allelopathic potential of green manure cover crops on

Research ArticleAllelopathic Potential of Green Manure Cover Crops onGermination and Early Seedling Development of Goose Grass[Eleusine indica (L) Gaertn] and Blackjack (Bidens pilosa L)

Joyful Tatenda Rugare 12 Petrus Jacobus Pieterse 2 and Stanford Mabasa 2

1Department of Agronomy Stellenbosch University Private Bag X1 Matieland 7602 South Africa2Department of Plant Production Sciences and Technologies University of Zimbabwe PO Box MP 167 Mount PleasantHarare Zimbabwe

Correspondence should be addressed to Joyful Tatenda Rugare rugarejoyyahoocouk

Received 22 June 2021 Revised 24 September 2021 Accepted 13 October 2021 Published 1 November 2021

Academic Editor Yong In Kuk

Copyright copy 2021 Joyful Tatenda Rugare et al is is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work isproperly cited

Green manure cover crops (GMCCs) which are recommended for improving soil fertility also have the potential of reducingweed populations in cropping systems through allelopathy e objective of this study was to evaluate the effect of eight GMCCson the germination and seedling development of two weeds of divergent morphology namely goose grass [Eleusine indica (L)Gaertn] and blackjack (Bidens pilosa L) Aqueous leaf stem and root extracts of hyacinth bean (Lablab purpureus L) redsunnhemp [Crotalaria ochroleuca (G) Don] showy rattlebox (Crotalaria grahamiana Wight amp Arn) common bean (Phaseolusvulgaris L) common rattlepod (Crotalaria spectabilis Roth) radish (Raphanus sativus L) tephrosia (Tephrosia vogelii L) andblack sunnhemp (Crotalaria juncea L) at 0 125 25 375 and 5wvminus1 were applied to weed seeds in Petri dishes to determinetheir effect on germination radicle and plumule growth and germination vigor index e experimental design was 3(tissuetypes) lowast 5(concentrations) treatment combinations replicated four times in a completely randomized design In the pot study 25seeds of either goose grass or blackjack were planted separately in approximately 400 g of soil mixed with cover crop tissue powderat 1 concentration per pot e experimental design was cover crop residues + control replicated four times in randomizedcomplete blocks ere was a significant (plt 005) extract lowast concentration interaction on all germination parameters across allGMCCs e different cover crop aqueous extracts differentially reduced all germination parameters of both weeds in the orderleafgt stemgt root extract except for radish root extracts being most inhibitory to all germination parameters of goose grass eleaf stem and root soil-incorporated residues of GMCCs significantly (plt 005) affected seedling emergence dry weight andvigor indices of both weeds Based on the results of this study it was concluded that the different GMCC tissues containallelochemicals that inhibit the emergence of both monocotyledonous and dicotyledonous weeds

1 Introduction

e widespread and improper use of synthetic herbicidesleads to environmental damage and a surge in the devel-opment of herbicide-resistant weed biotypes [1] is hascreated the need for alternative sustainable weed controlmethods Crop rotations with GMCCs mainly practiced inconservation agriculture (CA) and organic farming arearguably the most efficacious environmentally friendly andeconomically feasible alternative or complement to herbi-cides [2] Cover crops play an important role in integrated

weed management (IWM) either as smother crops mulchesor allelopathic crops [3]

Most of the cover crops that are being promoted in thesmallholder sector of Zimbabwe are fast growing andproduce a lot of biomass resulting in rapid canopy closurewhich smothers weeds when they are used as live mulches[4] Alternatively their dead residues may be used as surfacemulches or be incorporated into the soil Weed suppressionwhen cover crop residues are incorporated into the soil mayoccur via altered nutrient dynamics depending on severalother factors including type of tillage used carbon to

HindawiInternational Journal of AgronomyVolume 2021 Article ID 6552928 13 pageshttpsdoiorg10115520216552928

nitrogen ratio (C N) of the decomposing cover crop ma-terial soil type and the environment [5] In addition covercrops can suppress weeds by competing for resources [6]disruption of life cycles of crop bound and crop associatedweeds [7] through resource and light competition [8 9]creation of soil conditions that promote seed decay andpredation [10] and prevention of weed seed set and dispersal[5] Furthermore some cover crops such as stooling rye(Secale cereal L) barley (Hordeum vulgare L) and blacksunnhemp (Crotalaria juncea L) have demonstrated alle-lopathic activity [11ndash13] Ferreira and Reinhardt [14] re-ported the possibility of using cover crops to manageherbicide-resistant weeds

