productivity and sustainability of the rice-wheat cropping

C H A P T E R S I X

* Intery Punjz Inter

AdvanceISSN 0DOI: h

Productivity and Sustainability of

the RiceeWheat Cropping System in

the Indo-Gangetic Plains of the Indian

subcontinent: Problems,

Opportunities, and Strategies

Bhagirath S. Chauhan*, Gulshan Mahajany, VirenderSardanay, Jagadish Timsina* and Mangi L. Jatz

Contents
1. Introduction 316 2. Problems 319
natab Anat

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2.1. Diverse Agronomy of Rice and Wheat
320 2.2. Declining Water Availability and Groundwater Pollution 321 2.3. Deteriorating Soil Health 322 2.4. Disposal of Crop Residues 324 2.5. Weed Flora Shifts and Herbicide Resistance 326 2.6. Climate Change 328
3. Opportunities and Strategies
330 3.1. Conservation Agriculture 330 3.2. Crop Residue Management 338 3.3. Precision Land Leveling 340 3.4. Crop Diversification 341 3.5. Soil Nutrient Management 343 3.6. Reducing Water Requirements 344 3.7. Weed Management 346 3.8. Climate Change 352
4. Conclusions
354 Acknowledgments 355 References 355
ional Rice Research Institute (IRRI), Los Baños, Philippinesgricultural University, Ludhiana, Punjab, India

ional Maize and Wheat Improvement Center (CIMMYT), New Delhi, India

Agronomy, Volume 117 � 2012 Elsevier Inc.-2113, All rights reserved.://dx.doi.org/10.1016/B978-0-12-394278-4.00007-6

315

http://dx.doi.org/10.1016/B978-0-12-394278-4.00007-6

316 Bhagirath S. Chauhan et al.

AbstractRice and wheat are the staple foods for almost the entire Asian population andtherefore they occupy a premium position among all food commodities. The eraof the Green Revolution started during the early 1970s with wheat and rice andsince then the riceewheat cropping system of the Indo-Gangetic Plains hasplayed a significant role in the food security of the region. However, recentyears have witnessed a significant slowdown in the yield growth rate of thissystem and the sustainability of this important cropping system is at risk dueto second-generation technology problems and mounting pressure on naturalresources. Traditional cultivars and conventional agronomic practices are nolonger able to even maintain the gains in productivity achieved during thepast few decades. Demand for food is increasing with the increasing populationand purchasing power of consumers. The riceewheat cropping system is labor-,water-, and energy-intensive and it becomes less profitable as these resourcesbecome increasingly scarce and the problem is aggravated with deterioration ofsoil health, the emergence of new weeds, and emerging challenges of climatechange. Therefore, a paradigm shift is required for enhancing the system'sproductivity and sustainability. Resource-conserving technologies involvingzero- or minimum-tillage in wheat, dry direct seeding in rice, improved water-and nutrient-use efficiency, innovations in residue management to avoid strawburning, and crop diversification should assist in achieving sustainable produc-tivity and allow farmers to reduce inputs, maximize yields, increase profitability,conserve the natural resource base, and reduce risk due to both environmentaland economic factors. A number of technological innovation and diversificationoptions have been suggested to overcome the system's sustainability problemsbut some of them have not been fully embraced by the farmers as these areexpensive, knowledge-intensive, or do not fit into the system and have resultedin some other unforeseen problems. Different concerns and possible strategiesneeded to sustain the riceewheat cropping system are discussed in this reviewon the basis of existing evidence and future challenges.

1. Introduction

Rice and wheat crops have been grown in South Asia (India,Pakistan, Nepal, Bangladesh, and Bhutan) and China for more than 1000years. The riceewheat (RW) cropping system, that is, growing these cropsin a sequence in an annual rotation, has been developed through the intro-duction of rice in the traditional wheat-growing areas and vice versa (Parodaet al., 1994; Tran and Marathee, 1994). This cropping system is one of theworld's largest agricultural production systems, covering an area of26 million hectares (Mha) spread over the Indo-Gangetic Plains (IGP) inSouth Asia and China. It accounts for about one-third of the area ofboth rice and wheat in South Asia and produces staple food for more

Rice-Wheat Sustainability in Indian Subcontinent 317

than 20% of the world population. The RW system now comprises about13 Mha in area in the IGP, of which the Indian part of the IGP comprisesabout 10 Mha (Table 1; Timsina and Connor, 2001). More than 85% of theRW system practiced in South Asia is located in the IGP. About one-thirdand one-half of the total cereals of India and Pakistan, respectively, areproduced in this region. In India, the IGP cover about 20% of the totalgeographical area (329 Mha) and about 27% of the net cultivated area,and produce about 50% of the total food consumed in the country(Dhillon et al., 2010).

The IGP, spread from the Indus basin in Pakistan Punjab in the west,passing through India and Nepal to the Brahmaputra floodplains inBangladesh in the east, are the most important agricultural regions of theIndian subcontinent. The IGP comprise (i) the Trans IGP (Punjab inPakistan, and Punjab and Haryana in India), (ii) Upper and Middle IGP(western, central, and eastern Uttar Pradesh and Bihar), (iii) Terai (anextension of the IGP) in Uttrakhand in India and parts of Nepal, and (iv)Lower IGP (West Bengal in India and parts of Bangladesh). In Pakistan,the RW cropping system occupies about 10% of the total cultivated area(2.1 Mha), mainly in two zones, the Punjab RW zone, comprising about1.2 Mha, and the Sindh RW zone, occupying the remaining area(Aslam, 1998a).

The Indian portion of the IGP comprises Punjab, Haryana, Uttrakhand,western and eastern Uttar Pradesh, Bihar, and West Bengal. These plains arebelieved to be formed by alluvium soil brought from the Himalayas by theIndus and Ganges river systems about 7000 years ago (Pal et al., 2009).Except for a strip of the Shivalik hills, alongside its eastern border, theentire area is flat alluvial plain at 180e290 m above mean sea level. Thesoils of the plains region are generally deep alluvium, sandy loam to loamin texture, having moderate water-holding capacity, alkaline in reaction,and poor in organic matter content. In the plains of the Indus and Gangesbetween west and east, there is a gradual transition in physiography,

Table 1 Area under the riceewheat cropping system in various Asian countries

Country Area (Mha)

Area (%)

Total cereal productionRice Wheat

China 13.0 31 35 72India 10.3 23 40 85Pakistan 2.3 72 19 92Bangladesh 0.5 5 85 100Nepal 0.6 35 84 71

(Timsina and Connor, 2001)


climate, natural vegetation, and cropping patterns. Alluvial soils deposited bythe river systems are highly fertile and underlain by extensive aquifersresulting in the development of an extensive groundwater network forirrigation. The rice and wheat crops have become so important for thisregion that rapid expansion of a tubewell network in the northwest (NW)part of the IGP has led to the exploitation of even poor-qualitygroundwater aquifers for irrigation.

Until the mid-1960s, rice cultivation in some pockets of the IGP wasrestricted to basmati rice (tall-statured aromatic rice with good cookingquality) having low productivity of about 1 t ha�1 (Kumar andNagarajan, 2004). The introduction of dwarf wheat cultivars (Sonora 64and Lerma Rojo) from Mexico, which required lower temperature forgermination and tillering, led to a shift in the sowing time of wheatfrom mid-October to late October/early November, allowing thecultivation of kharif (rainy season) crops of longer duration. Thisprovided an opportunity for the cultivation of input-responsive, high-yielding dwarf cultivars of rice in the non-traditional areas of the IGP.In the subsequent years, rice and wheat in these areas replaced the lessremunerative coarse cereals and more risk-prone oilseed and pulsecrops. The Green Revolution technologies led to the emergence ofRW as the major cropping system in the IGP, which now ranks firstamong the 30 major cropping systems identified in India (Das, 2006).In the Trans and Western parts of the Upper Gangetic Plains, rice(indica-type monsoon rice)-wheat (spring) is the main cropping system,whereas rice-wheat-mungbean/cowpea/jute in the eastern part of theUpper Gangetic Plains and in the Middle and Lower Gangetic Plainsand the riceericeewheat cropping system in the Nepal Terai region arepredominantly followed.

After the mid-1960s, a substantial increase in area under rice and wheattook place in NW India, particularly in Punjab and Haryana states, despitesemi-arid climatic conditions. These two states constitute a highly produc-tive RW zone in the IGP contributing about 69% of the total food outputin the country (about 84% wheat and 54% rice) and this region is called the“food bowl of India.” This region has played a vital role in sustaining the foodsecurity of India by contributing about 40% of wheat and 30% of rice to thecentral stock of India every year during the last four decades (Hira andKhera, 2000). Average combined yields of rice and wheat in the RWsystem in the IGP area are 5e7 t ha�1 while yields of 8e10 t ha�1 areattained with greater fertilizer application and the adoption of betteragronomic practices (Timsina and Connor, 2001).

The soils of the IGP region have developed from alluvium broughtfrom the Himalayas by the Indus River system. Soil moisture regimes areudic, ustic, and aridic, whereas the soil temperature regime is mainly hyper-thermic. Soils under the RW system in the IGP, especially in the NW,


are porous, coarse, and highly permeable and are being used for raisingupland crops as well as wetland rice (Aulakh and Bijay-Singh, 1997).

Groundwater and rainfall are the major water resources. Mean annualrainfall in Punjab, Haryana, and Uttar Pradesh plus Uttrakhand is 61, 56,and 84 cm, respectively, with a coefficient of variation often exceeding20% (Parthasarthy et al., 1987). NW India and Bangladesh are perhapsthe only regions in the world where groundwater is used for rice (mainlytransplanted) cultivation. Of the net cultivated area, 97% of the area inPunjab, 83% in Haryana, and 68% in Uttar Pradesh is irrigated.

2. Problems

The spread of the RW system has brought forth several edaphic, envi-ronmental, and social implications. A number of problems have cropped upin the region with the cultivation of rice and wheat in a system mode forthe last four decades, threatening the sustainability of the system. The NWIGP region has paid a heavy price to earn the status of food bowl. Evidenceis accumulating that the RW system is now showing signs of fatigue andyields of rice and wheat in this region have reached a plateau or aredeclining, the soils have deteriorated, the groundwater table is recedingat an alarming rate, total factor productivity or input-use efficiency isdecreasing, cultivation costs are increasing, profit margins are decreasing,and the simple agronomic practices that revolutionized RW cultivationin the IGP are fast losing relevance (Hobbs and Morris, 1996). In India,rice production and productivity that increased at an annual compoundgrowth rate of 3.62% and 3.19%, respectively, in the 1980s, declined to1.61% and 1.34%, respectively, in the 1990s. During the same period,wheat productivity declined from 3.10% to 2.32%. This declining rate ofproduction has posed serious challenges before the nation to produceenough food for the increasing population and has raised some doubtsabout sustaining the productivity gains of rice and wheat in the region(Paroda et al., 1994).

The major factors that have stalled the productivity of the RW system are(i) overexploitation of groundwater resources leading to a decline in thegroundwater table (Hira, 2009; Hira et al., 2004; Humphreys et al., 2010),increased energy cost of pumping water, and deterioration of groundwaterquality; (ii) declining soil organic matter and increasing multiple deficienciesof major nutrients (N, P, K, and S) and micronutrients (Zn, Fe, and Mn) dueto their overmining from soils (Ladha et al., 2000; Tiwari, 2002); (iii)increasing salinity (Tiwari et al., 2009); (iv) the development of herbicideresistance and a shift in weed flora and pest populations (Hobbs et al.,1997); and (v) poor management of crop residues, leading to their burning.


The situation in the Pakistan Punjab is similar to that of the Indian Punjab,where waterlogging, soil salinity and sodicity, inadequate and unreliablewater supplies, poor-quality underground water for irrigation, andinadequate drainage are the major constraints (Aslam, 1998b). Theseproblems are expected to be further aggravated unless interventions aremade soon. Judicious use of fertilizers and water will determine thesustainability of RW and future food security of the IGP. About a decadeago, some challenges confronting the productivity and management of theRW system were described (Timsina and Connor, 2001). Since then,several new developments have occurred. In this article, the problemsconfronting the productivity of rice and wheat in the RW system areupdated and discussed and efforts are made to suggest technologicalinnovations to overcome these problems to sustain the productivity ofthese crops in the region.

