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Datasheet

Oryza rufipogon (wild rice)

Summary

  • Last modified
  • 22 June 2017
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Host Plant
  • Preferred Scientific Name
  • Oryza rufipogon
  • Preferred Common Name
  • wild rice
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Plantae
  •     Phylum: Spermatophyta
  •       Subphylum: Angiospermae
  •         Class: Monocotyledonae

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Identity

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Preferred Scientific Name

  • Oryza rufipogon Griff.

Preferred Common Name

  • wild rice

Other Scientific Names

  • Oryza aquatica Rosh.
  • Oryza cubensis Eckman ex Gotoh & Okura
  • Oryza fatua J. Koenig ex Trin., nom. nudum
  • Oryza formosana Masamune et Suzuki
  • Oryza glumaepatula Steud.
  • Oryza paraguayensis Wedd. ex E. Fourn.
  • Oryza sativa subsp. rufipogon (Griff.) de Wet
  • Oryza sativa var. abuensis G. Watt
  • Oryza sativa var. bengalensis G. Watt
  • Oryza sativa var. coarctata G. Watt
  • Oryza sativa var. fatua Prain
  • Oryza sativa var. paraguayensis Parodi
  • Oryza sativa var. rubribarbis Desv.
  • Oryza sativa var. rufipogon (Griff.) G. Watt
  • Oryza sativa var. savannae Körn.
  • Oryza sativa var. sundensis Körn.

International Common Names

  • English: brownbeard rice; common wild rice; perennial wild red rice; red rice; red-bearded rice; wild red rice
  • Spanish: arroz colorado; arroz rojo
  • French: riz rouge sauvage

Local Common Names

  • Australia: arrozrana; jingirra; wild rice
  • Brazil: arroz-preto; arroz-vermelho
  • Germany: Reis, Wilder roter
  • India: birhni; karga; reesa
  • Indonesia: padi burung; padi hantu
  • Laos: khao nok; khao pa
  • Malaysia: padi hantu; padi yang
  • Thailand: khao phee

EPPO code

  • ORYFA (Oryza fatua)
  • ORYRU (Oryza rufipogon)

Taxonomic Tree

Top of page
  • Domain: Eukaryota
  •     Kingdom: Plantae
  •         Phylum: Spermatophyta
  •             Subphylum: Angiospermae
  •                 Class: Monocotyledonae
  •                     Order: Cyperales
  •                         Family: Poaceae
  •                             Genus: Oryza
  •                                 Species: Oryza rufipogon

Notes on Taxonomy and Nomenclature

Top of page The genus Oryza is classified under the tribe Oryzeae, subfamily Oryzoideae. This genus has two cultivated species (O. sativa L. and O. glaberrima Steud.) and more than 20 wild species distributed throughout the tropics and subtropics. The genus Oryza probably originated about 130 million years ago in Gondwanaland and species were distributed amongst different continents when Gondwanaland split up. Cultivated species originate from a common ancestor with an AA genome. Perennial and annual ancestors of O. sativa are O. rufipogon and O. nivara (Khush, 1997).

There are wild, weedy and domesticated races of most crop plants. The wild races can survive without man, the weedy ones survive because of man (and in spite of his efforts to get rid of them) and the domesticated races demand care and cultivation for survival (Harlan, 1976). According to a review of Oryza species by Takeoka (1962), O. rufipogon is considered to be a progenitor of O. sativa, and together with O. longistaminata and O. sativa forms the O. sativa complex. Takeoka used the name O. barthii for O. longistaminata, but that name now refers to the annual wild rice species of West Africa. Takeoka (1962, 1963) made a comprehensive study of the group, resolving much of the confusion that had previously existed. Analysing awn length, lifespan and root type, characters that had previously been used, he characterized the O. sativa complex, and found that they were also divided geographically, such that:

- awn length: generally samples of plants of the species complex were not well separated by awn length; African samples had awn lengths never longer than 10 cm, usually 4-7 cm; American samples had awn lengths up to 16 cm; and Asiatic samples had awn lengths intermediate between those of American and African samples.

