Rice (
Oryza sativa L.) is a major cereal crop is regarded as staple food by majority of world’s population and about 90% of the world’s rice is produced and consumed in Asia only
(FAO, 2014). In India, it is grown on an area of about 43.66 m ha with a total production of 118.87 m t during 2019-20
(Anonymous, 2020-21). More than half of the world’s population depends on rice for their daily sustenance
(Chauhan and Johnson 2011a) and it is the primary source of income and employment for more than 100 million households in Asia
(Singh et al., 2016). To meet the world’s increasing demand of rice in a sustainable way with shrinking natural resources is a great challenge. To ensure good crop establishment rice is predominantly grown by transplanting after puddling and kept flooded for most part of the growing season
(Singh et al., 2016) and it has been a major traditional method of rice establishment. Puddling adversely affects soil physical properties and requires a large amount of water, labour and more energy to achieve proper soil tilth for succeeding crops. Excessive pumping of water for puddling in most part of the rice growing areas, water table has been declining. Water scarcity in most of the rice growing areas of India and labour costs have increased dramatically due to migration of labours from rural areas to cities as well as non agricultural sectors of rural economy
(Chauhan, 2012a and
Singh et al., 2016). Production of rice with conventional transplanting method has been limited by a number of factors such as water scarcity, high input and labour costs, shortage of skilled labour resulting into low rice yield. In DSR system, dry rice seeds are sown with or without tillage and irrigation is applied periodically to maintain soil at field capacity.
Faster and easier planting, improve soil health, higher tolerance to water deficit, less methane emission and often higher profit in areas with an assured water supply
(Kumar and Ladha 2011). Weed competition reduced multiple rice yield components and weed biomass in wet-seeded rice was six-fold greater that in rice transplanted into puddled soil and twice as much again in dry-seeded rice sown either after dry tillage or without tillage
(Singh et al., 2011). Weed problem is sought to be addressed from two basic points of view: weed control and weed management. Weed control approach only emphasizes on reduction of weed pressure and in contrary the management approach focuses on keeping weed infestation at economic threshold level which are compatible with environmentally and economically sustainable production. Different weed control options are available for rice such as physical, chemical and biological control. Physical control is eco-friendly but labour intensive and delayed in weeding due to unavailability of sufficient labour in time. Biological control by using different bio-agents and mycoherbicides are practiced in irrigated lowland rice, but these may not be effective under aerobic soil conditions. Chemical control, on the contrary, is the most effective, economic and practical way of weed management
(Hussain et al., 2008, Anwar et al., 2012a).
Though, to overcome these challenges, direct seeding of rice (DSR) seems promising under several ecologies and production systems and is considered as viable and potential alternative to PTR (Puddled Transplanted Rice).
Yield losses due to weeds in DSR
Direct seeded rice (DSR) is a cost effective rice establishment method where dry seed are directly drilled into the unpuddled soil. It provides opportunities of saving irrigation water by 12-35%, labour up to 60% and provides higher net returns with at par yield than transplanted rice
(Kumar and Ladha, 2011). In addition to higher economic returns, DSR is shorter duration, less labour intensive, consume less water
(Bhushan et al., 2007) and have less methane emissions
(Wassmann et al., 2004). Despite multiple benefits of dry direct seeded rice, weed control remains one of the major challenges for its success in South Asia
(Kumar and Ladha, 2011; Rao et al., 2007; Singh et al., 2008). Weed control is more difficult in dry- DSR than conventional tillage (CT)- Transplanted rice (TPR) because weeds emerge with rice seedlings in dry–DSR which are less competitive than 30-35 days old rice seedlings use in CT- TPR and initial flooding used in CT-TPR helps in weed control but it is lacking in dry- DSR
(Kumar and Ladha, 2011).
Weeds adversely affected yield, quality and cost of production as a result of competition for various growth factors in DSR system. Extent of loss due to weeds may vary depending upon cultural methods, rice ecosystems, management practices, cultivars, weed species and their association, growth rate, their density and duration of crop-weed competition. The weeds do compete with crop for various growth factors,
viz. nutrients, moisture, light and space resulting into greatest loss
(Walia 2006). Globally, actual yield losses due to pests have been estimated ~40%, of which weeds caused the highest loss (32%)
(Rao et al., 2007). Yield reduction due to weeds is more critical in direct-seeded rice than in transplanted rice
(Karim et al., 2004). Red rice (
Oryza sativa f.sp.
spontanea) is highly competitive and causes severe rice yield losses ranging from 15% to 100%
(Farooq et al., 2009). On average, estimated losses from weeds in rice are around 10% of total grain yield; however, can be in the range of 30 to 90%, reduces grain quality and enhances the cost of production
(Rao et al., 2007). Season-long weed competition in direct- seeded aerobic rice may cause yield reduction up to 80%
(Sunil et al., 2010). On an average, yield loss, due to weed competition ranges from 15 to 20 per cent, but in severe cases it may exceed 50 per cent
(Hasanuzzaman et al., 2009). In case of heavy infestation and extreme cases, weed infestation may cause complete failure of aerobic rice
(Jayadeva et al., 2011). Thus direct-seeded aerobic rice is highly vulnerable to weeds compared with other rice ecosystems
(Anwar et al., 2011).
