J. Agronomy & Crop Science 169, 33C^335 (1992)© 1992 Paul Parey Scientific Publishers, Berlin and HamburgISSN 0931-2250

University of Peradeniya, Sri Lanka

Effects of Types and Duration of Weedson Yield Parameters of Cocoyam {Xanthosoma sagittifolium)

U. R. SANGAKKARA

Author's address: Dr. U. R. SANGAKKARA, Faculty of Agriculture, University of Peradeniya, Sri Lanka.

With 3 tables

Received January 3, 1992; accepted May 22, 1992

Abstract

Weeds reduce yields of all tropical crops significantly, and these effects vary with different species. Thus, theinfluence of time of weed control and types of weeds on yield parameters of cocoyams {Xanthosomasagittifolium)^ a tropical tuber crop, was determined by two experiments conducted simultaneously. In oneexperiment, weeds were removed at different times during crop growth. In the other, different weed typeswere removed selectively to leave either one or two of the identified categories, namely grasses, broadleavesand sedges. The presence of weeds throughout the growth cycle of the crop reduced yields of cocoyams by60 %. Removal of weeds at early growth stages produced greater yields than weeds were present beyond18—22 weeks after planting. The presence of weeds at the time of tuber initiation had the greatest adverseeffect on yield parameters. Weeds had a delaying but not an inhibitory effect on cormel initiation. However,all other yield parameters were reduced by weeds. Broadleaved weeds, which have similar growth patterns ascocoyams reduced yields to a greater extent than grasses. Sedges had no significant impact on cocoyams. Thepresence of grasses and broadleaves together reduced growth and yields of this crop to a greater extent thanany other combination.

Introduction

Xanthosoma sagittifolium, commonly knownas cocoyams is predominantly grown in thetropical developing countries as a source ofcarbohydrates for humans. This species is apreferred food in the Pacific region, althoughits cultivation is being expanded into Africaand Asia (HORTON 1988, SAFO-KANTANKA et al.1991).

Cocoyam is generally cultivated is smallhol-der farming systems of the tropics (ONWUEME

1988). Thus they are planted in combinationwith many other crop species. However, thiscrop plays an important role in maintaining thelong term sustenance of farming systems dueto their requirement of a low level of manage-ment and long duration (ONWUEME 1987).

Smallholder farming systems of the develop-ing world generally lack good crop husbandry.Thus weeds become the most important pest intropical agriculture, although their role isunderestimated in most situations (MOODY andEzuMAH 1974). The most common practiceadopted by smallholder farmers to overcomethis problem is selective manual weeding. Thisoperation can have a significant impact onyields of most tropical crops (AKOBUNDO1990).

The influence of weeds on tuber crops hasbeen evaluated with the more prominentspecies such as Irish potato and cassava. How-ever, tuber crops vary in their response to theintensity and duration of weed infestation.Thus research (e.g. AKOBUNDO 1981) identify

U.S. Copyright Clearance Center Code Statement: 0931-2250/92/6905-0330$02.50/0

Effects of Types and Duration of Weeds 331

cassava as a crop tolerant to weeds, whileyams, which do not produce an extensivecanopy is susceptible to competitive effects byweeds. However, research does not highlightthe effects of weeds and weed control on theyields of cocoyams, which may be due to thelack of international significance of this crop,although it is a principal source of carbohy-drates to a large population of the world.

Thus experiments were conducted to deter-mine the influence of the duration of weedsand different types of weeds on the growth andyields of this tropical tuber crop.

Materials and Methods

The experiments were carried out simultaneously atthe experimental farm of the University ofPeradeniya, Sri Lanka (7° N, 80° E, 472 m above sealevel) on adjacent sites. The soil of the site was anAlfisol, with the following characteristics:

Texture — Sandy clay loamC E C — 18.91 meq/lOOgsoilpH (1 : 1.5 H2O) 6.34 ± 0.12Organic matter — 1.12 ± 0.03 %.

The climate of the experimental period lasting for 10months beginning from October was:

Rainfall 1567 mmHumidity 74.8 ± 3.4 %Mean monthly temp. °C 27.5 ± 2.3Daylength — 11—12 hours.

Each experiment was carried out as a randomizedcomplete block design, with three replicates.

The two experiments had similar cultural prac-tices. Cut setts, each weighing 200 ± 2.5 g (at 60 %moisture content on a fresh weight basis) wereplanted onto well prepared weed free plots (dimen-sions 4 X 4 m) at a spacing of 60 x 60 cm and adepth of 2.5 cm. The setts were lightly covered withsoil.

