Biology and management of barnyardgrass, red sprangletop and weedy rice

REVIEW

Biology and management of barnyardgrass, red sprangletop and weedy rice

DUONG VAN CH IN

Cuulong Delta Rice Research Institute,Omon,Cantho,Vietnam

About 400 weed species of 73 families have been reported to occur in upland and lowland

rice ?elds in Vietnam.T wo important families are Poaceae and Cyperaceae having 42% of

weed species with 21% each.The barnyardgrass,Echinochloa crus-galli(L.) Beauv.,is the most

important weed in both transplanted and direct-seeded rice in this country.The competi-

tion of 25 barnyardgrass plants/m2causes approximately 50% yield loss.Red sprangletop,

Leptochloa chinensis(L.) Nees.,is an emerging major weed in direct-seeded rice.W eedy rice

(Oryza sativa L.) was detected in 1994 and is now a new dangerous pest in rice ?elds.The

main reasons are:herbicides which can kill weedy rice completely are not available in the

market,non-chemical methods are laborious and costly,and weedy rice can develop a new

generation from contaminated seeds through rice sowing or emerging from soil seed bank.

Research results on the Vietnamese situation of barnyardgrass,red sprangletop and weedy

rice in terms of biology and management by chemical and non-chemical methods have been

reviewed in this paper.

Keywords:biology,Echinochloa crus-galli,Leptochloa chinensis,management,weedy rice

INTRODUCTION

Rice is the most important crop in Vietnam with more than 7million ha sown and 29 million tons produced every year.Paddy farming is the main source of employment for the majority of the rural population. There are 4.199million ha for rice production sharing 74.1% of arable land in the whole country.T otal production in the year 1998 was 29.141million tons (Statistical Publishing H ouse 1999).Vietnam produced 31.4million tons of rough rice and exported 4.5million tons of milled rice in 1999.Rice is grown in all eight agro-ecologic zones of the country,but the two main regions for rice production are the Mekong River delta in the south and the Red River delta in the north.The average rice yield is still low compared with many other countries.There are some abiotic and biotic factors contributing to the low yield.W eed infestation is a major biotic factor affecting rice growth and yield compared with insects and diseases.In order to understand the biology and ecology of rice weeds for the better management in the future,the biology and management of three important rice weeds,including barnyardgrass,red sprangletop and weedy rice,has been reviewed.

BARNYARDGRASS

Biology

Barnyardgrass is a very competitive annual grass with a height of up to 1–2m.The stem is stout and tufted. Culms are erect and ascending with wide phenotypic varieties.Its leaves are linear and lanceolate up to 40cm long and 5–15mm wide.Ligules are absent.The barnyardgrass in?orescence is a green-to-purplish apical panicle of 5–40 spike-like racemes.Spikelets are elliptical and pointed,3–4mm long,awned or awnless with wide variations.It ?owers all year round and propagates by seed.Barnyardgrass prefers wet soil with a high nitrogen content and sunny places.This weed

Correspondence to:Duong V an Chin,Cuulong Delta Rice Research Institute,Omon,Cantho,Vietnam

Email:duongvanchin@hcm.vnn.vn

Accepted 14 December 2000

Weed Biology and Management2001;1:37–41

D.V.Chin 38

grows in rice ?elds,water edges and marshes.It is one of the most damaging weeds of rice and other crops globally (Koo et al.2000).In direct-seeded rice in the Mekong delta,the relative densities were ranked into four classes:absent (0%),rare (<10%),common (10–70%) and dominant (>70%).The species E.crus-galli was dominant at more than 50% of total 197 sites surveyed and common at over 35% of the remaining sites (Chin et al.1998).