Allelopathy a phenomenon occurring in natural oragricultural communities of plants is defined as the in-hibitory or stimulatory effects of one plant species onanother through the release of chemicals called alle-lochemicals [15] Allelochemicals are released by liveplants directly into the environment as volatiles leachatesandor root exudates [16] e allelochemicals are alsoreleased when the residues of the plants decompose [17]According to Ayeni [18] allelopathy can be exploited toachieve reduced application of synthetic herbicides sincemost allelochemicals do not have residual effects and cantherefore be exploited for early season weed control inarable fields without affecting successive crops in croprotations Barberi and Lo Cascio [19] reported that themaize-cover crop rotations reduced weed density underfield conditions demonstrating the possibility ofexploiting allelopathic cover crops for effective weedcontrol in IWM

e fact that allelopathic effects from decomposingcover crop residues were reported to be more pronouncedon small seeds and early emerging species compared tolarge-seeded crops which offers an opportunity forexploiting them for selective weed control in arable cropproduction [20] Aqueous extracts of velvet bean (Mucunapruriens L) and Jack bean (Canavalia ensiformis L) re-duced weed germination and early seedling development[21 22]

Most of the cover crops being proposed for adoption insmallholder CA are legumes with a low carbon to nitrogen(C N) ratio resulting in a faster decomposition than cerealresidues that have higher C N ratios [23] As such whenallelopathic materials of leguminous cover crops are appliedin the field a considerable amount of nutrients are addedwhich may cause stimulation of weed growth due to min-eralisation [23 24] and hormesis e need for finding al-lelopathic cover crops whose phytotoxicity outweighs theirweed growth stimulation activity is therefore indispensable[25] is work aimed at the assessment of the allelopathicpotential of leaf stem and root aqueous extracts and in-corporated biomass of eight cover crops that are beingpromoted for the adoption in smallholder CA in Zimbabwee hypothesis tested in the present study was that aqueousextracts and soil-incorporated cover crop biomass suppressblackjack (Bidens pilosa L) and goose grass [Eleusine indica(L) Gaertn] germination emergence and early seedlinggrowth

2 Materials and Methods

21 Experimental Site e study was carried out at theUniversity of Zimbabwe (UZ) situated at 1778degS 3105degEwith an altitude of 1523m above sea level Laboratorybioassays were carried out in Petri dishes in the WeedScience laboratory and pot experiments were carried out ina greenhouse at the Department of Plant Production Sci-ences and Technologies between January 2015 and June2016 Average day and night temperatures in the glasshousewere 281degC and 152degC No artificial light was used in theglasshouse

22 Biomass Preparation e GMCCs used in the studywere grown under irrigation in a field at UZrsquos Department ofPlant Production Sciences and Technologies in September2014 e GMCCs were grown in soils with 18 clay 16silt and 66 sand with a pH (CaCl2) of 52 Exchangeablecations in milliequivalents percent (me ) of soil were 761417 and 032 for calcium magnesium and potassiumrespectively Green manure cover crops were grown in thefield at the University of Zimbabwe as described in Rugareet al [26] e preparation of dry biomass powder was donefollowing the method described by Rugare et al [21] withoutany amendments

23 Aqueous Extract Preparation Fifty grams of GMCCpowder were soaked in 1000ml distilled water to produce a5wvminus1 solution on a dry weight basis e solution wasthen stirred for 24 hours at room temperature (25degC) on anorbital shaker at 100 rpm e extracts were then strainedthrough four layers of cheese cloth before being centrifugedat 4000 rpm for 15 minutes e clear solution was pipettedto separate it from the supernatant is stock solution wasthen stored in parafilm sealed bottles at 4degC for 24 hoursbefore being used e stock solution was then diluted withdistilled water to produce 125 25 and 375 plantextracts prior to use Conductivity of the 5wvminus1 aqueousextract concentrations of the ten GMCCs was measuredusing a conductivity meter (Model SX713 version 20 2013-7-30) and the values obtained were used to calculate theosmotic potential using the following formula [27]

