wild and weedy rice in rice ecosystems in asia --- a review

8/8/2019 Wild and Weedy Rice in Rice Ecosystems in Asia --- A review

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Contents

Foreword v

Opening address vi

Rungsit Suwanketnikon

Opening remarks vii

Kozo Ishizuka

Integrated weed management and control of weeds 1

and weedy rice in Vietnam

Nguyen Van Luat

Collecting wild relatives of rice from the Mekong Delta, Vietnam: 5

distinguishing wild rice from weedy rice

Bui Chi Buu

Origin and evolution of wild, weedy, and cultivated rice 7

Yo-Ichiro Sato

Rice seed contamination in Vietnam 17

Vo Mai, Ho Van Chien, Vo Van A, Vo Thi, Thu Suong,

and Le Van Thiet

Red rice status and management in the Americas 21

J.A. Noldin

Weedy rice complexes: case studies from Malaysia, Vietnam, 25

and Surinam

H. Watanabe, D.A. Vaughan, and N. TomookaWild and weedy rice in China 35

Chao Xian Zhang

Distribution, emergence, and control of Korean weedy rice 37

J.Y. Pyon, W.Y. Kwon, and J.O. Guh

Wild and weedy rice in the Nepalese ecosystem 41

S.R. Gupta and M.P. Upadhyay

Weedy rice in Vietnam 45

Duong Van Chin, Tran Van Hien, and Le Van Thiet

Weedy rice (Oryza sativa L.) in Peninsular Malaysia 51

B.B. Bakar, M.A. Bakar, and A.B. Man

Wild and weedy rice in Thailand 55

P. Vongsaroj

Geographic distribution, ecology, and morphology 59of wild and weedy rice in Lao PDR

A. Appa Rao, V. Phetpaseuth, C. Bounphanousay,

J.M. Schiller, and M.T. Jackson

Sources of wild rice and their control in Myanmar 69

Saw Ler Wah Win and Khin Nwe Nwe Win

Weedy rice in the Philippines 75

A.M. Baltazar and J.D. Janiya


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Weedy rice in Sri Lanka 79

B. Marambe and L. Amarasinghe

Control of red rice 83

H. Sadohara, O. Watanabe, and G. Rich

Biological control of rice weeds using fungal isolates 87

K. Yamaguchi, K Ishihara, and S. Fukai

Management of weedy rice (Oryza sativa L.): 91

the Malaysian experience

M. Azmi, M.Z. Abdullah, B. Mislamah, and B.B. BakiWeedy rice: approaches to ecological appraisal and implications 97

for research priorities

M. Mortimer, S. Pandey, and C. Piggin

An economic framework for optimal management of weedy rice 107

on Asian rice farms

S. Pandey, A.M. Mortimer, and C. Piggin


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v

Foreword

Weedy rices constitute a major threat to irrigated rice production systems in Southeast Asia. Commonly

considered to be ecotypes ofOryza sativa, they display a range of undesirable agronomic traits damaging both

cultivated rice yield and quality. These proceedings represent the culmination of a review of the origins and

management of weedy rices in Asia. It was initiated under the auspices of the Asian Pacific Weed Science

Society (Professor B. Bakar) and Cuu Long Delta Rice Research Institute, Vietnam (Dr. D.V. Chin). An inaugural

workshop held in Vietnam in 1998 discussed a wide range of issues relating to weedy rices in which participants

summarized the findings in working papers. These proceedings comprise a selection of those presented that

have since been updated and reviewed as a collaborative partnership between the Asian Pacific Weed ScienceSociety, Cuu Long Delta Rice Research Institute and the International Rice Reseach Institute.

The editors are very grateful for the support from Bill Hardy and Tess Rola in the editing of these proceed-

ings and for financial support from the Department for International Development (UK) for production.

Dr. Duong Van Chin

Cuulong Delta Rice Research Instiute

Professor B. Bakar

University of Malaya, Malaysia

Dr. M. Mortimer

International Rice Research Institute


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vi

Honorable guests, distinguished delegates, ladies and

gentlemen:

On behalf of the Asian-Pacific Weed Science Society,

I would like to extend our thanks to all of you for your

attendance.

The APWSS meets every 2 years. To strengthenthe close relationship among weed scientists in the

Asian-Pacific countries, the Executive Committee of

the 15th APWSS supported several activities in the

periods between the biennial conferences. A recent

activity was the Northeast Asian Area Weed Science

Symposium of China, Korea, and Japan on 20-23 Aug

1996 at Harbin, Peoples Republic of China. In

addition, the Asian Tropical Weed Management

Training Courses were conducted at Kasetsart

University, Khamphaengsaen Campus, Thailand (13

Oct-9 Nov 1996 and 18 May-8 Jun 1997). The idea of

organizing this Weedy Rice Symposium came from Dr.

Kozo Ishizuka, the former president of the WeedScience Society of Japan and APWSS, and Dr. Hiroshi

Hyakutake of the Executive Committee of APWSS.

During last years meeting in Kuala Lumpur, Malaysia,

the Executive Committee of the 16th APWSS decided

to hold this symposium on weedy rice in Vietnam.

Rice is the worlds most important crop, and it

feeds most of the people in Africa, Asia, and Latin

America. Weeds are plant species that compete with

the rice crop for plant nutrients, water, and light.

Traditionally, weeds have been controlled in trans-

planted rice in puddled soil by water management and

hand weeding. However, rice farmers in many

countries have changed the method of planting from

transplanting to direct seeding. The easiest way to

control weeds in direct-seeded rice is by applying

herbicides. Herbicide use in rice worldwide grew more

rapidly than insecticide and fungicide use during the

past 20 years. In the next decade, herbicide use in rice

will increase dramatically.

Weedy rice is a serious weed in the rice fields of

more than 50 countries in Africa, Asia, and Latin

Opening address

America. It reduces rice yield and quality. In addition,

weedy rice infests several upland crops, such as jute,

maize, soybean, and vegetables. The spread of weedy

rice is almost always as a contaminant in rice seed

from cultivated varieties.

Hand pulling of weedy rice in cultivated rice is

impractical because it is difficult to distinguishbetween the two until heading occurs. Herbicides are

not effective due to the close relationship between

weedy and cultivated rice. Preplanting applications of

herbicides before land preparation or seeding can be

appropriate in some situations. Research on herbicide

safeners has been done to protect the crop, but no

commercial treatment has yet been found.

To try to solve weedy rice problems, the Execu-

tive Committee of APWSS welcomes this symposium.

We believe that this is an excellent time for weed

scientists who have been working in the area of

weedy rice to meet and exchange views as well as

research findings.On behalf of APWSS, I thank the Government of

Vietnam; the Ministry of Agriculture and Rural

Development, Vietnam; the Cuu Long Delta Rice

Research Institute; Dr. Ahmed Anwar Ismail of

MARDI, Malaysia; Dr. Baki Hj Bakar of the University

of Malaya (the respective past president and secre-

tary of APWSS); and Dr. Anis Rahman of the Ruakura

Agricultural Research Centre, New Zealand (the

treasurer of APWSS), for their kind support that made

this symposium a reality. Special thanks go to Dr.

Duong Van Chin and his Organizing Committee, Dr.

Kozo Ishizuka, and Dr. Hiroshi Hyakutake of the

Weed Science Society of Japan, whose contributions

ensured the success of the symposium.

I wish this symposium every success in under-

taking this activity and achieving the demanding

targets that it set.

Thank you.

Rungsit SuwanketnikonPresident, Weed Science Society of Thailand, and President, Asian-Pacific Weed Science Society


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vii

Dr. Rungsit Suwanketnikom, president, APWSS, Dr.

Nguyen Van Luat, director, CLRRI, distinguished

delegates, ladies, and gentlemen:

As one of those who proposed the holding of this

affair to the executive committee of the Asian-Pacific

Weed Science Society held in Malaysia 1997, I amhonored to be given the pleasant task of opening this

International Symposium on Wild and Weedy Rices in

Rice Agro-Ecosystems under the auspices of APWSS

and the Cuu Long Delta Rice Research Institute.

I extend my sincere gratitude to Dr. Duong Van

Chin, chair of the Organizing Committee of the

symposium. Without his energetic leadership, we

could not expect such a concentrated effort and

meticulous preparation for this activity.

I should emphasize also the great contributions

made by Dr. Baki Hj Bakar and Dr. Sombat Chinawong,

the respective secretaries of the 16th and 17th

APWSS Conferences, and Dr. Hiroshi Hyakutake ofthe Weed Science Society of Japan.