- rhizomes: these were seen only in plants from Africa. In Asiatic and American samples, the lower culms were submerged and had rootlets at the nodes.

- lifespan: lifespan was the poorest character to separate the species because habitat affects lifespan. In the tropics it is difficult to distinguish between annual and perennial plants because of the absence of distinct seasons. However, in a recent review of the genus, Khush (1997) suggests that the name O. nivara should be retained for the annual forms of the wild (shattering) rice in Asia.

The degree of shattering observed in wild and weedy rice is much higher than that exhibited by shattering (shedding) genes in cultivated rice. There are a number of loci controlling seed shedding with major and minor effects. When major genes are involved, shedding is dominant over non-shedding. The occurrence of shedding segregants from crosses between non-shedding parents suggests complementary action between loci (Tang and Morishima, 1988).

Features of wild rice (Angiras and Singh, 1985):

- the grains of wild rice ripen earlier and irregularly than those of cultivated rice and are extremely prone to shattering;

- the stem of wild rice is comparatively more brittle and round in cross section than that of cultivated rice;

- the surface of the leaf sheath of wild rice is softer and spongier than that of cultivated rice;

- leaves are generally narrower, deep green and occur at short intervals on the stem;

- wild rice plants generally have a spreading habit and flower earlier than cultivated rice plants.

Before Takeoka's work, the Asiatic individuals were known as O. fatua (O. sativa var. fatua, O. sativa forma spontanea) or O. rufipogon. American plants were listed as O. perennis. African individuals were referred to as O. longistaminata or O. perennis. However Takeoka redefined the complex and indicated that the Asiatic plants should be included in one species as indicated by Bor (1960) and the correct specific name for them is O. rufipogon. As the American plants had no clear distinction from the Asiatic plants in terms of awn length and rhizomes, he also called them O. rufipogon. However, African plants were separated from O. rufipogon as a different species, called O. longistaminata, on account of their different underground systems.

A key for separating the complex makes use of spikelet characters. O. sativa has persistent spikelets, O. rufipogon has deciduous spikelets and O. longistaminata is perennial with creeping and branched rhizomes (Bor, 1960; Takeoka, 1963). Second (1985), using isoenzyme analysis, described an O. rufipogon complex with geographical forms separated from South Asia, China, Papua New Guinea, Australia and the Americas.

O. rufipogon and O. sativa intercross and have a high rate of natural crossing. There are considered to be numerous intergradations between the two species (Chang et al., 1982). Sometimes hybrid swarms are produced and the hybrids show no sterility. In contrast to wild plants, domesticated rice cultivars are characterized by a low rate of seed shedding at maturity, a low degree of seed dormancy, synchronous heading, self-pollination and high grain yield (Oka, 1991). Hybridization and backcrossing between perennial wild rice and cultivated rice has created a highly variable range of weedy perennial wild rice types, including annual types, resulting in much taxonomic confusion.

Rao et al. (1997) found interspecific hybrids between cultivated rice and O. rufipogon and O. nivara in six populations in roadside ditches, isolated ponds, canals and rice fields in Laos. It is suggested that gene flow occurred from the cultivated to the wild species. Hybrids resembled the cultivated forms for most morphological characters until flowering, where they developed conspicuous panicle and grain characters, which resembled the wild species, and red or purple bristles of intermediary length. In China, high genetic diversity occurred in O. rufipogon populations from Guigang in Guangxi growing adjacent to cultivated rice fields, and it is suggested that this was due to the flow of genes from these neighbouring cultivated rices (Cai et al., 1996).

Khush (1997) reviewed the genus and defined O. rufipogon in the narrowest sense as a perennial, restricted to Asia from Pakistan to China and Indonesia, and tropical Australia. He uses the name O. nivara for the wild annual species, also of Asia, intermediate between O. rufipogon and O. sativa. All three species have the same AA genome and are not readily distinguished other than by their annual/perennial character and deciduous/non-deciduous spikelets. Following Khush's usage, many of the records and illustrations of O. rufipogon in the literature should be more correctly attributed to O. sativa (or O. nivara).