Osmotic potential (in MPa) conductivity (in mS)lowast minus0036Osmotic potential values for the different stock solu-tions (5wvminus1 concentrations) of the different GMCCtissue aqueous extract treatments ranging from minus026to minus009

In order to ascertain whether the inhibitory activityexhibited by different extract concentrations of the covercrop tissues was not due to differences in the osmoticpressure of the solutions germination bioassays wereconducted using polyethylene glycol (PEG) 6000 solutions at000 005 010 015 020 025 and 030MPa (ie the rangeof osmotic potentials of the 5wvminus1 of the cover croptissues)

2 International Journal of Agronomy

24 Effect of Cover Crop Aqueous Extracts on Weed SeedGermination and Early SeedlingGrowth e weed bioassayswere carried out in the Weed Science laboratory betweenJanuary and March 2015 e weed laboratory bioassays forindividual GMCCs were laid out as 3 lowast 5 factorial experi-ments with two factors in a completely randomized design(CRD) with four replications e factors were extract tissuetype at three levels (leaf stem and root) and extract con-centration at five levels (0 125 250 375 and 5) In all thelaboratory experiments distilled water was used as thecontrol Each weed species was considered a separate ex-periment and the experiments were repeated once Twenty-five seeds of the respective weeds were counted sterilised in1 sodium hypochlorite for 10minutes rinsed four timeswith distilled water and placed in 90mm diameter Petridishes lined with Whatman No 2 filter paper e seeds inthe Petri dishes were treated with 10ml of the respectiveextract concentration sealed with parafilm and placedrandomly on a table in the laboratory at room temperature(approximately 25degC day temperature) Data on germina-tion plumule and radicle length were collected from fiverandomly selected plants on day 10 and day 14 in blackjackand goose grass respectively Germination was consideredto have occurred when the radicle was 2mm long Ger-mination percentage (G) and germination vigor indices(VI) were calculated using the formulae given below

G a

b1113874 1113875lowast 100 (1)

where a is the number of germinated seeds and b is the totalnumber of seeds in each Petri dish

VIGermination () lowast (radicle + plumule length)[28]

25 Greenhouse Experiment e glasshouse study wascarried out using the method described by Rugare et al [21]

Each pot experiment was laid out as a completely ran-domized design (CRD)e effect of the different cover croptissues on the emergence and seedling growth of goose grassand blackjack were compared separately (ie leaves of thedifferent cover crops and the stems and roots were evaluatedseparately) No cover crop residues were put in the controlpots e pot experiments were replicated four times andonly goose grass experiments were repeated once e soilused in the goose grass bioassays was granite-derived sandsfrom Domboshava (17deg 37prime S 31deg 10prime E and 1560 metresabove sea level) whereas UZ red soils were used in theblackjack bioassay in order to mimic edaphic factors underwhich the two weeds exert their dominance [22] echemical and physical properties of the soils used are shownin Table 1

Pots measuring 90mm bottom diameter 105mm topdiameter and 65mm height were filled with 400 g of soil inwhich one Gram of compound D (7 N 14 P2O5 7K2O) was added e respective cover crop tissue powderwas thoroughly mixed with soil at a concentration of 1[cover crop biomass concentration adopted from Caamal-Maldonado et al [29] and Fujii [30]] aiming to provide aconcentration of the cover crop residues similar to whatwould be obtained by the cover crops in nutrient depletedsoils under dry land conditions ereafter 25 seeds of therespective weed species were counted and placed on the soilsurface in the pots after which they were covered with a thinlayer of soil A uniform amount of tap water of 150ml wasapplied to the pots daily using a perforated cup e numberof emerged goosegrass and blackjack seedlings in the potswas recorded daily until no further emergence was notede final emergence percentage was calculated using thefollowing formula

emergence percentage number of emerged seeds

total number of seeds in each pottimes 100 (2)

On day 21 after planting of the weed seeds the seedlingswere harvested and washed gently with tap water to removeany soil from the rootse uprooted weeds were oven-driedfor 72 hours at 70degC to obtain the dry weight of the weedse seedling vigor index (SVII) was calculated using theformula adopted from Abdul-Baki and Anderson [28] asfollows

SVII seedling emergence times seedling dry weight (g)