The fertilizer used for both experiments was abasal dressing containing nutrients equivalent to90 kg N and P2O5 and 150 kg K.O per ha. A topdressing of 25 kg N and 100 kg K2O per ha wasapplied at 90 days after planting. No supplementaryirrigation was carried out as planting was initiatedwith the onset of the rainy season, and there were noproblems of insect pests and diseases.

The treatments adopted for each experiment wereas follows:

Experiment 1 — Effect of duration of weeds onyield parameters of Cocoyams. Maintenance ofweed free conditions from planting, or from 2, 6,10, 14, 18, 22, 26 and 30 weeks after planting or

plots not being weeded until the final harvest at 40weeks (10 Treatments).

Experiment 2 — Effect of different types of weedson yields of Cocoyams. Removal of either allweeds, grasses, sedges or broadleaved weeds, gras-ses and broadleaved weeds, grasses and sedges orbroadleaved weeds and sedges from the plots untilthe final harvest at 40 weeks (8 Treatments).

The samplings carried out in the experimentswere:

Experiment 1 — Two plants per plot at 90 daysafter planting and thereafter at 4 weekly intervalsupto 40 weeks to observe the number of cormeisand yield per plant. All cormel weights were cor-rected to 60 % moisture content on a fresh weightbasis.At the time of weeding of the respective plots, theweeds from two randomly sited quadrats (0.1 m')were obtained, and seperated into grasses, broad-leaves and sedges. These were dried at 80 '̂ C for 48hours and weighed. The percentages of each cate-gory (on a dry weight basis) was calculated.

Experiment 2 — Two plants per plot were careful-ly uprooted beginning from 90 days after planting,at four weekly intervals. The shoot dry weights,cormel numbers, their weights and final yieldswere determined as per methods of Experiment 1.

The cormel weights at each sampling in both experi-ments and the shoot dry weights of the secondexperiment were utilized to calculate the bulkingrates and shoot growth rates by regression analysis asdescribed by GOMEZ and GOMEZ (1981).

Results and Discussion

Weeds were observed in the experimental plotswithin one week of planting cocoyams. Sam-pling at four weekly intervals beginning fromthe second week after planting illustrated thepattern of dry matter accumulation of weedsover time (Tab. 1). Rapid growth of weeds wasobserved from 2—10 weeks and the rate of drymatter accumulation decreased significantlythereafter. This phenomenon can be associatedwith the growth of cocoyams, which developsa good canopy, thereby suppressing weedgrowth. However the rapid weed growth atearly stages of most tropical crops (MOODY andEzuMAH 1974) is clearly observed, and thisphenomenon affects the associated cropspecies.

Grasses were the most dominant type ofweeds at the first few samplings (Tab. 1). Thecontribution of grasses to total weed biomass

332 SANGAKKARA

Table 1. Total dry weights (g/m^) and percentages of different weeds in experimental plots

Time of sampling

(Weeks after planting)

Percentage of total dry weight

Grasses Broadleaves Sedges

Total dry weight

26

101418222630Final harvest (40 weeks)

LSD (P - 0.05)

92.582.069.548.136.528.931.532.830.8

3.25

7.515.424.645.255.764.260.962.564.9

5.18

0.02.65.96.77.86.97.64.74.3

0.56

15.434.756.778.997.5

108.4128.5138.0189.5

11.49

decreased with time, in contrast to that ofbroadleaved species. The accumulation ofbiomass by the larger plant forms of broad-leaved weeds with time and their greater com-petitive ability (TRIPATHI 1968) could be con-sidered the causal factor of this phenomenon.In contrast, the presence of sedges such asCyperus species was not prominent. Their con-tribution to the total weed biomass increasedupto 18 weeks (Tab. 1) and decreased signifi-cantly thereafter. This suggests their relativeunimportance under the conditions of thisstudy. In addition, the lower competitive abili-ty of sedges when compared with tropical gras-ses and broadleaved weeds (ZIMDHAL 1980)could also be a contributory factor.

Weeds have a significant impact on cormelinitiation in cocoyams (Tab. 2). The presenceof weeds beyond 22 weeks prevents normalcormel initiation 10—12 weeks. In addition,weeds in plots from planting to 22 weeks alsoreduces cormel initiation. This reduction be-comes significant from 6—10 weeks afterplanting.