2.1. Diverse Agronomy of Rice and Wheat

Rice and wheat crops have diverse edaphic requirements. Rice is generallytransplanted in puddled soils and prefers continued submergence. Puddlingis achieved by repeated intensive tillage under ponded-water conditions,which serves to break down soil aggregates, reduce macro-porosity, reducesoil strength in the puddled layer, disperse fine clay particles, and form a densezone of compaction (i.e., plow pan) in subsoil. In contrast, wheat is grown inwell-drained soils having good tilth, obtainedwith repeated (5e6 operations)tillage in dry soil. Therefore, the dominating feature of the RW croppingsystem is the annual conversion of soil from aerobic to anaerobic conditionsfor rice and then back to aerobic conditions for wheat. This causes severalphysical, chemical, and biochemical changes in the soil, which regulate trans-formation and availability of nutrients, root penetration,moisture availability,and croperoot interactions (Ponnamperuma, 1985), and this may haveimportant implications for management practices, including cropestablishment and nutrient, water, and weed management in the RWsystem. Long-term cultivation of puddled rice results in the formation ofa hard pan with a consequent increase in bulk density and lowering ofhydraulic conductivity below the plow layer (Singh et al., 2009). The hardpan impedes root growth of subsequent upland crops, including wheat.The nutrient transformations due to flooding of rice fields increase theavailability of P, K, Si, Mo, Cu, and Co, and decrease those of N, S, andZn (Timsina and Connor, 2001).

The conventional method of land preparation for transplanting orsowing in the RW system not only disturbs the soil environment butalso leads to atmospheric pollution. It is estimated that 2.6 kg of carbondioxide (CO2) is released to the atmosphere for each liter of diesel fuelconsumed (Erenstein and Laxmi-Vijay, 2008). About 150 l of diesel are


consumed per hectare per annum to run a tractor for field preparation andto pump water for irrigation in a conventional system, which amounts toemissions of about 400 kg CO2 per hectare per annum. Thus, thefarming practices used for RW cultivation over the years have changedthe soil structure and nutrient availability, and increased dependence onsynthetic fertilizers and machinery. This has increased the cost ofcultivation and affected the immediate environment.

2.2. Declining Water Availability and Groundwater Pollution

A large amount of water is used to maintain flooding in rice fields. Ricegrown by traditional practices requires approximately 1500 mm of waterduring a season. In addition, around 50 mm of water is required to growseedlings to the transplanting stage. The actual amount of water appliedby farmers, however, is much higher than the requirement, especiallywhere rice is grown on light-textured soils in India and Pakistan(Timsina and Connor, 2001).

Availability of sufficient irrigation water has made the RW system in theNW IGP a typical example of a highly productive system in non-ideal ricesoils (porous, coarse, and highly permeable). Four decades of RW croppinghave depleted the water resources in this region to a great extent. In theIndian Punjab alone, there is an annual shortage of about 1.2 M ha metersof water (Hira, 2009). The excess demand for water is being met throughoverexploitation of groundwater, leading to a decline in the water table.Though the name Punjab signifies land of five rivers (Panj means fiveand Aab means water), only 23% of the area in the Indian Punjab isirrigated by canals. Tubewells have become a more reliable source ofirrigation for about 77% of the area in the state as their control is withthe farmers and water supplies from these tubewells can be regulated.

Rice and wheat crops have become so important for this region thatrapid expansion of the tubewell network in the NW IGP has led to theexploitation of even low-quality groundwater aquifers for irrigation ofthese crops. The problem in the central districts of the Indian Punjab,where about two-thirds of the total number of tubewells in the state (about1.28 million) have been installed, is alarming. The groundwater table in thisregion during 1993e2003 went down at about 0.55 m yr�1. In some areasof NW IGP, the water table is now being depleted at nearly 1 m yr�1.

The areas having a water table below 9 m increased from 3% in 1973 toabout 90% in 2004 and almost 100% in 2010 (Kumar et al., 2010). Thewater table in more than 70% of the area has now gone down to 21 mor more. In other words, 101 out of 143 blocks in the Indian Punjabhave been declared dark zones. This has resulted in an increasing numberof submersibles as the centrifuge pumps are no longer effective inpumping water. The cost of installing tubewells as well as the electricity


consumption to pump water have increased several-fold. About 30% of thetotal electricity in the state is being used for pumping water for irrigation. Arecent study by NASA (National Aeronautics and Space Administration,Washington, D.C., USA) suggests that 13e17 km3 of groundwater is lostpermanently every year from the aquifers in the northwestern plains ofPunjab, Haryana, and western Uttar Pradesh (Rodell et al., 2009).

The problem of groundwater pollution by nitrates does not normallyarise in areas where wetland rice is grown on fine-textured ideal rice soilsbecause nitrate is not normally formed under flooded conditions. Nitrate isformed when soil is allowed to dry, particularly in typical or ideal rice fieldsthat possess very low percolation rates; once these soils are puddled, nitratemay be promptly converted to nitrous oxide (N2O) via denitrification(Aulakh et al., 2009). Increasing irrigation rate (amount or depth) anddecreasing irrigation frequency cause more leaching of nitrate to thedeeper soil layer, whereas light and frequent irrigations cause much lessleaching of nitrate (Singh and Sekhon, 1976). Depending upon therooting habit of the plants, vegetation retards nitrate leaching from theroot zone by absorbing nitrate and water. The study of a 600-ha farm atthe Punjab Agricultural University, Ludhiana, revealed that NO3eNcontent varied from 1.3 to 11 mg l�1 in deep irrigation tubewells, from0.4 to 11 mg l�1 in shallow irrigation tubewells, and from 0.6 to 28 mgl�1 in hand pumps (Thind and Kansal, 2002).

To feed India's projected population of 1.35 billion in 2025, agriculturalproduction, especially that of rice and wheat (staple foods of India), wouldhave to increase by approximately 25%. Agricultural production in Punjab,Haryana, and western Uttar Pradesh might not be sustainable unless majorsteps are taken to improve management of groundwater (Hira, 2009). Thus,groundwater management holds the key to the future sustainability of RWin the region. The development of water-efficient cultivars and alternativemethods of irrigation as well as crop establishment methods that require lesswater will be the deciding factors for future rice cultivation. The time hascome to seriously think about rainwater harvesting.

2.3. Deteriorating Soil Health

Crop production removes varying amounts of mineral nutrients from thesoil, depending on production and nutrient-supplying capacity of thesoil, which in turn is influenced by soil type, soil organic matter content,amount of nutrients applied, and removal or recycling of crop residues inthe soil. Rice and wheat are heavy feeders of nutrients. The RW systemhas not only resulted in mining of major nutrients (N, P, K, and S) fromthe soil but also has created a nutrient imbalance, leading to deteriorationin soil quality. Deficiencies of N, P, and K are most extensive. One tonof wheat grains is estimated to remove 24.5, 3.8, and 27.3 kg N, P, and


K, respectively, whereas similar production of rice grains removes 20.1 kgN, 4.9 kg P, and 25.0 kg K (Tandon and Sekhon, 1988). Soils in the IGPcontain low organic matter content and are being consistently depleted oftheir finite reserve of nutrients by crops (Bijay-Singh and Yadvinder-Singh,2004). Excessive nutrient mining of soils is one of the major causes offatigue experienced by soils under the RW system. The quantities ofnutrients removed by rice and wheat are greater than the amount addedthrough fertilizers and recycled. Sulfur deficiency has been observedin about one-quarter of the samples tested in the NW region of India,particularly in soils that re coarse-textured, low in pH, and poorin organic matter (Sharma and Nayyar, 2004). The micronutrientrequirement of rice is more than that of wheat. Zn deficiency hasbecome widespread in the IGP (Nayyar, 2003) and it has become thethird most limiting nutrient after N and P, particularly in soils with highpH and those irrigated with poor-quality water. Forty-nine percent ofmore than 90,000 soil samples tested in India and 55%, 47%, and 36% ofthe soil samples in the trans-northern, central, and eastern parts of thecountry, respectively, have been found deficient in Zn (Shukla andBehera, 2011). Deficiency of Zn is more in rice and that of Mn is morein wheat. Deficiencies of other micronutrients such as Fe, Cu, and B arealso appearing.

Removal of all the straw from crop fields leads to K mining at alarmingrates because 80% to 85% of the K absorbed by the rice and wheat cropsremains in the straw. Removal of K by crops far exceeds that of N andP, resulting in a negative K balance in the soil (Tandon and Sekhon,1988). K removal by crop residues represents approximately five times asmuch as is supplied by fertilizers (Chander, 2011). Soils in the IGPgenerally contain adequate exchangeable K and K-bearing minerals(illite), which release exchangeable K for the crop. The absence ofa response of crops to the application of K at present is mainly due tosufficient release of K from K-rich illitic and biotic minerals, and burningof rice straw, which is rich in K. The continuous RW cropping system isexpected to deplete soil of K when residues are not returned to the soileven when optimum doses of fertilizer K have been applied (Meeluet al., 1995; Tandon and Sekhon, 1988). A negative K balance can besubstantially improved by returning wheat residues to the field (Bijay-Singh and Yadvinder-Singh, 2004), which are now being removedmainly for dry fodder.

The fertilizer use pattern in the RW system in the IGP varies greatly,with the highest doses applied in the Trans-IGP, particularly in the IndianPunjab, and lower doses in the Central and Eastern IGP (Timsina andConnor, 2001). Fertilizer use is consistently increasing and so is theN:P2O5:K2O ratio due to the imbalanced use of these nutrients.Application of nutrients through fertilizers is highly skewed toward N


with a very low rate of K application. The partial factor productivity of N,P, and K for food grain production has dropped from about 81 kg grain perkg of N, P, and K in 1966e67 to 15 kg grain per kg N, P, and K in2006e07 (Benbi and Brar, 2009). This decline is attributed to changesin physical and biochemical composition of soil organic matter anda decline in nutrient-supplying capacity of soil. Long-term application ofinorganic fertilizers at optimum rates has either maintained or increasedsoil organic carbon (Biswas and Benbi, 1997). In the RW system also,an increase in soil organic carbon content with the application ofrecommended amounts of fertilizers was reported in long-termexperiments after 13 years (Rekhi et al., 2000) and 25 years (Benbi andBrar, 2009). The response was due to a progressive increase inproductivity (making available more biomass) and submergence of soilsfor 3e4 months during rice cultivation, which reduced the oxidation ofsoil C.

The efficiency of applied nutrients has been about 50% for N, <25% forP, and 40% for K (Witt et al., 1999). Lower efficiencies are due tosignificant losses of nutrients by leaching, runoff, gaseous emission, andfixation by soil. These losses can potentially contribute to degradation ofsoil and water quality and eventually lead to overall environmentaldegradation (Prasad, 2005). There is a need to tackle the emergingconcerns of soil organic matter degradation and reduced nutrient-supplying capacity of the soils in the RW system. Inclusion of short-duration legumes between wheat and rice, balanced application ofnutrients, returning rice crop residues to the soil after harvest, and thepossibility of introducing some microbes for fast decomposition ofresidues to facilitate wheat sowing may help to restore soil fertility.

2.4. Disposal of Crop Residues

The RW system accounts for nearly one-fourth of the total crop residuesproduced in India (Sarkar et al., 1999). Wheat straw is consideredimportant as dry fodder for dairy animals and farmers do not mind itsharvesting and collection with the help of special cutting machinesthough this requires additional operation and investment. Traditionally,straw of rice and wheat (other than that used as dry fodder) and residuesof other crops are removed from fields for use as livestock bedding,thatching material for housing, and fuel but these form only a smallportion of the total quantity of crop residues produced by the system.The remaining rice and wheat stubbles are burned or incorporated aftercrop harvest (Bijay-Singh et al., 2008). There is an increasing trend ofharvesting of rice and wheat through combines, leading to theproduction of an enormous quantity of crop residues. According toa survey, 91% of rice area and 82% of wheat area in the Indian Punjab is


harvested by combines (ICAR, 1999), annually producing about 37 M tonsof crop residues and this practice is increasing in other regions of thecountry where the RW system is practiced. A combine harvester leavesa large quantity of straw residue on the field surface and farmers are notequipped to handle such a large mass of residues left in the field. Withthe increasing trend of combine harvesting, disposal of crop residues,especially of rice, has become a problem.

Crop residues, particularly from wheat and rice crops, have a wide C:Nratio of 70:1e100:1. About 30% to 40% of C added through crop residuesbecomes decomposed in about 2 months (Beri et al., 1992). As long asadded C remains in the soil, it causes immobilization of applied N (Toorand Beri, 1991). Thus, the effect of crop residues on grain yield dependsupon the amount, mode, type, and state of the applied residues(shredded or non-shredded), and the crop itself. A crop grownimmediately after the incorporation of residues suffers from N deficiencycaused by microbial immobilization of soil and fertilizer N in the shortterm (Mary et al., 1996). The magnitude of immobilized N is influencedby the decomposition period of rice straw prior to fertilizer application(Yadvinder-Singh et al., 2005).

