water with highefficiency, incurs low labour expenses and is conducive tomechanisation (Bhuiyan et al., 1995). DSR reduces the labourrequirement for establishment by transferring field activities toperiods when labour costs are comparatively lower (Pandey andVelasco, 1999). Substantial water savings are possible from DSR(Dawe, 2005). For example, experiments in Northwest India usingDSR into non-puddled soils found 35–57% water savings (Sharmaet al., 2002; Singh et al., 2002). In these trials, soils were kept nearsaturation or field capacity unlike the flooded conditions used inpuddle-transplanted systems. In small plot DSR trials, theirrigation requirement decreased by 20% (Gupta et al., 2003).DSR on raised beds decreased water use by 12–60%, and increasedyield by 10%, in trials at both experimental stations and on-farm,compared with TPR (Gupta et al., 2003).
Water productivity in DSR was 0.35 and 0.76, and TPR was 0.31and 0.57 during 2002 and 2003, respectively, indicating betterwater-use efficiency (Gill et al., 2006). Research into resourceconservation using non-puddled, zero-till and DSR showed thatTPR water requirements were 35–40% higher than no-till. DSR onraised beds had 13–30% water saving but was associated with a14–25% yield loss (Gupta et al., 2006). In this regard, Bhushan et al.(2007) opined that since economic profitability in terms of netreturns is high with no-tillage and DSR on raised beds, it can easilyreplace TPR.
Table 5Rice varieties and hybrids suitable for direct-seeded rice.
Regions Ge
Bangladesh BR
Bihar (India) RajaCambodia Kos
Eastern Uttar Pradesh (India) ND
Haryana, Indian Punjab, Western Pradesh (India) PusbJapan RS-
Nepal SondSouth Korea Jua
Pakistan KS–
Tarai of Uttaranchal (India) NideThailand IR5
Source: Rice–Wheat Consortium (2006).a Tong (2008).b Tanno et al. (2007).c Shah and Bhurer (2005).d Choi et al. (2007).e Naklang et al. (1996).
2.5. Varietal development
Cultivar selection based on an ideal type has contributedremarkably to increased rice yield. Rice cultivars with fewer tillers,lower panicle weights with thick roots and culms are suitable forDSR (Won et al., 1998). Early heading rice varieties with betterdrought tolerance are better suited for dry-seeded rice, such asIR36 with �105-day duration and good drought tolerance (Gineset al., 1978). Some varieties and hybrids suitable for DSR are listedin Table 5.
Increased plant density and avoiding transplanting shock byusing DSR (Dingkuhn et al., 1990) resulted in more biomass than inTPR, which confirmed the advantage of DSR under fully irrigatedlowland conditions (Dingkuhn et al., 1991). This suggests thatgenotypes with more panicles may have larger sinks (spikeletnumber per area) and higher grain yields. Photoperiod-insensitivecultivars for drought-prone areas may also perform well under theDSR system. Early planting of such cultivars would result in anearlier maturing crop, thus escaping late-season drought. Anopportunity exists to develop high-yielding cultivars fromphotoperiod-insensitive genotypes (Mackill et al., 1996).
2.6. Weed management
Weeds are the major constraint towards the success of DSR(Caton et al., 2003; Rao et al., 2007). Estimated losses from weeds inrice are around 10% of total production grain yield; however, suchlosses can be much higher (Rao et al., 2007). In wet-seeded anddry-seeded rice, weed growth reduced grain yield by up to 53 and74%, respectively (Ramzan, 2003), and up to 68–100% for direct-seeded Aus rice (cropping season in Bangladesh) (Mamun, 1990).More than 50 weed species cause yield losses in DSR (Gianessiet al., 2002). The DSR fields are more species-rich with greaterdiversity in weed flora than TPR (Tomita et al., 2003).