Now, let us think for a moment why we are having

this symposium on wild and weedy rice at this time

and at this place.

It is well known that Asia is one of the biggest

rice production areas in the world and rice is one of

the most abundant cereal crops in the region. Many

researchers in Asia have been engaged in wild rice

studies, focusing especially on the history or origin of

cultivated rice. Comparative studies between cultivars

of domesticated rice and lines of wild rice have been

carried out, mainly to enhance our understanding of

cultivated rice.

Recently, attention has been given to wild rice

from the point of view of germplasm utilization. The

objective is to incorporate useful traits of cultivated

ricee.g., adaptation to environmental stresses, pest

and disease tolerance, or productivity of tasty rices

by conventional and transgenic techniques.

It is equally important to take note of the

propagation and distribution of weedy rice in

cultivated rice fields. Frequently, weedy rice has been

Opening remarks

observed to spread from rice fields, levees, or road

sides. Because weedy rice has properties very similar

to those of cultivated rice, such as easy shattering, or

some perennial vegetative reproduction properties,

the invasion of weedy rice in cultivated rice fields

would become more severe.

For effective control of weedy rice by farmers, weshould find suitable and efficient techniques, be they

chemical, physical, or biological, that are firmly based

on fundamental weed science and technology.

When it comes to chemical weed control, we

have had quite a number of herbicides that show

selectivity among plant species even taxonomically

related to each other at remarkably low dosages.

One example is the control of barnyardgrass, one

of the most dominant weeds in rice fields. Taxonomi-

cally, it is closely related to rice. Our study on

propanil herbicide showed that its degradation

enzyme (arylacylamidase) occurred both in rice and

barnyardgrass, but it had mutually different substratespecificities. The one from rice combined well with

propanil, but the one from barnyardgrass did not.

These differences in substrate specificity resulted in

rice being tolerant of propanil and barnyardgrass

being susceptible to the same chemical. These two

species of plants show clearly different degrees of

tolerance for propanil because of the different

properties of their detoxifying enzyme.

Let us turn to another example. If you apply

pretilachor, for example, to rice seedlings, which are

relatively tolerant of the herbicide, the plants induce

isozymes of the detoxifying enzyme (glutathion-S-

transferase), which combine specifically with

pretilachor and detoxify it. These isozymes are clearly

induced in rice, but much less so in barnyardgrass.

The process is assumed to be a kind of intrinsic self-

protecting system of plants to xenobiotics. Such self-

defense systems vary from one species to another.

Differences in tolerance for some herbicides are

observed not only between rice and barnyardgrass;

they exist even among lines or cultivars of rice.

Simetryn or bensulfuron methyl showed a specific

Kozo Ishizuka

Past president, Asian Pacific Weed Science Society, Weed Science Society of Japan


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1

The rapid increase in population has raised the

demand for food and crop plants. Our ability to meet

the increased food needs of the population depends

on several measures, one of them being how well we

manage competition imposed by weeds on crop

plants. Since 1950, world rice production has in-

creased remarkably. However, there has not been anaccompanying decrease in crop losses due to pests,

including weeds, despite intensified crop protection

measures (Moody 1997). Integrated weed manage-

ment (IWM) is an approach to help crops combat

weeds and obtain higher productivity at the lowest

cost and in the most environment-friendly manner.

This method is similar to integrated pest management

(IPM) and integrated nutrient management. In

practice, increasing rice production inputs such as

fertilizers and intensive cropping provide favorable

conditions for weed development, especially weedy

rice. On the other hand, improving land preparation,

planting, harvesting, and postharvest operations andwater management techniques can either increase

crop yield or control weeds better. Therefore, weed

control methods should be improved with these

different techniques incorporated into the IWM

approach. Another component of IWM is the

judicious use of herbicides. Given the trend toward

increased herbicide use and the likely environmental

and health consequences of such a trend, IPM

involving a combination of several methods is

desirable (Pingali et al 1997).

The Asia-Pacific Weed Science Society has

conferences on weed science every 2 yr. The 16th

conference held in Malaysia in 1997 attracted about

300 participants from 28 countries and from different

international organizations and companies. Of more

than 100 presentations, several reports involved

IWM, including rice-duck, rice-fish, and rubber-sheep

farming systems. Nearly 50% of the technical reports

were on herbicide efficiency and nearly 10% were on

biological control and biotechnology solutions.

Integrated weed management is still on a lower rung

Integrated weed managementand control of weeds and weedy ricein Vietnam

as compared with IPM that employs resistant varieties

and natural enemies. The International Symposium on

Wild and Weedy Rice in Agroecosystems held in

Vietnam (10-11 Aug 1998, Ho Chi Minh City) was

organized by the Asia-Pacific Weed Science Society

and the Cuu Long Delta Rice Research Institute

(CLRRI) and Ministry of Agriculture and RuralDevelopment (MARD). Another conference on weed

science will be held in Thailand. These activities of

the weed science societies (Asia-Pacific or national)

focus on the importance of weed control to enhance

food production.

Weed control for rice productionin Vietnam

Vietnam has a long history of rice production. Rice

has been grown in Vietnam for more than 2,000 yr. An

ancient Vietnamese idiom says, cong cay la cong bo,

cong lam co la cong an,which means withoutseeding, without harvesting.

Vietnamese farmers also have a lot of experience

in weed control. Some of the more popular and

effective methods follow.

Land preparation

Dry and/or wet rototilling (plowing and/or harrowing,

sometimes drainage and leveling, first tillage) is done

to provide good conditions for weed and weedy rice

emergence and to eliminate them by second tillage.

If necessary, a herbicide spray or a third tillage

follows. This method requires more labor inputs.

Farmers in the north have applied it on their trans-

planted fields, which is the best nonherbicide

integrated method for controlling weeds and weedy

rice. In the south, however, especially in the Mekong

Delta, broadcasting is practiced on a large scale, in 3

million out of 3.6 million ha of rice area. Almost all

high-yielding varieties (HYVs) are broadcast under

several rice cropping patterns: two crops yr1, three

crops yr1, and seven rice crops every 2 yr (Luat

Nguyen Van Luat


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2

1991,1994). Because of labor shortages during times

of sowing and harvesting, minimum/no tillage (Luat

1996) or burning of rice straw is commonly practiced.

This induces the development of pests because, after

burning, the pests that survive develop much faster

than their natural enemies (Loc and Heong 1997).

Sowing method

Water seeding. In the Mekong Delta, farmers broad-

cast rice seed, generally pregerminated, when water in

the field is 1050 cm deep. This water-seeding method

has been applied mainly for the winter-spring crop

when flooding occurs and when floodwater recedes at

the end of the wet season. All weed and crop residues

are removed from the field. Algae and other aquatic

weeds are collected by net to allow contact between

the rice seed and the soil surface. Water seeding has

been practiced in large areas every year, mainly in the

Longxuyen Quadrangle and in the Plain of Reeds.

Water seeding has some advantages overconventional wet seeding. Water seeding can be

practiced earlier than wet seeding by 1020 d; farmers

can therefore save on water. In addition weed control

with the use of water is effective. The energy input

can be reduced by nearly half due to drainage saved

at the beginning of the cropping period and reduced

irrigation requirements from tillering to ripening.

Water-seeding input for weed control is also reduced

23.5 times that of wet and dry seeding. By applying

water seeding, the rice crop is harvested 1525 d

earlier and farmers now have more time to take part in

other activities, such as preparing the land or growing

one more short-duration crop such as mungbean,soybean, or sesame. Rice in rotation with upland

crops is the best integrated method to control weeds

and weedy rice. The main constraint of water-seeding

is the damage done to fish and the crab. In addition,

chemicals for control must be used safely (Luat 1996).

Row seeding. The CLRRI has recommended

broadcasting rice in row seeding, saving at least 100

kg seed ha1. Compared with the conventional

broadcasting method, row seeding can increase grain

yield from 0.3 to 1.5 t ha1. This method is also better

in terms of increased resistance to lodging, rats, and

pests, use of solar radiation for photosynthesis, and

controlling weedy rice inasmuch as it is easier to

differentiate cultivated rice plants in rows and weedy

rice between rows. Row seeding is done by using the

improved IRRI seeder (Luat 1997).