The addition of wheat straw at 4 t ha�1 before sowing wheat depressedwheat yield (Sidhu and Beri, 1989). An addition of N fertilizer along withresidue could only partly offset the immobilization process, whereasallowing adequate time for the decomposition of residues before plantingthe next crop can be more beneficial. Recycling of rice residues posesmore problems to succeeding wheat than wheat straw to the followingrice crop because of the shorter window between rice residueincorporation and wheat sowing, low temperature, and the slow rate ofdecomposition of rice straw due to high silica content. Sidhu and Beri(2005) observed that incorporation of rice residue resulted in lower Ncontent in wheat grain compared to a control as it increased organic C,labile C, water-soluble C, potentially mineralizable C, humic acid, andfulvic acid. Only a small amount, 6 to 9 kg ha�1, of N is released duringthe growing period of the wheat crop from incorporated residue, hardlyoffering any significant effect on the yield of wheat. Application of riceresidue to wheat generally has little effect on wheat yields during theshort term of 1e3 years (Bijay-Singh et al., 2008; Gupta et al., 2007;Yadvinder-Singh et al., 2005).

Rice straw takes a longer time for decomposition due to high silicacontent. Since rice straw has no economic value, farmers hesitate to investin cleaning the field by using a chopper. This practice also requires anotheroperation and increases cost. Farmers in NW India have discovered burningas the cheapest and easiest way of removing large loads of crop residuesproduced by rice and wheat grown in the cropping system mode. In a study,the yields of rice and wheat crops decreased with the incorporation of


crop residues, whereas burning increased yields significantly overthe residue removal treatment (Beri et al., 1995). Residue burning andresidue removal resulted in greater grain yields of rice and wheat thanresidue incorporation when residues of both crops were managedimmediately (Beri et al., 1995). In another study, the encouraging effectsof residue incorporation and burning on yield became apparent after 5years (Sidhu and Beri, 1989).

In the Indian Punjab, about 15 M tons of rice and wheat residues out ofa total of 37 M tons of residues are being burned in situ annually, leading toa loss of about 5 M tons of C equivalent to a CO2 load of about 18.3 Mtons per year and a loss of 69,000 tons of N (Benbi et al., 2011). Oneton of wheat residue contains 4.8 kg N, 0.7 kg P, and 9.8 kg K, whereas1 ton of rice residues contains 6.1 kg N, 0.8 kg P, and 11.4 kg K (Singhand Singh, 2003). Burning of rice straw causes gaseous emission of 70%CO2, 7% CO, 0.66% CH4, and 2.09% N2O (Sharma, 1998). Smokefrom the burning of crop residues pollutes the air with a mixture of gasesand fine particles, which can clog the lungs and cause breathingproblems. Asthmatic people have great difficulty in breathing under theseconditions. The peak in asthmatic patients in hospitals in India coincideswith the annual burning of rice residue in surrounding fields (Bijay-Singh and Yadvinder-Singh, 2003). Smoke particles can remain in theatmosphere up to several weeks. Smoke can also create haze that impairsvisibility. Thus, burning of crop residues not only results in a loss oforganic matter and nutrients but also causes atmospheric pollution due tothe emission of toxic and greenhouse gases such as CO, CO2, and CH4(methane) that pose a threat to human and ecosystem health.

2.5. Weed Flora Shifts and Herbicide Resistance

The main reason for the adoption of transplanted rice is the desirablecontrol of weeds due to continuous submergence of rice fields and efficacyas well as ease of application of herbicides such as butachlor. Echinochloacolona and E. crus-galli are the dominant weeds of rice in the RW system.Use of a single herbicide or the same group of herbicides for the controlof Echinochloa species in rice has led to the occurrence of a number ofweed species belonging to the grass, broadleaf, and sedge family in differentecological regions throughout the NW IGP. Cynodon dactylon, Ischaemumrugosum, Leptochloa chinensis, and Paspalum distichum among grass weeds,Ammannia baccifera, Ludwigia hyssopifolia, Ludwigia octovalvis, Caesulia axillaris,Commelina benghalensis, and Eclipta prostrata among broadleaf weeds, andCyperus difformis, Cyperus iria, Cyperus rotundus, and Fimbristylis milaceaamong sedges have become important and dominant weeds in rice. Themajor problem preventing the success of direct-seeded rice (DSR), anemerging crop establishment method in Asia, is competition from weeds


such as L. chinensis, Digitaria sanguinalis, Dactyloctenium aegyptium, E. colona,and Cyperus spp. (Chauhan, 2012; Chauhan and Johnson, 2009b, 2010a,c, 2011b; Chauhan et al., 2011; Mahajan et al., 2009). In general, theweed flora of direct-seeded upland rice is quite different from that of trans-planted lowland rice and therefore requires different approaches, includingchoice of herbicides for weed management. Successful cultivation of DSRrequires an intensive use of herbicides for weed control. Currently, aceto-lactase synthase (ALS) inhibitor herbicides that may have more selectionpressure are being widely advocated for weed management in DSR. Evolu-tion of resistance in weeds to ALS inhibitor herbicides has been reportedmore frequently than for any other herbicide (Kumar et al., 2008). Thisis because these herbicides exert strong selection pressure as a result ofhigh activity on susceptible biotypes as well as persistent soil residualactivity (Tranel and Wright, 2002).

In wheat, Phalaris minor, Coronopus didymus, Melilotus spp., Vicia sativa,Lathyrus aphaca, Chenopodium album, Anagallis arvensis, and Polygonum spp.are the dominant weeds. The most dominant and problematic weed is P.minor, which was introduced in India with wheat seed imported fromMexico in the late 1960s and soon became a major weed of wheat inthe RW cropping system in the fertile and irrigated conditions of theNW IGP. The morphological similarity of P. minor with the wheat plantand narrow row spacing inhibited its mechanical control. Its prolific seedproduction and earlier maturity than wheat resulted in its dominance inwheat fields over other weeds. The herbicide isoproturon (substituted-urea group) introduced for its control in the 1970s proved to be a “magicherbicide” that ruled the wheat fields for more than two decades due to itsbroad spectrum of weed control (almost complete control of P. minor andsome other prominent grass and broadleaf weeds), low cost, and postemer-gence application. However, faulty spraying techniques, the use of lowerthan recommended rates, the use of less quantity of spray solution, andrepeated use of the herbicide year after year eventually resulted in theevolution of biotypes of P. minor resistant to isoproturon.

The first sign of P. minor developing resistance to isoproturon in theNW IGP occurred during the mid-1990s. This seriously affected theproductivity of wheat on many farms; severe cases compelled growers toharvest the wheat crop at anthesis along with weeds for stock feed, some-times resulting in complete crop failure with a potential to make the RWcropping system unsustainable in the region. The causes and the extent ofdominance of P. minor in this cropping system in the IGP have been high-lighted in previous reports (Harrington et al., 1992; Malik et al., 1998;Malik and Singh, 1995). The isoproturon resistance mechanism is metabolicdegradation, mediated by P-450 mono-oxygenase enzymes (Singh, 2007).This type of resistance could become serious and lead to the evolution ofmultiple resistances to herbicides of different modes of action. There are


now confirmed reports of cross-resistance of P. minor to other substitutedherbicides such as clodinafop, fenoxaprop-p-ethyl, methabenzthiazuron,and metoxuron (Mahajan and Brar, 2001).

Chhokar and Sharma (2008) reported that the multiple herbicide-resistant populations of P. minor had low sulfosulfuron resistance but highresistance to clodinafop and fenoxaprop (ACCase inhibitors). Theclodinafop-resistant populations also showed higher cross-resistance tofenoxaprop (fop group) but low cross-resistance to pinoxaden (dengroup). Some populations of P. minor resistant to four groups ofherbicides (phenylureas, sulfonylurea, aryloxyphenoxypropionate, andphenylpyrazolin), however, were susceptible to the triazine (metribuzinand terbutryn) and dinitroaniline (pendimethalin) herbicides. The studiessuggest that RW monoculture has resulted in the dominance of someweeds in rice and others in wheat.

2.6. Climate Change

The global climate witnessed warming of 0.74�C between 1906 and 2005due to an increase in concentration of greenhouse gases CO2, CH4, andN2O. CO2 concentration increased from 280 ppm in the pre-industrialera to 379 ppm in 2005 (IPCC, 2007a). The IPCC (2007b) has furtherprojected a rise in temperature from 0.5 to 1.2 �C by 2020 and from0.88 to 3.16 �C by 2050 in the South Asia region depending uponfuture developments. Timsina and Humphreys (2006a, b) extensivelyreviewed the evaluation and application of the CERES-Rice andCERES-Wheat models for estimating yields of these crops under currentand future change scenarios. They concluded that the projected increasein temperatures due to global warming would decrease the growthduration and yield of both crops. Other crop models (WTGROWS andINFOCROP) have also predicted a decline in rice productivity by 0.75 tha�1 with a 2 �C increase in mean air temperature (Wassmann et al.,2009). A group of scientists from the Climate Research Unit, Universityof East Anglia, UK, have predicted a similar reduction in wheat yields inNW India (Ortiz et al., 2008).

The frequency of extreme events such as high temperature is predictedto increase in the future (IPCC, 2001). Heat stress severely restricts plantgrowth and productivity and is regarded as one of the major abioticadversities for many crops (Georgieva, 1999; Hassan, 2006), particularlywhen it occurs during the reproductive stage, which may lead toa substantial yield loss in wheat (Hays et al., 2007). In the RW croppingsystem, crop damage due to heat stress under late-planting conditions hasbecome an important factor limiting wheat yields (Aslam et al., 1989).High temperatures during early crop development and particularly afteranthesis may limit yield (Hunt et al., 1991). Temperature fluctuations


during grain filling in wheat were found to cause deviations from expecteddough properties (Blumenthal et al., 1991). The rise in daily averagetemperature, up to about 30�C, increased dough strength, whiletemperatures above this threshold value (35e40 �C), even only for fewdays, tended to decrease dough strength (Borghi et al., 1995; Corbelliniet al., 1997).

Increases in temperature reduce crop duration, increase crop respirationrates, reduce crop yield, reduce the number of grains formed, inhibitsucrose assimilation in grains, affect survival and distribution of pest popu-lations, hasten nutrient mineralization in soils, decrease fertilizer-use effi-ciency, and increase evaporation. In a simulation study, an increase intemperature by 2�C led to a 10e20% decrease in grain yield of both riceand wheat but, beyond that, the yield reduction was very high in wheat(Hundal and Kaur, 2007). Pandey et al. (2007), using the CERES-Wheatmodel, reported that the simulated grain yield of wheat underincremental units of maximum temperature (1 to 3 �C) graduallydecreased from 3546 to 2646 kg ha�1 (8% to 31% less than the baseyield) under optimal moisture conditions. Similarly, under suboptimalconditions, yield declined from 2841 to 2398 kg ha�1 (9% to 23% lessthan the base yield). The reduction in wheat yield with an increase inmaximum temperature was mainly because of a reduction in duration ofanthesis and grain filling with a rise in ambient temperature and viceversa (Aggarwal and Kalra, 1994).

An increase in atmospheric CO2 concentration, though, has a favorableeffect on the productivity of C3 plants (rice and wheat are C3 species) in thearid region, but any increase in temperature might offset such benefits ofenhanced CO2 supply and might even increase the evapotranspirationaldemand of water (Timsina and Humphreys, 2006a). Rising temperaturescaused heat-induced spikelet sterility or increased crop respiration lossesduring grain filling. Such reductions in the grain-filling capability (termedcrop “sinks”) of rice also have implications for greenhouse gas emissionsfrom rice paddies and the use efficiency of fossil fuel-based N fertilizer.Thus, any anticipated rise in temperature may jeopardize foodproduction in the region.

Next to CO2, CH4, and N2O are two very active trace gases in theatmosphere. In addition to other sources (coal mining, oil and natural gasflarings, domestic ruminants, sewage, etc.), rice fields and residue burningare the important anthropogenic sources of CH4. Methane is producedby methanogenesis, the process that occurs when easily degradable organicmatter is devoid of oxygen and other suitable electron acceptors (Chenget al., 2006). The C for CH4 production leading to methanogenesis inthe rice ecosystem mainly comes from root exudates and debris of therice plant growing in the field. Such release of C represents a loss ofvaluable assimilates that would have otherwise been incorporated into


plant tissue or grain (Cheng et al., 2006). N2O is produced as anintermediate by-product in the processes of nitrification and denitrification.