(3)

e inhibition of different cover crop plant parts wasevaluated using the method of Hong et al [24] where in-hibition magnitudes were ranked based on the mean inhi-bition of leaf stem and root soil incorporated biomass in

descending order e average inhibition percentages werethen grouped into three categories where more than 8050 and 20 were classified as the first second and thirdstrongest inhibitory degree respectively e inhibitionpercentage was calculated as follows

inhibition percentage 1 minustreatmentcontrol

1113876 1113877 lowast 100 (4)

26 Data Analysis e data were tested for normality usingthe ShapirondashWilk test and subjected to analysis of variance(ANOVA) using Genstat 18th edition e seedling vigorindex (SVII) data did not meet the assumptions of ANOVAand were radic (x+ 05) transformed before being analysed

International Journal of Agronomy 3

using Genstat version 18 Significantly different means wereseparated using Fischerrsquos protected least significance dif-ference (LSD) at 5 significance level

3 Results

31 Germination and Early Seedling Growth of Blackjack andGoosegrass in PEG 6000 Solutions e seedling vigor indexof goose grass seeds was significantly (plt 005) affected bythe different PEG solutions (Table 2) On the other handthere were no significant differences in the seedling vigorindex of blackjack in PEG 6000 solutions of different os-motic potentials (Table 2)

32 Effect of GMCC Aqueous Extracts on Goose Grass andBlackjack Germination Parameters e interaction of ex-tract tissue and concentration was significant (plt 005) onthe germination percentage of goose grass and blackjackacross all the GMCCs (Table 3) Leaf extracts reduced thegermination of both weeds significantly better than the stemand root extracts Similarly the interaction between extracttissue and concentration was significant (plt 005) on radicleand plumule length of goosegrass and blackjack across allGMCCs (Tables 4 and 5 respectively) GMCC aqueousextracts exhibited phytotoxic activity on radicle growth inthe order leafgt rootgt stem Consequently leaf extractssignificantly (plt 05) reduced the germination vigor indexbetter than the other tissue extracts across all the GMCCsexcept radish whose root extracts showed greater phytotoxicactivity than the other extract tissues (Table 6)

33 Effect of Cover Crop Soil-Incorporated Residues onEmergenceDryWeight andSeedlingVigor IndexofBlackjacke emergence of blackjack was significantly (plt 005) af-fected by different cover crop leaf stem and root soil-in-corporated residues (Table 7) Leaf stem and root soil-incorporated biomass inhibited blackjack seedling emer-gence by 42ndash96 49ndash94 and 46ndash93 respectivelyGenerally the leaf tissues were more inhibitory than thestem and root tissues Overall tephrosia and hyacinth beanresidues inhibited blackjack emergence significantly(plt 005) better than the other cover crops with a percentagemean inhibition of above 80 Leaf stem and root soil-incorporated residues of all cover crops significantly(plt 005) suppressed the dry weight of blackjack (Table 7)Results showed that among the eight cover crop speciescommon bean hyacinth bean and tephrosia exhibited thegreatest average suppression on dry weight of blackjack witha mean inhibition above 80 over the control A showy

rattlebox showed the least mean inhibition of 46 eseedling vigor index of the blackjack was significantly(plt 005) reduced by different tissue residues of the covercrops (Table 7) Percentage inhibition ranged from 82ndash9976ndash100 and 73ndash97 over control where leaf stem androot residues were used respectively Overall all the covercrops were ranked to be in the first level of allelopathicpotential with mean inhibition above 80

34 Effect of Cover Crop Soil-Incorporated Biomass onEmergence Dry Weight and Seedling Vigor Index of GooseGrass Leaf biomass of all the cover crops except showyrattlebox exhibited significant (plt 005) phytotoxic activityon the emergence of goose grass with inhibition percentagesthat ranged from 24ndash56 (Table 8) Most of the cover cropscaused a weak inhibition of goose grass emergence exceptjack bean hyacinth bean red sunnhemp and commonrattlepod that reduced emergence by more than 50 overthe control Similarly stem residues apart from the showyrattlebox exhibited significant (plt 005) allelopathic activityon the emergence of goose grass with the highest percentagereduction of 70 being observed where black sunnhempstem residues were mixed with the soil Except for showyrattlebox and common bean soil incorporated root residuesof cover crops significantly (plt 005) reduced the emergenceof goose grass Radish and hyacinth bean caused the highestinhibition percentage over the control compared to the othercover crops Results in Table 8 show that the effect of thecover crop leaf residue type on the dry weight potminus1 of goosegrass was significant (plt 005) e soil-incorporated stembiomass of the cover crops significantly (plt 005) reducedgoose grass dry weight by 57ndash97 except showy rattleboxresidues that stimulated weed growth by 12 above the

