J- Agronomy & Crop Science 175, 1—5 (1995)© 1995 Blackwell Wissenschafts - Verlag, BerlinISSN 0931-2250

Department of Crop Science, Faculty of Agriculture, University of Peradeniya, Sri Lanka

Impact of Different Weed Types on Growth and Yield of Mungbean(Vigna radiata L. Wilczek)

U. R. S A N G A K K A R A , B . M E Y L E M A N S and P. VAN D A M M E

Authors' addresses: Dr. U. R. SANGAKKARA and Dr. B. MEYLEMANS, Department of Crop Science, Universityof Peradeniya, Peradeniya, Sri Lanka; Dr. P. VAN DAMME, Faculty of Agricultural Sciences, University ofGhent, B 9000, Belgium.

With 3 tables

Received September 19, 1994; accepted November 26, 1994

Abstract

The influence of different weed types (grass, broadleaved and sedge species) on growth and yield parametersof mungbean {Vigna radiata L. Wilczek) was evaluated under field conditions. The presence of all types ofweeks had the most significant effect on growth and yields of mungbean. Broadleaved species alone had agreater adverse effect than grasses, and the influence of sedges was the lowest. The adverse effect of weedswas most prominent on vegetative growth. The influence on yield components reduced with time. However,the adverse impact of weeds on yields was similar to that on vegetative growth. A significant correlationbetween weed biomass and yield loss was observed. The study indicated the critical competitive period to bein the vegetative phase. Some possible implications for selective weeding are presented.

Key words: Mungbean, weeds, grasses, broadleaved species, sedges, growth, yields

Introduction

Food legumes are important components infarming systems of most developing countries.Their ecological and nutritional value arisesfrom their ability to produce protein rich foodand high quality organic matter and fixatmospheric nitrogen (APO 1982, SiNGH1985). However, most legumes grown in thedeveloping world are cultivated on marginalsoils, unsuited for cereals or other industrialcrops, due to their wide adaptabihty (WOODand MYERS 1987).

Due to the planting of food legumes in a widerange of environments, the success of mostcultivations is hindered by weeds (MoODY1986). Weeds can reduce yields of legumes sig-nificantly, at times leading to crop failure. Thisproblem is most significant under tropicalsmallholder farming conditions where the crops

are grown under sub-optimal conditions(SWARBRICK and MERCADO 1987).

Current trends in weed management of cropsemphasize the need for integrated approaches(ZANIN et al. 1993). The acceptance of a cenainquantity of weeds in a crop is now a commonphenomenon, although weed populations aremaintained within precise limits (COUSENS1985, 1986). Farmers are thus encouraged toadopt suitable weed control measures to over-come competitiveness rather than clean weedplots, especially on marginal lands. This wouldnot only reduce expenditure, but makes taskseasier and protects soil from erosion. Thus, theconcept of weed control thresholds has beendeveloped (COBLE and MORTENSEN 1992).

Tropical weeds are classified as grasses,broadleaved species and sedges (SWARBRICKand MERCADO 1987). However, studies on theeffects of these categories on growth and yieldsof tropical food legumes have not been widely

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S A N G A K K A R A , M E Y L E M A N S and VAN

reported. This is due to the application ofchemical herbicides, which tend to removemost species at early stages. In contrast, todevelop sustainable management systems, thecompetitiveness of different weed types needsto be identified beforehand to enable the fore-casting of potential weed control measures(ZANIN and SATTIN 1988). A field study wascarried out to define the influence of differentweed types on the growth and yields of mung-bean, a popular food legume in the tropics.

Materials and Methods

The experiment was carried out at the AgriculturalResearch Unit of the University of Peradeniya, SriLanka, over the dry season of 1993 (May to August).The mean climatic parameters of the season were476 mm rainfall, a mean daily temperature of 29.1 °C(±1.6''C) and humidity of 78.4%. The soil of thesite, which had been cultivated for several seasons,was an Ultisol (Rhodudult/Tropudult) with a sandyclay loam texture, pH (1:2.5H2O) 6.18 ±0.17,organic carbon content of 0.96 % ± 0.02 % and aCEC of 12.36 m.eq 100 g"' soil.

At the onset of the season, the soil was well pre-pared and all weeds were removed. Plots (dimensions3 X 4 m) were laid and mungbean {Vigna radiata L.Wilczek — variety MI 5) was planted at a distance of20 X 10 cm and fertilized as recommended by theDEPARTMENT of AGRICULTURE (1989). Irrigation wasprovided during the latter pan of the season, in theabsence of 10 mm rainfall for 4 consecutive days.

Weeding was carried out manually, as in mostsmallholder fanning systems, at 15, 30 and 45 daysafter planting and at the final harvest. The treatmentsimposed during the weeding were:

removal of broadleaved species and sedgesremoval of grasses and sedgesremoval of grasses and broadleaved speciesremoval of all weedsno weed control.