In large-scale farmer participatory trials in India, Singh et al.(2005c) had success with DSR by using the stale-seed bedtechnique combined with a pre-emergence herbicide, pendi-methalin, applied within 2 days after seeding (DAS). Several pre-emergence herbicides including butachlor, thiobencarb, pendi-methalin, oxadiazon, oxyfluorfen and nitrofen, alone or supple-mented with hand weeding, resulted in good weed control asexpressed by reduced weed density and improved yields (Moorthyand Manna, 1993; Pellerin and Webster, 2004). Oxadiazon appliedat pre-emergence controlled major weeds but one manualweeding session was needed to control joyweed (Alternanthera
sessilis L.), rice flat sedge (Cyperus iria L.) and knotgrass (Paspalum
notypes for DSR
RI Dhan–33, BRRI Dhan–39, BRRI Dhan–44, Zata
shree, MTU–7029, Satyam, Rajendra Mahsuri–I, NDR–359, Prabhat
hihikari and W42
R–359, Sarjoo–52, Muhsoori, Swarna, MTU–7029, Moti, Pusa–44, KRH-2
a–1121, Pusa–2511, PRH–10, Pusa Basmati–1, Pant Dhan–12, Sharbati, PHB–71
15, RS-20
a Masuli, Hardinath, cRadha–4, Radha–11, cChaite 2
n1
282, hybrid rice
hi, UPRI–92–79, Narendra–359, PD–4, Sarvati, PR–113, HKR–120, Sarjoo–52
7514-PMI-5-B-1-2, IR20
M. Farooq et al. / Soil & Tillage Research 111 (2011) 87–9892
distichum L.) weeds in DSR (Mazid et al., 2003). Barnyard grass(Echinochloa crus-galli L.) was controlled effectively (up to 97%) 27days after treatment with either pyrazosulfuron-ethyl + molinateor cyhalofop-butyl + azimsulfuron + molinate. However, thioben-carb applied five days before seeding followed by pyrazosulfuron-ethyl + molinate fully controlled barnyard grass (Kuk et al., 2002).Effective control of weeds in dry-seeded DSR varies. It depends ontiming and method of land preparation (Maity and Mukherjee,2008), effectiveness of herbicides (Pellerin et al., 2004; Sinha et al.,2005), relative to the dominant weed species and soil conditions atthe time of application (Street and Mueller, 1993), effect ofweather on weeds (Maity and Mukherjee, 2008), and effect ofcombining herbicides and manual weed control (Rao et al., 2007).
Sharma (1997) suggested that pre-emergence application ofthiobencarb at 2.0 kg ha�1, hand weeding 20 DAS, or post-establishment intercrop cultivation 37–42 DAS effectively con-trolled weeds and increased yield by 32.7–34.7%, 36.7% and 28.7–83.9%, respectively. Rao et al. (2007), while reviewing weedmanagement in DSR, described tillage practices for land levelling,choice of competitive rice cultivars, mechanical weeders, herbi-cides and associated water management as component technolo-gies essential to control weeds. Johnson et al. (2004) studied themanagement effects of 10 weeds in five farmers’ fields in DSR andreported average yield losses of 49% if weeds were not controlled.Critical periods for 95% weed control were estimated to be 29–32DAS in the wet season and 4–83 DAS in the dry season.
3. Challenges
Several challenges confront the wide-scale adoption of DSR byfarmers, such as weed infestation, stagnant yield, availability ofpurposely developed varieties, panicle sterility, nutrient availabil-ity, pests and diseases and water management (Nguyen andFerrero, 2006). An account of each is given below.
3.1. Weeds
High weed infestation is the major bottleneck in DSR especiallyin dry field conditions (Harada et al., 1996; Rao et al., 2007). TPRseedlings have a competitive advantage over newly emergedweeds compared with emerging DSR seedlings. In addition, earlyweeds in TPR are controlled by flooding, unlike in DSR (Rao et al.,2007).