Table 2 Effect of different osmotic potentials of polyethyleneglycol (PEG) solutions on the seedling vigor index of blackjack andgoose grass

PEG solution osmotic potential (MPa) Goose grass Blackjack000 16342b 58072005 24797a 60883010 24333a 56240015 17726b 60466020 15429b 59266025 24710a 52018030 20888ab 60051p value 0013 0202LSD 65263 NSMeans followed by different letters in the same column are significantlydifferent at plt 005

Table 1 Physical and chemical properties of soil that was used in the study

Soilproperties Clay Silt Sand Ca me Mg me K me Na me CEC me H2O

Organiccarbon

pH(CaCl2)

Organicmatter

UZ red soil 18 16 66 761 417 032 022 1232 298 166 520 332Domboshavasand soil 7 5 88 46 02 023 046 281 176 041 005 061

me ()milliequivalents percent


control Red sunnhemp common bean common rattlepodand black sunnhemp showed strong allelopathic potentialwith mean inhibition percentages that were above 80 Twocover crops (red sunnhemp and common rattlepod) sig-nificantly (plt 005) suppressed seedling vigor of goose grassby more than 80 over the control

4 Discussion

Results obtained demonstrated that all the GMCC extractsused in this study inhibited the germination and earlyseedling growth of goosegrass and blackjack e degree ofinhibition varied greatly amongst the different cover cropsand tissues within each cover crop with leaf extracts showing

the greatest phytotoxic activity followed by the stems exceptradish root extracts that were more efficient on goosegrassthan the other tissue extracts e inhibition of germinationof both weeds especially by leaf extracts of all the cover cropsexcept radish on goosegrass suggests the presence of alle-lochemicals in these GMCCs e reduced seedling growthcaused by the cover crop extracts could also imply reducedearly seedling growth which would give the crops a headstart and competitive advantage over the weeds resulting inreduced crop-weed competition during the early stages ofcrop development when crop seedlings would be sensitive toweed pressure [21]e seedling vigor index gives the overalleffect of the treatments on seedling germination and growth[31] In this study leaf extracts of all the GMCCs and root

Table 3 Effect of aqueous extracts of eight green manure cover crops on germination percentage of goose grass and blackjack

Extracttissue

Extract concentration (w vminus1) Extract concentration (w vminus1)0 125 25 375 5 0 125 25 375 5