A comparison of the number of cormeis perplant at 90 days after planting and at the finalharvest confirms the above observation. Thepresence of weeds delays the normal process ofcormel initiation in cocoyams, which generallyoccurs around 10—12 weeks after planting(ONWUEME 1978). However, the process ofcormel initiation occurs m all plants, lrrespec-

Table 2. Effect of time of weeding on yield parameters of cocoyams

Weed free period

From plantingFrom 2 weeksFrom 6 weeksFrom 10 weeksFrom 14 weeksFrom 18 weeksFrom 22 weeksFrom 26 weeksFrom 30 weeksNo weeding

LSD (P = 0.05)

Cormeis per plantat 90 days after

planting

8.58.37.86.44.13.61.5———

0.54

Cormeis per plantat harvest

11.410.69.48.68.18.28,37.47.67.2

1.03

Bulking rate(g/plant/week)

41.140.938,434.728.525.522.021.520.921.6

2.09

Yield per plant(g/plant)

1499138612481090897784648625596604

47.65

Effects of Types and Duration of Weeds 333

tive of the presence of weeds, and the numberof cormeis decreases with increasing durationof weeds.

The effects of weeds on cormel numberscould be categorized into three stages. Thepresence of weeds from planting to 6 weeksreduces the cormel numbers by approximately12 %, when compared with plants grown inplots kept weed free from establishment.When plots are kept unweeded from 6—22weeks after planting, cormel numbers per plantare reduced by 27 % with no significant differ-ences between weeding intervals during thisperiod. The most significant impact is ob-served when plots are left unweeded fromplanting to 26 weeks or longer, and cormelnumbers are reduced by 35 % when comparedwith plants in plots kept weed free from estab-lishment. However, as the reduction in cormelnumbers is lower than effects on cormel initia-tion as observed at 90 days after planting, thestudy reveals that weeds delay but not inhibitthe process of cormel formation in cocoyams.In contrast to cormel numbers, bulking ratesof the initiated cormeis per plant are signifi-cantly reduced by the presence of weeds(Tab. 2). The presence of weeds, especiallyduring the early growth stages depresses thebulking rates to a greater extent than at laterstages. However, the presence of weeds upto 6weeks from planting does not depress bulkingrates significantly. As cormeis begin to form at10—12 weeks from planting, the presence of

weeds prior to this phenomenon does not seemto affect the bulking rates. The decrease of 7 %in bulking rates when weeds are removed atearly stages (2—6 weeks) may be attributed tothe adverse effects on the growth of the de-veloping plants of cocoyams.

Removal of weeds at 10 weeks depressesbulking rates of cormeis by 14 % when com-pared with that of plants in weed free plots.This clearly suggests that the presence of weedsat cormel initiation not only depresses cormelnumbers but also affects their growth. Thesignificant decline in bulking rates continueswith the presence of weeds upto 22 weeks afterplanting, when cormeis are in the rapid de-velopment phase (ONWUEME 1978). The declinein bulking rates is approximately 50 % lessthan that observed when plants are grownunder weed free conditions. There is no fur-ther decline in bulking rates with continuedpresence of weeds at later stages, until the finalharvest.

The adverse effects of weeds on yield com-ponents of cocoyams is culminated in yieldsper plant. LJnder weed free conditions, coco-yams produce a significant quantity of cor-meis. Thus under the recommended densities,this crop produces 12—14 MT of cormeis.However, weeds, which are common in allfarming systems reduces yields significantly.The presence of weeds upto 2 weeks fromplanting reduces yields by 7 %. This reductionis primarily due to lower cormel numbers

Table 3. Effect of different types of weeds on growth and yield parameters of cocoyams

Types of weeds

All weedsGrasses onlyBroadleaves onlySedges onlyGrasses and BroadleavesSedges and BroadleavesGrasses and SedgesNo weeds

LSD (P = 0.05)

Shootgrowth''

0.01850.16450.11870.22870.10870.11610.14880.2415

0.0014

Cormeisper plantat 90 days

—

4.51.58.4—

1.6

4.6

8.9

0.95

Cormeisper plantat harvest

7.18.67.8

10.27.07.58.5

10.5

1.54

Bulkingrate

(g/plant/week)

28.436.131.443.930.132.634.544.6

2.78

Yieldper plant(g/plant)

5981099724

1392656716

10481423

58.99

''' Shoot growth and Bulking rates calculated by regression analysis using equation Loge: Y = A -h Bx whereY and X are weights (dry weights of shoots and weights of developing cormeis at 60 % moisture content)and time (days for shoot weights and weeks for bulking rate) respectively.