Broadcasting of seedlings. This method has

been introduced from China. Seedlings are prepared in

plastic plateswith holes 2 cm in diameter. Two or three

rice seeds are put in each hole, covered with soil, and

irrigated by spraying water. Seedling broadcasting

can be practiced by broadcasting or putting seedlings

in rows. The method has the row-seeding advantage

mentioned above; it also enjoys transplanting

benefits as a result of shortened growth duration in

the field. It is even better than transplanting in termsof protecting seedling root systems from pulling

damage (Luat et al 1998).

Rice production in Vietnamand role of weed control

Present status

Rice in Vietnam accounts for more than 90% of

national food production. From 1990 to 1997, food

production increased yearly by 1.3 million t; however,

the annual increase in demand was estimated to be

0.30.4 million t. Therefore, some 34 million t of

milled rice yr1 had to be exported. The cultivated ricearea is 4.2 million ha, but rice-growing area is more

than 7 million ha. As rice area becomes limiting, rice

yield can be increased quickly. Average grain yield is

still low (3.83.9 t ha1); the yield gap is attributed to

technological and/or sociological constraints that can

be managed or eliminated.

Future outlook

Vietnam aims to meet its food production goal of 32

million t in 2000 and 3840 million t in 2010. Rice

production will still be 90% of total food production

and milled rice exports will remain at 3.54 million t yr1

(MARD 1998). Vietnam has attained great progress in

rice production after establishing a market-oriented

economy. Many constraints need to be overcome,

however, to achieve the national objectives. One of

these constraints is weed damage.

Role of weed control

According to studies on yield loss from biotic and

abiotic stresses, rice yield loss attributed to weeds is

4.3% in the irrigated area and 8.4% in the rainfed area

(Thanh et al 1998). If this yield loss could be reducedeven by half, rice production in the Mekong Delta

would increase by half a million t. The worldwide loss

in rice yield from weeds has been estimated to be

around 10% of total production (Moody 1991).

Besides rational herbicide use, fertilizer applica-

tion and intensive rice cropping, which favor either

rice production or weed development,should be


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3

improved. Better land preparation and seeding

techniques, which increase rice yield and grain quality

and control weeds and weedy rice, should also be

developed.

References

Loc NT, Heong KL. 1997. Effect of crop residue burning on

rice predators: a case study in Vietnam. Paper

presented at the workshop on increasing efficiency of

fertilizer application, 29-30 Jul1997, CLRRI-Mega

Project/IRRI, Cuu Long Delta Rice Research Institute,

Can Tho, Vietnam.

Luat NV. 1991. Process of increasing rice yield in the MRD

from the point of view of agricultural science and

technique. Sci. Activ. Monthly J. p 2-5.

Luat NV. 1994. Progress obtained in research and produc-

tion programmes on rice production in Vietnam

(country report). In: Proceedings of the Eighteenth

Session of the International Rice Commission. Rome

(Italy): Food and Agriculture Organization.Luat NV. 1996. Low-input rice-based cropping systems for

sustainable agriculture. In: Proceedings of the 2nd

Asian Crop Science Conference, 21-25 Aug 1995.

Japan: Asian Crop Science Society.

Luat NV. 1997. Effect of broadcasting and row seeding with

different seed rate on rice yield. Paper presented at a

workshop on increasing efficiency of fertilizer

application, 29-30 July 1997, CLRRI-Mega Project,

Cuu Long Delta Rice Research Institute, Can Tho,

Vietnam.

Luat NV, Loc NT, Nhan NT. 1998. Study on seedling

broadcasting as compared to seed broadcasting. Paper

presented at the regional workshop on methods of rice

seed multiplication, 25 Jul 1998, Cuu Long Delta Rice

Research Institute, Can Tho, Vietnam.

MARD (Ministry of Agriculture and Rural Development).

1998. Annual report for 1997. Vietnam: MARD.

Moody K. 1991. Weed management in rice. In: Handbookof pest management in agriculture. 2nd ed. Boca Raton,

Florida: CRC Press Inc. p 301-328.

Moody K. 1997. Priorities for weed science research. In:

Research in Asia: progress and priorities. Evenson RE,

Herdt RW, Hossain M, editors. Wallingford, Oxon

(UK): IRRI-CAB International. p 277-290.

Pingali PL, Hossain M, Gerpacio RV. 1997. From manual

weeding to herbicide use. In: Asian rice bowls, the

returning crisis? Wallingford, Oxon (UK): IRRI-CAB

International. p 245-256.

Thanh DN, Thuy NTL, Hossain M. 1998. Yield gap

production losses and priority research problem areas

in the Mekong Delta of Vietnam. Paper presented at

the workshop on prioritization of rice research in Asia,20-22 Apr 1998, International Rice Research Institute,

Los Baos, Philippines.


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viii

herbicidal activity toward some indica cultivars,

different from that observed in japonica types. These

differences are not as big as those noted among

selective herbicides being developed for cultivated

and weedy rice, but such differences would indicate

many possibilities. I suppose that this kind of

research would be one of the topics that will bediscussed in this symposium.

We should keep in mind that any differences in

properties between cultivated and weedy rices, be it

agronomical, ecological, genetic, morphological,

physiological, or biochemical, could lead to funda-

mental approaches for controlling weedy rice in rice

fields.

Today, securing food production to solve global

food problems is an urgent matter. In sustainable

agriculture, weed management continues to gain more

importance. Relevant weed management measures

have been taken under various types of cultivation

e.g., direct seeding, transplanting of very youngseedlings, rotational cropping, limited tillage, and so

forth. Meanwhile, development in the use of herbi-

cides has been accompanied by development of

cultivation methods, adapted to suit local conditions.

Progress in weed technology, including herbicide

technology, has actually had a great influence on the

improvement of agriculture itself.

In the past several decades, so-called high

technologies have been used in agriculture. Care

must be taken such that these are introduced to

complement the economic and social activities of

people as a whole. Concerns on environmental

conservation and sustainable development should beconsidered. Technology in general should not simply

be evaluated by how efficient it functions. We must

use utmost discretion in evaluating newly introduced

technologies and we must know what their role would

be in society.

Based on these considerations, we should put

results of research on wild and weedy rice to good

use to enhance our understanding and to identifyways to control them.

We in the Asian-Pacific region deal directly with

wild and weedy rices. Much of our research has

focused on this important subject. In addition, we

have the responsibility to make as big contribution as

possible to our own agriculture. These are the major

reasons for holding this kind of symposium here in

Vietnam.

Now, please allow me to share with you my

thoughts on future APWSS activities. As one of the

members of APWSS, I am happy to join you all in this

well-organized symposium, in the rice bowl of

Southeast Asia. It is my long-cherished wish to havemore Society activities in addition to the biennial

conferences that we regularly hold. It could be a

symposium, a seminar, or a training course on

subjects considered important by Asian weed

scientists. These activities should enable APWSS to

establish a strong presence in the Asian-Pacific

region. Another way of doing this is putting up our

own communication channela refereed journal or

periodicalthat could promote mutual exchange of

information in weed science and technology in the

region.

Finally, I pray for the success of this symposium,

and I hope that all of you will have a good time here.Thank you.


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1

The rapid increase in population has raised the

demand for food and crop plants. Our ability to meet

the increased food needs of the population depends

on several measures, one of them being how well we

manage competition imposed by weeds on crop

plants. Since 1950, world rice production has in-

creased remarkably. However, there has not been anaccompanying decrease in crop losses due to pests,

including weeds, despite intensified crop protection

measures (Moody 1997). Integrated weed manage-

ment (IWM) is an approach to help crops combat

weeds and obtain higher productivity at the lowest

cost and in the most environment-friendly manner.

This method is similar to integrated pest management

(IPM) and integrated nutrient management. In

practice, increasing rice production inputs such as

fertilizers and intensive cropping provide favorable

conditions for weed development, especially weedy

rice. On the other hand, improving land preparation,

planting, harvesting, and postharvest operations andwater management techniques can either increase

crop yield or control weeds better. Therefore, weed

control methods should be improved with these

different techniques incorporated into the IWM

approach. Another component of IWM is the

judicious use of herbicides. Given the trend toward

increased herbicide use and the likely environmental

and health consequences of such a trend, IPM

involving a combination of several methods is

desirable (Pingali et al 1997).

The Asia-Pacific Weed Science Society has

conferences on weed science every 2 yr. The 16th

conference held in Malaysia in 1997 attracted about

300 participants from 28 countries and from different

international organizations and companies. Of more

than 100 presentations, several reports involved

IWM, including rice-duck, rice-fish, and rubber-sheep

farming systems. Nearly 50% of the technical reports

were on herbicide efficiency and nearly 10% were on

biological control and biotechnology solutions.