Goose grass BlackjackHyacinth bean

Leaf 3000bc 2850bcd 1950de 350fg 000g 9500a 7000d 4900e 2600f 400g

Stem 3100bc 3750b 2700cd 900fg 000g 9600a 8100bcd 8100bcd 7100d 4200e

Root 3600bc 5350a 4950a 2050d 1100ef 9200ab 9100abc 8200bcd 8500abc 7900cd

LSD 9276 12710Red sunnhemp

Leaf 375ab 460a 355ab 145c 70c 9600a 7300de 4500f 2800g 2000g

Stem 305b 335b 370ab 305b 275b 9100ab 8300bcd 8600abc 7300de 6300e

Root 320b 360ab 295b 375ab 300b 9400ab 8600abc 8200bcd 7800cd 6200e

LSD 10601 12260Common rattlepod

Leaf 3650def 4100cde 2200g 1150h 400h 10000a 7500cd 3800e 1600f 000f

Stem 3613def 4750abc 4850abc 3200efg 2450g 9400ab 9100abc 8800abc 8800abc 6800d

Root 2950fg 5200ab 4450bcd 5650a 4500bcd 9100abc 9800a 7900bcd 6900d 6800d

LSD 10274 1635Common bean

Leaf 4250ab 4850a 3550abc 1250defg 700fg 9700a 9100a 7500bc 4700e 1800f

Stem 4350ab 2450cd 2300cde 3000bc 800efg 9700a 9000a 5500de 2100f 1700f

Root 4850a 4700a 4900a 2200cdef 450g 9400a 8600ab 9300a 8400ab 6600cd

LSD 15856 13090Radish

Leaf 3850bcde 5450a 2900e 1200fg 200g 9900a 6800c 3500d 1200e 000e

Stem 3400cde 4100bcd 4650ab 3750bcde 1550f 9200ab 9300ab 8900ab 8200bc 7800bc

Root 3850bcde 4400abc 3450cde 3050de 550fg 9900a 9300ab 8800ab 6700c 4100d

LSD 10619 15980Showy rattlebox

Leaf 3250cd 3150d 1350e 300f 150f 9600a 6400ef 2900g 1700g 100h

Stem 3100d 4400ab 4200abc 3750abcd 1050ef 8200bcd 8600abc 7300de 6400ef 2800g

Root 3200cd 3550bcd 3400bcd 4600a 3750abcd 8700ab 9200ab 7400cde 7000de 5700f

LSD 10044 13390Tephrosia

Leaf 3250bc 3450abc 3400abc 2950c 2900c 9800a 9000abcd 8400cde 5000g 2600h

Stem 3850ab 1950d 850e 000f 000f 9200abcd 9500abc 8900abcd 7600ef 6400f

Root 3250bc 3350bc 4150a 3250bc 4150a 9400abc 9700ab 9000abcd 8100de 8500bcde

LSD 7821 12216Black sunnhemp

Leaf 3200abcd 2500de 2100e 3000cde 200e 9300a 4600d 1900f 600g 000g

Stem 3800abc 4250a 3950abc 3050cde 3250abcd 8000b 8000b 6400c 6000c 3000e

Root 3150bcde 3000cde 3650abc 4150ab 3750abc 8800ab 8200b 7900b 8400ab 6700c

LSD 10582 9834Means followed by the same letter for each GMCC are not significantly different at plt 005


extracts of radish reduced the seedling vigor index dem-onstrating their effectiveness in suppressing germinationand early seedling growth of monocotyledonous and di-cotyledonous weeds

Generally the inhibitory effects of aqueous solutions inPetri dish experiments could be attributed to factors such asallelopathy and variations in osmotic potential [27 32] Inthis study the osmotic potentials of the different cover cropaqueous extracts that were equivalent to those of PEG so-lutions used in the study did not affect seed germination andseedling development and as such it is highly unlikely thatthe osmotic potentials observed could have caused anyinhibitory effect on germination and early seedling growth of

plants [33] When osmotic potential values show little effectamongst treatments they probably have no effect on thegermination traits evaluated [27] erefore the inhibitoryactivity of extracts observed in this present study could beattributed to the presence of phytotoxic allelochemicals inaqueous extracts of the cover crops [34]

e inhibition of germination and early seedling de-velopment of both goosegrass and blackjack that was ob-served in this study indicated that the green manure covercrops possess allelochemicals with strong phytotoxic ac-tivity e inhibition that was observed was concentrationdependent e findings corroborate with the earlier find-ings of Runzika et al [35] who reported weed germination

Table 4 Effect of aqueous extracts of eight green manure cover crops on the radicle length of goose grass and blackjack

Extracttissue

Extract Concentration (w vminus1) Extract Concentration (w vminus1)0 125 25 375 5 0 125 25 375 5


