Rice is the staple food for more than half of the world population. Asia accounts for 90% of the world’s total rice area and production. In India, rice is grown on approximately 45 million ha annually with a production of 104 million tonnes. India’s rice demand is estimated to rise to 122 million tonnes in 2025, which is equivalent to an overall increase of 22% in the next 10 years. But, the current evidence shows declining factor productivity and a plateau in rice yields due to fatigued natural resources, declining water table, increasing labour scarcity and energy shortage, escalating fuel prices and changing climatic conditions. Therefore, yield of gains have to be achieved by using less water, labour, land and energy.
In India, rice is most commonly grown by transplanting rice seedlings into puddled soil (wet-tillage). Both transplanting and puddling, however, require a large amount of water, labour and energy. All these resources are becoming increasingly scarce, making rice production more expensive and less profitable. Therefore, interest has increased in shifting from puddled transplanted rice to direct seeded rice (DSR). DSR is more rapidly and easily planted, less labour intensive, consumes less water, matures 7 to 10 days earlier and has less methane emissions. Despite these benefits, one of the major threats associated with the introduction of DSR is the evolution of weedy rice, one of the most difficult-to-control weed species of rice in the world
(Azmi et al., 2000).
Weedy rice
Weedy rice is one of the most persistent and noxious weeds found in rice-growing ecosystems worldwide
(Cao et al., 2007). It can be defined as a weedy population of genus
Oryza.
(1) It is similar to cultivated rice but has greater seed dormancy and longevity, high susceptibility to seed shattering, red pigmentation of pericarp.
(2) Grows unintentionally and vigorously in and around cultivated rice.
(3) Very diverse phenotypically and genotypically.
(4) Highly competitive and difficult to control.
(5) Reduces farmers’ income by reducing grain yield, quality and profit.
Weedy rice known as
O.
sativa f.
spontanea is the result of mutation or natural crosses between cultivated species of
O.
sativa with the annual
O.
nivara/O.
rufipogon (Gupta and Upadhyay, 2000). Weedy rice is known by different names in different countries. Examples are “Padi Angin” in Malaysia, “Lua Lon” in Vietnam, “Lutao” in China, “Akamai” in Japan, “Sharei” in Korea, “Khao Pa” in Laos, “Khao Nok” in Thailand and “Jhora Dhan” in Bangladesh
(David Johnson, 2011).
Origin and distribution
It is widely hypothesized, that weedy rice may have a variety of origins
(Vongsaroj, 2000). Weedy rice is a complex of
Oryza morphotypes widely distributed in the commercial rice fields in more than 50 countries of Asia, Africa, Latin America and South east Asia, epecially in areas where farmers have switched to direct seeding. The Indian weedy rice belongs to indica group. In India
Oryza sativa f.
spontanea is considered as weedy species in cultivated rice. Its problem in Eastern India (Eastern U.P., Bihar, Orissa, Manipur and West Bengal) and Southern India (Kerala, Karnataka and AP)
(Abraham et al., 2012).
Botany of weedy rice
Common name : Weedy rice
Scientific name :
Oryza sp
Family : Poaceae
Weedy rice has variable, erect, stout or slender annual rice with varying height and form.
Stem
Tufted, erect, hollow and slender or stout, smooth and hairless, 80-120-cm tall.
Leaf
Blade flat with parallel veins, 15-30 cm long. Ligule and auricle present; Ligule usually 10-20 mm long.
Inflorescence
An erect or nodding loose panicle, spikelet about 7-mm-long, flat, one-flowered, with or without awns of varying length.
Weedy rice characters
Weedy rice is variable in height, panicle form, grain size, awn length, flag leaf and growth duration. Weedy rice can be a serious problem because it is very competitive with cultivated rice, its seeds have dormancy and its grains shatter easily. Weedy rice tends to have vigorous vegetative growth, comparatively early maturity, easy shattering and, in some variants, the grains have long awns and the grain (preicarps) color after milling is red
(David Johnson, 2011).