More than 50 weed species infest direct-seeded rice, causingmajor losses to rice production worldwide (Rao et al., 2007; Tomitaet al., 2003). When farmers change from TPR to DSR the weed florachanges dramatically (Azmi and Mashhor, 1995; Rao et al., 2007).Weed species such as barnyard grass and Asian sprangletop(Leptochloa chinensis L.) become more prevalent within a few yearsof adopting direct-seeded, wet-sown rice (Allard et al., 2005). Inthe 1970s, when DSR was introduced into Malaysia and Vietnam,barnyard grass, Asian sprangletop and aromacca grass (Ischaemum
rugosum L.) were not common in rice fields (Azmi et al., 1993) butdominated rice fields by the 1990s (Azmi et al., 2005; Chin et al.,2000). DSR systems also favour variable flat sedge (Cyperus
difformis L.) and water plant (Sagittaria montevidensis L.) inAustralia and USA, and Lindernia spp. in Asia (Gressel, 2002). InIndia, densities of barnyard grass, climbing dayflower (Commelina
diffusa L.) and purple nut sedge (Cyperus rotundus L.) increased inDSR compared with TPR in field experiments from 2000 to 2004(Singh et al., 2005a). In Pakistan, the frequency of bermuda grass(Cynodon dactylon L.), climbing dayflower, purple nut sedge, riceflat sedge and most importantly horse-purslane (Trianthema
portulacastrum L.) increased substantially in DSR. In many cases,horse-purslane led to complete failure of DSR rice crop (Farooq’spersonal observation).
DSR is subjected to more severe weed infestations than TPRbecause, in dry-seeded rice, weeds germinate simultaneously withrice, and there is no water layer to suppress weed growth (Fukai,2002). In DSR, weedy rice (Oryza spp.) becomes another majorweed to control (Ferrero, 2003). Phenoxy and sulfonylureacompounds are widely used herbicides in Malaysia, Vietnamand Thailand to control broad-leaved weeds and sedges in DSR.Incidences of weeds becoming resistant to those herbicides are onthe rise (Watanabe et al., 1997); for example, there is evidence thatweed species such as Shorea zeylanica (Thwaites) P. Ashton, dwarfclover (Marsilea minuta L.), and globe fringerush (Fimbristylis
miliacea L.) have developed resistance to phenoxy herbicides(Watanabe et al., 1997). Further studies on genotypic variation inweed tolerance traits including early seedling vigour andallelopathy would be worthwhile (Fukai, 2002).
3.2. Diseases and insect pests
Rice is susceptible to various diseases, rice blast being one of themost devastating, in both aerobic and direct-seeded cultures(Bonman and Leung, 2004; Farooq’s personal observation).Nonetheless, the severity of rice blast increases under water-limited conditions (Bonman, 1992; Mackill and Bonman, 1992). InBrazil, blast resistance is the most important target trait forbreeding programs in aerobic rice (Breseghelo et al., 2006). Waterdeficit and shift from transplanting to direct seeding favours neckblast spread. Studies suggest that the level of water supplyinfluences several processes such as spore liberation, germinationand infection in rice blast epidemics (Kim, 1987). Watermanagement directly affects the crop microclimate particularlydew deposition, which affects the lifecycle of the pathogen (Sahand Bonman, 2008), and indirectly affects crop physiology, therebyinfluencing host susceptibility (Bonman, 1992). Poor watermanagement practices result in moist or dry soil instead offlooded or wet conditions favouring dew deposition and makingthe environment susceptible for host and blast development(Savary et al., 2005).
Sometimes the attack of arthropod insect pests is reduced in DSRcompared with TPR (Oyediran and Heinrichs, 2001), but a higherfrequency of ragged stunt virus, yellow orange leaf virus, sheathblight and dirty panicle have been observed in DSR (Pongprasert,1995). Savary et al. (2005) also reported increased attack of brownspot disease and plant hoppers in DSR compared with TPR. The soil-borne pathogenic fungus Gaeumannomyces graminis var. graminis
has been observed in dry-seeded rice without supplementalirrigation in Brazil (Prabhu et al., 2002). Root-knot nematodes havealso been observed when switching from flooded to waterconservation rice production systems (Prot et al., 1994).
3.3. Panicle sterility
Rice is more drought-sensitive than other cereal crops duringflowering (Liu and Bennett, 2010; Saini and Westgate, 1999). Riceplants grown in both DSR and TPR systems are sensitive to waterdeficit. Dry seeding is the most common method of cropestablishment in DSR; rice grows on marginal soil moisturecompared with TPR (which has continuous standing water in thefield most of the time). Thus any short episode of drought, inparticular during the reproductive phase, may be more devastatingfor plants raised in DSR compared with TPR. Time to anthesisreduces when panicle water potential decreases, resulting in large-scale panicle sterility. Panicle transpiration resistance increasedrice spikelet fertility during flowering when water stressed(Garrity et al., 1986). Reduced starch levels have been observedin anthers of plants exposed to water stress (Lalonde et al., 1997);which may reduce pollen viability (Garrity et al., 1986; Lalonde
M. Farooq et al. / Soil & Tillage Research 111 (2011) 87–98 93
et al., 1997); and hence panicle fertility. The number of sterilespikelets increased, as well as abortive, opaque and chalky kernelsin DSR compared with TPR (Farooq et al., 2006a,b, 2007, 2009c).