Critical period of crop-weed competition
The critical period for crop-weed competition refers to the period from sowing of crop upto which it has to be maintained in a weed free environment for remunerative crop production. Weed control during the critical period is essential to reduce the weed competition and for effective utilization of available resources for enhanced productivity and profitability. Due to alternate events of wetting and drying and emergence of weeds and rice simultaneously, crop-weed competition is more severe in DSR than in transplanted rice. The severity of competition depends not only on competing species but also on its density, duration and the fertility status of the soil. In DSR, it is important to minimize the crop-weed competition during the early stages of the crop before it forms a closed leaf canopy
(Singh 2008). In DSR, the critical period of weed competition has been reported to be 14- 41 days after sowing
(Chauhan and Johnson, 2011b).
Azmi et al., (2007) reported that critical period for weed control under mixed weed infestation in DSR was from 12 to 60 DAS. In aerobic rice cultivation, weed free condition during the initial crop growth period (upto 35 DAS) is critical which has the potential to reduce grain yield drastically.
Major weed flora in DSR
The DSR crop faces severe challenges from weeds and effective weed management is essential for successful rice production from DSR. About 350 species have been reported as weeds of rice, of which grasses are ranked as first posing serious problem followed by sedges and broad-leaf weeds causing major losses to rice production worldwide
(Singh et al., 2016). In DSR, fields are dominated by different grasses, sedges and broad leaf weeds (BLW) and weed flora will vary depending on the season, climatic and edaphic conditions. Among major grassy weeds
Echinochloa colona (L.),
Echinochloa crusgalli (L.),
Leptochloa chinensis (L.),
Oryza sativa L. f.
spontanea Roshev and
Ischaemum rugosum Salisb causes major yield loss.
Echinochloa colona and
E.
crusgalli are the most serious weeds affecting DSR. The density of these weeds in DSR depends upon moisture condition in the field.
E.
colona requires less water, so it is more abundant in DSR
(Singh et al., 2016). Sedges causing major yield losses are
Cyperus iria,
Cyperus difformis L.,
Schoenoplectus juncoides. Broad-leaves weeds (BLW) causing major yield losses are
Eclipta prostrata (L.),
Spheoclea zeylanica Gaertner and
Ludwigia hyssopifolia (G. Don.) Excell.
Raj et al., (2013b) reported that in Kuttanad, during
kharif season BLW predominate (39.3 per cent) followed by sedges (38.9 per cent) and grassy weeds (21.8 per cent) while in
rabi season sedges predominate (96.8 per cent) followed by BLW (2.7 per cent) and grasses (0.5 per cent). During the first 30 days after sowing, broad-leaf weeds dominated the grassy weeds and sedges, contributing more than 62% of the total weed population. At later stages, grasses dominated over non-grasses and sedges, contributing more than 90% of the total weed population at 75 DAS, at which
E.
colona alone contributed more than 80% of the total weed population at 60 DAS and beyond
(Singh 2008). Changes in crop establishment, from transplanting to direct seeding also resulted in marked changes in the composition of weed flora
(Singh et al., 2008). Direct seeding increased the population of annual grasses such as
Echinochloa crusgalli,
Echinochloa colona and
Leptochloa chinensis, perennial sedge
Cyperus rotundus and BLW such as
Commelina diffusa and
Caesulia axillaris (Singh et al., 2008). Adoption of direct seeding technology may result in weed flora shifts towards more difficult to control and competitive grasses and sedges
(Kumar and Ladha, 2011).
Weed management strategies in DSR
Weed prevention methods
This method restricts introduction and spread of weeds and is the most basic of all weed control methods
(Buhler, 2002). It includes using weed-free seeds, maintaining clean fields, borders and irrigation canals and cleaning farm equipments
(Datta and Baltazar, 1996). Rice seeds contaminated with weed seeds may introduce problematic weed species to a new field and enrich the soil weed seed bank. In addition to clean crop seed, the machinery used for tillage, sowing, harvesting, or threshing operations should also be cleaned before moving it from one field to another. Bunds and irrigation canals free from weeds may also help to reduce the spread of weed seeds through irrigation water
(Hossain et al., 2016).