Weedy rice has variable seed dormancy, but commonly a large proportion of the seeds will germinate within 3 months of shedding. This can lead to severe problems in a subsequent crop where rice is double-cropped
(David Johnson, 2011). They come to flowering much earlier than cultivated rice and produce grains having awns, with varying coloration of ear head. The seeds mature within a short period and shatter immediately facilitating the buildup of weed seed bank before the farmer gets a chance to remove the seeds along with the harvest of rice crop
(Abraham et al., 2012). Weedy rice is taller than common rice, grain has awns, grain shatters early, grain has a dark pericarp color, plants flower early and grains are smaller
(Mai et al., 2000).
Weedy rice grows taller than cultivated rice and it has longer and wider leaves. Weedy rice reached a height of 140-150 cm, whereas cultivated rice grew to 115-120 cm. Because it is tall, weedy rice lodged and its panicles drooped and it germinated and emerged at the same time as cultivated rice, but it flowered earlier and its seeds matured 4-5 days earlier than did those of cultivated rice. The grains shattered at or even before maturity. Weedy rice also produced fewer leaves, tillers, panicles per plant and grains per panicle than cultivated rice. The grains were smaller and the pericarp shorter than those of cultivated rice
(Ramachandiran and Balasubramanian, 2012). Currently the wide spread incidence of weedy rice in the rice wing tracts of Kerala has become a major threat to the rice cultivation in the state. A survey conducted by AICRP on weed control in the rice growing tracts of Kole lands, Thrissur and Kuttanad of Alleppy and Kottayam districts revealed the presence of different biotypes of weedy rice variation in height from (70 cm to 130 cm), tiller number (1 to 32), panicle number (1 to 12), awn length (1.8 to 6.6) and seed colour varied from golden yellow, brown and black. The initial germination (just after harvest) ranged from 8-12%. Above 50% of the seed germinated after 5 months of storage, 5-10% seeds germinated during the intervening period
(Girija et al., 2010). Chin (2001) reported that one major feature of weedy rice is its shattering trait. This would result in higher population densities in the soil seed bank, thus rendering control more difficult. The other major characteristics of weedy rice are tallness, low tillering ability and a high percentage of red rice (weedy rice) and unfilled grains.
Dispersal of weedy rice
Weedy rice spreads rapidly from infested fields to new non-infested areas. Knowledge of the sources responsible for the dispersal of weedy rice can help in preventing its spread to non-infested areas. The use of contaminated weedy rice seeds is the most important source of its spread to new areas. The use of weedy rice-contaminated agricultural equipment/machinery also plays a vital role in its dispersal
(Olguin et al., 2009). It is therefore important to use weedy rice-free certified seeds and machinery should be cleaned before moving it to new areas to prevent weedy rice spread. In addition, weedy rice can be dispersed from one field to another through irrigation channels or irrigation water, heavy winds or storms and flooding.
Extent of infestation
Weedy rice was first documented in USA as early as 1846 and has continued to affect the rice production areas. Weedy rice is a superior competitor to crop cultivars due to its early vigour, greater tillering and height of plants. Yield losses largely depend on season, weed species, weed density, rice cultivar, growth rate and density of weeds and rice. Weedy rice at 35% infestation caused about a 60% yield loss and, under serious infestation, yield loss of 74% was recorded in direct seeded rice
(Mispan and Baki, 2008). Yield of weedy rice infested plots at the rate of 10, 100 and 1,000 weedy seeds per square meter were 4.05, 2.75 and 0.43t ha
-1, respectively, compared to the check yield of 4.53 t ha
-1 (Chin and Mortimer, 2002). Short varieties are usually more susceptible to weedy rice competition than tall ones and interference duration is also a yield deciding factor
(Shivrain et al., 2009). Weedy rice usually coexisted with cultivated rice and was highly competitive in rice fields of China
(Xia et al., 2011). According to
Chauhan (2012), the important weedy traits of weedy rice are early shattering of seeds, variable seed dormancy, high seed persistence in soil and high nitrogen use efficiency.