Wetland rice is a major source of atmospheric CH4 due to extendedflooding periods resulting in anaerobic decay of organic material. About10% of the rice cultivated area in the world is close to natural wetlandsand constitutes a significant source of CH4. Irrigated rice cultivated inEast Asia accounts for about 97% whereas South and Southeast Asiaaccount for about 60% of the CH4 emissions from all kind of rice ecosys-tems (Wassmann et al., 2000). Rainfed rice and deepwater rice contributeabout 24% of the CH4 emissions in South Asia and 16% in SoutheastAsia. In the IGP, particularly northern India, where rice is grown inrotation with wheat on coarse-textured soils of high percolation ratesrequiring frequent irrigations, constant inflow of oxygen into soilsrestricts CH4 emissions (Jain et al., 2000). Factors that contribute to anincrease in rice production such as the application of organic manures,crop residues, and inorganic fertilizers also increase CH4 emissionswhereas fertilizer N applications regulate the production of N2O.Denitrification is a significant N-loss process in wetland rice amountingto about one-third of the applied N (Aulakh et al., 2001). About 15 kgN2OeN ha�1 is emitted from the RW system.

Substantial production of CH4 occurs even in non-flooded soilsamended with rice straw (Sass et al., 1992). Anaerobic microsites cancreate congenial conditions for CH4 production. Emission of CH4resulting from rice straw burning were in a similar range of strawincorporation into the soil (Miura and Kanno, 1997).

3. Opportunities and Strategies

3.1. Conservation Agriculture

Conservation agriculture is a concept designed for optimizing crop yields,and reaping economic and environmental benefits. “Conservation agricul-ture is recognized as agriculture of the future, the future of agriculture”(Pretty et al., 2011). The key elements of conservation agriculture areminimum disturbance of soil, rational organic soil cover using cropresidues or cover crops, and the adoption of innovative and economicallyviable cropping systems and measures undertaken to reduce soilcompaction through controlled traffic (Hobbs et al., 2008). Theseconservation agriculture principles are not “site-specific” but represent“unvarying objectives” that are practiced to extend conservationagriculture technologies efficiently across all production environments.Therefore, other than the key elements described above, conservation


agriculture systems include best management practices of componenttechnologies (weed, water, nutrient, integrated pest management, etc.).The way crop management is practiced in different ecologies (e.g., plainsand sloping lands) may vary the importance of the ‘unvarying objectives’according to local situations, resource endowments of farmers, andfarming systems. Thus, conservation agriculture is an innovation processof developing appropriate conservation agriculture implements, cropcultivars, etc., for iterative guidance and fine-tuning to modify cropproduction technologies. This only suggests that conservation agriculturesystems should be “divisible” in nature and “flexible” in operation,allowing farmers to benefit from them under diverse situations.Conservation agriculture-based crop management technologies are an“open” approach, easier to mainstream and be able to quickly respond tocritical needs that address the concerns faced by South Asian agriculturetoday: i.e., farm economics and climate change.

Conservation agriculture practices have been widely adopted in tropical,subtropical, and temperate regions of the world for rainfed and irrigatedsystems. The area of conservation agriculture is increasing steadily world-wide. Recent estimates revealed that conservation agriculture-basedresource-conserving technologies that include laser-assisted precision landleveling, no-till, reduced tillage, direct drilling into residues, DSR,unpuddled mechanical transplanted rice, raised-bed planting, diversifica-tion/intensification, etc., are being practiced over nearly 3.9 M ha in SouthAsia (Jat et al., 2011a).

Several studies conducted across the production systems under variedecologies of South Asia revealed potential benefits of conservation agricul-ture-based crop management technologies (listed above) in resourceconservation, use efficiency of external inputs, yield enhancement, soilhealth improvement, and adaptation to changing climates (Table 2)(Gupta et al., 2010b; Gupta et al., 2003; Gupta and Sayre, 2007; Guptaand Seth, 2007; Jat et al., 2010; Malik et al., 2005b). The details ofindividual conservation agriculture-based crop management technologiesare described below.

3.1.1. No-till systemTillage contributes significantly to the labor and fuel cost of intensive agri-culture in any crop production system, resulting in lower economic returnsand soil degradation. There are indications of non-sustainability of the RWcropping system with the traditional practice of planting rice seedlingsmanually in random geometry after repeated dry and intensive wet tillagefollowed by conventionally tilled seeding of wheat. A shortage of water,labor, and energy resources, prohibitive costs of diesel, inappropriate cropmanagement practices, adverse effects of conventional tillage on thecarbon-based sustainability index, and declining profit margins are forcing

Table 2 Effect of different conservation agriculture-based crop management technologies on crop yields, water savings, and waterproductivity

TechnologiesLocation,country

Crop/croppingsystem

Yield gain overconventionalpractices (kg haL1)

Water savingsover conven-tional practices(ha-cm)

Increase inwater produc-tivity (kgmL3) Reference

Laser leveling Meerut, India Riceewheat 750 26.5 0.06 Jat et al. (2009a)Karnal, India Riceewheat 810 24.5 e Jat et al. (2009b)Ludhiana, India Rice 750 22.0 e Sidhu (2010)

No-till Karnal, India Wheat 150e400 2e4 0.10e0.21 Malik et al.(2005a);Saharawatet al. (2010)

Meerut, India Wheat 610 2.2 0.28 Gathala et al.(2011)

Delhi, India Corn 150 8.0 0.21 Parihar et al.(2011)

No-till withsurfaceresidue

Karnal, India Riceewheat 500 61 0.24 Gathala et al.(2010)

Meerut, India Wheat 410 10 0.13 Jat et al. (2009c)Direct-seededrice

Ghaziabad, India Rice 120 25 0.08 Jat et al. (2006a)Ludhiana, India Rice 510 13 0.09 Gill et al. (2006)Karnal, India Rice 62 18 0.10 Gupta and Jat

(2010)Raised-bedplanting

Meerut, India Corn 324 12 0.80 Jat et al. (2006b)Meerut, India Wheat 310 16 0.58 Jat et al. (2011a)Kaithal, India Wheat 270 5.0 0.50 Chandra et al.

(2007)

332Bhagirath

S.Chauhan

etal.


farmers of the IGP to switch over to no-till practices. No-till in the RWsystem has helped to save fuel and water, reduce the cost of production,and improve system productivity and soil health (Gupta et al., 2003; Guptaand Sayre, 2007; Gupta and Seth, 2007; Jat et al., 2009a; Malik et al., 2005b;Saharawat et al., 2009; Saharawat et al., 2010).

Declining soil health has become an important constraint to produc-tivity in the region. Traditionally, land preparation constitutes the majorcost of production of wheat in the transplanted RW production systemdue to repeated plowings needed to obtain desirable tilth and this practicealso delays wheat sowing, thus affecting its productivity. The adoption ofno-till, which is based on the principle of no or minimum soil disturbance,is considered vital for maintaining the productivity of the RW system. Anumber of benefits of a no-till system such as lower production costsachieved through savings in fuel, labor, and irrigation water, early sowingof wheat, positive effects on soil health, better quality of environment,lower weed (P. minor in wheat) infestation, reduced lodging, etc., havebeen documented (Gupta et al., 2005; Khan et al., 2001; Ladha et al.,2003; Malik et al., 2005a). No-till cultivation of wheat was once practicedon over 2.0 M ha in India, with potential of a further increase in easternIndia (RWC-CIMMYT, 2005). A similar trend was witnessed in PunjabProvince of Pakistan (Gill, 2006; Khan et al., 2001). Gill (2006) hasreported problems and constraints faced by farmers in the use of a zero-till drill for wheat sowing in Pakistan. There are contrasting reports of anincrease in incidence of rice stem borer in the RW system due to theuse of zero-till in Pakistan (Gill, 2006; Inayatullah et al., 1989).However, in India, an increased problem of pink stem borer and rats inwheat has been observed (Jaipal et al., 2005). Contrary to expectations,a significant reduction has occurred in area under no-till cultivation ofwheat in the last few years. In fact, a zero-till seed drill works well whenthere is no rice residue in the field, a condition normally achieved bymanual harvesting of rice. Besides saving time and facilitating sowingimmediately after the rice harvest, sowing in residue-free fields leads toreduced biotic stresses in terms of weeds, insects, and diseases andgenerally does not result in a decline in crop yields when compared withother crop residue management options (Samra et al., 2003).

The introduction of no-till and other resource-conserving technologiesinto the rice phase of the RW system may further increase long-term prof-itability as there is considerable scope to reduce the cost of cultivation andsoil health hazards in aerobic rice systems and the subsequent negativeeffects on the succeeding wheat crop. It is possible to significantly reduceCH4 emissions from rice paddies in the RW system through the adoptionof resource-conserving technologies (Samra, 2004). Transplanting of rice inthe NW IGP is becoming increasingly difficult due to the non-availabilityof labor and uneconomical due to high wages, the high cost of fuel, and


decreasing availability of irrigation water. A no-till system omits prior landpreparation and the seed is placed into the soil by a seed drill. No-till DSRand no-till transplanted rice have shown promising results and they havethe potential for large-scale adoption. In spite of promising benefits fromthe adoption of no-till cultivation of DSR or transplanted rice, farmersmay not be able to adopt the technology until the problems of no-tillcultivation in wheat and DSR are solved and the technologies are fine-tuned to suit farmers' conditions.

3.1.2. The furrow-irrigated raised-bed system (FIRBS)For the past one and a half decades or more, efforts have been in place topopularize FIRBS of planting rice and wheat in the IGP. It is a system inwhich the crop is sown on ridges or raised beds of 15e20-cm height.Usually, the bed is 37.5 cm wide with furrow width of 30 cm (total bedsize of 67.5 cm) to accommodate two rows of wheat. Beds of bigger size(90 or 120 cm wide) could also be prepared. The system aims at saving irri-gation water and increasing nutrient-use efficiency. The FIRBS is alreadypopular in irrigated wheat-corn systems in northwest Mexico (Meisneret al., 1992; Sayre and Hobbs, 2004), where area under bed plantingincreased from 6% in 1981 to 75% in 1994 (Sayre and Moreno Ramos,1997), and for the production of irrigated non-rice crops on heavy claysoils of Australia since the 1970s (Maynard, 1991). Potential agronomicadvantages of beds include improved soil structure due to reducedcompaction through controlled trafficking, reduced waterlogging, bettersurface drainage, improved fertilizer placement, feasibility of mechanicalweed control, reduced irrigation time, and improved yields on fine-textured soils prone to waterlogging. There are several reports of reducedirrigation amounts or time, with similar or higher yields, for wheat onbeds compared with conventional-tilled wheat. Although the potentialbenefits of bed planting for wheat in corn-wheat systems have beenestablished in the IGP for some time (Bhardwaj et al., 2009; Dhillonet al., 2000; Hari Ram et al., 2012), the evaluation of beds for rice andpermanent beds in the RW system commenced more recently (Connoret al., 2002).

Connor et al. (2002) listed advantages of the FIRBS: (i) reduced tillageand direct seeding on permanent beds reduce the costs of labor, diesel, andmachinery in comparison with the costs of initial bed formation andmaintenance; (ii) it arrests soil structural degradation and hard panformation from intensive rice; (iii) it provides feasibility of mechanicalweeding and inter-culture operations in wheat and other non-rice cropsto reduce herbicide or labor costs and reduce the development ofherbicide resistance and this could also extend to rice on beds; (iv)mechanical placement of fertilizers below the soil surface and near theroot zone, leading to improved fertilizer-use efficiency; (v) saving up to


30e40% of irrigation water in wheat and rice; (vi) reduced cost incurredin digging, maintenance, and deepening of wells, power for pumping,and labor for irrigation; electricity for pumping is generally not availablein the hot summer season prevailing during rice seasons due to peakdemand from other sectors; (vii) reduced seed requirement of a rangeof crops compared with flat surfaces; (viii) increased opportunitiesfor crop diversification in both the wet and dry seasons; and (ix)lower groundwater pollution and CH4 emissions from non-puddledintermittently irrigated rice on beds compared with puddled rice on flatsurfaces.