Integrated weed management is still on a lower rung

Integrated weed managementand control of weeds and weedy ricein Vietnam

as compared with IPM that employs resistant varieties

and natural enemies. The International Symposium on

Wild and Weedy Rice in Agroecosystems held in

Vietnam (10-11 Aug 1998, Ho Chi Minh City) was

organized by the Asia-Pacific Weed Science Society

and the Cuu Long Delta Rice Research Institute

(CLRRI) and Ministry of Agriculture and RuralDevelopment (MARD). Another conference on weed

science will be held in Thailand. These activities of

the weed science societies (Asia-Pacific or national)

focus on the importance of weed control to enhance

food production.

Weed control for rice productionin Vietnam

Vietnam has a long history of rice production. Rice

has been grown in Vietnam for more than 2,000 yr. An

ancient Vietnamese idiom says, cong cay la cong bo,

cong lam co la cong an,which means withoutseeding, without harvesting.

Vietnamese farmers also have a lot of experience

in weed control. Some of the more popular and

effective methods follow.

Land preparation

Dry and/or wet rototilling (plowing and/or harrowing,

sometimes drainage and leveling, first tillage) is done

to provide good conditions for weed and weedy rice

emergence and to eliminate them by second tillage.

If necessary, a herbicide spray or a third tillage

follows. This method requires more labor inputs.

Farmers in the north have applied it on their trans-

planted fields, which is the best nonherbicide

integrated method for controlling weeds and weedy

rice. In the south, however, especially in the Mekong

Delta, broadcasting is practiced on a large scale, in 3

million out of 3.6 million ha of rice area. Almost all

high-yielding varieties (HYVs) are broadcast under

several rice cropping patterns: two crops yr1, three

crops yr1, and seven rice crops every 2 yr (Luat

Nguyen Van Luat


15/123

2

1991,1994). Because of labor shortages during times

of sowing and harvesting, minimum/no tillage (Luat

1996) or burning of rice straw is commonly practiced.

This induces the development of pests because, after

burning, the pests that survive develop much faster

than their natural enemies (Loc and Heong 1997).

Sowing method

Water seeding. In the Mekong Delta, farmers broad-

cast rice seed, generally pregerminated, when water in

the field is 1050 cm deep. This water-seeding method

has been applied mainly for the winter-spring crop

when flooding occurs and when floodwater recedes at

the end of the wet season. All weed and crop residues

are removed from the field. Algae and other aquatic

weeds are collected by net to allow contact between

the rice seed and the soil surface. Water seeding has

been practiced in large areas every year, mainly in the

Longxuyen Quadrangle and in the Plain of Reeds.

Water seeding has some advantages overconventional wet seeding. Water seeding can be

practiced earlier than wet seeding by 1020 d; farmers

can therefore save on water. In addition weed control

with the use of water is effective. The energy input

can be reduced by nearly half due to drainage saved

at the beginning of the cropping period and reduced

irrigation requirements from tillering to ripening.

Water-seeding input for weed control is also reduced

23.5 times that of wet and dry seeding. By applying

water seeding, the rice crop is harvested 1525 d

earlier and farmers now have more time to take part in

other activities, such as preparing the land or growing

one more short-duration crop such as mungbean,soybean, or sesame. Rice in rotation with upland

crops is the best integrated method to control weeds

and weedy rice. The main constraint of water-seeding

is the damage done to fish and the crab. In addition,

chemicals for control must be used safely (Luat 1996).

Row seeding. The CLRRI has recommended

broadcasting rice in row seeding, saving at least 100

kg seed ha1. Compared with the conventional

broadcasting method, row seeding can increase grain

yield from 0.3 to 1.5 t ha1. This method is also better

in terms of increased resistance to lodging, rats, and

pests, use of solar radiation for photosynthesis, and

controlling weedy rice inasmuch as it is easier to

differentiate cultivated rice plants in rows and weedy

rice between rows. Row seeding is done by using the

improved IRRI seeder (Luat 1997).

Broadcasting of seedlings. This method has

been introduced from China. Seedlings are prepared in

plastic plateswith holes 2 cm in diameter. Two or three

rice seeds are put in each hole, covered with soil, and

irrigated by spraying water. Seedling broadcasting

can be practiced by broadcasting or putting seedlings

in rows. The method has the row-seeding advantage

mentioned above; it also enjoys transplanting

benefits as a result of shortened growth duration in

the field. It is even better than transplanting in termsof protecting seedling root systems from pulling

damage (Luat et al 1998).

Rice production in Vietnamand role of weed control

Present status

Rice in Vietnam accounts for more than 90% of

national food production. From 1990 to 1997, food

production increased yearly by 1.3 million t; however,

the annual increase in demand was estimated to be

0.30.4 million t. Therefore, some 34 million t of

milled rice yr1 had to be exported. The cultivated ricearea is 4.2 million ha, but rice-growing area is more

than 7 million ha. As rice area becomes limiting, rice

yield can be increased quickly. Average grain yield is

still low (3.83.9 t ha1); the yield gap is attributed to

technological and/or sociological constraints that can

be managed or eliminated.

Future outlook

Vietnam aims to meet its food production goal of 32

million t in 2000 and 3840 million t in 2010. Rice

production will still be 90% of total food production

and milled rice exports will remain at 3.54 million t yr1

(MARD 1998). Vietnam has attained great progress in

rice production after establishing a market-oriented

economy. Many constraints need to be overcome,

however, to achieve the national objectives. One of

these constraints is weed damage.

Role of weed control

According to studies on yield loss from biotic and

abiotic stresses, rice yield loss attributed to weeds is

4.3% in the irrigated area and 8.4% in the rainfed area

(Thanh et al 1998). If this yield loss could be reducedeven by half, rice production in the Mekong Delta

would increase by half a million t. The worldwide loss

in rice yield from weeds has been estimated to be

around 10% of total production (Moody 1991).

Besides rational herbicide use, fertilizer applica-

tion and intensive rice cropping, which favor either

rice production or weed development,should be


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3

improved. Better land preparation and seeding

techniques, which increase rice yield and grain quality

and control weeds and weedy rice, should also be

developed.

References

Loc NT, Heong KL. 1997. Effect of crop residue burning on

rice predators: a case study in Vietnam. Paper

presented at the workshop on increasing efficiency of

fertilizer application, 29-30 Jul1997, CLRRI-Mega

Project/IRRI, Cuu Long Delta Rice Research Institute,

Can Tho, Vietnam.

Luat NV. 1991. Process of increasing rice yield in the MRD

from the point of view of agricultural science and

technique. Sci. Activ. Monthly J. p 2-5.

Luat NV. 1994. Progress obtained in research and produc-

tion programmes on rice production in Vietnam

(country report). In: Proceedings of the Eighteenth

Session of the International Rice Commission. Rome

(Italy): Food and Agriculture Organization.Luat NV. 1996. Low-input rice-based cropping systems for

sustainable agriculture. In: Proceedings of the 2nd

Asian Crop Science Conference, 21-25 Aug 1995.

Japan: Asian Crop Science Society.

Luat NV. 1997. Effect of broadcasting and row seeding with

different seed rate on rice yield. Paper presented at a

workshop on increasing efficiency of fertilizer

application, 29-30 July 1997, CLRRI-Mega Project,

Cuu Long Delta Rice Research Institute, Can Tho,

Vietnam.

Luat NV, Loc NT, Nhan NT. 1998. Study on seedling

broadcasting as compared to seed broadcasting. Paper

presented at the regional workshop on methods of rice

seed multiplication, 25 Jul 1998, Cuu Long Delta Rice

Research Institute, Can Tho, Vietnam.

MARD (Ministry of Agriculture and Rural Development).

1998. Annual report for 1997. Vietnam: MARD.

Moody K. 1991. Weed management in rice. In: Handbookof pest management in agriculture. 2nd ed. Boca Raton,

Florida: CRC Press Inc. p 301-328.

Moody K. 1997. Priorities for weed science research. In:

Research in Asia: progress and priorities. Evenson RE,

Herdt RW, Hossain M, editors. Wallingford, Oxon

(UK): IRRI-CAB International. p 277-290.

Pingali PL, Hossain M, Gerpacio RV. 1997. From manual

weeding to herbicide use. In: Asian rice bowls, the

returning crisis? Wallingford, Oxon (UK): IRRI-CAB

International. p 245-256.