Weedy rice infestation across the world
The infestation has spread across the globe and is now a problem particularly to America, the Carribean, South Asia and South east Asia. There are many reports on weedy rice infestation although per cent area infested varies. Infestation of weedy rice has also been reported in China, Thailand, Sri Lanka, Vietnam, the Philippines and in many states of India. An yield loss of 74% in direct seeded rice has been reported in Malaysia. Weedy rice densities of 35-40 plants/m2 can reduce yields of tall rice cultivars by 60% and short cultivars by 90%, indicating losses greater than grass weeds
(Rathore et al., 2013).
Weedy rice infestation across the India
The problem has now invaded India with an infestation of 5-60% across different states. A paddy field in
Kharif 2012 of Jabalpur district, Madhya Pradesh infested with weedy rice where labours are engaged in hand weeding to remove the same. In 2009, agronomists from West Bengal also detected and revealed their concerns regarding weedy rice to the State authorities. Rice agricultural fields in Kerala were found to have weedy rice as a major problem
(Abraham et al., 2012). Heavy infestation in the fields of Kerala during recent years has caused a reduction in yield by 30-60% depending on severity of infestation (3-10 mature plants of weedy rice/m
2).
Recently, Jharkhand recorded 24-32% infestation of weedy rice across Ranchi, Khunti and East Singhbhum areas with as estimated yield loss of 10-45%. Heavy infestation of weedy rice is seen in eastern and southern India where direct seeding of rice is common, though it is yet not a problem in Haryana and Punjab where paddy is cultivated through transplanting method
(Rathore et al., 2013).
Management strategy of weedy rice
Weedy rice is very difficult to control with a single method and an integrated weed management (IWM) strategy is needed for its effective control in DSR systems
(Singh et al., 2013). IWM consists of preventive, cultural, chemical, mechanical and biotechnological approaches.
Preventive methods
Prevention is one of the most important steps that can be implemented immediately to minimize the infestation of weedy rice in DSR fields.
Use of weedy rice-free seeds
It is one of the best preventive measures to control weedy rice. The use of self-saved rice seeds is considered one of the major causes of the spread of weedy rice to new areas
(Abraham, 2012). Seed replacement and the selection of pure, good-quality certified seed is the best method to prevent the introduction or dissemination of weedy rice in new areas
(Kathiresan and Vishnudevi, 2021).
Use of clean agricultural implements
Agricultural implements can disperse weedy rice seeds from one area to another and proper cleaning of harvesting/ sowing equipment prior to its use in new areas can prevent the spread of weedy rice seeds
(Chauhan, 2013).
Increasing awareness for farmers
There is a need to increase awareness for farmers about the risk imposed by weedy rice
(Nadir et al., 2017). Closer watch on the species in new areas is needed to avoid its invasion and such plants should be rogued out upon their initial appearance in the field
(Svizzero, 2021).
Cultural methods
Cultural strategy of weedy rice control includes the use of weed suppressing varieties and submergence tolerant varieties. Tall and long cycle varieties usually have shown a greater competitiveness than modern early and semi dwarf varieties
(Marambe, 2009). Non chemical means of weed control in rice should be centered on land preparation, varietal selection of crop, water management and fertilizer management
(Svizzero, 2021).
California Department of Food and Agriculture (2001) reported that in the absence of effective selective post emergence chemical control, techniques to minimize weedy rice infestations necessarily focused on
(1) Lowering the chance of emergence of weedy rice seedlings at crop establishment.
(2) Preventing subsequent seed return to the soil from surviving plants at maturity.
Stale seedbed technique
The field is irrigated and left unsown for two to three weeks to allow weedy rice seeds to germinate and then emerged weedy rice seedlings are killed by either a non-selective herbicide (paraquat or glyphosate) or by shallow tillage prior to rice sowing
(Nadir et al., 2017). The duration of the stale seedbed must be long enough to allow the weedy rice seedlings to grow up to 2 to 3 leaf stage and must not delay the sowing of the main crop. This technique not only reduces the weedy rice population in the crop but also decreases the weedy rice seed bank in the soil
(Marambe, 2009). The rice seed should be sown with minimum disturbance to avoid bringing new seeds buried at deeper depth to the upper germination zone of the soil and also to avoid exposure of weedy rice seeds to light and other stimuli that encourage germination and emergence of weedy rice
(Kathiresan and vishnudevi, 2021).