In the FIRBS, irrigation savings range from 18% to 50% (Gupta et al.,2005; Jat et al., 2005; RWC-CIMMYT, 2003). Farmer and researcher trialsin the IGP suggest irrigation water savings of 12% to 60% for DSR andtransplanted rice on beds, with similar or lower yields for transplantedrice on beds compared with puddled flooded transplanted rice, and usuallyslightly lower yields with DSR on beds (Balasubramanian et al., 2003). TheRW system on permanent beds has been intensively studied in the IndianPunjab and the results showed that there was no yield advantage of growingcrops on beds compared with flats but there was little advantage in watersavings (Humphreys et al., 2008a; Humphreys et al., 2008b; Kukal et al.,2008; Kukal et al., 2010; Yadvinder-Singh et al., 2008b; Yadvinder-Singhet al., 2009b). Another study in the NW IGP also showed little effect ofrice on beds on water productivity (typically around 0.30e0.35 g kg�1)as the decline in water input was accompanied by a similar decline inyield (Sharma et al., 2002).

With few exceptions (e.g., Limon-Ortega et al., 2000; Maynard, 1991),the beds are heavily cultivated and reshaped or completely reformed eachyear, at considerable expense. The acceptance of this technology by RWsystem farmers has been low owing to the significant cost of makingbeds and their subsequent reshaping on a regular basis. In addition, bed-planters in the NW IGP with large farm size require tractors with highhorsepower (>45) and custom hiring is still not a popular practice. Farmershave also reported difficulty of movement through these narrow furrowswhile spraying herbicides. Permanent raised beds prepared with no-tillseeders in situations where wheat is rotated with crops other than rice,however, still look promising. There is a further need to refine the FIRBStechnology to fit in the RW cropping system so that irrigation water in theIGP region can be saved.

3.1.3. Direct-seeded riceThe common method of establishing rice in South Asia is manual trans-planting with random geometry after repeated wet tillage, the processcalled puddling. The puddling process consumes 30% of the total waterrequirement in rice culture to form a dense clay layer, which controls


water losses through percolation. Puddled transplanted rice consumes 120to 180� 20 cm-ha of water in the Eastern and Western IGP. In recentyears, declining water tables, increasing labor shortage, and deterioratingsoil health due to edaphic requirements make the RW system uneco-nomical and unsustainable. The fixed support price of rice and committedprocurement policy of the government of India do not encourage farmersto shift away from the rice crop. As a result, the water table in this regionis declining at a fast rate and efforts are being made toward crop diversi-fication, which is considered crucial to conserve natural resources.Preliminary research investigations conducted over a period of a decadeclearly reveal the possibility of cultivating aerobic rice, which requiresdiscernibly low irrigation water (Mahajan et al., 2011c; Sudhir-Yadavet al., 2010).

The alternative establishment techniques for rice, such as dry DSR, haveshown promise in different production environments in South Asia andmay alleviate many of the problems associated with puddled transplantedrice. In South Asia, DSR (wet- and dry-seeded) is already practiced inmany medium-deep and deepwater rice ecologies of the Eastern GangeticPlains of India and Bangladesh, on terraced and sloping lands in the North-east and Western Himalayan region, and the Ghats along the west coast ofIndia. The area under DSR in India, Pakistan, and Bangladesh is 14.2 Mhaof the total rice area of 55.3 Mha (Pandey and Velasco, 2005). In the past,the common factors contributing to poor yields of DSR were supra-optimal seeding rates (60e100 kg ha�1), increased weed competition,insufficient fertilizer use, and a lack of improved cultivars selected forgood stand establishment with direct seeding.

Recently, some of these problems have been overcome and agro-tech-nology with respect to cultivars, sowing time, seeding rate, crop geometry,and weed, nutrient, and water management has been developed anddeployed in the RW system, which resulted in similar or higher outputs(system productivity), with 25% less water and labor and timeliness inplanting (Gathala et al., 2011). Sowing in the first fortnight of June,using 15e30 kg seed ha�1 at 2e3-cm soil depth (Mahajan et al., 2010)and 20e25-cm row spacing gives rice productivity of 6e8 t ha�1

(Gansham and Singh, 2008). A seeding rate of 30 kg ha�1 is adequate ifweeds are properly controlled. In Pantnagar, India, no differences in theyields of dry DSR were observed at seeding rates ranging from 15 to125 kg ha�1 when grown under weed-free conditions (Fig. 1) (Chauhanet al., 2011). Since the conditions are not conducive for perfect seedlingemergence in the farmers' fields, the seeding rate should be kept higherthan 15 kg ha�1 to compensate for damage by rats and birds, to partiallyovercome the adverse effects of herbicides, and to compensate for poorstand establishment if rains occur immediately after sowing (Chauhanet al., 2011).

Figure 1 The relation of rice grain yield to rice seed rate when grown under weed-freeconditions in 2008 at Pantnagar, India (Chauhan et al., 2011). A quadratic model wasfitted to the data and the 95% confidence intervals are depicted by the dotted lines.


Irrigation in dry DSR is applied after 3e5 days when the ponded wateris infiltrated into the soil. Mahajan et al. (2011c) found that appliedirrigation water increased by 28% in puddled transplanted rice comparedwith dry DSR. Bouman et al. (2005) reported that, on average, anaerobic field used 190 mm less water for land preparation and had250e300 mm less seepage and percolation than a puddled field. Gopalet al. (2010) claimed that dry DSR saved irrigation water by eliminatingthe need to puddle the field. Similarly, Bhushan et al. (2007) realizedirrigation water savings of 25% with no-till dry DSR compared withpuddled transplanted rice when irrigation for both establishment methodswas scheduled based on the appearance of hair-line cracks.

The promotion of dry DSR is the appropriate option of crop diversifi-cation for high yield realization and efficient use of resources. The impact ofDSR on the long-term productivity and sustainability of the RW system,however, requires careful evaluation within the context of productionsystems. In dry DSR, major challenges in addition to weed managementare nutrient management (including iron deficiency) and water manage-ment for successful crop establishment because most of the soils in NWIndia, including Punjab, are coarse-textured. Mahajan et al. (2011a)reported that application of N in four splits resulted in higher grain yieldof dry DSR than its application in three splits as is recommended fortransplanted rice. With the application of N in three splits in dry DSR,grain filling was limited by pre- and post-anthesis assimilate, indicatingthat fertilizer application schedules for transplanted rice are not suitablefor dry DSR. This study revealed that application of N at sowing time indry DSR can be skipped because the N may not be immediately used bythe emerging seedlings and application at anthesis may further boost theproductivity of dry DSR. Cultivars having characteristics of anaerobic


germination, weed competitiveness, and efficiency in Fe uptake from thesoil are needed for wider adoption of direct seeding in the near future.

3.2. Crop Residue Management

Crop residues play a significant role in building soil organic matter, nutrientrecycling, and improving the soil physical environment. The long-termsustainability of a cropping system depends on its carbon inputs, outputs,and carbon-use efficiency. Crop residues are the principal source of carbonand the way they are managed has a significant effect on soil physical,chemical, and biological properties (Kumar and Goh, 2000). Incorporationof crop residues into soil is known to improve soil structure, reducebulk density, and increase the porosity and infiltration rate of soil. Thismay help reduce the adverse effects of hard pan in the RW system andbenefit the wheat crop (Singh et al., 2005). Crop residues still play animportant role in the cycling of nutrients despite the dominant role ofchemical fertilizers in crop production. Crop residue management regulatesthe efficiency with which fertilizer, water, and other reserves are used ina cropping system.

Soil organic carbon influences physical, chemical, and biological func-tions and serves as an important sink and source of main plant nutrients.Soil organic carbon content, aggregation status, total porosity, pore sizedistribution, bulk density, dispersion ratio, and soil strength were corre-spondingly improved with crop residue incorporation (Bhatnagar et al.,1983; Bijay-Singh et al., 2008). Soil organic carbon content also influencesthe crop response to applied nutrients. Beneficial effects of rice straw (5 tha�1), applied in situ 20 days before wheat sowing, in conjunction with120 kg N ha�1 (C:N ratio of 15:1) have been observed on wheat yield(Sidhu and Beri, 2005). Organic matter decomposition releases nutrientsinto the mineral nutrient pool and reactive C (CO2) and N (N2O, NO)compounds to the atmosphere, and surface water and groundwater(NO3, NH4). Therefore, practices that increase organic matter additionto the soil through increased productivity, exogenous supply, or on-farmrecycling improve soil fertility and crop productivity. Each ton of organiccarbon in the plow layer is equivalent to 4.75 kg fertilizer N ha�1 (Benbiand Chand, 2007).

The addition of organic materials in sufficient amounts to soil is impor-tant for improving soil health as the organic substrates influence the micro-bial population in the soil, which is responsible for nutrient transformationsresulting in the availability of nutrients, particularly N, P, and S. Incorpo-ration of crop residues increased the population of aerobic bacteria andfungi while burning decreased it (Beri et al., 1992). These organicsources also reduce gaseous and leaching losses of N (Beri et al., 1995;Beri et al., 1992).


With increased production of rice and wheat, straw production has alsoincreased. Burning crop residues due to a lack of efficient and user-friendlytechnologies for in situ recycling (Jat et al., 2004) has significantimplications, leading to a loss of organic matter and precious nutrients,especially sulfur contained in crop residues, deterioration in the quality ofair, and increased emission of greenhouse gases (Samra et al., 2003).Increasing mechanization has led to increased availability of rice residuesto further aggravate the situation, which may contribute substantially toair pollution. Burning not only leads to the loss of a considerable amountof N, P, K, and S but also contributes to the global NO2 and CO2budget (Grace et al., 2002) and the destruction of beneficial microflora ofthe soil (Jat and Pal, 2000; Timsina and Connor, 2001). The decline insoil organic matter owing to residue burning has been implicated as oneof the key factors in non-sustainability of the system. The incorporationof crop residues alters the soil environment, which in turn favorablyinfluences microbial population activity in the soil and subsequentnutrient transformation (Kumar and Goh, 2000).

Rice straw can be managed successfully in situ by allowing sufficienttime between its incorporation and sowing of the wheat crop(Yadvinder-Singh et al., 2004). The magnitude of immobilized N wasinfluenced by the decomposition period of rice straw prior to fertilizerapplication (Yadvinder-Singh et al., 2005). Application of rice residue towheat typically has a small effect on wheat yields during the short termof 1e3 years (Bijay-Singh et al., 2008; Yadvinder-Singh et al., 2005) butthe effect appears in the fourth year with the incorporation of straw(Gupta et al., 2007). Crop residues can lower P sorption capacity andenhance P availability. Both organic and inorganic P contents andavailable soil K increased with straw incorporation compared with strawremoved (Beri et al., 1995; Gupta et al., 2007). The release of K fromrice straw occurs at a fast rate and more than 70% of total straw-K isreleased within 10 days after incorporation. Soil treated with cropresidues contained 5e10 times more aerobic bacteria and 1.5e11 timesmore fungi than soil on which residues were either burned or removed(Sidhu and Beri, 2005).

The emerging residue management options in the IGP are to mulchwith rice straw (5e7 t ha�1) in no-till wheat and incorporate combine-har-vested or even manually harvested (as in the Middle and Lower IGP) wheatstraw and stubble (1e2 t ha�1) in rice (Bijay-Singh et al., 2008). Riceresidue can also be used as a mulch for a wheat crop established aftertillage; however, this option is more feasible for farmers with smalllandholdings and sufficient labor (Bijay-Singh et al., 2008), as thisinvolves temporarily removing residue from the field and then returningit after the wheat crop has been planted. Residue management in no-tillsystems (surface retention) helps in improving soil health (Sharma et al.,


2008), reduces greenhouse gas emissions equivalent to nearly 13 t ha�1

(Mandal et al., 2004), regulates canopy temperature at the grain-fillingstage to mitigate the terminal heat effects in wheat (Gupta et al., 2010a;Jat et al., 2009c), and significantly improves the C sustainability index (Jatet al., 2011b).

It has not been possible to manage crop residues in no-till systems withtine-type openers. The use of new-generation planters (Happy seeder,rotary-disc drill, double-disc drill, and punch planter) may lead to wideradoption of conservation agriculture in the region (Sidhu et al., 2007).The Happy seeder works well for direct drilling in standing as well asloose residues, provided the residues are spread uniformly. In the CerealSystems Initiative for South Asia (CSISA) project being implemented inthe four South Asian countries, including India, a straw spreader fitted toa combine was developed and tested. These straw spreaders spreadresidues uniformly, facilitating the direct drilling of wheat immediatelyafter the rice harvest. The major demerit of the Happy seeder is itsinability to work in the early morning and late evening hours when thestraw is wet with dew, thus limiting its practical use to only a few hoursof the day during the peak period of sowing of wheat. Its high cost isanother prohibitive factor.