3.4. Stagnant yield
In DSR, occasional yield decline of aerobic rice has beenobserved (Kreye et al., 2009; Vermeulen, 2007), but is not fullyunderstood. It could be related to ‘soil sickness’, potentially thecombined effects of allelopathy, nutrient depletion, buildup of soil-borne pests and diseases, and soil structural degradation (Venturaand Watanabe, 1978). In Brazil, for example, plant autotoxicity isthought to be the cause of yield decline by continuously raising thecrop following dry seeding system (Fageria and Baligar, 2003); yetthe presence of G. graminis var. graminis was noted in dry-seededrice fields (Prabhu and Filippi, 2002).
Rice yields declined significantly when upland rice cultivarswere sown consecutively for more than two years on the Oxisols ofcentral Brazil (Fageria and Souza, 1995). Such yield reductions maybe related to allelopathic residual effects or autotoxicity (Jensenet al., 2001). Similarly, in the Philippines, residual effects ofallelochemicals on yield reductions of subsequent rice crops wereobserved (Olofsdotter, 2001). Chou (1980) reported a 25%reduction in rice yield of a second crop in Taiwan, which wasattributed primarily to phytotoxins released during decompositionof rice residues on the soil.
3.5. Varietal development
Varietal development for DSR has been neglected which mayexplain why it has not become popular. The quest for short-duration, short-statured, long-rooted, resistance-to-lodging andblast, and early vigour varieties remains (Ikeda et al., 2008).
Cultivars with improved resistance to adverse soil conditionssuch as mineral toxicity and deficiency, short mesocotyls forherbicide tolerance, and resistance to rice blast need to beidentified and developed. To reduce weed competition in DSRcrops, early vegetative vigour in combination with early maturityis also important, particularly for high grain yields in intensifieddouble cropping systems (Coffman and Nanda, 1982). Yun et al.(1997) studied growth and yield of cultivars of japonica–indicacrosses under DSR upland fields. They concluded that indicacultivars had higher yields than japonica cultivars and must beincluded in breeding programs for DSR.
3.6. Nutrient dynamics
Puddling in continuously flooded rice limits percolation lossesin the field and retains a saturated soil profile, which inhibitsestablishment and growth of many weeds (Sahid and Hossain,1995), and has positive consequences for nutrient availability(Wade et al., 1998). Reduced oxygen in the rhizosphere for longperiods prevents oxidation of NH4
+ and retains this form of Nagainst leaching (Kirk et al., 1994). High pH favours NH3
volatilisation, and increase stocks of plant available K, Ca, Si andFe in soil for rice growth (Surendra et al., 2001). Some evidencesuggests that chemical changes in flooded soils increase Pavailability (Neue and Bloom, 1987; Willet, 1991). Continuousremoval of nutrients by the crop results in decreased availability ofNH4, P, K, Ca, Mg, Mn, Zn and Cu. However, there is a small increasein Fe availability in soil with increased periods of submergence andcrop growth (Pandey et al., 1985).
Land preparation and water management are the principalfactors governing the nutrient dynamics in both DSR and TPRsystems. As most often in DSR, land is prepared in dry and soilremains aerobic throughout the season, nutrient dynamics are
altogether different than the TPR, where land is prepared instanding water and soil is kept flooded during most of the season.Micronutrient deficiencies are of concern in DSR – imbalances ofsuch nutrients (e.g. Zn, Fe, Mn, S and N) result from improper andimbalanced N fertiliser application (Gao et al., 2006; Saleque andKirk, 1995). Reasons for Zn deficiency in rice fields include lowredox potential, high carbonate content and high pH (Mandal et al.,2000). Zinc deficiency in rice grown on calcareous soil is primarilydue to bicarbonates (Forno et al., 1975); possibly due to inhibition(Dogar and Hai, 1980) and immobilisation in roots, which restrictsits translocation to shoots. In aerobic soils, Fe oxidation by root-released oxygen reduces rhizosphere soil pH and limits release ofZn from highly insoluble fractions for availability to the rice plant(Kirk and Bajita, 1995). Saleque and Kirk (1995) showed that a pHbelow neutral in the rhizosphere increases solubility of P and Znand hence their availability (Kirk and Bajita, 1995). The timing andsource of Zn application may influence Zn uptake in DSR (Giordanoand Mortvedt, 1972). Therefore, a shift from TPR to DSR may alsoaffect Zn bioavailability in rice (Gao et al., 2006).