Cultural methods
Cultural practices play significant role to determine the crop-weeds competition for above ground and below ground resources and hence influence weed management
(Grichar et al., 2004 and
Dass et al., 2016). Some cultural practices like selection of cultivars, stale seedbed technique, tillage, seeding density, crop residue, crop rotation, intercropping, cover crops, fertilizer management and water management etc to be used in DSR could be a good alternative to control weeds.
(A) Land leveling
Land levelling is essential to achieve uniform crop stand before sowing. Levelling is usually done by using a wooden or metal board. However, laser land levelling ensures better crop establishment
(Jat et al., 2009), precise water control and increased herbicide use efficiency
(Chauhan, 2012a). Laser land levelling reduces the weed population by up to 40 per cent and the labor requirement for weeding by 75 per cent
(Rickman, 2002) and 86.7 per cent compared to traditional method of land levelling
(Banerjee, 2015).
(B) Selection of cultivars
Cultivars to be used in DSR could be a good alternative to control weeds. Rice cultivar having strong weed competitiveness is deemed to be a low cost safe tool for weed management in DSR
(Gibson and Fischer, 2004). In general, tall and traditional cultivars with high tillering ability, high early growth rate, high leaf area index and specific leaf area, long leaves and droopy plant type are more weed suppression ability than short-statured early maturing, modern cultivars with erect leaves Early maturing rice cultivars and rice hybrids also have a smothering effect on weeds due to improved vigour and having the tendency of early canopy cover
(Mahajan et al., 2011). Rice hybrids are more competitive and usually have better vigor than inbreds and effectively suppressed the infestation of
Echinochloa spp. and helped reduce herbicide dependency
(Gibson et al., 2001). Rice cultivars having good mechanical strength in the coleoptiles to facilitate early emergence of the seedlings under crust conditions (generally formed after light rains), early seedling vigour for weed competitiveness
(Zhao et al., 2006), higher tiller number
(Fischer et al., 1997), rapid canopy ground cover
(Lotz et al., 1995) and yield stability over planting times are desirable traits for DSR. Rice cultivars having allelopathic effect can contribute to weed suppression.
Kumar et al., (2013) reported that variety Gautam was highly competitive in suppressing the
Echinochloa spp compared to the Prabhat and Krishna Hamsa.
(C) Stale seedbed technique
Stale seedbed technique is an important cultural practice and a method of weed control where weeds are allowed to germinate and the emerged weed seedlings are killed by shallow tillage or using a non selective herbicide that can be used to reduce the weed seed bank. Those weed species that have low initial dormancy and are present in the top soil layers are more conducive to be controlled by this practice, such as
Eclipta prostrate,
Leptochloa chinensis,
Ludwigia hyssopifolia and
Digitaria ciliaris. The use of the stale seedbed practice could also help to reduce the problems of some hard weeds, such as
Cyperus rotundus, weedy rice and volunteer rice seedlings
(Chauhan, 2012).
Singh et al., 2009 reported 53% lower density in Dry- DSR after a stale seed bed than without this practice. Stale seedbed combined with herbicide (paraquat) and zero-till results in better weed control of
Cyperus iria,
Cyperus difformis,
Fimbristylis miliacea (L.) Vahl,
Leptochloa chinensis and
Eclipta prostrata because of low seed dormancy of weeds and their inability to emerge from a depth greater than 1 cm
(Chauhan and Johnson, 2008; Chauhan and Johnson 2010).
(D) Tillage
Tillage can affect weed species through the changes and disturbances in weed seed distribution in the soil. Primary tillage can reduce annual weed species, especially when planting is delayed to allow weed seeds to emerge before final tillage
(Buhler and Gunsolus, 1996). On the other hand, zero tillage favors weed infestation. Conservation tillage has been criticized particularly in relation to lower yields and perennial weed problems which results in an increase in herbicide application
(Singh et al., 2011). Zero tillage DSR had more equivalent yield than DSR CT. Another study recorded that the direct-seeded ZT gave at par yield as compared with transplanted (TP) rice
(Singh et al., 2008). Shallow tillage before crop emergence and post plant tillage after crop establishment help to remove annual weeds and inhibit the growth of perennial weeds
(Buhler, 2002).