Thanh DN, Thuy NTL, Hossain M. 1998. Yield gap

production losses and priority research problem areas

in the Mekong Delta of Vietnam. Paper presented at

the workshop on prioritization of rice research in Asia,20-22 Apr 1998, International Rice Research Institute,

Los Baos, Philippines.


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5

South Asia is a rich source of diversity for rice and its

relatives. The primary area of diversity for rice extends

from the foothills of the Himalayas in Nepal to north-

ern Vietnam (Chang 1976). Previous collecting efforts

for wild Oryza species in southern Vietnam concen-

trated on the Mekong Delta, especially in Dong Thap

Muoi.

Wild rice species and their habitats

Table 1 shows some of the agronomic traits of wild

rice species found in the Mekong Delta of Vietnam.

Oryza granulata

This species can grow on steep, well-drained slopes

where little light penetrates to the forest floor,

especially in deciduous secondary forests. So far, it is

found only in Muong Te, Lai Chau, northern Vietnam.

Oryza granulata has not been crossed successfully

with cultivated rice and is in a distantly relatedsection of the genus, section granulata.

Oryza officinalis

This species (genome CC) can be found in the

Mekong Delta, mostly at the edge of fruit orchards or

under shade in citrus plantations in Tien Giang and

Can Tho provinces. Oryza officinalis is usually found

in moist habitats, such as banks of canals. Though

usually rhizomatous, it has smaller spikelets and more

panicle branches of equivalent length from the lower

panicle nodes. Farmers use this species to prevent

soil erosion in their citrus orchards. Because of the

difference in genome, O. officinalis does not cross

successfully with O. sativa, except in cases of embryo

rescue to exploit the latters resistance to brown

planthopper. This specieshas a wide range of

resistance to pests (Heinrichs et al 1985).

Oryza nivara

This wildannual species has been exploited for its

resistance to viral diseases. O. nivara was found

many years ago in Dong Thap Muoi and Ho Lac,

Vietnams highlands. It can be outcrossed with

cultivars because it has the same genome, AA.

Oryza rufipogon

Very diverse populations of this perennial species

(genome AA) have been found from north to south,especially in the Mekong Delta. Widespread and well-

established populations ofO. rufipogon, together

with annuals such as O. nivara and weedy rice O.

spontanea, are usually found along borders of rivers

and canals of the delta, as well as occasionally in rice

fields or marshes. Wild and weedy rice species are

most common in abandoned fields and village ponds.

Fortunately, O. rufipogon can be found in the

protected sanctuary of Tram Chim (Dong Thap Muoi).

The cultivated rice species (sativa) hybridize with O.

rufipogon or O. nivara; then the hybrids backcross in

either direction and produce morphological inter-

grades. These are known as O. sativa f. spontaneatypes. They invade cultivated fields and pose as

weeds. Oryza rufipogon grows in swamps, often

suspended in water or procumbent on the ground.

The panicles are lax and the spikelets are long (710

mm) and slender (2.2.5 mm wide), well-filled with

anthers, and shatter easily. The shattering is also

noticed in hybrids (O. rufipogon/O. sativa) and

weedy rice, creating problems in rice fields.

References

Chang TT. 1976. Exploration and survey in rice. In: Frankel

OH, Hawkes JG, editors. Crop genetic resources fortoday and tomorrow. Cambridge (UK): Cambridge

University Press.

Heinrichs EA, Medrano FG, Rapusas HR. 1985. Genetic

evaluation of resistance in rice. Manila (Philippines):

International Rice Research Institute. p 159-165.

Collecting wild relatives of ricefrom the Mekong Delta, Vietnam:distinguishing wild rice from weedy rice

Cuu Long Delta Rice Research Institute, Omon, Can Tho, VietnamBui Chi Buu


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Table 1. Agronomic characteristics of wild rice found in the Mekong Delta, Vietnam.

Characteristic O. rufipogon O. officinalis O. nivara Remarks

Vegetative stage

Blade pubescence 3 1 1 1 = glabrous, 3 = pubescent

Blade color 060 060 060 060 = green

Basal leaf sheath color 999 999 060 060 = green, 999 = mixture (green and light purple)

Leaf angle 1 1 1 1 = erect

Ligule shape 2 3 2 2 = V-shaped, 3 = truncated

Ligule color 011 011 011 011 = whitish

Collar color 080 999 060 060 = green, 080 = purple, 999 = mixture

Auricle color 999 061 061 061 = light green, 999 = mixture

Reproductive stage

Culm angle 5 5 1 1 = erect, 5 = open

Node color 060 060 060 060 = green

Internode color 999 999 060 060 = green, 999 = mixture

Culm strength 3 1 3 1 = strong, 3 = moderately strong

Flag leaf angle 3 1 3 1 = erect, 3 = intermediate, 5 = horizontal

Panicle type 9 9 1 1 = compact, 9 = open

Secondary branching 0 0 0 0 = absent, 1 = light

Panicle exsertion 1 1 1 1 = well exserted

Panicle axis 1 1 1 1 = sraight, 2 = droopy

Awning 9 9 9 9 = long and fully awned

Awn color 070 020 070 020 = straw, 070 = red

Apiculus color 070 100 070 010 = white, 070 = red, 100 = black

Stigma color 080 080 080 010 = white, 080 = purple

Sterile lemma color 020 020 020 020 = straw

Leaf length 3 4 3 3 = intermediate (4160 cm), 4 = long (6180 cm)

Leaf width 2 3 2 2 = intermediate (12 cm), 3 = broad (>2 cm)

Culm length 7 7 4 4 = (91110 cm), 7 = (>150 cm)

Culm number 2 2 2 1 = spare (


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Asian cultivated rice (Oryza sativa L.) is believed to

have been domesticated in the Himalayan foothills

(Chang 1976) or the Assam-Yunnan area, which

stretched from northeastern India to southwestern

China (Watabe 1977). This hypothesis has been

supported by several genetic studies showing high

genetic diversity among native varieties in theseareas. For the phylogenetic origin, many researchers

assume a common ancestor for various varietal

groups, although the phylogenetic relationship

remains a controversial subject (Ting 1961, Oka and

Chang 1962, Chang 1976).

Yet, recent archaeological studies in China

suggest that rice cultivation originated in the middle

and lower basins of the Yangtze River. Molecular

genetic analysis of cytoplasmic and nuclear DNAs

suggests differential parentage of the two major

groups of cultivars, indica and japonica. Here, a new

hypothesis based on evidence of the geographic

origin and phylogenies of cultivated rice is described.It is assumed that japonica comes from a progenitor

having the japonica-like nuclear and cytoplasmic

genomes in eastern China, whereas indicais the result

of natural hybridization somewhere else in the tropics.

Phylogenetic relationships of wild, weedy,and cultivated rice

Ancestor of cultivated rice

Hypotheses concerning the phylogenetic origin of

cultivated rice are based on the assumption of either

single or plural ancestry. The monophyletic hypoth-

esis has been widely accepted following the work of

Oka and colleagues. One way of differentiating indica

and japonica from the common ancestor (indica-

japonica differentiation) was illustrated by Oka and

Chang (1962). Progenies having genetic features of

japonica emerged in japonica wild crosses, while

those having indica features were seen in indica wild crosses, suggesting that the monophyletic origin

of indica and japonica is acceptable (Oka and

Origin and evolution of wild, weedy,and cultivated rice

Morishima 1982). The nontendency of indica-japonica

differentiation to occur among wild rice strains

(Morishima and Oka 1981) also supports this conten-

tion. Oka and his school thus concluded that indica-

japonica differentiation occurred after domestication.

Indica-japonica differentiation was considered as a

result of adaptation to different environments.Hypotheses based on diphyletic origin have

been also proposed. Here, japonica cultivars are

considered to have originated in China. Such consid-

eration was based on the literature describing the

existence of wild rice in ancient times (Chou 1948).

Similar arguments have frequently been forwarded but

have never been widely accepted due to a lack of

biological evidence. The diphyletic hypothesis based

on biological data was originally proposed by Second

(1981) and Dally and Second (1990). Both indica and

japonica varieties were considered to have differenti-

ated from different ancestral species by analysis of

more than 40 polymorphic isozyme loci. Cultivarshaving atypical features were regarded as the results

of several natural hybridizations but were not

primitive cultivars (Second 1981).