On the basis of RFLP analysis of nuclear DNA, Sun et al. (1997) suggested that common wild rice from China could be classified into three types: primitive types, indica-like types and japonica-like types. The genetic diversity found among Chinese wild rice accessions was related to their geographical distribution.

Juliano et al. (1998) compared the morphological variation of 26 diploid O. glumaepatula accessions from South America and Cuba, held in the International Rice Genebank at IRRI, with that of O. rufipogon and O. nivara from Asia. Sixteen spikelet and grain, eight leaf and culm, and four panicle characters were analysed using principal component analysis and hierarchical agglomerative cluster analysis. Most of the accessions from South America were quite distinct from O. rufipogon, with which they have often been grouped as a single species in some taxonomic treatments. Their study supports a distinct taxonomic status of a group of diploid wild rices from South America as O. glumaepatula. Naredo et al. (1988) confirmed O. glumaepatula as an independent species.

O. rufipogon is a C3 plant having a chromosome base number, x = 12, 2n = 24 and 48 (Watson and Dallwitz, 1992). O. rufipogon distributed throughout southern and south-eastern Asia, southern China and Australia, has the chromosome number 2n = 24 (Hore, 1997).

The great morphological variation in the genus Oryza causes taxonomic difficulties. Therefore, because of the widespread misuse of the name O. rufipogon and the difficulty of distinction between closely related taxa, this data sheet interprets the name in the broadest sense to include the many forms of annual 'red rice', 'black rice' and 'wild rice', as well as the strictly perennial O. rufipogon of South-East Asia.

Description

Top of page General

O. rufipogon in the strictest sense is an erect, perennial tufted grass, 150-400 cm tall, with culms spongy below, the lower parts floating and rooting at the nodes, the upper parts sub-erect, culm nodes glabrous and hollow. In the broadest sense, as used for the purposes of this data sheet, O. rufipogon sensu lato includes a range of annual types intermediate between O. rufipogon sensu stricto and O. sativa. These, however, have much the same morphology, other than being annual. Studies on the germplasm of 202 wild rices shows that there is an annual type in China. The characteristics investigated include 13 morphological characters, ratooning ability from node cuttings, mode of reproduction and the germination of seed harvested in the current year or stored for 2-3 years (Pang and Wang, 1996).

Leaf blades linear, involute in bud (also when dry), acute, flat, somewhat glaucous, scabrid on margins and main nerves, 15-18 cm x 10-25 mm. Leaf sheaths loose, cylindrical, glabrous with distinct auricles at the junction with the blade. Auricles 1-7 mm long, narrow, curved, glabrous or lined with long hairs to 2 mm long.

Ligule, triangular, an unfringed membrane up to 17 mm long, divided into acute points.

Roots, fibrous, often with rhizomes.

Inflorescence

The inflorescence is terminal and paniculate (axes usually wavy, the spikelets adpressed) well exserted, up to 20 cm long, initially concealed in the spathe-like sheath of the upper leaf, ultimately nodding; the main axis is long and slender, laterally compressed, flexuous; branches angular, rough on the angles.

O. rufipogon has many bisexual spikelets, always awned, easily shed, articulate on top of the stalk, which is more or less distinctly 2-lobed, 7-9 x 2-2.5 mm; each on a pedicel up to 2 mm long, one flowered; lower glume lanceolate, upper glume similar to lower but narrower, 2.4 mm long, lemma 7 mm long, boat-shaped, oblong, rounded, 3-nerved, with a rough awn, up to 7 cm long, often reddish, jointed on the lemma, but half as broad with similar texture, 3-nerved, bristly, roughish outside the midline, with two short basal processes (mucro) and an apical awn, 6 mm long. Lemma and palea green to yellowish, often dark red at apex, covered with stiff transparent hairs. Six stamens; anthers 5 mm long, linear, yellow or brown. Two styles, free; two stigmas, plumose, laterally exserted from the spikelets, blackish-purple or brown. Caryopsis narrow, red-brown, enclosed by a stiff lemma and palea (Clayton et al., 1974; Soerjani et al., 1987; Watson and Dallwitz 1992; California Department of Food and Agriculture, 2001).