(E) Seed rate and spacing
Seed rate and spacing influence the weed population. Higher seeding rate of rice has been advocated not only for weed control but also for avoiding higher risk of poor seedling establishment
(Anwar et al., 2011). Higher seeding rates would be beneficial if no weed control is planned or if only partial weed control is expected. A quadratic model predicted that seeding rates of 48- 80 kg/ha for the inbred varieties and 47-67 kg/ha for the hybrid varieties were needed to achieve maximum grain yield when grown in the absence of weeds, while rates of 95-125 kg seed/ha for the inbred varieties and 83-92 kg seed/ha for the hybrid varieties were needed to achieve maximum yields in competition with weeds. Such high seeding rates may be prohibitive when using expensive seed and maximum yields are not the only consideration for developing recommendations for optimizing economic returns for farmers
(Chauhan et al., 2011). Weeds grown in wider rows may have greater biomass than weeds grown in narrow rows. Therefore, a direct-seeded crop should be grown using narrow row spacing to obtain faster canopy closure and less penetration of light and ultimately less weed growth
(Chauhan, 2012b).
(F) Crop residue and Sesbania co-culture (Brown Manuring)
Crop residues have a chemical and physical effect on the growth of crops and weeds by reducing sunlight and modifying soil temperature. Seedlings of many weed species can be suppressed by using crop residue as mulches
(Chauhan, 2012b). Growing
sesbania as a green manure with rice is called brown manuring (
sesbania co-culture). After 25-30 days of growth
sesbania is killed with 2, 4-D ester at 0.5 kg ha
-1. This co-culture technology can reduce the weed population by nearly half without any adverse effect on rice yield
(Singh et al., 2007 and
Dhyani et al., 2007). These residues may not only help in suppressing weed emergence but also add fertility to the soil.
Sesbania co-culture was more effective against BLW and sedges and less effective on grasses. So, it is recommended to use pendimethalin as pre-emergence to overcome the problem of grass weeds in this technique
(Kumar and Ladha, 2011).
(G) Seed priming and seed quality
Quality seeds which are free from any contaminants might be an important approach to manage weeds in DSR systems. A robust seedling stand obtained from primed seeds enhanced rice competitiveness against weeds
(Anwar et al., 2012). Juraimi et al., (2012) reported that priming treatments also produced the most vigorous seedlings with 50 per cent more vigour index compared to unprimed seeds. Higher and synchronized emergence of primed seeds can ensure vigorous crop stand with rapid canopy development giving rice plants a preliminary advantage over weeds
(Anwar et al., 2012). Relative yield loss was reduced by around 10 per cent due to priming
(Juraimi et al., 2012). This was mainly due to the fact that priming reduces the risk of poor stand establishment and crop losses due to weeds.
(H) Crop rotation
Crop rotation considered as a vital tool of weed management and helps in breaking the weed seed cycle as well as facilitating the identification of weedy rice and it leads to better control. Effective control of weedy rice can be obtained by rotating the rice crop with other crops, such as soybean, mungbean, cotton, maize,
etc., which allow using other herbicides and cultural practices that cannot be used in rice
(Singh et al., 2013). Planting mung bean in dry season in Northern India also reduced weed growth and weeding time and increased herbicide performance
(Mahajan et al., 2012).
(I) Intercropping and cover crops
Intercropping can reduce both weed density and biomass to a great extent due to decreased light transmission through the canopy Intercropping for 30 days were found effective in controlling weeds in DSR
(Singh et al., 2007). Incorporation of cover crop into the soil may add allelo-chemicals to the soil to prevent germination and establishment of weeds and decreased light transmission to the ground
(Buhler, 2002). Intercropping with
Sesbania for 30 days were found effective in controlling weeds in DSR
(Singh et al., 2007a).
(J) Fertilizer management
Fertilizer management can definitely alter the competitive balance between crops and weeds and should be aimed at maximizing nutrient uptake by crop and minimizing nutrient availability to weeds. Manipulation of crop fertilization is a promising approach to reduce weed infestation and may contribute to long-term weed management
(Blackshaw et al., 2004). Weeds become less competitive when N was applied at early growth stages of crop compared with later application and weeds are found to be more responsive to added N than that of crop
(Blackshaw et al., 2000 and
Buhler, 2002). When weeds were controlled, rice crop responded to higher amount of N application but under weedy and partially-weedy conditions, grain yield reduced drastically with higher amount of N fertilization
(Mahajan and Timsina, 2011).
(K) Water management
Every weed species has an optimum soil moisture level, below or above which its growth is hampered and therefore time, depth and duration of flooding could play an important role in suppressing weeds in DSR system. Good water management together with chemical weed control offers an unusual opportunity for conserving moisture and lowering the cost of rice production
(Rao et al., 2007).