The experiment was laid out as a randomized blockdesign, with three replicates per treatment.

The establishment of mungbean was determmedwith a fixed quadrant of 1 x I m. Thereafter, at 7 dayintervals, four plants were carefully removed fromeach plot until flowering (Rl growth stage) and drymatter was determined to calculate relative growthrates as described by FAGERIA (1992). Yield com-ponents and yields of mungbean were determined on10 tagged plants per plot.

At the final harvest (R8 growth stage), the popu-lation of weeds in the different treatments wereremoved from an area of 3 x 2 m from each plot anddry weights determined. The species of plants in plotsnot weeded were classified into grasses, broadleavedspecies and sedges prior based on morphologicalcharacteristics (SwARBRICK and MERCADO 1987)prior to drying. All dry weights were determinedafter desiccation at 80 °C for 48 h.

The data was subjected to statistical analysis usinga general linear model as described by GOMEZ andGOMEZ (1981) to determine the significance of treat-ment differences.

Results and Discussion

Weeds did not affect the establishment ofmungbean seedlings (Table 1) due to the com-plete removal of weeds prior to planting. Asmungbean seedlings emerge and establishwithin 8-10 days, the emerging weeds did notaffect this process. In contrast, the presence ofall categories of weeds over the vegetative phasereduced relative growth rates of mungbean by70 % when compared with that of plants inweed free plots. Competitive relationshipsbetween species develop in the early growthstages (GOLDBERG 1990) and thus the presenceof weeds over this phase had the most signifi-cant impact on mungbean. Among the differentcategories of weeds, broadleaved species hadthe most significant impact on relative growth

Table 1. Growth of mungbean as affected by types of weeds

Treatment

GrassesBroadleavesSedgesClean weedingNo weedingLSD (P = 0.05)CV %

93.092.094.594.091.0NS

18.54

Growth Rate

0.1060.0840.1450.2160.0650.0149.96

Branches/Plant

5.24.95.35.73.41.34

17.52

Impact of Weeds on Mungbean

Table 2. Effect oi

Treatment perplant

F weeds on yield components of

Flowers Pods % Pod

mung

set

bean

Seed/Pod 100 Seed wt (g)

3

Yield (g)

GrassesBroadleavesSedgesClean weedingNo weedingLSD (P = 0.05)CV%

15.511.221.024.0

8.14.05

11.49

12.79.5

16.619.56.22.14

15.42

81.984.879.080.576.45.99

10.94

7,26.19.29.05.40.48

14.98

4.944.745.485.824.040.197.42

5.183.418.449.032.331.42

21.06

rates (a 60 % reduction in comparison to thatof weed free plots). Sedges had the least impact.As competitive effects between similar speciesoccur at greater intensity (AARSEN and EPP1990), the presence of broadleaved species alsohad the greatest adverse effect on mungbean.

The impact of weeds on branching habit wassimilar to that on relative growth rates,although the influence was less prominent.While the presence of all weeds or broadleavedspecies significantly reduced the branching ofmungbean when compared to plants in cleanweeded plots, the other species alone did notaffect branch numbers, which is an importantcharacteristic in yielding behavior in mungbean(LAWN and A H N 1985).

Weeds influenced yield components andyields of mungbean significantly. The presenceof all weeds lowered flower and pod numbersby 68 % (Table 2). Broadleaved weeds reducedthese parameters by some 50 %. The impact ofsedges on these parameters was lower than ofgrass weeds. However, the percentage pod setwas not affected by the presence or absence ofweeds.

Weeds reduced seed numbers and 100 seedweights, although the impact was not assignificant as on relative growth rates, flowernumbers and pod set. For example, the presenceof all weeds reduced seed numbers by 40 % andseed weights by 30 % when compared with thatof plants in clean weeded plots. Amongst thedifferent categories, the broadleaved specieshad the most significant impact, while thegrasses, and especially the sedges, had a minimalimpact on these parameters. Thus, in thepresent study, the impact of sedges was verylow on growth and yield components of mung-bean.

Table 3. Dry matter content of weeds in thedifferent treatments at final harvest

Treatment Weed dry matter (g6 m~ )̂ ± SE

GrassesBroadleavesSedgesAll weeds'-"

197.58 ± 8.46252.95 ±20.4086.52 ± 14.59

527.96 ± 26.24

'•" The dry matter of all weeds included224.94 g± 10.84 grasses, 68.85 g± 7.29 sedgesand 234.16 g ±21.24 broadleaved species respect-ively.