The IGP have an acute power shortage and the available hydro- andthermo-power plants have not been able to meet the rising demand.Several power plants that run on biofuel have been started in different areason a pilot basis and have been running satisfactorily. It is expected thatsurplus rice straw will find its end use for running these plants in thenear future. Bailing of rice straw with the help of balers for use in the card-board and paper industry seems to be another viable option in the futureprovided balers are available at an affordable price or custom hiring isencouraged.

3.3. Precision Land Leveling

Land leveling is a precursor to good agronomic, soil, and crop managementpractices and the levelness of the land surface has a significant influence onall farming operations. Unevenness of the soil surface influences farmingoperations, energy use, aeration, crop stand, and yield mainly throughnutrient-water interactions. Traditionally, farmers perform land levelingby using plankers (wooden boards) drawn by draft animals or tractors.Traditionally leveled fields have frequent dikes and ditches within the fieldsand, even with best efforts by conventional leveling practices, field slopesvary from 1� to 3� in transects I and II (Pakistan Punjab, Indian Punjab,Haryana, and Western Uttar Pradesh) to 3� to 5� in transects IV and Vof the IGP (Jat et al., 2006a). Undulating land hampers seedbedpreparation, seed placement, and germination and also requires more


power for machines, which leads to the consumption of more energy, andultimately to more cost of production and low productivity (Jat et al.,2006a; Jat et al., 2009a). In practice, it has been observed that land-leveling methods used by farmers in the IGP often result in low input-use efficiencies (nutrient and water) and low crop yields. Poormanagement and uneven fields lead to 10e25% irrigation water lossduring application (Kahlown et al., 2002), which results in lower cropyields, higher irrigation costs, and poor resource-use efficiency (Jat et al.,2006a).

Laser land leveling is one of the few mechanical inputs in intensivelycultivated irrigated farming that meets the objectives of saving irrigationwater, improved input-use efficiencies, and achieving a better crop stand.Hill et al. (1991) rated the development of laser technology for precisionland leveling as second only to the breeding of high-yielding varieties. Itis a process of leveling the land surface within þ2 cm of its averagemicro-elevation using a laser-equipped drag scrapper. Laser leveling hasbecome an important component of conservation agriculture-based cropmanagement technologies. Laser-assisted land leveling improves cropyields and input-use efficiency, including water and nutrients (Jat et al.,2006a; Jat et al., 2010). Precision land leveling has been shown toimprove water use and obtain up to 50% savings of irrigation water (Jatet al., 2006a; Jat et al., 2009b; Jat et al., 2010; Rickman, 2002). Cropyields depend on optimum seedling emergence, better crop stand, andearly crop vigor. Laser land leveling is also reported to improve cropstand and crop productivity (up to 30%) and reduce the laborrequirement for weeding (from 21 to 5 days ha�1) in rice (Bell et al.,1998; Jat et al., 2006a; Jat et al., 2009b; Rickman et al., 1998). Kahlownet al. (2002) reported that precision land leveling improved theperformance of RW and water productivity in non-puddled soil, withno-till surface seeding and seeding on permanent beds compared withconventional tillage. Currently, laser-assisted land leveling is beingpracticed over 1.5 Mha in South Asia (Jat et al., 2009a; Jat et al., 2009b;Jat et al., 2011a).

3.4. Crop Diversification

Crop diversification is of paramount importance in mitigating the bioticand abiotic problems arising on account of RW monoculture. Diversifica-tion with high-value crops will encourage the export of farm produce,bringing more profits. Inclusion of certain crops in sequential and inter-cropping systems has been found to reduce nutrient and water needs andthe population of some obnoxious weeds to a considerable extent, therebyreducing herbicide needs to a great extent in areas where such weeds haveassumed alarming proportions. Continuous cultivation of crops having


similar management practices allows certain weed species to become domi-nant in the cropping system and, over time, these weed species becomehard to control. In addition, continuous cropping can negatively interactwith conservation agriculture systems and shift the weed flora towarda troublesome composition. Rotating crops having divergent and distinctmorphologies, growth habits, life cycles, differing cultural practices, andnutrient and water needs can all potentially affect the community compo-sition and distribution of weeds.

Increased fertilizer N-use efficiency through improved management ina diversified crop can reduce the potential for nitrate contamination ofgroundwater. Inclusion of legumes in cropping systems has been foundto be effective in reducing nitrate leaching in lower profiles. Legumescan play an important role in conserving groundwater and soil water.However, profitability of legumes has remained too low in comparisonwith rice and wheat (Joshi, 2003). Similarly, there is a chronic shortageof edible oils in the country and massive spending of money is madeevery year to import 5e6 million tons of edible oils. In spite of the hugedemand for pulses and edible oils in the country, farmers are nottempted to cultivate them because of lower yields in comparison to riceand wheat and their sensitivity to biotic and abiotic stresses. Cultivationof sunflower or corn during spring holds some promise but this willreplace wheat rather than rice. Moreover, the water requirement of thesecrops (8e9 irrigations for sunflower and spring corn) is much higherthan for wheat (4e5 irrigations). Soybean cultivation during the rainy/kharif season has also failed to make any impact because of establishmentproblems in soil with low organic carbon, low yields, damage by whitefly, and lack of taste due to beanie flavor. Consistent efforts bypolicymakers and scientists to convince farmers to diversify the presentRW system have failed to attract farmers due to a lack of profit, pricesupport, and marketing bottlenecks for the alternative crops. Thesituation is unlikely to change unless some breakthrough is achieved interms of developing high-yielding (2.5e3.0 t ha�1) cultivars or hybrids ofoilseeds and pulses. The cost of cultivation of groundnut, soybean, andmany other crops in the region is much higher than in central orsouthern India, where the same yields are achieved under rainfedconditions. Moreover, the quality of these crops as well as wheatproduced in these areas is better (higher protein content). Raised-bedplanting technology provides an opportunity for diversification throughintensification and it saves on water (Jat et al., 2005). Crop diversificationwithin the RW system can be achieved by replacing part of the areaunder rice with basmati rice and bread-wheat with durum-wheat (Gillet al., 2008). There is an urgent need for the government andpolicymakers to make stronger efforts to convince farmers to diversifyRW monoculture with other crops.


3.5. Soil Nutrient Management

Crop requirement for nutrients varies with crop potential, field history,cropping sequence, sowing time, season, location, and growing conditions(Timsina and Connor, 2001). Results of long-term experiments haveshown a linear response of rice to applied nutrients, suggesting scope fortheir higher application (Ladha et al., 2003). Site-specific nutrientmanagement, which takes these factors into consideration, is needed toensure balanced and optimum nutrient use, improvement in cropproductivity, and higher nutrient-use efficiency. The use of a chlorophyllmeter and leaf color chart is being advocated to apply the requiredamount of N (Balasubramanian et al., 1999; Balasubramanian et al., 2000;Peng et al., 1996; Turner and Jund, 1991). In the Indian Punjab, a valueof 37.5 was found as the critical SPAD value for rice grown sequentiallywith wheat, whereas, in wheat, yield increased with a topdressing of30 kg N ha�1 when the SPAD reading at maximum tillering was lessthan 44 (Singh et al., 2002). Wheat yield increased by 20% when 30 kgN ha�1 was applied at a SPAD value of 42 at maximum tillering (Singhet al., 2002). Application of 50% of the recommended N dose with pre-sowing irrigation resulted in significantly higher wheat yield than itsapplication at sowing (Sidhu et al., 1994). Probably N applied with pre-sowing irrigation was transported to the deeper layers of the soil and thuswas not prone to loss via ammonia volatilization. On coarse-texturedsoils, application of N in three equal splits at sowing and at first andsecond irrigation resulted in more efficient use of N.

For P and K management, nutrient omission technology that deter-mines the soil supplying capacity and crop requirement for P and K in indi-vidual fields is suggested (Dobermann and Fairhurst, 2000). It is estimatedthat only 20e25% of the applied P is used by cereal crops and the rest isretained in the soil as residual P. In general, high water solubility of Pwill be required for alkaline and calcareous soils and for wheat more thanfor rice. P-use efficiency in the case of its placement below the soilsurface and into the root zone of wheat was 1.5 times higher than whenit was broadcast (Vig and Singh, 1983). Balanced fertilization is the keyto increasing the use efficiency of plant nutrients as it ensures theapplication of fertilizers in adequate amounts and correct ratios foroptimum plant growth. Efficient nutrient management holds the key inRW double- or triple-cropping systems in which there is hardly anyscope for keeping fields fallow between two crops. Also, nutrientrecycling through organic manures is important, as shown for highlyintensive triple-cropped riceewheatecorn or riceewheatemungbeansystems in Bangladesh (Panaullah et al., 2006; Saleque et al., 2006;Timsina et al., 2006). Whenever feasible, inorganic fertilizers should beused in conjunction with organic manures.


Since nutrient-supplying capacity and rate of mineralization of organicsources of nutrients vary greatly, these may have a short- to long-termeffect on crop yields and soil fertility. The rate of mineralization of Nfrom poultry manure was much faster than from farmyard manure(Yadvinder-Singh et al., 2009a). Poultry manure contains high amountsof uric acid and urea substances that readily release NH4

þeN on a parwith urea N and the efficacy of poultry manure N was similar to that ofurea N in increasing yield and N uptake of rice (Bijay-Singh et al.,1997). Application of 6 t ha�1 of poultry manure or 12 t ha�1 offarmyard manure to rice also showed residual effects in wheat, whichwere equivalent to 30 kg N and 30 kg P2O5 ha�1 (Yadvinder-Singhet al., 2009a). Regular application of pressmud cake at 5 t ha�1 for 4years increased the organic carbon content and available P content ofsoil, with its long-term application having the potential to completelyreplace the P-fertilizer need of the RW system (Yadvinder-Singh et al.,2008a).

Sufficient time is available between the wheat harvest (in April) and ricetransplanting/planting to grow a green manure crop, which has the poten-tial to substitute 50% of the N needs of rice. Sesbania aculeata and Crotolariajuncea are the most commonly grown green manure crops in the IGP,which produce 4e5 t ha�1 dry biomass and 80e100 kg N ha�1 in50e60 days (Beri and Meelu, 1981; Beri et al., 1989; Yadvinder-Singhet al., 1991). S. aculeata is more tolerant of salinity, acidity, and excess mois-ture conditions than C. juncea, which performs better in low rainfall andlimited soil moisture. Vigna radiata and Vigna unguiculata are even betteroptions as their grain is used as a pulse for human consumption and plantbiomass can be incorporated into the soil.

3.6. Reducing Water Requirements

The Indian Punjab now occupies about 2.8 Mha under rice and 3.5 Mhaunder wheat. In neighboring Haryana State, rice and wheat cover about1.2 and 2.5 Mha, respectively. In Uttar Pradesh, wheat is grown on9.5 Mha and rice on 6.0 Mha. Some 30e35 irrigations (1500e1800 mmwater) are given to rice, whereas wheat is irrigated 5e6 times during thecrop season. Some simple measures that can save a substantial amount ofwater in rice are the cultivation of short-duration (early-maturing) cultivars,appropriate crop establishment methods, transplanting not before 15 June(Mahajan et al., 2008), submergence of fields only for the first 2 weeksfor seedling establishment and application of subsequent irrigations only2e3 days after complete removal of water (Sandhu et al., 1980), the useof tensiometers for scheduling irrigations (Kukal et al., 2005), stopping/withdrawing irrigation about two weeks before crop harvest, etc.Humphreys et al. (2010) have also suggested several ways of saving water


in rice in the RW system, including laser land leveling, alternate wettingand drying (AWD), delayed rice transplanting, shorter duration ricevarieties, cultivation on raised beds, and replacing part of the rice areawith other crops. The AWD practice can save 15% to 30% of irrigationwater, without any adverse effect on rice yield (Sandhu et al., 1980).Adoption of dry DSR in the IGP can save a substantial amount of waterrequired for rice (Mahajan et al., 2011c; Bouman et al., 2005; Gopalet al., 2010; Bhushan et al., 2007).