Distribution

Top of page A native of Asia. O. rufipogon is widely distributed in the tropics and subtropics except Africa (Takeoka, 1963).

According to Hall (1990), O. rufipogon, which occurs in the Florida Everglades is the only known population of O. rufipogon in the USA. This weedy form of rice differs from weedy forms of O. sativa in having a pronounced rhizome and being perennial. The red rices that occur in Louisiana, Arkansas and California, USA, are annual forms of O. sativa. No known populations of perennial wild rice have ever been found in California. Previous populations of perennial wild rice hybrids in the Sacramento Valley have been eradicated (California Department of Food and Agriculture, 2001).

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

Asia

AfghanistanPresentHolm et al., 1979
BangladeshRestricted distributionHolm et al., 1979; Moody, 1989; EPPO, 2014
CambodiaPresentMoody, 1989
China
-FujianPresentPang, 1992
-GuangdongPresentPang, 1992
-GuangxiPresentPang, 1992
-Hong KongPresentHolm et al., 1979
-HunanPresentPang, 1992
-JiangxiPresentPang, 1992
-YunnanPresentPang, 1992
India
-Andhra PradeshPresentChang, 1975
-AssamPresentChang, 1975
-BiharPresentChang, 1975
-KarnatakaPresentChang, 1975
-Madhya PradeshPresentChang, 1975
-OdishaPresentChang, 1975
-Tamil NaduPresentChang, 1975
-Uttar PradeshPresentChang, 1975
-West BengalPresentChang, 1975
Indonesia
-Irian JayaPresentChang, 1975
-JavaPresentChang, 1975
-KalimantanPresentChang, 1975
-SulawesiPresentChang, 1975
IranPresentHolm et al., 1979
IraqPresentHolm et al., 1979
Korea, DPRWidespreadHolm et al., 1979
Korea, Republic ofWidespreadHolm et al., 1979
LaosPresentMoody, 1989
MalaysiaPresentMoody, 1989
-Peninsular MalaysiaPresentChang, 1975
MyanmarPresentChang, 1975; Moody, 1989
NepalPresentMoody, 1989
PakistanPresentMoody, 1989
PhilippinesPresentChang, 1975; Moody, 1989
Sri LankaPresentChang, 1975
TaiwanPresentChang, 1975
ThailandPresentChang, 1975
VietnamPresentMoody, 1989

Africa

EgyptPresentHolm et al., 1979
SenegalPresentHolm et al., 1979
SwazilandPresentParker and Dean, 1976
TanzaniaPresentChen, 2001

North America

USAPresentSmith, 1981
-FloridaPresentWestbrooks and Eplee, 1988
-HawaiiPresentHolm et al., 1979
-LouisianaPresentSmith, 1981
-North CarolinaPresentSmith, 1981
-South CarolinaPresentSmith, 1981
-TexasPresentSmith, 1981

Central America and Caribbean

Costa RicaWidespreadHolm et al., 1979
HondurasPresentHolm et al., 1979
JamaicaPresentHolm et al., 1979

South America

BrazilPresentAbud, 1981
-Espirito SantoPresentLorenzi, 1982
-GoiasPresentLorenzi, 1982
-Mato GrossoPresentLorenzi, 1982
-Mato Grosso do SulPresentLorenzi, 1982
-Minas GeraisPresentLorenzi, 1982
-ParanaPresentLorenzi, 1982
-Rio de JaneiroPresentLorenzi, 1982
-Rio Grande do SulPresentLorenzi, 1982
-Santa CatarinaPresentLorenzi, 1982
-Sao PauloPresentLorenzi, 1982
ColombiaPresentTakeoka, 1963
EcuadorPresentTakeoka, 1963
GuyanaPresentTakeoka, 1963
PeruWidespreadHolm et al., 1979
VenezuelaPresentTakeoka, 1963

Oceania

AustraliaPresentTakeoka, 1963
-Australian Northern TerritoryPresentCalder et al., 1999
-QueenslandPresentNeldner et al., 1997
Papua New GuineaPresentTakeoka, 1963

Habitat

Top of page O. rufipogon is perennial, tufted wild rice. It grows in shallow water, irrigated fields, pools, ditches and sites with stagnant or slow, running water. It occurs at altitudes from 0 to 1000 m.