Vegetative growth influences yields of allcrops (FAGERIA 1992). This is confirmed inmungbean by the study data. The presence ofall weeds reduced per plant yields of mungbeanby 71 % when compared to that of cleanweeded plots. This reduction is similar to thatobserved in relative growth rates (Table 1).Broadleaved species reduced yields by 60 %when compared with that of clean weededplots, which again was similar to the reductionin growth rates. Grasses and sedges reducedyields by 42 % and 6 %. This clearly indicatedthe impact of different categories of weeds ongrowth and yields of mungbean.

Analysis of the weeds (Table 3) highlighteda greater quantity of biomass in broadleavedspecies. This is followed by the grass biomass,which IS a general characteristic of most tropicalcropping systems, where these two categoriesare common weeds (SWARBRICK and MERCADO1987). However, weed species and types dochange with location, management and theenvironment.

SANGAKKARA, MEYLEMANS and VAN DAMME

The weed biomass of the non-weeded plotshad different quantities of a given type whencompared to plots in which that type was main-tained. The biomass of grasses was greater inthe non-weeded plot while those of other cat-egories were lower than in plots in which thesetypes were maintained with the mungbeancrop. This indicated competitive effectsbetween weed species. Grasses, due to theirearly emergence and more rapid growth, couldsuppress the development of broadleavedspecies and sedges when grown together. Incontrast, the biomass of sedges was significantlylower m the unweeded plots.

The reduction in growth and yields of mung-bean due to weeds could be significantlycorrelated (Y = -0.574 x + 39.65, r = 0.672)with the weed biomass. The non-weeded plotshad the lowest yields while the clean weededplots had the highest. Thus, the greater accumu-lation of weed biomass suppressed crop growthand yields to a greater extent than a lowerquantity of weeds. This is a resultant effect ofcompetitive relationships between the weedsand crop (ZiMDHAL 1980).

An overall analysis of the results illustratedthat weeds affected vegetative growth of mung-bean to the greatest extent. This had a signifi-cant and similar impact on yields, whichdetermined the success of the crop. Hence,the highest degree of competitive relationshipsdeveloped in the vegetative phase, as the effectof different weeds was lower at the later stagesas shown by yield components. The removalof weeds during the vegetative phase, whichcould be termed the critical competitive period,would thus increase yields significantly.

Smallholder farmers in most tropicalcountries who cultivate food legumes adoptmanual weed control measures after planting.The general trend in these farms is to removeall weeds, resulting in the loss of soil cover andincreased erosion, especially on marginal lands.The study illustrated that the most significantyield loss is caused by broadleaved species dueto the similar growth habits of these species andmungbean. The presence of sedges had the leastadverse impact in this study. Thus thresholdweed populations, which consist of less harmfultypes, could be retained within the crop with-out major crop losses, which in turn wouldinfluence the functioning of the ecosystem dueto the presence of botanic diversity. Studies arethus required to evaluate the impact of clean

weeding which is practised in most smallholderfarms of the tropics in comparison to thoseretaining some weeds. Identification oi thecritical competitive periods and the economicthreshold values of weeds and types to obtainoptimal production on a sustainable basis withminimal disturbance of the fragile ecosystemsis also required. The results of this study indi-cate the possibihties of retaining some weedswith a very low impact on yields in mungbean.

Zusammenfassung

Einflufi unterschiedlicher Unkrauttypen aufWachstum und Ertrag von Mungbohne(Vigna radiata L. Wilczek).

Der Einflul^ verschiedener Unkrauttypen(Gras, breitblattrige und Seggen-Arten) auf dasWachstum und Ertragsparameter von Mung-bohne {Vigna radiata L. Wilczek) wurden unterFeldbedingungen untersucht. Das Vorhan-densein aller Unkrauttypen hatte den starkstenEinfluf̂ auf Wachstum und Ertrag von Mung-bohne. Breitblattrige Unkrauter hatten einenstarkeren ungunstigen Einfiuf̂ als Graser; dieWirkung von Seggen war am schwachsten. Dieungunstige Wirkung war hinsichtlich des ve-getativen Wuchses am starksten ausgepragt.Der Einfiuf̂ auf die Ertragskomponenten redu-zierte sich im Laufe der Zeit. Die ungunstigeWirkung der Unkrauter auf die Ertrage warenden Wirkungen auf das vegetative Wachstumvergleichbar. Eine signifikante Korrelationwurde zwischen der Biomasse der Umkrauterund der Ertragseinbul^e beobachtet. Die Unter-suchung weist auf eine kritische Konkur-renzphase in der vegetative Entwicklung hin.Es werden Hinweise auf eine selektiveUnkrautbekampfung gegeben.

Acknowledgements

Gratitude is expressed to the OVERSEAS DEVEL-OPMENT CORPORATION ( A B O S ) of the Kingdom ofBelgium for funding this study, and to Mr S.BuTAWATTE for research assistance.

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