Barnyardgrass is a very strong competitor with rice. Results from pot experiments at the Cuulong Delta Rice Research Institute (CLRRI) showed that dry matter accumulation of two,four,six and eight plants of the weed were 123.0,172.0,168.0 and 262.0g/pot re-spectively.They competed with rice causing lower yield.The corresponding data of rice yields were 79.0, 48.6,55.0 and 34.3g/pot compared with the weed-free control of 117.6g/pot.In another investigation Trung et al.(1995) reported that in transplanted rice,the infestation of less than ?ve plants/m2of barnyardgrass did not cause yield reduction.Yield losses range from 7 to 13% in ?ve to 10 plants/m2to 23–27% in 15–35 plants/m2.In wet-seeded rice during the summer–autumn season,populations of seven,11,99 and 258 plants/m2(corresponding to dry weights of 64,126, 233 and 484g/m2) caused yield losses of 10.5,15.8, 38.8 and 86.8% respectively.The corresponding data in the winter–spring season are that 87,254 and 340 plants/m2(corresponding to 188,623 and 943g/m2) caused yield losses of 19.0,50.0 and 63.5% respectively. The germinating ability of barnyardgrass seed is affected by water depth.Its seeds need 4.2days to germinate under 1–3cm water,but 10days to germinate at 10cm depth (Son 2000).W ater depths of 5–12.5cm can suppress the germination and emergence of grasses including barnyardgrass successfully (Chin 1995). Keeping water depths of 10–20cm during the ?rst 30days after sowing (DAS) was found to inhibit the emergence of barnyardgrass.After harvesting of summer–autumn rice,the total number of barnyard-grass seeds 0–15cm deep in soil was 8171 seeds/m2. The number of seeds is highest in 0–5cm soil depth (3587 seeds/m2) followed by those in the depths of 5–10cm (2803 seeds/m2) and 10–15cm (1781 seeds/m2).The germination percentage is highest 1–2 months after burying.It continue to reduce from the third month (30%) onwards and remains at only about 10% germinated seeds after 12months.The percentage of viable seeds differs at the two soil depths of 5cm or 10cm.The viable seed percentage tested by tetrazolium chloride staining shows that this percentage tends to reduce each month and remains at only 14% after 12 months buried compared with 98% before burying (Hach 1999).

Management

In Vietnam,the majority of farmers (81.3%) prepare seed stock themselves.The rest exchange seeds with neighbors or buy certi?ed seeds from companies.The amount of certi?ed seeds used for rice production in Vietnam is less than 5% of the total rice seeds needed every year.The quality of self-supplied seeds is very poor.In the south of Vietnam,the number of weed seeds,including barnyardgrass,per kg of rice seeds is 466,46.6 times higher than the permitted level (Mai et al.1998).In order to check this detrimental trend of using contaminated seeds,last year the government of Vietnam launched a national program of rice seed production with a total budget of 1.07million $US to substantially increase the amount of certi?ed seeds for farmers.In the south of Vietnam,weed infestation in dry-seeded rice is more than that of wet-seeded due to insuf?cient and ?uctuating water at the early stage of crop establishment.Early introduction of water at 4DAS resulted in reduction of barnyardgrass infestation in rice yields.Rice yields under treatments of early water introduction at 4 DAS are 30.3% and 14.8% higher than those in late irrigation treatments at 12 DAS in both the summer–autumn and winter–spring seasons (H ach 1999).Good land preparation provides weed-free conditions at planting and also provides a favorable environment for the growth and development of the crop.In transplanted rice in the Red River delta, ploughing followed by harrowing before transplanting resulted in signi?cant reduction of the number of weed species and also the level of perennial weed infestation. Rice yields under plowed twice (4.63–4.77tons/ha) are higher than those plowed once (4.3–4.35 tons/ha) (Son 2000).In wet-seeded rice in the Mekong delta,one long-term experiment with seven treatments was conducted to ?nd out the direct and cumulative effects of tillage practices on weed infestation and soil seedbank.The seven treatments were:(i) zero-tillage with straw burning;(ii) zero-tillage without straw burning;(iii) dry ploughing +harrowing;(iv) dry ploughing +harrowing +puddling;(v) dry rotary cultivation +puddling;(vi) dry rotary cultivation + puddling +levelling;(vii) wet rotary cultivation + puddling +levelling.Results revealed that under zero-tillage treatment without straw burning the dry matter accumulation by weeds,including barnyardgrass,is highest.The infestation of perennial weeds is also high