334 SANGAKKARA

rather than bulking rates, which in turn can beattributed to the effects of weeds on earlygrowth of the crop. The presence of weedsupto 6, 10 and 14 weeks reduces yields further(i.e. 1 6 % , 2 7 % and 4 0 % respectively). Incontrast, reductions in yields when plots areleft unweeded beyond 18 weeks after plantingIS not as marked. This clearly indicates thatweed control at early stages of cocoyamgrowth, especially upto 10—14 weeks is im-portant in obtaining high yields. This isprimarly due to the adverse effects of weeds onearly plant growth, cormel initiation and de-velopment. Weed control at later stages of cropgrowth does not produce high yields incocoyams.

Different types of weeds have varying effectson crops (ZIMDHAL 1980). This phenomenon ishighlighted in this study, as removal of allweeds, grasses, broadleaves or sedges in theselected combinations affect shoot growth andyield components differently (Tab. 3). How-ever, a particular combination of weeds hassimilar effects on shoot growth and yields.This demonstrates a direct relationship be-tween shoot growth and yield parameters incocoyams.

The absence of weeds produces the highestyields which is equivalent to that of the experi-ment on the effects of duration of weeds(Tab. 2). This confirms the high yielding abili-ty of this crop. The presence of all types ofweeds throughout the growth cycle of the cropreduces yields per plant by 58 %, which againis similar to results of the associated trial(Tab. 2), confirming the adverse effects ofweeds on cocoyams.

Sedges have the least impact on cocoyamgrowth. The presence of these weeds alone donot affect any of the measured parameters sig-nificantly, thus revealing that these weeds,which are present in small numbers (Tab. 1) donot affect yields of cocoyams.

Grasses and broadleaved species affect thegrowth and development of cocoyams signifi-cantly. Amongst these two types, broadleavedspecies have a greater impact than grasses, andshoot growth, number of cormeis at 90 daysand at the final harvest. The reductions inbulking rates too are similar (i.e. a 31 % and20 % reduction in bulking rates) due to broad-leaves and grasses respectively, when com-pared with plants in weeded plots. These ef-

fects are culminated in per plant yields andbroadleaves and grasses when left unweededreduce the harvestable product by 49 % and23 % respectively.

A comparison of the effects of binary com-binations of weeds (Tab. 3) also confirms thegreater adverse effects of the broadleaved andgrass weeds. Thus a comparison of per plantyields indicate significant reductions whengrasses and broadleaved species are left to-gether in a single plot when compared withplots where these types are present alone. Incontrast, the presence of sedges with the othertypes does not affect yields greatly when com-pared to those of plots containing either gras-ses or broadleaved weeds alone. Thus, sedgescan be considered a group of weeds that do nothave a significant impact on cocoyam yields,either alone or when present with other weeds.

The degree of competition intensifies whenthe associated species have similar growth pat-terns (HARPER 1977). This is observed in thedata, where broadleaved weeds have a greateradverse effect on yields of cocoyams. The slowrate of establishment of the broadleaved weeds(Tab. 1) and the coincidence of the rapidgrowth phases of these and cocoyam can de-velop intensive competition. In contrast, gras-ses which develop rapidly during the initialphases of growth of cocoyams offers less com-petition. The ability of the broadleaved weedsto successfully compete with grasses at laterstages and the shading effect by cocoyams dueto canopy development may also be contribu-tory factors to the lower impact of grasses onthe crop yield.

Conclusions

The study reveals that the time of weed remov-al has a significant impact on the growth andyield of cocoyams. Control of weeds upto14—18 weeks from planting, to coincide withthe time of cormel initiation and their earlygrowth ensures a higher yield than when weedsare removed at later stages. Weed control at thelatter stages does not increase yields signifi-cantly when compared to unweeded plots.

Broadleaved weeds have the most impact oncocoyams, followed by grasses. The removalof these two types increases yields of coco-yams, by producing a greater number of welldeveloped cormeis. In contrast, sedges do not

Effects of Types and Duration of Weeds 335

have a significant detrimental effect on coco-yams, and their eradication is not vital forobtaining high yields. Thus, selective weedcontrol at the appropriate time can producehigh yields of cocoyams under conditionsfound in most tropical countries.