Leaf 251a 200abc 104d 00e 00e 336a 1000e 440f 220fg 000g

Stem 206abc 203abc 161c 64d 00e 3555a 1505cd 1115de 1170de 885e

Root 238ab 212abc 188bc 66d 58d 341a 2700b 2665b 1720c 1695c


Leaf 2273def 1833ef 916g 215h 215h 3010a 1585e 485g 405g 360g

Stem 2141def 3234ab 2950bc 2430cde 1729f 2965a 2060d 2410c 1290e 915f

Root 2648bcd 3625a 2995bc 2492cd 1736f 2800ab 2545bc 2010d 1445e 915f


Leaf 3763a 2778b 1923de 1668ef 298h 4895a 2155ef 1090g 505hi 000i

Stem 2823b 2265cd 1981de 1043g 873g 5340a 3930b 3135c 3035cd 1850f

Root 3853a 3373a 2598bc 1365fg 1018g 4870a 3495bc 2590de 2105ef 655gh


Leaf 3315a 3280a 2595ab 1390cd 209fg 3970b 1310ef 1140f 495gh 275h

Stem 3113a 1065de 820def 1310cd 000g 3870b 1985d 1705de 905fg 475gh

Root 3275a 1905bc 955def 395efg 365efg 4535a 2975c 1885d 1770de 1320ef

LSD 7671 5167Radish

Leaf 2175bcd 2698ab 515fg 239g 013g 4695a 1125ef 405g 123g 000g

Stem 1960cd 3245a 3070a 2780ab 1113ef 4455a 3325b 3075b 2340c 1760d

Root 1540de 2858ab 2283bc 2315bc 144g 4850a 985ef 925f 1405de 1135ef


Leaf 2258cd 2120cd 1898d 848ef 056f 4510a 1460ef 400gh 240gh 005h

Stem 1881d 3495a 2725abcd 2416bcd 988e 2855bc 2260cd 2390bcd 990fg 185h

Root 1898d 3109ab 2805abc 3515a 3527a 4110a 3040b 2785bcd 2085de 2080de


Leaf 264cd 306abc 283bcd 293abcd 327abc 3725a 1705cd 1250d 1148d 188e

Stem 229de 175e 010f 000f 000f 3803a 2295bc 1975bc 1295d 1125d

Root 195e 305abc 286abcd 346ab 352a 3980a 3485a 2570b 2505b 2410b


Leaf 312abc 215c 226c 223c 031d 3770a 1240de 605f 100g 000g

Stem 336ab 339ab 367a 245bc 243bc 3840a 2170b 1680cd 1120e 965ef

Root 285abc 362a 342ab 332ab 341ab 4005a 2470b 2110bc 2015bc 1075ef



and plumule and radicle growth inhibition of weed seedstreated with whole plant aqueous extracts of these covercrops In this study the different tissue aqueous extractsshowed variable inhibitory activity suggesting differences inconcentrations of potent allelochemicals in the differentplant parts or different active compounds Leaf extractsexhibited higher germination inhibition suggesting thepresence of more potent allelochemicals in the foliage ofcover crops than the other tissues Gulzar and Sidiquie [36]working with different plant parts of Eclipta alba (L) Hasskreported that foliar extracts were generally more potent thanstem and root extracts probably due to the greater metabolicactivity in the foliage

Results showed that only radish roots exhibited phytotoxicactivity on goosegrass whereas its leaf residues were morephytotoxic to germination and seedling growth of blackjackConsistent with the current results Ali [37] reported the in-hibition of germination of other annual grasses includingwheat (Triticum aestivum L) barley (Hordeum vulgare L)canary grass (Phalaris canariensis L) and black mustard(Brassica nigra L) using aqueous root extracts of radish epresence of protein synthesis inhibitors namely vanilic acidand ferulic acid in the roots of radish has previously beenreported [38] e lack of the phytotoxic activity of radish rootextracts on black Jack demonstrated in this study contradictsthe findings of Zhou and Yu [39] who reported the inhibitory

Table 5 Effect of aqueous extracts of eight green manure cover crops on the plumule length of goose grass and blackjack