It seems imperative to assess the dynamics of macro- andmicronutrients in DSR culture and develop appropriate manage-ment strategies in order to harvest maximum crop returns on asustainable basis.
3.7. Lodging
Lodging, the permanent vertical displacement of the stem of afree-standing crop plant (Berry et al., 2004), has been observed moreoften in DSR than TPR fields in recent years (Farooq’s personalobservation). Rice plants with a thicker band of sclerenchyma at theperiphery of the stem are more lodging-resistant (Ramaiah andMudaliar, 1934). Moreover, lodging-tolerant rice varieties havemore vascular bundles, both peripheral and in the inner section ofthe outer layers (Chaturvedi et al., 1995). DSR is more susceptible tolodging during ripening than TPR (Setter et al., 1997).
Lodging results in substantial yield reductions due to decreasedphotosynthesis by self-shading, and hampered grain quality due toincreased colouring and decreased taste (Matsue et al., 1991;Setter et al., 1997). In addition, mechanical harvesting of a lodgedcrop is extremely cumbersome. In this regard, intermediate plantheights, large stem diameters, thick stem walls and high lignincontents are lodging resistant characteristics (Mackill et al., 1996).
4. Opportunities
Despite several challenges confronting DSR, many opportu-nities exist to tackle these issues; some of which are discussedbelow.
4.1. Management options
Management options start from the selection of a goodgenotype, site selection, seedbed preparation, sowing time, plantprotection, nutrient management, through to crop harvesting. InDSR systems, soil type (Tripathi, 1996), weed management (Raoet al., 2007) and land levelling (Kahlown et al., 2002) are of primaryimportance. Early canopy closure helps to reduce evaporation aftercrop establishment (Tuong et al., 2000). This is achieved by properplant density and growing rice varieties with good seedling vigour(Tuong et al., 2000). These measures also help rice plants to bettercompete with weeds (Rao et al., 2007). Other managementstrategies are detailed below.
4.1.1. Integrated weed management
Weeds pose a serious threat to DSR by competing for nutrients,light, space and moisture throughout the growing season. Tillage
M. Farooq et al. / Soil & Tillage Research 111 (2011) 87–9894
may help to control weeds temporarily by burying ungerminatedweed seeds at a depth that stops germination; but it may allowother, once deeply buried, seeds to germinate (Stoskopt, 1985). Anintegrated approach involving cultural practices, crop rotation,stale seedbed practices, selection of suitable competitive varieties,and use of herbicide mixtures is essential in response to changes inweed community structure in DSR (Maity and Mukherjee, 2008;Sharma, 1997; Yaduraju and Mishra, 2004). However, culturalmethods of weed control are preventive, since they enhance cropgrowth by precision agronomy, and in doing so maximise cropcompetition against weeds (Radosevich et al., 1997; Zimdahl,1999). Crop residues such as mulch, which may selectivelysuppress weeds by covering the soil surface (Teosdale et al.,1991) should be part of an integrated weed management programin DSR. The development of new and improved herbicides for dry-seeded rice is also needed (Gupta et al., 2003).
Allelopathic plant extracts may also be beneficial in the weedmanagement program. Allelopathic crops suppress obnoxiousweeds when exploited in the field by using strategies such as croprotation (Wu et al., 1999), cover or smother crops, intercropping,crop residues, mulching (Khanh et al., 2005) and allelopathic cropwater extracts (Jabran et al., 2008).