Zusammenfassung

Einflusse des Typs und der Dauer von Un-krautern auf Ertragsparameter von Co-coyam {Xanthosoma sagittifolium)

Unkrauter reduzieren signifikant den Ertragtropischer Kukurpflanzen, wobei die Wirkungartspezifisch zu beurteilen ist. Es wurden da-her der Einfluf̂ des Zeitpunktes der Unkraut-kontrolle und des Typs der Unkrauter aufErtragsparameter von Cocoyam {Xanthosomasagittifolium), einer tropischen Knollenfrucht,in zwei simultan durchgefiihrten Experimen-ten untersucht. In einem Experiment wurdendie Unkrauter zu unterschiedlichen Zeitpunk-ten wahrend des Wachstums der Kulturpflanzeentfernt; in dem anderen Experiment wurdenunterschiedliche Unkrauttypen selektiv ent-fernt, wobei ein oder zwei der identifiziertenKategorien, namlich Graser, breitblattrige Un-krauter sowie Riedgraser beriicksichtigt wur-den. Die Gegenwart von Unkrautern wahrenddes gesamten Wachstumsverlaufs der Kultur-pflanze reduzierte den Ertrag von Cocoyamum 60 %. Die Entfernung der Unkrauter inemem friihen Wachstumsstadium erbrachtehohere Ertrage im Vergleich zu einer Verun-krautung iiber 18—22 Wochen nach der Pflan-zung. Die Gegenwart von Unkrautern zumZeitpunkt der KnoUenanlage hatte die stark-sten ungiinstigen Wirkungen auf die Ertrags-parameter. Unkrauter verzogerten, unterban-den aber nicht die Spro^anlage. Alle anderenErtragsparameter wurden durch die Verun-krautung reduziert. Breitblattrige Unkrautermit einem zu Cocoyam vergleichbaren Wachs-tumsmuster reduzierten den Ertrag starker alsGraser. Riedgraser hatten keinen signifikantenEinfluE auf Cocoyam. Die Gegenwart vonGrasern und breitblattrigen Unkrautern ge-meinsam reduzierten Wachstum und Ertrag

der Kulturpflanze starker als jede andere Kom-bination.

Acknowledgements

Gratitude is expressed to Messers H. H. RATNAY-

AKE and E. R. PlYADASA for research assistance.

References

AKOBUNDO, I. O., 1981: Weed control in cassavacultivation in the subhumid tropics. Tropical PestManagement 26, 420—426.

y 1990: The role of weed control in integratedpest management for tropical root and tuber crops.In: S. K. HAHN and F. E. CAVENESS (eds.). Inter-grated Pest Management for Tropical Root andTuber Crops, 23—29. IITA, Idaban, Nigeria.

GOMEZ, A. A., and K. A. GOMEZ, 1981: Statisticalprocedures for agricultural research with emphasison rice. IRRI, Philippines, 294 p.

HARPER, J. L., 1977: Population biology of plants.Academic Press, London, U.K., 698 p.

HORTON, D., 1988: Underground crops: Long termtrends in production of roots and tubers. WinrockInternational, Morrilton, U.S.A., 20 p.

MOODY, K., and H. C. EZUMAH, 1974: Weed con-trol in major root and tuber crops — A review. Pestarticles and news summaries 20, 292—299.

OmvxiEME, I. C , 1978: The tropical tuber crops.John Wiley, London, U.K., 234 p.

, 1987: Strategies for increasing cocoyams inthe Nigerian food basket. In: A. ARENE et al. (ed.),Cocoyams in Nigeria, 5—9. Nigeria.

, 1988: Technological advances and potential ofroot and tuber crops in Africa. In: R. H. HOWELER(ed.). Proceedings of 8th Symposium of the Inter-national Society for Tropical Root Crops, 20—31.Thailand.

SAFO-KANTANKA, O . , V. BoETENG, and V. CHAM-

BERS, 1991: Growth and development oi Xantboso-ma and Colocasia under different spacings. In: F.OFORI et al. (ed.). Proceedings of 9th Symposiumof International Society for Tropical Root Crops,in press. Ghana.

TRIPATHI, R. S., 1968: Comparison of competitiveability of certain common weed species. TropicalEcology 9, 37—41.

ZiMDHAL, R. L., 1980: Weed crop competition — Areview. International Plant Protection Centre,Oregon, U.S.A., 195 p.