39 Barnyardgrass and others

under this treatment.Zero-tillage as well as dry rotary practices caused high weed infestation resulting in low yield of rice.W et rotary cultivation +puddling + levelling is an excellent land preparation method for minimizing the damage of weeds in general and barnyardgrass in particular and increasing rice yield (Hach 1999).Hand weeding continues to be practised in rice production in Vietnam.The work is more dif?cult in direct-seeded rice as compared with transplanted rice.It is very dif?cult for farmers practising direct-seeded rice cultivation to differentiate between cultivated rice and barnyardgrass seedlings at early vegetative stage.In the case of no herbicide application,farmers usually hand weed three times per season at 10–15,25–30 and 40–45 DAS.In total,it takes about 150–200 person-days/ha to keep rice crop free of major weeds (Trung et al.1995).The majority of farmers in direct seeding areas of south Vietnam use herbicides (82.2%) but they also do hand weeding (85%) as a supplemental method.Normally,women (75.8%) do this laborious and tedious work (Mai et al.1998).The competition of only barnyardgrass with a density of 175.8 plants/m2and a dry weight of 150g/m2 caused yield reduction of 60% as compared with land that had been hand weeded twice.The data of rice yields under those treatments are 2.16 and 5.46 tons/ha,respectively.The trend of using herbicides to control weeds in rice has increased in Vietnam.

H erbicide application is one essential component in integrated weed management.There are 35 compounds or proprietary mixtures formulated in commercial products with 79 trade names which currently are being used for rice in Vietnam (Ministry of Agriculture and Rural Development 2000).Out of these,the 16 most popular rice herbicides are:thiobencarb,bensulfuron methyl,bispyribac-sodium,butachlor,butachlor + propanil,cyhalofop-butyl,2,4-D,ethoxysulfuron, fenoxaprop-P-ethyl,fenoxaprop-P-ethyl +2,4-D + MCPA,metsulfuron methyl,metsulfuron methyl + bensulfuron methyl,pretilachlor,pyrazosulfuron ethyl, oxadiazon,propanil,thiobencarb +propanil.Barnyar-dgrass is susceptible to almost all these 16 popular compounds except:2,4-D,ethoxysulfuron,metsulfuron methyl.Marginal bene?t–cost ratios in the treatments of herbicides or herbicides plus hand weeding are higher than that of hand weeding alone.H erbicide-resistant barnyardgrass has not been reported in this country so far.Bioherbicide research has made very signi?cant progress in Vietnam.Promising fungi which have been found to control barnyardgrass,the most important weed in rice,are Exserohilum monoceras and Cochliobolus lunatus.At the rate of 7 ￥107spores/m2of E.monoceras and C.lunatus,100% death after 14 days was recorded with a dew period of 48h.Rice varieties IR50404 and CR203 were not affected by the fungi.