Nature of indica-japonica differentiation

Many genes for adaptive traits as well as neutral

molecular markers show nonrandom association in

their frequency among cultivars. Cultivars having the

Ph allele (positive reaction to phenol) frequently

show a susceptible reaction to KClO3

and have short

apiculus hairs (Oka 1958, Sato et al 1990). This

nonequilibrium state of allele composition at different

loci shapes two major varietal groups, the indicas and

japonicas. Japonicas tend to have long awns,

pigmentation at the apex of spikelets, more secondary

branches of panicles, etc., while indicas tend to have

the opposite characters (Cheng 1985). In the F2

population derived from an indica japonica cross,

the genes combined in an equilibrium manner. In the

F5

population, however, a nonequilibrium was seen

among the same genes and characters as in cultivars

Yo-Ichiro SatoFaculty of Agriculture, Shizuoka University, Shizuoka City, 422-8529, Japan


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recovered, although the level of the nonequilibrium

was lower than those observed among cultivars. The

state of nonequilibrium of genes and characters

among cultivars was at least partly caused by gametic

selections but not by linkage of the relevant genes or

zygotic selections (Sato et al 1990).

Alleles at independent isozyme loci also showednonequilibrium among cultivars (Glaszmann 1987,

Sano and Morishima 1992). The two varietal groups

shaped by such nonequilibrium show a good

agreement for indica and japonica groupings.

Restriction fragment length polymorphisms (RFLPs)

in nuclear DNAs, however, were identical between

indica and japonica. Two major varietal groups

defined by a computer-generated dendrogram were in

good agreement for indica and japonica (Wang and

Tanksley 1989, Kawase et al 1991). On the other hand,

conventional indica and japonica types as defined by

the size and shape of spikelet and morphological

characters do not fit with varietal groups illustratedby RFLP (Wang and Tanksley 1989). The defined

indica and japonica likely illustrated the phylogenetic

relationship among cultivars vis--vis IRRIs conven-

tional indica and japonica types as far as isozyme and

genomic DNAs are concerned.

Evidence of indica-japonica differentiation in wild

rice

A deletion is known in the ORF 100 region of chloro-

plast DNA (cpDNA) in rice. This deletion is seen not

Table 1. Distribution of cpDNA with and without the deletion at ORF 100 region in species of Oryza

(after Chen et al 1993).

Region or No. of cpDNA typea % of on deletion

Species area Genome samples type

D ND

O. rufipogon South China AA 7 2 5 71

India AA 7 3 4 57

Nepal AA 1 1 0 0

Myanmar AA 2 0 2 100

Vietnam AA 15 2 13 87

Malaysia AA 5 2 3 60

Lao PDR AA 8 7 1 14

Cambodia AA 9 6 3 33

Thailand AA 23 16 7 30

Indonesia AA 1 0 1 100

Sri Lanka AA 2 2 0 0

Philippines AA 1 1 0 0

Oceania AA 1 1 0 0

Subtotal 82 43 39

O. glumaepatula Central and AA 3 1 2 77

South America

O. barthii Africa AA 26 1 25 96

O. longistaminata Africa AA 3 0 3 100

O. meridionalis Australia AA 3 0 3 100

Total 117 45 72

aD = deletion cpDNA type, ND = nondeletion cpDNA type.

only in cultivars but also in wild rice strains. This

deletion is frequently found in annual strains ofO.

rufipogon (or O. nivara), but seldom in other species,

including O. glumaepatula perennis), O. longistami-

nata perennis), and O. meridionalis. The deletion has

been found in the O. rufipogon complex (Chen et al

1993) and was occasionally transferred to indicacultivars (Table 1).

RFLPs in genomic DNAs of wild rice suggest that

indica-japonica differentiation occurred before

domestication began. A comparative cluster analysis

using wild and domestic strains showed indica-

japonica differentiation but not wild-domestic

differentiation. Some of the wild strains collected from

tropical regions were grouped into a cluster of indica

cultivars, whereas others from China, northern

Thailand, Indochina, and upper Myanmar were

grouped into the japonica cluster.

The number of nucleolar-organizing regions

(NORs) in domestic and wild strains conforms withthe hypothesis of diphyletic origin. Indica cultivars

are frequently quadrinucleolar, whereas japonica ones

are mostly binucleolar (Shinohara 1960). Nucleolar

regions are formed at the site of ribosomal DNA

(rDNA), and their numbers are precisely countable by

fluorescent in situ hybridization. Strains of the

japonica cluster tended to have two NORs, while

many strains of the indica cluster have four NORs

(Fukui et al 1994). These facts support the diphyletic

origin of cultivated rice.


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Differential length of introns of the rDNA region

has been observed between indica and japonica

cultivars, as well as indica- and japonica-like wild rice

(Sano and Sano 1990). Such polymorphism in

molecular size of introns suggests the independent

occurrence of two types, judging from the

distranscription feature of introns.The tendency for indica-japonica differentiation

is also detected in the mitochondrial plasmid-like

DNAs among cultivars and wild strains (Kanazawa et

al 1992, Miyata et al 1995). These works indicated that

indica cultivars and annual wild strains tend to fall

into the same group, separate from japonica cultivars

and strains of perennial O. rufipogon. This trend was

also seen in the presence or absence of the deletion

of cpDNA.

Genetic diversity in indica cultivars

Domestication usually leads to a reduction in genetic

diversity within populations. However,indicacultivars show more diversity than japonica cultivars

in isozyme and molecular markers. The mean genetic

diversity among indica cultivars was higher than

among japonicas (Tang and Morishima 1988). Indicas

also showed four times higher genetic diversity in 43

RFLP markers than did japonicas (Zhang et al 1992).

The banding pattern of a polymerase chain reaction

(PCR) marker, ALPHA (Nakamura et al 1990), was

polymorphic in indica but monomorphic in japonica.

Such genetic diversification in indica cultivars

suggests their probable defuse origin. The origin of

the deletion of cpDNAs is unknown. This deletion

has been detected in O. rufipogon and in a few strainsofOryza, but this was not common in other Oryza

species (Chen et al 1994). The cpDNAs of indica

might have emerged later than those of japonica. The

fact that almost all indica cultivars have the deleted

cpDNAs, regardless of nuclear genomes, suggests

restricted derivation; they might be derived from the

hybridization between the progenitor of indica (eu-

indica, as female) and the primitive japonica cultivars

or wild strain(s) having a japonica-type nuclear

genome (as male, see Figure 1).

Differentiation of perennial and annual habits in

wild rice

In the floodplains of big rivers in tropical monsoon

Asia, water levels change remarkably from season to

season. Land higher than the maximum water level is

occupied by terrestrial plant communities, while that

lower than the minimum level is occupied by aquatic

plant species. Population differentiation in response

to habitat variation across the toposequence is

therefore likely.

Life history traits are diversified in wild rice

populations; many strains are classified into annual

and perennial types (Morishima et al 1984). The

annual type tends to have high seed productivity,short anther length, high selfing rate, and short

stature. It has been called O. nivara by some authors.

The perennial type tends to have opposite features

(Oka 1983) (Table 2). The annual strain tends to be

found in more heavily disturbed conditions, such as

fringes of ponds, abandoned fields, etc., while

perennial ones prefer undisturbed places such as the

inside of ponds (Oka 1983). Accordingly, annual types

are adapted to habitats exposed to drought in the dry

season.

On the other hand, the perennial type tends to

inhabit more stable conditions, such as deep water or

marshy areas. Distantly related perennial species alsoprefer stable conditions. Strains ofO. redleyi and O.

minuta are adaptive to shaded conditions in forest

areas.

A significant correlation was observed between

annual-perennial and indica-japonica scores as

defined by isozyme loci among wild rice strains.

Perennial and annual strains tended to have japonica-

and indica-type nuclear genomes, respectively

(Morishima and Gadrinab 1987). Annual and perennial

types have deleted and nondeleted cpDNAs,

respectively (Sato et al 1994). This suggests that the

progenitors of indicas and japonicas had annual and

perennial features, respectively.

Weedy rice and its origin

Some autogamous strains exist as weeds in or within

the fringes of rice fields. These strains of weedy rice

are recognized as harmful weeds in lowland fields,

particularly in direct-seeded habitats. Several weedy

rice accessions are known in Thailand, Malaysia,

Fig. 1.Fig. 1.Fig. 1.Fig. 1.Fig. 1. Three-dimension phylogenetic tree showing the

relationship among wild, weedy, and cultivated strains of

Oryza sativa.