Physical methods
Manually or mechanically control of weeds are called physical control of weeds. Hand weeding is very easy and environment-friendly but tedious and highly labor intensive and thus is not an economical for the farmers
(Juraimi et al., 2013). Mechanical weeding using hand pushed weeders is feasible only where rice is planted in rows; however, weeds emerging within rows are difficult to remove with these weeders
(Chauhan, 2012b).
Chemical methods
Due to scarcity of labour during the critical period of weed competition, high labour cost and unfavourable weather at weeding time manual and mechanical methods could not find much place among farmers and not economical. Under such situation, most effective, economical and practical way of weed management is the use of weedicides
(Hussain, 2008). Herbicides may be considered to be a viable alternative to hand weeding due to scarcity and high wages of labour
(Chauhan and Johnson, 2011a; Anwar et al., 2012a). Singh et al., (2004) reported that a ready mix formulation of metsulfuron methyl + chlorimuron ethyl was very effective against diverse weed flora.
Singh et al., (2010) found effective control over the density of
C.
rotundus with the application of azimsulfuron + Metsulfuron-methyl. Application of penoxsulam at 20, 22.5 and 25 g/ha have better control over the density of grasses and broad-leaf weeds in DSR
(Singh et al., 2012). Lowest population of
E.
colona was recorded with application of pendimethalin at 2.0 kg while of
C.
axillaris was with combined application of bentazone with pendamethalin
(Singh et al., 2005). Singh et al., (2016) also reported that treatments with pendimethalin PRE fb bispyribac-sodium + azimsulfuron POST had lower weed biomass at 45 days after sowing (DAS). Highest grain yield of scented basmati rice (3.43 t ha
-1) was recorded with the sequential application of pendimethalin PRE fb bispyribac-sodium + azimsulfuron POST. Applying oxadiargyl by mixing with sand onto flooded field was less effective than spray applications in non-flooded field. Other herbicides that are found effective in DSR are pyrazosulfuron and oxadiragyl as pre-emergence and azimsulfuron, penoxsulam, cyhalopfop-butyl and ethoxysulfuron as post-emergence
(Rao et al., 2007).
Due to narrow spectrum activity of single herbicides these are seldom furnishes satisfactory and season long weed control. Herbicides with different mode of action when mixed together, bind to different target sites in weeds and prevent the probability of target site resistance in susceptible species
(Paswan et al., 2012). Chauhan and Yadav (2013) opined that in future, the combination of two or more herbicides may become a part of an effective and integrated approach to achieve more satisfactory control of complex weed flora in DSR.
Lap et al., (2013) revealed that combination products containing penoxsulam and cyhalofop butyl increased the rice productivity in direct seeded systems.
Raj et al., (2013a) reported that the application of bispyribac sodium + metamifop 14 per cent SE @ 70 g ha
-1 + PIW -111 wetter, 10 -15 DAS resulted in enhanced rice yield in wet DSR.
Biological methods
It involves utilization of natural living organisms such as insects, pathogens and competitive plants to limit the weed infestation. Different herbivorous bio-agents such as fish, tadpoles, shrimps ducks and pigs are used to control weeds in irrigated lowland rice but these cannot be used in the area where there is no standing water. The main objective of the biological control is reduction and regulation of the weed population below the level of economic injury. Weed control by micro herbicides are now being studied to reduce herbicide dependency. COLLEGO, a powder formulation of
Colletotrichum gloeosporioides (Penz.) Sacc.
f.
sp.
Aeschynomene for the control of
Aeschynomene virginica (L.) in rice. The endemic fungus
Colletotrichum gloeosporioides f.
sp.
jussiaeae (C.g.j.) controlled >80 per cent of water primrose in rice after four weeks
(Boyette et al., 1979). Thi et al., 1999 reported that myco-herbicides using most promising fungi
e.
g.
Exserohilum monocerus and
Cocholiobolus lunatas found to control barnyard grass.
Setosphaeria sp. and
C.
rostrata were also found to control
Leptochloa chinensis effectively without causing any damage to rice plant.
Integrated weed management
Various agronomic tools have been evaluated for managing weeds. But, all the agronomic tools may not work perfectly with every crop or weed species. Integration of higher seed rate and spring-applied fertilizer in conjunction with limited herbicide use managed weeds efficiently and maintained high yields
(Blackshaw et al., 2005). Adoption of IWM approach for sustainable rice production has been advocated by many researchers
(Sunil et al., 2010, Jayadeva et al., 2011). Sequential application of pre-emergence herbicides such as pendimethalin, in dry seeded rice or early post-emergence application of anilofos/thiobencarb for the control of annual grasses in wet-seeded rice and post-emergence application of 2, 4-D against sedges and non-grassy weeds in wet and dry-seeded rice may be a better option than the use of one herbicide.