RED SPRANGLETOP

Biology

Red sprangletop is a strongly tufted annual grass with height up to 1m.Its culm is slender,erect or ascending from a branching base.The leaves of this weed are smooth and linear,10–20cm long,?at,acute,thin, membranous,rough on the upper surface.Ligule length is around 1–2mm.Its in?orescence is a narrowly ovate panicle.The main axis is 10–40cm long.Branches are simple,numerous,spreading and 5–15cm long. Spikelets are subsessile with three to seven pale green or reddish ?owers.This weed is propagated by seeds.It is commonly found in direct-seeded rice ?elds (Koo et al. 2000).The dry matter accumulation by two,four, six and eight plants of red sprangletop were 63,89,150 and 159g/pot.Rice yields were 43.8;22.0;21.0 and 13.8g/pot respectively compared with a weed-free control (97.7g/pot) (Chin 2000a).In one experiment at CLRRI the population of only red sprangletop in untreated controls at 56days after sowing was 298.9 plants/m2as compared with that of an area which had been hand weeded twice (1.7 plants/m2).The corres-ponding data on weed dry weights are 31.6g/m2and 0.2g/m2.Red sprangletop infestation in the summer–autumn season is higher than that of the winter–spring season because in the latter there is plenty of water in the rivers and farmers can pump it into the ?elds to suppress this weed.Good land preparation and well levelling is very important for checking of red sprangletop infestation.In that situation,water depth can be uniform throughout the ?eld to suppress weed and interact with herbicides to get better results.In the case of bad levelling,some high land pockets exist in the ?eld.A small area of dry conditions is a niche for the weed to germinate,emerge and grow profusely.One ef?cient method for checking this weed is to use cyhalofop-butyl at very high rate of 160–200g active ingredient (g.a.i.) per ha as spot treatment at 15–25 DAS.Even very old plants of red sprangletop at the end of vegetative or nearly ?owering stages also can be killed completely by this herbicide (Chin 2000b).Rice is very safe at this very high rate.Spraying 106spores/mL of the fungus Setosphaeria rostrata at seven,14,21 and 28 DAS resulted in dead weed percentage of 99.4,97.7,95.0 and 89.2% observed at seven,14 and 21 days after application (DAA) (Thi et al.1999).

D.V.Chin 40

WEEDY RICE

Biology

The scienti?c name of weedy rice (red rice) is Oryza sativa(L.).It is an annual herb 0.5–1.7m high or more. The in?orescence,panicle,spike are yellowish.W eedy rice has single ?ower with six stamens and usually husks are awned.Its caryopsis sticks strongly on husks with red pericarps and is easily shattered.Normally,this weed is taller than cultivated rice.Its seeds contaminate rice seeds and increase seeds in the soil seed bank.W eedy rice may come from degraded cultivated rice or crossing between cultivated rice and wild rice such as Oryza nivara or Oryza ru?pogon.It is an important weed in direct-seeded rice in Vietnam (Koo et al.2000). Results from a regional survey of south Vietnam in 18 provinces with 4397 respondents revealed that farmers produce seed stock for themselves from rice products. Farmers (63.8%) said that they observed weedy rice existence in their ?elds.

The most severe period is the summer–autumn season. W eedy rice causes yield loss and reduces the quality of milled rice (79.8%).The average loss ranges from 15 to 17% and the highest one reached 70%.The farmers recognized weedy rice by characteristics such as:taller plants (90.6%),awned seeds (89.5%),easy scattered seeds (73.6%) and small seeds (76.8%).The monitoring data of 100 lines of weedy rice were directly collected in one province in the coastal area of central Vietnam. Results revealed that there were 34.3 weedy rice plants/m2and 252 cultivated rice plants/m2.The corresponding data is 86.9 tillers/m2and 519.6 tillers/m2,respectively.Some characteristics of weedy rice,which is generally larger than cultivated rice,are: taller plants (112.5cm compared with 74.3cm in cultivated rice),longer ?ag leaves (30.8cm cf.21.9cm), longer panicles (20.1cm cf.17.2cm) and higher numbers of grains per panicle (53.1 cf.34.4).Normally, cultivated rice has no awn whereas the average awn length of weedy lines is 2.57cm (Chin 1997).The dormancy period of weedy rice ranges from 50 to 65 days after harvesting (DAH).W eedy rice seeds lose their seed viability faster in moist conditions as compared with submerged ones.The percentage of viable seeds of weedy rice after 4 months of burying is higher than that of cultivated rice in both moist and submerged conditions (Chin et al.2000).The number of con-taminated weedy rice seeds is tremendously high at 314 seeds/kg rice seeds (Mai et al.1998).Field experiments conducted at two provinces in the Mekong delta revealed that the number of weedy rice tillers produced in the ?elds does not correspond to the seeds sown.Under treatments of 10 seeds/m2,the number of tillers is only 14.0 and 10.3 tillers/m2in both locations.The corresponding data for the treatment of 1000 seeds/m2are 487.0 tillers/m2and 422.8 tillers/m2respectively.Yields of cultivated rice start to reduce at the level of 12.2 tillers/m2.The competition of weedy rice from one to ?ve tillers/m2is not strong enough to cause rice yield reduction.Under the average level of 454.9 tillers/m2or 552g/m2of weedy rice,yields of cultivated rice are only equal to one-sixth of those in weedy rice-free control (Chin et al.2000).