Gene flow

O. sativacomplex

JaponicaIndica

Time

O. rufipogon

Weedy

Indica Japonica


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Vietnam, China, Korea, Brazil, the United States, and

Japan (Ting 1949, Arashi 1974, Morishima et al 1984b,

Suhet al 1997). Weedy rice has become differentiated

from the natural hybridization between cultivars

(Arashi 1974) or between perennial wild and domestic

types (Morishima et al 1984a). In the latter case, gene

flow occurs commonly from the domestic to theperennial wild type, but such an occurrence is rare in

the opposite direction because the perennial type has

less fertile pollen grains and a high outcrossing rate.

The recurrent gene flow of this direction might raise

produce a weedy type having an indica-type nuclear

genome and japonica-type cytoplasmic genome

(Fig. 1).

Table 2. Differential characteristics of annual and perennial types of wild rice O.

rufipogon (after Oka 1998).

Asian forms African speciesAttribute

Perennial Annual Perenniala Annualb

Mode of variation Continuous Different species

Reproductive barrier None F1

inviability and

partial sterility

Resource allocation

Seeds whole plant1 Small Large Small Large

Awns seed1 Small Large Small Large

Pollen seed1 Large Small Large Small

Propagation

Regenerating ability High Low High Low

of stem segments

Seed productivity Low High Low High

Seed dormancy Weak Strong Weak Strong

Awn development Low High Low High

Buried seeds Few ManyOutcrossing rate High Low High Low

Mortality

Seedling Medium High

After tillering Medium Low

Phenotypic plasticity

Seedling growth High Low

Panicle development Low High

Competitive ability High Low

(with a rice cultivar)

Photoperiod sensitivity High Low

Tolerance for

Deepwater (floating) High Low No difference

Drought (seedling) Low High High Low

Submergence (seedling) Low High No difference

Population structure

Between-population Small Large Small Large

varianceWithin-population High Low High Low

polymorphism

Heterozygosity High Low High Low

Sterile plants Many Few Many Few

Habitat Allopatric

Water condition Deep Shallow No difference

Disturbance Low High No difference

Biomass Large Small Large Small

Companion plants Perennial Annual Perennial Annual

aO. longistaminata = O. perennis subsp.barthii. bO. breviligulata.

Geographic origin and distribution ofwild and cultivated rice

Chang-Watabe hypothesis

It has been commonly accepted that Asian culti-

vated rice (O. sativa L.) emerged in an area stretch-

ing across Assam District of India, upper Myanmar,

northern Thailand, northern Lao PDR, and thesouthwestern provinces of China (Fig. 2). Watabe

(1976) and colleagues examined old spikelets mixed

in bricks used in constructions in South and

Southeast Asia and considered that cultivated rice

was domesticated in the Assam-Yunnan area.

Chang (1976) quite independently proposed a

similar hypothesis. Changs hypothesis was


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11

formulated from a standpoint not only of genetics but

also of history, ethnology, ethnobotany, archaeology,

and geography.

The basic ideas on the geographic origin of rice

as proposed by these two workers (otherwise known

as the Chang-Watabe hypothesis) were acceptable to

ethnologists and ethnobotanists. They assumed the

Fertile Crescent as a center of diversity of cultural

components common in East and Southeast Asia,

such as the use of fermented soya paste (miso),cereals having waxy endosperm, silk, Japanese

lacquer (urushi), bamboo, etc. A primitive stage of

agriculture was practiced through the slash-and-burn

method in upland fields. This area was previously

covered by evergreen forest, consisting of

Castanopsis, Quercus, Camellia, and others.

Evergreen forests, compared with deciduous forests,

were regarded as having less capability to produce

edible nuts and bulbs, and were less suitable for

hunting and gathering. Ethnobotanically, it is

considered that rice was initially cultivated with

various varieties of millet under such upland fields. In

this sense, rice was regarded originally as an upland

crop.

Contrary to the Chang-Watabe hypothesis, an

assumption of a diffuse origin of rice (Harlan 1975)

has been proposed by several authors (de Candolle

1886, Chou 1948, Randhawa 1980). These reports

suggest China and India as the center of origin.However, these works have limited genetical evidence

or are supported only by prior literature and as such

are unsubstantive.

Center of genetic diversity

The Chang-Watabe hypothesis has also been

supported by several genetic studies, based on the

arguments of Vavilov (1926). Vavilovs hypothesis

postulates that a high level of genetic diversity in

Fig. 2.Fig. 2.Fig. 2.Fig. 2.Fig. 2. A map showing the origin of cultivated rice.


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cultivated plants will be observed in a particular area

where the species was domesticated from its ancestralspecies. This hypothesis has been widely accepted

by many geneticists as an indication of the likely

geographic origin of a cultivated plant species.

In rice, comprehensive work searching for the

center of genetic diversity was done by Nakagahra

and his group. High genetic diversity was observed in

esterase isozymes among indigenous cultivars

collected from the Fertile Crescent and its surround-

ing areas (Nakagahra 1977). Genetic diversity was

rather poor in areas far from the Fertile Crescent. After

Nakagahra (1977), many reports have been published

that show consistent and convincing evidence that

the Fertile Crescent was indeed the center of genetic

diversity despite critical arguments about the

parallelism between center of origin and center of

genetic diversity.

Remains of ancient rice

According to the Chang-Watabe hypothesis, rice

cultivation should have spread out from the Fertile

Crescent to surrounding areas. In China, rice cultiva-

tion has been shown to diversify from the west to the

east. On the other hand, recent archaeological recordssuggest an opposite direction of dissemination

(Wang 1986). The oldest rice relics have been found

in the middle and lower basins of the Yangtze River

(Fig. 3). Of the archaeological sites excavated, the

Homedu relics, located in Zhejiang Province (approxi-

mately 30 N) and dating back to 5000 BC, are world-

famous because of the massive number of intact

remains, including spikelets and straws of the rice

plant.

Sato (1991) examined rice grains from this source

having tough awns with many well-developed

needles. At the base of these grains, a trace of

abscission layer was observed, suggesting that those

seeds had naturally disseminated after maturation, a

feature representative of wild plants. Such seeds are

likely to be those of wild rice. This fact suggests that

the lower basin of the Yangtze River may be the center

of origin of cultivated rice.

Remains of old rice, on the other hand, are not

recorded in the Fertile Crescent. As far as the avail-

able data are concerned, rice cultivation began about

Fig. 3.Fig. 3.Fig. 3.Fig. 3.Fig. 3. The oldest archeological sites of rice cultivation in the middle and

lower basins of Yangtze River.


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2000 BC. Rice cultivation in China apparently began in

the east and spread to the west. The latest reports

suggest that older rice remains (50006000 BC) were

found in the middle basin of the Yangtze River.

Current archaeological data disagree with the Chang-

Watabe hypothesis.

Several archaeological relics in the tropics that

have traces of rice cultivation have been detected. In

India, many relics have been excavated throughout

the country, particularly in the Ganga Basin, that were

not older than 2200 BC (Randhawa 1980). In Thailand

and Vietnam, some relics are now known that were not

older than 3000 BC. Judging from these records and

based on available data from excavation work, rice

cultivation began not earlier than 3000 BC in the

plains of the tropics (Fig. 2).

Distribution of wild rice now and in ancient times

The geographic distribution of wild progenitors

should be taken into account to determine the origin

of cultivated species. The geographic distribution of

O. rufipogon in Southeast Asia is shown in Figure 4.

It is commonly seen in flooded plains and surround-

ing areas of big rivers in the tropics (Harlan 1975, Oka

1988). In China, well-organized surveys (Anonymous

1984) showed that O. rufipogon is distributed in the

southern region, mainly south of the Tropic of Cancer

(23.5 N). The most northern natural population ofO.

rufipogon is seen in Jiangxi Province (approximately

28 N) (Fig. 4).

The northern limit of the distribution of wild rice

during the Homedu period has been estimated to be

Yangtze River and was recorded several times

Fig. 4.Fig. 4.Fig. 4.Fig. 4.Fig. 4. Distribution of wild rice in Asia.


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rufipogon having japonica-type features (Fig. 5),

judging by the high genetic diversity in genomic

DNAs as well as isozymes.

High genetic diversity in the Fertile Crescent is

explained by the introduction of indica and japonica

cultivars (introduced from the tropics and eastern

China, respectively) and by the consequent naturalhybridization between them. Genetic variation has

been preserved without erosion because of the

complex conditions of climate, geography, and human

races. Sustainable agriculture may certainly play an

important role in their preservation.

Future work involving biological analysis of plant

and animal remains (bioarchaeology) will play an

important role in achieving a more comprehensive

understanding of the origin of rice as well as various

cultivated plant species.