Similarly, the use of laser land leveling in dry DSR can save a substantialquantity of water. In other parts of the world, savings in irrigation waterwere reported when rice was mulched with crop residue (Bijay-Singhet al., 2008). Rice yields were comparable for mulched and non-mulchedrice crops, but water-use efficiency was higher for the mulched crops.Experts in the Indian Punjab have suggested reducing the area under riceby about 1 Mha (Hira, 2009). Bt (Bacillus thuringienesis) cotton, kharifcorn, soybean, and groundnut, which require 2e5 irrigations, can bea viable alternative to rice (Kaur et al., 2010). Hira (2009) stated that, incotton, a broad bed with spacing of 135 cm and planting cotton infurrows in paired rows increased yield by 44% and saved 40% ofirrigation water compared with row spacing of 67.5 cm in flat-bedsystems. Planting of potatoes on both sides of a narrow bed increasedtuber yield (24 t ha�1) by 25% and saved 20% of irrigation watercompared with the ridge-planting method (20 t ha�1).

In wheat, desirable productivity can be achieved with only 4 irrigationsin comparison to 5e6 irrigations with better management practices. Theirrigations can be applied only at the critical stages. A modeling study usingthe DSSAT CERES-Wheat model showed that irrigation for wheat grownafter rice in Punjab, India, should be applied based on the atmosphericdemand and soil water status and not on the growth stage (Timsina et al.,2008). The use of rice mulch in no-till wheat may also help in reducingevaporation and, ultimately, water requirement. As discussed earlier,compared with conventional tillage, the FIRBS can help in savingirrigation water from 18% to 50% (Gupta et al., 2005; Jat et al., 2005;RWC-CIMMYT, 2003).

Some area under wheat can also be replaced by oilseed (rapeseed-mustard) crops, which require only 2e3 irrigations. Oilseed crops offer anexcellent opportunity for maximizing productivity under limited moistureavailability (Reddy and Suresh, 2008). Sunflower crop productivity can beincreased by more than 60% with limited irrigation at critical stages. In thewestern zone of Uttar Pradesh, growing of riceemustardemungbean for1 year followed by riceewheatemungbean in the succeeding 2 yearsregistered an 11% savings of water, clearly favoring diversification of theRW system. In Bihar, a riceepotatoesunflower cropping system recordedhigher rice equivalent yield, net returns, benefit:cost ratio, land-use


efficiency, and production efficiency than a traditional riceewheategreenmanuring (S. aculeata) cropping system. At Jalander (Punjab) also,riceepotatoesunflower registered higher rice equivalent yield, economicefficiency, land-use efficiency, irrigation water productivity, and nutrientproductivity than the RW system. Punjab and Haryana have becomemajor rice- and wheat-producing states for the last three decades with thedominant RW cropping system. To reduce fatigue of the RW system,alternate crops such as oilseeds can be grown instead of wheat withouthampering the profitability of the system. A modeling study usingORYZA2000 and Hybrid Corn models showed very high yield potentialof winter corn after rice in all four South Asian countries (Bangladesh,India, Nepal, and Pakistan). Potential yield of winter corn in Punjab,India, exceeded 16 t ha�1 and that of summer corn exceeded 12 t ha�1

(Timsina et al., 2011; Timsina et al., 2010). At least half a million hectaresof rice in Punjab could be shifted to soybean, kharif corn, or pigeon peain comparatively upland and less irrigated area (dry area). The area that isplanted with wheat in the late season due to the late harvest of basmatirice could be shifted to sunflower or winter/spring corn. In Haryana, thebasmati rice area is invariably planted to late-sown wheat. It is suggestedthat the basmati area be sown under sunflower or corn in place of wheat.In Uttar Pradesh, where the RW system is important, it is possible to shiftthe part of the rice area located in higher landscapes to corn, or soybean,or pigeon pea. However, soybean as a pulse crop is not preferred bypeople in the region because of the typical “bean flavor” produced bysoybean due to the lipoxygenase enzyme. Moreover, soybean and pigeonpea are susceptible to standing water, a situation normally experiencedduring the rainy season at any stage of growth of these crops and theircultivation also leads to late sowing of wheat.

3.7. Weed Management

Labor and water shortages are likely to become major constraints to futurerice production. DSR has the potential to reduce water and labor require-ments to grow rice. The success of DSR, however, depends on successfulweed management. An integrated strategy of weed management is neededfor sustainable production of DSR (Chauhan, 2012; Chauhan and Johnson,2010c). The development of rice cultivars with weed-smothering charac-ters, optimizing seeding rate, crop geometry, use of residue, and N manage-ment are some important aspects in managing weeds in DSR (Chauhan,2012; Chauhan and Johnson, 2010a, c, 2011a, b; Chauhan et al., 2011;Mahajan and Chauhan, 2011).

Weed pressure during the early crop growing season can be reduced byadopting the stale seedbed technique, in which the seedbed can be preparedat least 7e10 days in advance of seeding with moisture ensured either by


irrigation or rain to stimulate germination and emergence of weeds, whichare then destroyed either by shallow cultivation or the use of non-selectiveherbicides such as paraquat or glyphosate. The use of herbicide may havethe advantage of destroying weeds without disturbing the soil, thusreducing possibilities of bringing more seeds to the upper soil surface. Ina study in India, the use of non-selective herbicide in a stale seedbed wasmore effective than mechanical weeding in DSR (Renu et al., 2000).This suggests that rice should be sown with minimum soil disturbanceafter destroying the emerged weeds. A reduction of 59% in the densityof E. colona and 78% in fresh weed weight of Cyperus spp. wererecorded after the stale seedbed technique in the Philippines (Moody,1982). Weed species that require light to germinate and have low initialdormancy and are present in the top layers of soil are sensitive to thestale seedbed practice (Chauhan, 2012; Chauhan and Johnson, 2008a, c, d).

The use of weed-competitive crop cultivars can also help to suppress weedsin the RW system. A quick-growing and early canopy cover enables a cultivarto compete better against weeds. Research studies have shown that traditionaltall cultivars, such as NERICA rice, exert an effective smothering effect onweeds (Prasad, 2011). Further, it has been observed that early-maturingcultivars and hybrids of rice also have a smothering effect on weeds due tovigor and they have a tendency of early canopy cover (Mahajan et al.,2011b). Compared with shoot traits, little research has been done on therole of root competition for nutrients and water in riceeweed interactions(Chauhan, 2012; Chauhan and Johnson, 2010b). There is a need to furtherexplore the subject of weed-competitive cultivars taking into account bothaboveground and belowground plant traits.

The impact of weed on rice can be reduced by optimizing crop-plantarrangement; however, this approach should be considered as an aid inthe context of integrated weed management programs, rather thana stand-alone control strategy (Chauhan, 2012). Mahajan and Chauhan(2011) observed that a paired-row planting (15e30-15 cm row-to-rowspacing) pattern in dry DSR had a great influence on weeds comparedwith a normal row planting system (23-cm row-to-row spacing). Paired-row planting greatly facilitates weed suppression by maintaininga dominant position over weeds through modification in canopystructure. In other dry DSR studies, reduced crop row spacing increasedthe crop's ability to compete with weeds for light (Chauhan, 2012;Chauhan and Johnson, 2011b). Chauhan and Johnson (2011b) reportedthat the critical periods (days) for weed control were fewer for a dryDSR crop in 15-cm rows than in 30-cm rows (Table 3). In anotherstudy, E. colona and E. crus-galli emerging up to 60 days after riceemergence in 20-cm rows produced less shoot biomass and seeds thanwhen they were grown in 30-cm rows (Fig. 2; Chauhan and Johnson,2010a).

Table 3 The effect of row spacing on the critical periods for weed control to obtain80%, 90%, 95%, and 98% of maximum yield in a weed-free plot, during the wetseason

Row spacing

Critical periods (days) to obtain different percent ofmaximum yield

80% 90% 95% 98%

15 cm 30e37 23e45 18e52 11e6030 cm 24e42 19e51 15e58 10e66

(Chauhan and Johnson, 2011b)

a)

Sh

oo

t b

io

mass

(%

red

uctio

n o

f 30-cm

ro

w)

0

20

40

60

80Echinochloa colona

Echinochloa crus-galli

b)

Weed emergence time

(days after rice emergence)

0 15 30 45 60

Seed

p

ro

du

ctio

n

(%

red

uctio

n o

f 30-cm

ro

w)

0

20

40

60

80

100

Figure 2 Effect of narrow row spacing (20-cm) over wide row spacing (30-cm) on thereduction (%) of shoot biomass and seed production of Echinochloa colona and E. crus-galliwhen weeds emerged at different times after rice emergence. (Chauhan and Johnson,2010a)


Tillage systems influence vertical weed seed distribution in the soilprofile, and this differential distribution could affect weed seedling emer-gence (Chauhan, 2012; Chauhan et al., 2006a, b, c; Chauhan and Johnson,2009b). A study in dry DSR in the Philippines showed greater seedling

Table 4 Seedling emergence pattern of different weed species in conventionaltillage and zero-till systems (Chauhan and Johnson, 2009b). A three-parametersigmoid model was fitted to the seedling emergence data. Emax is the maximumseedling emergence (%), T50 is the time to reach 50% of maximum seedlingemergence (d) and Erate indicates the slope around T50

Weed species

Conventional tillage Zero-till

Emax Erate T50 R2 Emax Erate T50 R2

Eclipta prostrata 1.8 4.8 26.0 0.98 12.5 6.5 24.1 0.99Portulaca oleracea 4.6 6.6 17.2 0.92 21.8 5.0 16.0 0.99Digitaria ciliaris 2.4 3.7 22.0 0.99 15.8 3.2 21.7 0.99Echinochloa colona 3.6 5.7 28.7 0.99 26.2 2.4 24.0 0.99Eleusine indica 2.4 3.3 16.7 0.99 13.5 3.2 13.9 0.98


emergence of Digitaria ciliaris, E. colona, E. prostrata, Eleusine indica, andPortulaca oleracea in zero-till plots than in conventional-tillage plots(Table 4; Chauhan and Johnson, 2009b). The greater emergence inzero-till plots was due to the small seed size of these weed species andthe light requirement for germination. Despite these results, zero-tillsystems may help in reducing the weed seed bank if weeds arecontrolled effectively from the surface layer in the crop because soil isnot disturbed and weed seeds are not brought to the soil surface(Chauhan, 2012). The large weed seed bank on the soil surface can beburied with a deep tillage operation after growing a few crops undera zero-till system. Seedling emergence of some weeds in zero-till fieldscan be reduced further by using residue as mulch. In recent studies,Chauhan and Johnson (2008b, c, d, 2009a, c, d) observed that,compared with no residue, rice residue amounts at 1e6 t ha�1 reducedseedling emergence of different weed species (Fig. 3).

Dry DSR warrants an intensive use of herbicides. Sequential applicationof pendimethalin (1 kg a.i. ha�1) as pre-emergence followed by post-emer-gence application of bispyribac-sodium (25 g a.i. ha�1) proved effective forweed control in Punjab, India (Mahajan and Timsina, 2010). However,the continuous use of ALS-inhibitor herbicides (e.g., bispyribac-sodium)may increase the problem of herbicide resistance against weeds in DSR inthe near future. Therefore, appropriate and economical weed managementtechnologies are required for the sustainable cultivation of DSR, whichmay include the stale seedbed technique, proper land preparation,preventive measures (such as using clean seeds), zero-till systems, the useof competitive cultivars, optimum seeding rate, paired-row planting, waterand nutrient management, mulching, rotation of crop establishmentmethods, and the use of suitable chemicals at the right time.

Weed species

A. spinosus

D. ciliaris

E. colona

E. pro

strata

E.indica

P. olera

cea

Seed

lin

g e

me

rg

en

ce

(%

red

uctio

n co

mp

ared

w

ith

n

o resid

ue)

0

20

40

60

80

1001 t ha-1

2 t ha-1

4 t ha-1

6 t ha-1

Figure 3 Effect of rice residue amounts on percent reduction in seedling emergence ofdifferent weed species compared with no residue. (Chauhan and Johnson, 2008b, c, d,2009a, c, d)


The development of herbicide-resistant rice would enable early-seasonweed control, could help in promotingDSR systems (with orwithout tillage),andwould reduce flooding of fields to control weeds (Olofsdotter et al., 2000).To date, most research has been conducted with imidazolinone-resistant rice(non-transgenic). Rice cultivars resistant to this chemical have been releasedfor the first time in Asia (in Malaysia). The post-emergence application ofherbicides in herbicide-resistant rice will allow growers to adjust dosesaccording to the amount of weed infestation. The availability of herbicide-resistant rice will also facilitate wider adoption of conservation agriculture inthe RW system as has been observed for soybean and cotton in the U.S.(Fawcett and Towery, 2003).