According to
Azmi and Johnson, (2001) the success of the stale seed bed method depended on the way the soil is prepared, the water management and its duration. Wet tillage after weed germination destroyed weedy rice seedlings and promoted new emergence.
Sindhu et al., (2011) pointed out that soil flooding during the application of the stale seed bed reduced emergence from the soil in comparison to dry or moist soil, but favoured the evenness of the germination that in turn made the control easier. The duration of this technique in temperate climate conditions should be about 25-30 days.
Soil solarisation
Soil solarisation was reported as an advanced non chemical field technology for weed management
(Marambe, 2009). The process significantly increased the soil temperature to 10-15°C above the normal temperature. This technique was practiced in the warmest months for a duration of 4-6 weeks using thin transparent polyethylene films of 19-25 micro meter. The efficiency of soil solarisation on the control of weed seed bank was reported by
Abraham and Jose (2014).
Tillage
Tillage plays an important role in controlling weedy rice populations in DSR. Weedy rice seeds emerge from shallow soil depths; weed seeds present in the upper soil layer can be buried deep by deep ploughing
(Mansor et al., 2012). Under zero-till conditions, weedy rice seeds present in upper soil layers can be depleted before rice crop sowing by employing the stale seedbed technique. Residue should be retained on the soil surface as mulch under zero-till conditions can help in reducing weedy rice recruitment
(Nadir et al., 2017). In different direct seeding systems, tillage is performed in dry or wet soil conditions. In dry-seeded rice systems, soil is cultivated in dry conditions, whereas, in wet-seeded rice, soil is cultivated in wet conditions
(Kathiresan and vishnudevi, 2021). Through land preparation has been well known to control weeds
(Marambe, 2009). Repeated tillage operations may help reduce the seed bank and seedlings of weedy rice in rice fields. Weedy rice seedlings that emerge after cultivation can be killed by subsequent cultivation operations
(Svizzero, 2021).
Different tillage operations could be used to bury weedy rice seeds deep so that they cannot emerge from that depth. Most weedy rice seedlings emerge from the top 6-8 cm of soil burial depths. Such information indicates that weedy rice emergence could be suppressed by deep tillage that buries seed below 8 cm
(Saha et al., 2014). In such situations, however, subsequent tillage operations should be shallow in the next few seasons to avoid bringing back the buried and viable seed to the soil surface. Seed brought to the soil surface may reinvest the field.
Selection of rice cultivars
In any crop or seeding system, weed-competitive cultivars should be involved as a tool in integrated weed management strategies
(Mansor et al., 2012). Tall cultivars are usually more competitive against weeds, but they have lower yield potential than short-statured modern cultivars. In addition, tall cultivars may not be used widely in the future
(Svizzero, 2021). Because of the ever-increasing population, there will be more pressure in future farming to produce more rice on less land. In such situations, growers may need to increase nitrogen fertilizer rates and high nitrogen is known to cause crop lodging
(Wongtamee et al., 2017). Overall, information on the effect of weed-competitive cultivars on weedy rice is very limited in Asia. In the absence of such information, the use of cultivars with the traits of early vigor and quick canopy closure may help suppress weedy rice growth
(Saha et al., 2014). Short-duration cultivars that mature earlier than weedy rice may also help in reducing the weedy rice seed bank. Selecting cultivars with vigorous initial growth plays an important role in suppressing weedy rice
(Marambe, 2009). In Malaysia, cultivars having tall stature, more tillering capacity, more vegetative growth in the initial crop growth phase and early maturity reduced the weedy rice population. Coloured-stem cultivars are effective in identifying weedy rice at an early stage and thus in its management. In eastern and southern India, coloured-leaf (purple) rice cultivars help in differentiating cultivated rice from weedy rice
(Kathiresan and vishnudevi, 2021). Similarly, farmers in Himachal Pradesh adopted purple-leaf rice cultivars in weedy rice- infested fields.