Host Plants and Other Plants Affected

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Plant nameFamilyContext
Oryza sativa (rice)PoaceaeMain

Biology and Ecology

Top of page O. rufipogon is a perennial, tufted grass which grows in shallow water. It is a close relative of cultivated rice, O. sativa, and shares many of the characteristics if the crop. It is tolerant of flooding and acid soils (Mandal and Gupta, 1997). Vital differences from the crop are the tendency for the seed to shatter as soon as they are mature and their often prolonged dormancy. Like the crop, the seeds are unable to germinate in saturated soil.

Chen (2001) reported that the number of wild rice seeds in a soil sample 1m² x 15 cm deep ranged from 10 to 30,000. After harvest and before disk harrowing, 84.4% seeds remained in the 0-3 cm surface layer of soil. Disk cultivation helped move the seeds downward to the 3-15 cm soil layer resulting in serious infestations and control difficulties.

O. rufipogon seeds are typically dormant at maturity. Dormancy is partly due to the presence of inhibitors in the seed coat. The seed may remain dormant and viable for up to 3 years or more under field conditions, depending on the biotype and environment. Many seeds decay during long periods of flooded conditions. Germination typically occurs between 15 and 40°C. Seeds often germinate slightly sooner and at lower temperatures than commercial rice seeds. Some biotypes emerge from soil depths of up to 12 cm (California Department of Food and Agriculture, 2001). Chen (2001) reported that in a pot experiment, 98% of O. rufipogon seeds germinated within the 0-4 cm soil layer and only 0.8% seeds germinated in the 4-15 cm soil layer. A field investigation further proved that seeds of O. rufipogon germinated almost exclusively in the 0-4 cm layer, with very few seeds germinating below a depth of 4 cm.

Chen (2001) reported that the growth duration of O. rufipogon lasted about 130 days, 30-40 days shorter than rice cultivar IR8. The period from the start of inflorescence to seed maturation lasted only 14-15 days and the starch transport stage lasted only 10 days. In a pot experiment, a single O. rufipogon plant produced 86 tillers, 38 panicles and over 1000 seeds.

O. rufipogon has similar ecological requirements to the crop and hence tends to benefit from most of the conditions created by farmers for their rice crops.

Propagation

Reproduces by seed and vegetatively from rhizomes. Seeds fall near the parent plant or disperse to greater distances as rice seed contaminants, with human activities, water, soil movement, and possibly birds (California Department of Food and Agriculture, 2001). Although O. rufipogon is carefully controlled in rice fields, it reproduces heavily in irrigation canals, shedding its seeds in irrigation water and, thus, re-infesting commercial fields (Rojas and Aguero, 1996).

Impact

Top of page O. rufipogon is a vigorous, strongly competitive plant, which is difficult to eradicate (Lazarides, 1980). Infestations of wild rice reduce yield and lower the grade of cultivated rice. It is a noxious weed in the southern USA (Smith, 1981; Westbrooks and Eplee, 1988) and is considered a serious problem in Brazil, particularly where dwarf rice is grown (Abud, 1981). Yield reductions of 50-60% due to wild rice have been reported in Pakistan (Tiwari and Nema, 1967) and Tanzania (Chen, 2001). It is also causing severe problems in direct-seeding rice areas in South-East Asia (Hyakutake et al., 1990).

It can be a severe problem in rice because it is so similar to the crop vegetatively that it cannot be identified and removed before it flowers, by which time it will have been competing with the crop for many weeks. It then sheds most of its seed before harvest and contributes little or nothing to the yield. Grains of wild deepwater rice shed within 20 days of pollination. Such wild forms, unless thoroughly rogued out, may seriously contaminate the field and deprive the farmer of a reasonable yield (Zaman, 1981). Sometimes wild rice seedlings are more vigorous than commercial rice seedlings but they are usually difficult to distinguish.