In the IGP, one of the reasons for the slowdown in the growth of wheatproductivity during the 1990s was the widespread development of herbicideresistance in P. minor. The introduction of herbicides with different chemistry(clodinafop, fenoxaprop, and sulfosulfuron) in place of substituted-urea herbi-cides in 1997e98 together with no-till cultivation of wheat regained the lostconfidence of wheat growers and wheat yields showed a sign of increase in1999e2000 (Malik et al., 2002). The planting date of wheat determines thedegree of competition with weeds, depending on their emergenceperiodicity. Weed emergence is governed by soil temperature and availablemoisture. P. minor germination is retarded by ambient maximumtemperature of �30 �C (Singh and Dhawan, 1976). P. minor emergence isgreater at lower temperature (15e20 �C), most prevalent in the wintermonths of November and December in northern India. Late-sown wheat inDecember suffers from severe competition from P. minor compared with


early planting (October to November); however, the sowing date depends onthe availability of fields after the harvest of the previous crop (rice and cotton).The study ofMahajan andBrar (2001) revealed that a crop sownon25Octobercaused a 27% reduction in drymatter accumulation byP.minor and 22%highergrain yield than a crop sown on 10 November. Therefore, early planting ofwheat in late October or early November offers a competitive advantage towheat against P. minor, giving it a substantial head start (2e4 weeks). Thecombination of the adoption of early sowing and new herbicides has beenthe key ingredient in the effective management of herbicide-resistant P.minor in the IGP.

Wheat varieties with early canopy cover and greater dry matter accumula-tion can shade grass weeds. In general, taller varieties competemore vigorouslywith grass weeds. Varieties WH-147 and HD-2285 were found to be morecompetitive with Avena fatua than HD-2009, WH-291, and S-308; the lattervarieties had more tillers, but that advantage did not translate into higher grainyield (Balyan et al., 1991). Greater competition by PBW-343 than by PDW-233 was due to more tillers and thus a greater suppression effect on weeds(Mahajan et al., 2004). Under timely-sown wheat, PBW-343 and WH-542were equally competitive with P. minor (Chahal et al., 2003; Kaur et al.,2003; Mahajan et al., 2002); PBW-343 also maintained its competitivenessover P. minor in delayed plantings compared with other varieties (Kaur et al.,2003). Growth of P. minor can also be curtailed by increasing the seedingrate of wheat. Within limits, yield tends to increase at higher seeding rates,which may be due to less dry matter production of weeds in a denselyplanted crop (Walia and Brar, 2007). To improve the competitive ability ofwheat for applied fertilizers, particularly N, placement should be close to theseed or crop plant so that a maximum quantity of applied fertilizers can be atthe disposal of the crop plant and be a minimum for weeds. Walia and Kuar(2004) revealed that grain yield improved significantly in the crop receivinga half dose of N with the side placement method and the remaining halfwith broadcasting, which may be due to a significant reduction in dry matterproduction by weeds because of more availability of nutrients to crop plantsthan to weeds.

Strawmanagement is a major factor in weedmanagement in wheat grownafter rice. Many farmers burn straw to reduce thatch and also to burn weedseeds, notably those of P. minor dispersed on the soil surface after the puddlingprocess followed in rice. However, because of environmental pollution, strawburning is being banned inmany states. Straw of the previous crop has a signif-icant effect on weeds emerging in wheat. The density of P. minor was 1.2 and1.4 times greater in plots where 6 and 12 t ha�1 rice straw, respectively, wasburned compared with its removal (Singh, 1996). In the isoproturon-treatedplots, weed density was 1.9 and 2.8 times greater with a 6 and 12 t ha�1

straw burning treatment over straw removal treatment, respectively. Thiswas due to the adsorption of isoproturon to ash contents.


Efforts to use rice straw in generating energy and for composting arebeing investigated. The turbo seeder (drill) has been found effective becauseit places straw in front of the tines and places it in between rows. Strawmulch placed between two wheat rows suppresses the emergence of weedsand also adds organic matter to the soil. Weeds already emerged at plantingtime in zero-till fields need to be controlled by non-selective herbicides(paraquat or glyphosate). The early-emerged weeds might affect the effi-cacy of herbicides due to their advanced stage at spraying. Another drill,a rotovator, can also be useful where straw is less and weeds have emergedbecause it can uproot the emerged weeds, incorporate surface straw/stubble, and do seeding in a single pass, thus reducing the cost of non-selec-tive herbicides before wheat planting. However, rotovators are not suitableon heavy soils, or where there is low soil moisture or high amounts of cropresidues. Rotovators also create a hard pan below the plow layer, leading towater stagnation, resulting in yellowing of wheat after irrigation. In theRW cropping system in the IGP, there is a need to develop ecologicallybased weed management strategies, and the use of these strategies willhelp reduce reliance on herbicides.

As mentioned earlier, crop rotation may prevent one particular weedspecies from becoming unmanageable because of different managementrequirements. Diversification of the area under a riceewheat croppingsystem not only brings changes in the weed spectrum, but also makessoil conditions unfavorable for P. minor emergence and growth. In theIGP region, fewer resistance cases in P. minor were found where growersincluded sugarcane, sunflower, and vegetables in rotation than in a ricee-wheat cropping system (Malik and Singh, 1995). When replacing wheatwith other crops such as berseem clover, potato, sunflower, Indianmustard, and oilseed rape for 2e3 years in a riceewheat croppingsystem, the population of P. minor decreased significantly (Brar, 2002).Some crops such as Indian mustard, barley, and fodders can suppressweed growth significantly through their ability to grow faster. However,the adoption of a crop rotation will depend on the market price of the crop.

The use of herbicides is both efficient and economical and therefore thefirst choice of farmers. Metsulfuron has been found quite effective againstRumex spp. and it also provides good control of all broadleaf weeds, whichcannot be controlled by 2,4-D. Herbicides with new chemistry such aspinoxaden, metsulfuron, and sulfosulfuron þ metsulfuron have been rec-ommended for the control of P. minor.

3.8. Climate Change

Water management in rice holds the key for potential trade-off betweenrice production and CH4 mitigation options. Irrigated rice is the largestsource of CH4 but it also offers the most options for mitigating these


emissions. Optimizing irrigation scheduling in the field by introducingpractices such as additional midseason drainage accounted for 7e80% ofCH4 emissions (Wassmann et al., 2000). Continuous flooding emittedmore CH4 than alternate flooding and drying (Mishra et al., 1997). Asingle midseason drainage reduced seasonal CH4 emissions from ricefields but increased emissions of N2O (Bronson et al., 1997).

High-yielding plants use assimilates more efficiently and spare lesscarbon for microbial CH4 production. The development of rice cultivarsthat can maintain spikelet number and continue grain filling under highertemperatures with comparatively less wasteful maintenance respiration los-ses would not only sustain rice production but also reduce greenhouse gasemissions from rice fields. The development of cultivars with higher netassimilation rates and increased sink size (leading to increased net removalof atmospheric CO2), improved N-use efficiency (reduced fossil fuel useto produce inorganic fertilizers) would lead to more stable yields underrising atmospheric CO2 levels (Krishnan et al., 2007). To achieve this,research needs to identify the genetic controls and physiological processesinvolved in rice adaptation to higher temperature. Thrust is needed todevelop early-maturing upland rice cultivars not requiring standing waterconditions without a major sacrifice in yield and grain quality. Researchon the development of high-temperature- and drought-tolerant high-yielding cultivars of both rice and wheat needs to begin in the targetenvironments. The genetic resources, especially land races, from areaswhere past climates mimicked the projected future climates foragriculturally prime areas should be used in a breeding program todevelop desired cultivars. A combination of conventional and non-conventional biotechnology tools will be required to achieve these goals.

On several occasions in the last decade, South Asia witnessed adverseeffects of climatic variations in terms of terminal heat stress on wheatproductivity. For example, despite favorable weather conditions duringthe winter of 2009e10, an abrupt rise in night temperature during thegrain-filling stage in wheat adversely affected wheat productivity in theIGP and other northern states of India (Gupta et al., 2010a). Surveysindicated that terminal heat stress in the granary of Punjab in 2009e10led to an average yield penalty of 5.8% compared with the previous year.However, yield losses were as high as 20% in a few districts of Punjaband other transects of IGP. The magnitude of losses varied depending onplanting time, varieties, and other management practices.

In spite of better understanding and scientific knowledge of climatechange and its implications, there is a lack of functional partnershipbetween scientists, the private sector, policy planners, and farmers in orderto move beyond rhetoric to real actions. To adapt to effects of climatechange, immediate solutions could be altered management practicesinvolving a conservation agriculture approach for buffering soil and canopy


temperatures. Farmers' participatory field trials conducted across the IGP fordeveloping, defining, and validating innovative solutions for overcomingterminal heat stress in wheat production (Jat et al., 2009c) included (i)advancing planting time using no-till, (ii) cultivar choices that adapt to/escape terminal heat, (iii) no-till with mulching using crop residues, (iv)irrigation management, and (v) balanced plant nutrition (using site-specific nutrient management). Results of farmers' participatory field trialsacross the IGP revealed that no-till systems help in timely planting andterminal heat effects are less in no-till compared with conventional tilleven under late planting (from 21 Nov. till 20 Dec.). No-till drilling inresidues keeps canopy temperature lower by 1e1.5 �C during the grain-filling stage (cooling due to transpiration) owing to sustained soilmoisture availability to plants (Jat et al., 2009c). In the Eastern IGP, lateharvest of rice and 7e10 days of additional window required for plantingafter conventional tillage leading to delayed planting of wheat, relativelyshorter growing period, and terminal heat effects cause lowerproductivity than in the NW IGP. Significant tillage x genotype xcropping system interactions were also observed, suggesting a need toredefine recommendations for cultivar choices. Further, rescheduling ofirrigation and plant nutrient applications had further positive effects inadapting wheat to terminal heat effects. Therefore, implementing theseinnovative management practices can be an immediate solution foradapting to the effects of climate change (Gupta et al., 2010a).

4. Conclusions

As one of the world's largest agricultural production systems spreadover 26 Mha in the IGP of South Asia and China, the RW cropping systemmeets the staple food requirements of more than one billion people. In thethree decades between 1970 and the end of the twentieth century, theannual increase in rice production in the IGP was 4.1%, much greaterthan that achieved over the entire Indian rice belt (2.1%) over the sameperiod. The annual rate of increase in wheat production during the sameperiod was 4.4% in the IGP compared with 3.4% for the entire country.This increase in wheat and rice production in the IGP resulted from theintroduction of input-responsive high-yielding crop varieties along withmatching production and protection technologies, infrastructure develop-ment, and a protective minimum support price policy (assured purchaseby the government). As long as the government supports a guaranteedpurchase of wheat and rice at guaranteed prices, the economic well-beingof Indian farmers in the IGP will remain dependent on the RW croppingsystem. To ensure food security to meet any eventuality of low production,


the Indian government maintains a buffer stock of 54 M tons of food grain.With recent legislation in the form of the National Food Security Bill, thegovernment of India aimed to provide access to food for all at an affordableprice and a buffer stock of 62 M tons will have to be maintained in thecountry.

With increasing pressure on natural resources, particularly land andwater, decreasing labor availability, changing economic and social obliga-tions, impact on environment of the current farming practices, increasingmechanization, etc., the RW system is under pressure to fulfill theincreasing food needs of the rising population of more than nine billionpeople forecast by 2050. There is little scope for further expansion ofarea under RW, an increase in cropping intensity, and reclamation ofdegraded soils. The productivity gains of the past have, in fact, sloweddown and there is an urgent need for a technological breakthrough tohalt this downward trend and sustain productivity in the region. Cropmanagement practices are changing fast. The region has experiencedincreasing farm mechanization over the last 20 years. Seedbed preparationand harvesting in particular are now commonly undertaken with farmmachinery.

This article has identified several key issues and opportunities relevant tocurrent production and future sustainability of RW that warrant furtherresearch efforts. Currently, the RW system faces a number of problemsthat are system-specific and even region-specific. Attempts to importtechnologies generated elsewhere and for other crops or systems have notfound favor. Since conventional breeding is not delivering the expectedgains in productivity, there is an urgent need to develop inbred varietiesor hybrids that suit conservation agriculture, are more competitive againstweeds, and have improved disease and pest resistance. Efforts are neededto improve nutrient-use efficiency and total factor productivity bymaintaining or improving soil health, making crop residue managementmore efficient, and improving water-use efficiency. With the rising costand non-availability of labor, mechanization will become even moreimportant in the future.

ACKNOWLEDGMENTS

The authors are grateful to Dr Bill Hardy, International Rice Research Institute, Philippines,for providing comments on the manuscript.

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