High crop density and row-seeded crops
A dense crop may help to suppress weeds in rice. Growers in many regions in Asia use high seeding rates to reduce weeds and to compensate for poor seed quality and crop establish-ment. Growers also increase seeding rates to compensate for losses due to birds, rodents, insects, nematodes and snails
(Saha et al., 2014). In Asia, growers use high seeding rates (up to 150 kg seed ha
-1) mainly in a broadcast rice crop but, in other parts of the world (e.g., South America), high seeding rates are also used in a mechanized row-seeded rice crop. The use of high seeding rates in weedy rice-infested areas, for example, in Malaysia, helped to reduce the problem of weedy rice
(Chauhan, 2013). High seeding rates may not increase rice yield in weed-free environments, but may help to reduce yield losses due to weed infestation in weedy or partially weedy environments. Recent studies in the Philippines suggested that increasing rice crop interference could substantially reduce tiller and leaf numbers, leaf area and shoot biomass of different weedy rice biotypes
(Marambe, 2009). Nonetheless, the surviving weedy rice plants may produce enough seed to cause infestation in subsequent cropping seasons. Therefore, there is a need to combine other weed management approaches with the use of high seeding rates
(Chauhan, 2013).
Water management
Water management can play an important role in weedy rice control. Early flooding 20-30 days before land preparation would help to control red rice
(Espinoza et al., 2005). After seeding of pre germinated seeds, water management was critical to successfully suppress weedy rice. There were two management strategies for irrigation after seeding:
(1) Water was maintained at a depth of 5-10 cm until drainage at harvest (continuous flooding).
(2) Water was drained, soil is kept saturated for 3-5 days and flooding is returned gradually. Excessive drainage exposed the soil to air and increased oxygen concentration in the soil, thus stimulating weedy rice germination.
Azmi et al., (2000) reported that farmers resorting to transplanting rice culture in weedy rice infested areas had minimal or no recurrent problems with weeds. Puddling combined with the presence of a thin layer of water over the well levelled soil maintained the anaerobic conditions in the top soil and prevented weedy plants from becoming established.
Ferrero (2001) have also found that flooding in well levelled soils limited weedy rice germination. He reported that combination of water seeding and the use of weedy rice free seeds had led to the virtual disappearance of the weed in California.
Water seeding
Rice plants require oxygen, moisture and optimum temperature for germination and one of these missing may cause a reduction in rice germination
(Chauhan, 2013). In the water-seeding technique, pre-germinated rice seeds are shown on well-levelled flat soils having minimum residue with clear standing water by the broadcasting method to decrease the weedy rice population
(Ajaykumar and Sharmili, 2020). Seedling broadcasting is also an innovative technology adopted in some DSR-growing countries, such as Vietnam, China and Sri Lanka, for controlling weedy rice where 12-15-day-old seedlings are broadcast on 5-7 cm of standing water and this layer of water controls the emergence of weedy rice
(de Avila et al., 2021).
Shifting from direct seeding to transplanted culture
Transplanting is an established and efficient method for controlling weedy rice. Alternating transplanting (preferably, mechanical transplanting) with DSR after 3-4 years of DSR can minimize the problem of weedy rice and help in reducing the weedy rice seed bank in the soil
(de Avila et al., 2021). Transplanting also helps to improve competitiveness against weedy rice germination, establishment and growth.
In transplanted systems, rice seedlings are more competitive against the newly emerged weedy rice seedlings and it is easy to distinguish weedy rice seedlings from cultivated rice seedlings
(Saha et al., 2014). Standing water present in the field at the time of transplanting also helps to suppress the emergence and growth of weedy rice seedlings. Growers in many countries, such as Korea, Malaysia and Vietnam, have been successful in reducing weedy rice infestation by introducing transplanted rice. In many Asian countries, however, there is a need to develop “custom-hiring” systems as many growers cannot afford to buy transplanters
(de Avila et al., 2021). In Sri Lanka and Vietnam, seedling broadcasting is practiced in the absence of availability of effective transplanters
(Mansor et al., 2012). In this method, seedlings are grown in a nursery and thrown / broadcasted randomly into puddled soil. Standing water in the field at the time of seedling broadcast helps to suppress weedy rice. In Malaysia, too, seedling broadcasting was found effective in reducing weedy rice infestation.