Although red rice does not change the taste or nutritional value of rice, consumers view it as foreign particles in the white rice (Klosterboer, 1979). Red rice affects the appearance and market value of milled rice because of its red kernels. More severe milling is required to remove the red pericarp, increasing breakage of cultivated white rice, reducing head rice and total milling yield. In addition to milling losses, the farmer also suffers a loss in cultivated rice yield by competition from and preharvest shattering of weedy rice, the extent of loss depending upon the level of weedy rice infestation in the field. A further important characteristic is the dormancy of the seeds, which ensure that it survives repeated tillage. The relatively weak stems may also result in lodging of both the weed and crop (Ampong-Nyarko and De Datta, 1991).

According to Moody (1989), O. rufipogon occurs in different rice cropping systems in South and South-East Asia including: dry seeded and deep water rice in Bangladesh; dry seeded, wet seeded, seedling nursery, transplanted and upland rice in India; transplanted rice in Malaysia and the Philippines; wet seeded rice in Sri Lanka; deep water, transplanted and wet seeded rice in Thailand; and dry seeded and transplanted rice in Vietnam.

Uses

Top of page All the wild species in the Oryza genus serve as a valuable gene pool that can be used to broaden the genetic background of cultivated rice in breeding programmes. O. rufipogon has been used in breeding with O. sativa to confer resistance to stem rot disease, caused by Magnaporthe salvinii (Tseng and Oster, 1994), resistance to rice tungro viruses (Angeles et al., 1998) and submergence tolerance (Mandal and Gupta, 1997). It is also resistant to rice blast, caused by Magnaporthe grisea (Reimers et al., 1993) and bacterial leaf blight, caused by Xanthomonas oryzae pv. oryzae (Sun et al., 1992; Kaushal et al., 1998).

Martinez et al. (1998) reported that introgression of certain genes from O. rufipogon may contribute to yield increase in improved rice cultivars. Also, segregation for resistance to the white leaf virus was detected. According to Xiao et al. (1996), O. rufipogon alleles at marker loci RM5 on chromosome 1 and RG256 on chromosome 2 were associated with enhanced yield. The phenotypic advantage of lines carrying O. rufipogon alleles at these loci was estimated to be 1.2 and 1.1 t/ha, respectively.

In deepwater rice fields in Khulna, Bangladesh, an annual weed type of wild rice 'Jhora-dan' is one of the major weeds. It is sometimes used as food. There are two methods of collecting seeds. One is collecting panicles from the standing plants in fields before harvesting cultivated rice and another is collecting the fallen seeds by sweeping the parched ground after harvesting (Morishima et al., 1991). O. rufipogon is also eaten in times of drought or famine (Baksha et al., 1979).

In the Northern Territory, Australia, the Alawa tribe grind O. rufipogon seed, mix it a little water to form a paste and bake it in hot sand. Seed and the baked product can be stored in caves on paperbark (Melaleuca quinquenervia) sheets for up to 4 years (Wightman et al., 1990).

Uses List

Top of page

Genetic importance

  • Gene source for salt tolerance
  • Progenitor of
  • Related to

Human food and beverage

  • Emergency (famine) food

Similarities to Other Species/Conditions

Top of page Takeoka (1962) showed that O. sativa, O. rufipogon and O. longistaminata were all very closely related, and noted that O. sativa has persistent spikelets whereas the spikelets in O. rufipogon are deciduous, and that O. longistaminata is perennial with creeping and branched rhizomes. O. rufipogon and O. longistaminata are also geographically separated; O. rufipogon in Asia and O. longistaminata in Africa.

Prevention and Control

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Introduction

The more weeds ecologically resemble the crop plant, the worse they are. Perhaps the worst weeds of rice are wild species of rice that shed their seeds before the crop is ripe and have seeds with dormancy (Cook, 1990). O. rufipogon infestations are difficult and expensive to control. There is no single technique that will eliminate the problem. Hand weeding is still practised, mainly in developing nations, but with hand weeding, workers are faced with the dilemma of distinguishing between weeds and the crop. The closer the weed resembles the crop, the more likely it is to be overlooked during weeding. Chemical control of O. rufipogon in rice is difficult because of the close genetic relationship between the weed and the crop.