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historically (from AD 230 to AD 1613) in old literature

(Oka 1988). Although some authors might have

confused wild and weedy rice, it is possible that true

wild rice inhabited the area in ancient times. Wild rice

might have disappeared quite recently in the Yangtze

River Basin.

Conclusion

Asian cultivated rice has a diffuse origin, based on

data discussed in this paper. Japonica cultivars

probably emerged in the middle and lower basins of

the Yangtze River about 8,00012,000 years ago. They

are considered to have emerged from a type of

rufipogon having nondeleted cpDNA. At that time,

wild rice may have had a wider distribution under a

warmer and more humid climate than now. Perhaps

domestication began when strains ofO. rufipogon

having relatively high seed productivity were

discovered in drier habitats. Japonica cultivars werediversifying all over China before unification in the

Hang dynasty. Some strains may have reached the

tropics by crossing the Yunnan Mountains. Perhaps

these japonica strains hybridized with indigenous

annual types of wild rice to produce indica cultivars.

The center of origin of indica is still unknown.

Systematic excavations in the tropics, particularly in

Southeast Asia, will be required to obtain a clear

picture of the geographic origin of indica cultivars

and their dissemination process. Although excava-

tions in the tropics lag behind those in China,

cultivation of indica probably began later than that of

japonica. It is also probable that indica emerged as aresult of gene flow from the primitive japonicaor

Fig. 5.Fig. 5.Fig. 5.Fig. 5.Fig. 5. Phylogenetic origin of indica and weedy rice.

Nondeletedcp DNA

Deletedcp DNA

O. nivara

Weedy

IndicavarietiesO. sativajaponica

Natural hybridization

O. rufipugon

X

Japonicanuclear

nivaranuclear

Nu c l e a r r e c o m b i n e d


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Sano Y, Sano R. 1990. Variation of the intergenic spacer

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Rice seed contamination in Vietnam

Rice seed contamination by weed and weedy rice

seeds can result in overall higher production costs

because weed control entails additional expense. At

the same time, it lowers the quality of the harvested

rice crop. Weedy rice is a recent emerging problem in

Vietnam and has serious repercussions on the

countrys rice production. This paper discussesresults of research that analyzed farmers rice seed

contamination by weed and weedy rice and assesses

farmers perception of weeds and weedy rices.

Methodology

Questionnaires were sent to farmers in 18 provinces in

south Vietnam to obtain responses to questions

about weeds and contamination of their rice seeds. In

every province, 20 farmers were randomly chosen for

interview. Basic data were recorded and analyzed

statistically. Rice seed samples (2 kg) were collected

from each interviewee. These samples were analyzed

at the Regional Plant Protection Center of south

Vietnam. About 1 kg of each sample was examined for

the presence of weed seeds. Weed species, including

weedy rices, present as seed in grain samples were

identified and the quantity recorded.

Results

Seventy-three rice varieties were reported to be used,

30 of which were widely grown in the region (Table 1).

Eighty-one percent of farmers kept a seed stock for

the next cropping period. Some of them (about 19%)

exchanged seeds with other farmers or bought seedsfrom other sources. The seed rate used for direct

seeding was 200 kg ha1. The majority of the farmers

(82%) practiced rice monoculture, the remainder

alternating rice with other crops.

Occurrence of weeds and weedy rices

About 95% of the farmers reported that weed

infestations occurred in their fields every season.

Seventy-four percent reported the occurrence of

grass weeds, 68% indicated sedges, 41% reported

broadleaf and other weeds. About 76% of the farmers

said they had rice off-types in their rice fields and a

similar percentage considered weed infestation to bethe most important cause of yield loss and seed

contamination. When questioned about the origin of

rice seed contamination, insects were considered as

causal agents by 22% of farmers, diseases by 42%,

and weeds by 75.5% of those asked. Farmers also

stated that weed seeds came from the soil (65.0%),

were mixed with the rice seed (83.0%), but also came

from other sources (61.6%). All farmers agreed that

seed contamination by weed seed caused yield loss.

About 70% of the farmers said that grass weeds were

the most common and the most damaging weeds in

rice fields.

Weed management practices

Eighty percent of the farmers believed that rouging

rice off-types before harvest was an effective measure

to produce a clean crop seed. No less important was

removal of weed flower heads before seed set. Most

farmers (94 %) winnowed seed after harvest and many

Table 1. Rice varieties commonly grown in the south

Vietnam region in 1997.

OMCS 94 IR66707 IR9729-67-3

IR50404 IR64 (Mutation) IR13240-10-1IR56279 OM1305 IR29723

HT94 OM95-5 IR49517-23

IR64 OMCS 90-9 IR56279

OM1706 OMCS 96 IR59656

OM997-6 OM1303 OMCS1270

CL 7 (Cuu long) IR35546 IR42

Tai-nguyen TN128 OMCS576

OM1633 OMCS97 CL 8 (Cuu long)

Vo Mai, Ho Van Chien, Vo Van A, Vo Thi, Thu Suong, and Le Van ThietPlant Protection Department of South Vietnam.,Ho Chi Minh City, Vietnam


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Table 2. Results of the analysis of farmers rice seed, south Vietnam, 1997.

% contaminant grain type1 Grains kg1 seed

No. Province Samples Germination Moisture

(no.) rate (%) content (%) Empty Off-type Deformed Discolored Weed Weedy

grain grain grain grain rice

1 Binh Thuan 23 94 14.32 2.77 14.64 2.40 19.40 275 212

2 Dong Nai 20 84 14.67 6.65 1.74 5.20 11.40 417 194

3 Binh Phuoc 19 61 14.25 4.65 4.01 2.00 22.40 194 80

4 Binh Duong 9 86 13.96 3.03 2.25 2.40 14.80 157 100

5 Tay Ninh 12 88 14.05 8.27 2.36 2.00 22.40 518 450

6 Tp. HCM 25 83 14.37 4.39 1.68 3.20 17.40 558 489

7 Long An 18 55 14.68 7.38 2.69 3.80 24.60 345 312

8 Tien Giang 20 87 13.90 4.97 3.84 4.20 28.80 445 4169 Ben Tre 19 65 14.90 5.25 11.40 3.00 19.40 598 467

10 Dong Thap 28 81 13.76 6.90 1.11 3.20 33.00 697 400

11 Vinh Long 16 79 15.00 4.70 1.85 5.00 14.60 1096 500

12 Tra Vinh 23 77 15.10 5.40 2.03 2.60 17.40 610 480

13 Can Tho 20 80 13.79 4.91 1.48 1.00 13.00 825 462

14 Soc Trang 24 85 14.90 6.58 1.73 3.00 16.60 318 220

15 An Giang 22 87 13.75 6.17 1.66 4.20 17.20 353 250

16 Kien Giang 19 91 15.10 5.09 4.89 3.20 10.00 491 273

17 Bac Lieu 14 73 15.20 3.36 3.27 2.20 20.60 302 200

18 Ca Mau 20 72 14.60 2.68 1.42 2.80 14.40 200 150

Total 351 Av 80 Av 14.46 Av 5.17 Av 3.35 Av 3.13 Av 18.74 Av 466Av 314.16

(86%) also practiced re-winnowing before seeding but

only a small percentage (14%) indicated that winnow-

ing was completely effective in removing contami-

nants. Avoidance of seed mixing was also considered

as an effective way to prevent weed seed contamina-

tion. About eighty-five percent of farmers attempted

removal of weed seeds before seed soaking for directseeding.

Weed control measures varied within the survey

group: herbicides were used by 82% of farmers but

68% also weeded by hand, and water management for

weed control was mentioned by 17%. Most (93%)

farmers kept their fields inundated with water to a

depth of 15 cm from 710 d after sowing (DAS).

Hand weeding was done mostly by women (76%), this

being done 2030 DAS. On average, 1.5 herbicide

applications were made to each crop, with the first

application being made at 0-10 DAS; 80% of the

farmers followed this practice. Ten different kinds of

herbicides were widely used in the region; of these,the most popular were pretilachlor and 2,4-D.

Farmers perception of weedy rice

Nearly 70% of the farmers interviewed said they haveseen weedy rice on the fields, mostly during the

summer-fall crops and 64% indicated harvested rice

was contaminated with weedy rice. Prevalence of

weedy rice was most associated with dry seeding

(35.5% respondents), wet seeding (32.0%), seeding

without soil preparation (18.8%), and transp

wild and weedy rice in rice ecosystems in asia --- a review

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