4.1.2. Nutrient management
In shifting from TPR to dry-seeded aerobic rice, factors such asincreased redox potential (Gao et al., 2002), pH changes (Liu, 1996),precipitation of Fe as Fe(OH)3, oxidation of organic matter, andrestricted Zn movement towards plant roots due to reduced watercontents (Yoshida, 1981) occur.
Productivity in DSR systems approaches TPR systems when N-fertiliser is supplied at high rates (McDonald et al., 2006). Nutrientmanagement practices such as deep placement and use ofcontrolled-release fertilisers performed well under rainfed condi-tions (Zeigler and Puckridge, 1995). These approaches in DSR inrainfed lowlands depend upon rainfall patterns, methods of directseeding, soil type and soil/crop management. Rice soil in aerobicand reduced phases greatly differs in physical and chemicalcharacteristics.
For efficient use of N in flooded rice production, it is importantfor N to be quickly converted into NH4
+ which plants shouldassimilate as early as possible. With improved management,farmers should be able to double their present average recovery ofN fertiliser to 50% (Cassman et al., 1998). One method ofmaintaining soil N as NH4
+ is to add nitrification inhibitors alongwith the fertilisers, which also increase NUE and crop yield. Forexample, dicyandiamide is a commercially available nitrificationinhibitor, which is used with solid chemical fertiliser in rice fields.It is produced and marketed both in Japan and Germany.
Greater fertiliser N efficiency in rice can be achieved by using Nefficient varieties, improving timing and application methods andbetter incorporation of basal N fertiliser application withoutstanding water (Ali et al., 2007). Split application of N has beenreported as the best method to improve N fertiliser use efficiency(Ali et al., 2007; Bufogle et al., 1997), reduce denitrification losses,synchronise with plant demand (Bufogle et al., 1997), and improveN uptake, straw and grain yield, and harvest index in DSR (Bufogleet al., 1997; Wilson et al., 1989).
Silicon (Si) is the second most abundant element in soils and amineral substrate for most of the world’s plant life. Its role must beexplored in DSR culture. Rice–legume rotation should be tested atvarious locations. In addition, rhizobial strains may be used toimprove availability and efficiency of nutrients.
4.1.3. Water use and water use efficiency
New water cannot be created; thus, we have to conserve andmake judicious use of every drop. Two possible options are to
minimise water losses through better management thus ensuringmore water for crop production, and improve water use efficiency,i.e. increase in production per unit of water. Soil type influences theneed for irrigation water, e.g. coarse-textured soils have higherpercolation losses. Land levelling also facilitates uniform waterapplication in less time and helps in weed control.
There are few reports evaluating mulching for rice, apart fromthose from China, where 20–90% input water savings and weedsuppression occurred with plastic and straw mulches in combina-tion with DSR compared with continuously flooded TPR (Lin et al.,2003). Extensive research is needed to improve water productivityand WUE in DSR systems.
4.1.4. Lodging
There are several management options available to overcomethe problem of lodging in DSR, such as changes to nitrogenousfertiliser application, seeding rates, sowing times, hill seeding,seeding depth and seeding methods, and sowing lodgingresistant cultivars. Although incidence of lodging can be reducedby low dose N-fertilisation, which restricts culm elongation, yieldmay still be reduced in DSR (Ogata and Matsue, 1998). Breedingand biotechnological approaches may be effective to developlodging-resistant rice genotypes better adapted to direct-seedingcultures.
4.1.5. Physiological approaches
Seed priming tools have the potential to improve emergenceand stand establishment under a wide range of field conditions.These techniques can also enhance rice performance in DSRculture. Variation exists within Oryza species, varieties/genotypes/hybrids and rice types in response to various priming treatments.Further research into the following areas would be useful:
� u
sing salts, plant growth regulators, jasmonates, osmolytes atvarying concentrations and durations for seed priming � d etermining optimal water potential, temperature range andrequirement for oxygenation during seed priming
� u sing commercial fertiliser as priming and seed coating agents � te sting invigorated seeds under a range of field conditions � e valuating pre-sowing thermal seed treatments with alternatecycles of low and high temperature for better stand establish-ment
� u sing osmoprotectants and plant growth regulators to improvethe performance of direct-seeded culture.