Effective control of O. rufipogon and other weedy rice species in rice depends upon a rigorous weed management programme. Integrated weed control systems, involving the use of certified seed (or good quality weed-free seed), good land preparation, the use of stale seedbeds to encourage weed germination before seeding, careful crop and water management, herbicides and crop rotation are needed. In crop rotation, rice may be rotated with other crops in alternate seasons and an appropriate herbicide can be used to destroy weedy rice seedlings in these crops. Such programmes are recommended for rice in Asia and the Americas (Grist, 1986; Smith and Hill, 1990; Ampong-Nyarko and De Datta, 1991; Moody, 1994).

Cultural Control and Sanitary Methods

Cleanliness of cultivation

To avoid unnecessarily introducing the weed, the use of weed-free crop seed, the removal of red rice seed from irrigation water i.e. control of red rice growth in irrigation ditches, and the use of clean cultivation equipment are recommended. Wrigley (1969) commented on the difficulty of separating seeds of O. rufipogon from rice seeds by winnowing. Weeds and off-types of rice that synchronously flower and mature with the cultivated variety should be hand rogued to reduce crop seed contamination.

Crop rotation with control of red rice in all crops

Crop rotation is a very effective method of controlling difficult weeds in rice. Rice should be rotated with other grain or legume crops such as sorghum or soyabean. During the years in which the alternate crop is grown, cultivation and herbicide treatments should be used to control red rice thoroughly, and provide a clean bed in which to sow rice in the third or fourth year of the rotation. When the alternate crop is grown, pre-plant soil incorporating herbicides such as metolachlor, either alone or tank mixed with trifluralin, pendimethalin, metribuzin or imazaquin, may be used. Post-emergence treatments include the use of fluazifop, quizalofop or sethoxydim, or directed sprays of paraquat to control red rice missed by pre-plant treatments.

Crop and water management

Transplanting rice has multiple benefits; germination of the weed should be considerably reduced, and those that do germinate can still be removed by weeding. Even if the weeds are not removed they will be much less competitive and produce less seed than they would in a direct-sown crop. Trebuil et al. (1983) reported that rice is transplanted when the sown field has a high incidence of wild rice. It has become increasingly difficult to combat wild rice over the years as its morphological characteristic become closer to those of cultivated varieties because of strong selection pressuredue to long and careful weeding and possible natural hybridization with cultivated varieties.

The following practices are also effective: plant spacing, where crop competition can be used to reduce weed growth; high seedling rate of cultivated rice to reduce tillering of wild rice; and burning straw after harvest to kill wild rice seeds. O. rufipogon seeds that are buried will not germinate in flooded or water-saturated soil, but under these conditions the plants will propagate by stem cuttings or stem bases.

Continuous flooding reduces perennial wild red rice seed survival and attracts ducks that feed on the grains. Seeds do not survive ingestion by waterfowl (California Department of Food and Agriculture, 2001).

In Khulna, Bangladesh, an early flowering deepwater rice cultivar Ashina is cultivated when the rice field becomes badly infested with Jhora-dan. Since Ashina flowers 2-3 weeks earlier than Johra-dan, weeding of Johra-dan can be easily done after harvesting Ashina (Morishima et al., 1991).

Salimath (1921) recommended rotating different rice cultivars with different coloured stems. He recommended growing the white-stemmed cultivar Mugad for 2 years and weeding out all the red-stemmed plants and then in the succeeding 2 years growing the red-stemmed cv. Antarsali and weeding out all the white-stemmed plants. Roy (1921) recommended the use of purple-leaved cultivars, and land preparation (stale seedbed and puddling) for the control of red rice. For effective control of wild rice, Thakur (1969) recommended the growing of BR 11 or BR 12, which are purple cultivars. Srivastava et al. (1987) recommend the use of certified seed, regular removal pre- and post-flowering and cultivation of purple-leaved cultivars continuously for 2 or 3 years for the control of wild rice. The rice seedlings are, therefore, easy to distinguish from the green wild rice seedlings.