Hand weeding/roguing
The control of weedy rice plants is sometimes carried out manually, but this practice is costly and time consuming
(Mansor et al., 2012). Hand weeding is quite impractical up to 30-40 days after crop emergence as it is very difficult to distinguish the cultivated varieties from the weedy rice in the early stages
(Chauhan, 2013). Hand weeding of weedy rice plants can sometimes carried out for light infestations and frequently it is used together with other means of control (chemical) when the latter has given poor results, so as to avoid grain dispersal and also in seed production plots.
Crop rotation
Continuous growing of rice with similar management practices allows weedy rice to become dominant in the cropping system
(Saha et al., 2014). Crops with different management practices, however, may help in disrupting the growth cycle of weedy rice. Therefore, a crop rotation is considered as the most effective control measure for weedy rice and other problematic weed species
(Ajaykumar and Sharmili, 2020). The cropping system was also considered to be a very important factor influencing weedy rice infestation Based on the interviews, cultivation of upland crops seemed to contribute to control of weedy rice infestation. Farmers who introduced mungbean cropping in the dry season found that weedy rice decreased a few years after serious infestation. Multiple cropping systems were found to be effective in reducing the weedy rice population in this region
(Watanabe, 2000).
Mechanical methods
Inter-cultivation using mechanical weeders
Mechanical weeding is possible in row-seeded rice
(Singh et al., 2013). Mechanical weeders, such as the cono-weeder and inter-cultivation implements (tractor-drawn, bullock-drawn, or manual) remove weeds from between the rows
(Saha et al., 2014). Line sowing also reduces seed cost as a lower seed rate is used when seeds are drilled than when seeds are broadcasted.
Chopping
Chopping is applicable for controlling weedy rice plants, which are taller than cultivated rice
(Chauhan, 2013). The panicles are chopped from the weedy rice plants before seed setting takes place
(Ajaykumar and Sharmili, 2020). In many countries, weedy rice panicles are cut with the help of a machete or a special knife attached to a stick
(Singh et al., 2013). In developed countries, a combine harvester cutting device is mounted on the front of the tractor and used to cut the weedy rice panicles
(de Avila et al., 2021).
Chemical control of weedy rice
Herbicide based weed management is generally the most popular method for weed control in the direct seeded rice fields
(Braun et al., 2019). However, it is very difficult to control weedy rice by the use of selective herbicides because weedy rice was essentially the same biological species as cultivated rice
(Mansor et al., 2012). Close anatomical and physiological similarity of weedy rice to the crop made selective post emergence herbicidal control of weedy rice plants very difficult
(Chen et al., 2004).
Use of herbicides before sowing
Use of antigerminative herbicides, such as metolachlor at 3.5 kg ai ha
-1, alachlor at 3.5 kg ai ha
-1, applied in soybean as pre emergence resulted in weedy rice control of about 90 percent
(Espinoza et al., 2005). Ferrero (2001) could obtain good control of weedy rice (often higher than 75 percent) in European rice conditions with pretilachlor and dimethenamid used alone or in combination at 1.5 kg ai ha
-1 and 0.48 kg ha
-1, respectively. To avoid any phytotoxicity risks, both herbicides need to be applied at least 25 days before rice planting. Pre-plant incorporation of thiocarbamate herbicides like molinate and butylatealso controlled weedy plants
(Rao et al., 2007).
Jose et al., (2012) found that weedy rice was completely controlled by thiobencarb at 2.1 kg ha
-1 and oxadiazon at 0.24 kg ha
-1. Molinate (6.5 kg ha
-1), however, gave 26-67% control when applied 6 days before rice seeding. Thiobencarb application as a preplant surface treatment at the rate of 4.4 kg ha
-1 in combination with reflooding within 3 to 5 d after drainage is recommended to control red rice in the United States
(Yamaguchi et al., 2008).