Management

Vietnamese farmers have the tradition of removing weedy rice plants during cropping season.A proportion of farmers (23.2%) practise it during the vegetative stage,but the majority (80.6%) do the work at the booting to ?owering stage.This activity minimizes the contamination of weedy rice seeds into rice seeds and reduces dropped seeds onto soil seed bank.H owever, this method is only partly effective (Chin et al.2000). The national program with the budget of US$1.07 million for the production of large amounts of certi?ed seeds to satisfy the demand of farmers was launched in 1999 throughout the whole country.W ater at a depth of more than 5cm can suppress the germination and emergence of grass including weedy rice.Adoption of wet seeding at a water depth of 20–40cm in acid sulfate soil areas during the winter–spring season leads to low infestation of weedy rice.On the Ca Mau peninsular,traditionally,farmers practise dry seeding in the summer–autumn season.Recently,some of them have shifted to wet seeding by keeping rain water in the ?eld at the beginning of the monsoon season to a depth of 10–15cm and puddled the soil before broadcasting (Chin 2000a).The other method,namely modi?ed water seeding,has been introduced.In this method,pre-germinated seeds are sown onto well puddled soil without excess water but the ?eld will be watered as soon as possible after sowing.The introduction of water at 4days after sowing (DAS) results in less weed infestation than that of 12 DAS (H ach 1999).The method of row seeding can save above 100kg seeds/ha and minimize the damage due to insects,diseases and lodging.W eedy rice and volunteer rice plants grown between two rows of cultivated rice can be differentiated and removed easily by farmers during the vegetative stage.This work could not be done previously in randomly broadcasting (Luat et al.1998).Under row seeding,rice yields increase from 0.5 to 1ton/ha particularly in the summer–autumn season (Quan

41 Barnyardgrass and others

1999).The method of seedling broadcasting was introduced into Vietnam recently.The 12- to 15-day old seedlings are broadcast by hand randomly in the ?elds submerged in water at 5–7cm depth.This layer of water suppresses weeds and weedy rice at the beginning of crop establishment (Chin et al.2000).Growing a short duration upland crop in rotation with rice is a good method for checking the menace.Suitable moisture in the soil for upland crops encourages weedy rice seeds to germinate,emerge and be killed by herbicides used for upland crops or by other weeding methods.Farmers who introduced mungbean cropping in the dry season, observed a decrease in weedy rice a few years after serious infestation.They found that many weedy rice plants had emerged and died due to insuf?cient soil moisture during the mungbean season (W atanabe et al. 1998).Some herbicides have the ability to minimize the damage due to weeds.They are as follows:application of thiobencarb @ 300g.a.i./ha within 3–5days after drainage and before the reintroduction of water results in good weedy rice control (Sadohara et al.1998). Oxadiazon @ 240g.a.i./ha applied at 6days after rice seeding controlled weedy rice with slight rice injury under drain conditions (Pyon et al.1998).Molinate @ 4.500g.a.i./ha incorporated into the soil right before sowing gave satisfactory control of weedy rice in wet-seeded rice (Azmi et al.1998).Experimental results at CLRRI showed that oxadiazon @ 250g.a.i./ha, oxadiargyl @ 72 and 100g.a.i./ha caused signi?cant reduction in dry matter accumulation by weedy rice plants.Rice yields under treated plots are signi?cantly higher than those of untreated controls (Chin et al. 2000).

ACKNOWLEDGMENT

W e wish to thank the Japan International Research Center for Agricultural Science (JIRCAS) for sup-porting us to go to Japan to present this paper on the occasion of launching the ?rst all-English version of Weed Biology and Management.

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