O. rufipogon is not a weed problem in California, USA, because of a seed certification programme. Certified rice seed is used by practically all the farmers in the state and O. rufipogon is not permitted in certified seed. In the 10 years before 1932, 28% of California rice seed samples had O. rufipogon present at an average of 95 seeds/kg, the highest count being 1060/kg (Bellue, 1932).

Mechanical Cultivation

It is recommended that rice is sown in rows so that wild rice can be recognized by its presence between the rows and can be removed by hand or cultivation. Early ploughing of land after harvest to encourage the germination of O. rufipogon and control of these emergent weeds by grazing cattle, cultivation with spike tooth harrow or herbicide application are effective.

Early ploughing after harvest followed by flooding in the first 3 weeks aids control of the weed. Early season cultivation and harrowing stimulate germination of O. rufipogon and may allow the mechanical destruction of several flushes of wild rice growth before rice or rotational crops are planted.

Chemical Control

In the rice crop, infestations are reduced by applying molinate pre-plant incorporated (Smith and Khodayari, 1985), water seeding the rice, and maintaining the flood water, or keeping the soil moist by frequent irrigation, for several weeks after seeding. According to Hyakutake et al. (1990), O. rufipogon from Thailand, Malaysia, Sri Lanka and Brazil was tolerant to thiobencarb, while that from India, Myanmar and Guyana was susceptible. All were susceptible to simetryn regardless of origin.

Thiobencarb can also be surface applied, pre-planting, just before bringing on the flood (Smith and Khodayari, 1985). Although thiobencarb has been recommended for use on O. rufipogon in rice, it is recommended that the crop seed be treated with a protectant or antidote, such as NA (1,8-naphthalic anhydride), as a safeguard (Wirjarhardja and Susilo, 1979; Smith and Hill, 1990). Chemicals are more commonly used pre-sowing to destroy the rice weed before the susceptible crop is present, for example, chemicals such as metolachlor either alone or tank mixed with trifluralin, pendimethalin, metribuzin or imazaquin are pre-sowing treatments recommended by Smith and Hill (1990). Chen (2001) also obtained effective control of O. rufipogon by applying atrazine or atrazine + metolachlor in maize or grain sorghum grown in rotation with rice.

The method of weed control chosen will depend upon the cropping system and the benefit to cost ratio. Recommendations for control of O. rufipogon in the developing world are detailed in Moody (1994) and those for the Americas are reviewed by Smith and Hill (1990). Chen (2001) used the following steps to obtain 96% control of O. rufipogon.

1. A seedbed was finely prepared by disking and tine harrowing about 1 month before sowing.
2. This was irrigated two to three times to keep the surface soil moist for 25-30 days to stimulate germination of wild rice seeds.
3. When the wild rice seedlings reached the 3-4 leaf stage (95% of seeds in the 0-4 cm soil layer had germinated), a mixture of paraquat and oxadiazon was applied.
4. Rice seeds were direct seeded to a depth of 1-2 cm by drilling under zero-tillage to avoid turning up of wild rice seeds from the deeper soil layers.
5. The field was irrigated after sowing to promote germination of rice seeds.
6. The field was flooded from the 3.5 leaf stage of rice to check emergence of wild rice and other weeds.

A new approach to chemical control of wild and red rice is the use of herbicide-tolerant crop cultivars, which can be safely treated with otherwise non-selective herbicides such as glufosinate (Sankula et al., 1997). There is, however, concern that the tolerance genes will be transferred by out-crossing to wild rice, thus eventually reducing the effectiveness of the treatment. For example, Langevin et al. (1990) reported morphological convergence between cultivated and weedy O. sativa, with hybrids demonstrated to be more vigorous than pure weeds. A genetic barrier to outcrossing should be introduced into the herbicide-resistant crop to prevent the transferring of herbicide resistance to the weed species.

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