Use of seed protectants
Coating of dry seeds with 20% calcium peroxide using 4% PVA solution and broadcasting them in field with 10-15 cm standing water for 10 days can control weedy rice
(Abraham and Jose, 2014). Later, this technology was found not practicable due to increased water fowl attack on coated seeds and lankiness of the crop plants due to continued submergence
(de Avila et al., 2021).
According to the experiments carried out in Central and South America, the best weedy rice control could be achieved by applying molinate at 7.2 kg ha
-1 and butylate at 4.2 kg ha
-1 with seed protectants such as oxabetrinil at 1.5 g
kg-1 and flurazole at 2.5 g kg
-1 (Ziska
et_al2015).
Use of herbicides during the crop season
Chemical control in crop post planting should only be considered as a salvage operation and it mainly relies on difference in size or growth stage between weedy rice and commercial rice
(Singh et al., 2013). Weedy rice that had grown taller than rice could be treated with foliar systemic herbicides such as glyphosate or cycloxydim, at 20 and 5 per cent concentrations, respectively, by using wick/wiper applicators
(de Avila et al., 2021).
The equipment can be mounted on self moving machines, the front of a tractor or handheld equipment. A plant growth regulator, maleic hydrazide, had been used in Brazil to control seed production of red rice
(Abraham et al., 2012). Rice cultivars must be earlier and head at least 10-15 days before red rice. Maleic hydrazide sprayed at the rice milk stage and prior to or during red rice heading stage reduces the production of red rice seed. It was noticed that maleic hydrazide reduced seed viability and so it should not be used on rice seed production fields
(Abraham and Jose, 2014). The application of maleic hydrazide complements other methods for reducing seed production of red rice and, consequently, minimized this problem.
Biotechnological approach: Herbicide-resistant rice
The use of herbicide-resistant rice cultivars is another strategy advocated by researchers for selective control of weedy rice in cultivated rice. One of the main reasons for the development of herbicide-resistant rice was to obtain effective and selective control of weedy rice in the rice crop
(Olofsdotter et al., 2000). Three major herbicide-resistant rice systems developed are imidazolinone (IMI)-resistant rice (IMI-rice), glyphosate-resistant rice and glufosinate-resistant rice, which conveys resistance to the imidazolinone group of herbicides (imazethapyr, imazomox, imazapyr,
etc.), glyphosate and glufosinate, respectively
(Abraham et al., 2012).
Out of these three herbicide-resistant rice systems, IMI-rice is nontransgenic, whereas both glyphosate- and glufosinate-resistant rice are transgenic and are known as genetically modified crops. Glyphosate and glufosinate are non-selective, broad-spectrum and post-emergence herbicides with no soil or residual activity. The imidazolinone group of herbicides is also broad-spectrum, effective at low doses, can be applied pre- and post-emergence, has soil or residual activity and has a favourable environmental profile
(Singh et al., 2013). IMI-rice is the only herbicide resistant rice that has been commercialized. In Asia, IMI-rice cultivars have been released only in Malaysia. Although herbicide-resistant rice technology offers opportunities for selective control of weedy rice, the risk of gene flow from herbicide resistant rice to weedy rice poses a serious threat for the long-term utility of this technology
(Abraham and Jose, 2014). Therefore, it is important to assess the potential risks imposed by this technology before its introduction in the region.
The risk of gene flow from herbicide-resistant rice to weedy rice will be minimal in the northwestern Indian states of Punjab and Haryana as wild and weedy rice are not present in this region
(de Avila et al., 2021), but continuous direct seeding of rice could potentially lead to the evolution of weedy rice in this region, too. The risks of gene escape are higher in the eastern and southern Indian states where many wild and weedy relatives are present
(Olofsdotter et al., 2000). In summary, these methods can target many phases of the weedy rice life cycle as an integrated weed management strategy for weedy rice control.