Early vigour is important in DSR systems (Balasubramanian andHill, 2002; Cairns et al., 2009). Rapid early growth increases standestablishment and weed competitiveness, both of which areimportant components for high yields in DSR (Zhao et al., 2006).Developing rice cultivars with more seedling vigour may help toreduce crop losses due to competition from weeds and drought.
4.1.6. Greenhouse gas (GHG) emission
In wetland rice systems (both water seeding and transplantingin flooded soils), large quantities of CH4 are emitted, which accountfor 8.7–28% of total anthropogenic emissions (Mosier et al., 1998).Emission of GHG from rice fields is very sensitive to managementpractices, so rice is an important target in this regard (Wassmannet al., 2004). Direct seeding has the potential to decrease CH4
emissions (Ko and Kang, 2000). For example, in a field experimentin the Philippines, DSR reduced CH4 emissions by 18% comparedwith TPR (Corton et al., 2000). Wassmann et al. (2004) proposedthat CH4 emissions may be suppressed by up to 50% if DSR fieldsare drained mid-season. The net effect of direct seeding on GHGemissions also depends on N2O emissions, which increase underaerobic conditions.
M. Farooq et al. / Soil & Tillage Research 111 (2011) 87–98 95
Wassmann et al. (2004) opined that measures to reduce CH4
emissions often lead to increases in N2O emissions, and this trade-off between CH4 and N2O is a major hurdle in reducing globalwarming risks. Strategies must be devised to reduce emissions ofboth CH4 and N2O simultaneously. In a recent evaluation ofconventional and no-tillage systems in combination with fertiliserapplication for global warming potential in DSR, the no-tillagesystem was a better option to reduce GHG emissions from paddyfields (Ahmad et al., 2009).
4.2. Genetic and biotechnological approaches
The use of molecular markers and genomics platforms offerunique opportunities to develop early maturing and high-yieldingrice varieties with resistance to lodging. Dissecting quantitativetraits into single genetic components, so-called QTLs (quantitativetrait loci), is a more direct method for accessing valuable geneticdiversity of physiological processes that regulate a plant’s adaptiveresponse (Kirigwi et al., 2007). Genomics-assisted improvement ofrice genotypes to direct-seeding environments increasingly relieson the QTL approach.
Improvement of genetic resistance to biotic stress is anotherimportant and effective breeding approach to water-savingcultivation of rice. Rice blast disease, a destructive disease of riceunder water-limited conditions, is a major problem (Bonman,1992). Likewise, production of transgenic herbicide-resistant riceis a pragmatic approach to popularise DSR culture. Although thereare research efforts to develop herbicide-resistant rice transgens,so far there has been little success (Rao et al., 2007). Approaches toimprove NUE are also being investigated to incorporate thenitrogenase enzyme into the rice plant chloroplast and to engineerplants to nodulate with N-fixation bacteria (Ladha and Reddy,2000).
5. Conclusions
On the face of global water scarcity and escalating labour rates,when the future of rice production is under threat, direct seededrice (DSR) offers an attractive alternative. A successful transition ofrice cultivation from transplanting system (TPR) to DSR culturedemands breeding of special rice varieties and developingappropriate management strategies. Despite controversies, ifproperly managed, comparable yield may be obtained from DSRcompared with TPR. As the extent and nature of weed flora changesas a result of this transition, sustainable integrated weedmanagement tools must be identified. This shift also changesthe dynamics of mineral nutrients; the availability of mostmicroelements is reduced in DSR.
If not managed, weeds may cause partial to complete failure ofDSR crops. In DSR culture, WUE and productivity may increase ifappropriate soil types from levelled land are selected. Early cropvigour, short stature and short duration may also improve WUE.Poor stand establishment is another hindrance in the wide-scaleadoption of DSR. Development of effective seed priming techni-ques has helped to resolve this issue; but more practical seedpriming techniques are needed. Although methane emissions aresubstantially reduced in DSR, NO2 emissions increase; methods toreduce its emission for a safer environment are needed. Lodgingand blast attack are threats in DSR that need attention;biotechnological and genetic approaches may help resolve theseissues.
Acknowledgements
The authors acknowledge the financial support provided by TheUWA Institute of Agriculture, The University of Western Australia
and University of Agriculture, Faisalabad, Pakistan in completingthis review.
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