Extracttissue



Leaf 178a 172a 111cd 18e 00e 2285bc 2075bc 1405de 530fg 055g

Stem 181a 163ab 138bc 99d 00e 2350b 2245bc 1675cd 1355de 940ef

Root 178a 157ab 181a 134bc 105cd 2295bc 3090a 3015a 1905bcd 2110bc


Leaf 1623b 1930ab 1690ab 1028c 331d 2820bc 3025abc 1355f 145g 450g

Stem 1625b 1824ab 1875ab 1788ab 1648b 2860bc 3190ab 3600a 2110de 1505ef

Root 1660ab 1873ab 1598b 2028a 1675ab 3005abc 3320ab 2840bc 2440cd 1960def


Leaf 1820b 1778bc 1683bc 1378c 458d 2135b 2860a 1530cd 655e 000f

Stem 1843b 1870b 1775bc 1611bc 1685bc 2005bc 3265a 2935a 2895a 2055bc

Root 2590a 2435a 2303a 1830b 1498bc 2005bc 2955a 2995a 2725a 1308d


Leaf 1818ab 2023a 1930ab 1280cde 605f 2538abcd 2310bcde 2055def 1575fg 680i

Stem 1858ab 965ef 1450bcde 1205de 000g 2655abc 2690ab 2175cde 1225gh 875hi

Root 1944ab 1795abc 1653abcd 990ef 518fg 2515abcd 2840a 1965ef 2040def 1825ef

LSD 5242 5039Radish

Leaf 1748abc 1970a 1473bc 538e 025f 2090bcd 2025cd 1245e 555f 000f

Stem 1574abc 1773abc 1780abc 1853ab 1001d 1755cde 3235a 3265a 3205a 2325bc

Root 1725abc 1745abc 1425c 1445c 219ef 2075bcd 3005a 2655ab 2260bc 1595de


Leaf 1759ab 1805ab 1521bc 1075c 356d 2900cde 2955bcde 1180f 510gh 020h

Stem 1753ab 1850ab 2030a 1725ab 1118c 3145bcd 3420abc 2890de 2490e 875fg

Root 1803ab 1832ab 1925ab 1925ab 1808ab 2875de 3675a 3475ab 2865de 2700de


Leaf 167c 184abc 180abc 182abc 162ab 2210cde 1785ef 1535f 805g 285h

Stem 192ab 091d 024e 000f 000f 2230bcde 2415bcd 2125de 1435f 1325f

Root 175bc 192ab 188abc 199a 198ab 2285bcde 3005a 2735ab 2660abc 2475bcd


Leaf 157abcd 149bcd 144cd 137d 028c 3020bc 2520cd 895e 270f 000f

Stem 166abcd 170abcd 175abcd 148bcd 183abcd 2975bcd 3745a 2950bcd 2395d 1430e

Root 154abcd 190abc 200a 193ab 187abc 3040bc 3535ab 3130b 3025bc 2445cd



activity of radish root extracts on other broadleaved annualplants e inhibitory activity of radish leaf extracts could beattributed to the presence of isothiocynates that are released bythe foliage of this crop [40] Uludag et al [41] reported theinfestation of Johnsongrass [Sorghum halepense (L) Pers] incotton (Gossypium hirsutum L) grown after the harvest ofgarden radish demonstrating the selective activity of alle-lochemicals produced by radish erefore the insensitivity ofblackjack seeds to root extracts of radish underscores that thebroad spectrum weed control may best be achieved usingwhole plants as mulch or sources of extracts

Aqueous extracts of all the Crotalaria species used in thisstudy exhibited phytotoxic activity on the germination ofgoosegrass and blackjack in the order leafgt stemgt root

indicating the presence of more potent allelochemicals in theleaves than any other plant part ese results concur withthose of Adler and Chase [42] who reported the germinationinhibition of smooth amaranth (Amaranthus hybridus L)bell pepper (Capsicum annuum L) and tomato (Solanumesculentum L) where extracts and mulches of black sunn-hemp were used Black sunnhemp roots leaves stems andseeds are known to contain several dehydropyrrolidizinealkaloids such as junceine trichodesmine isohemijunceinesA B C and acetyl isohenmijunceines [8] Pilbeam and Bell[43] identified the nonprotein 5-hydroxy-2-aminohexanoicacid as the allelochemical responsible for the phytotoxicactivity exhibited by black sunnhemp ere are no reportsof any phytotoxic allelochemicals isolated from common

Table 6 Effect of aqueous extracts of eight green manure cover crops on the seedling vigor index (SV1) of goose grass and blackjack

Extracttissue



Leaf 707a 691abc 591cd 000g 000g 859ab 765c 651d 518e 077f

Stem 705ab 718a 518de 475ef 000g 864a 800bc 770c 745c 660d

Root 725a 756a 735a 595bcd 3708f 856ab 856ab 843ab 799bc 800bc


Leaf 722abc 741abc 674d 492e 423f 863a 810abc 655f 474g 489g

Stem 698cd 737abc 744ab 713abcd 671d 857ab 838ab 855ab 775cde 729e

Root 721abc 752a 726abc 739abc 700bcd 860a 852ab 828abc 801bcd 746de


Leaf 751ab 750ab 655de 576f 447g 886a 823ab 653c 485d 000e

Stem 740bc 754ab 745b 660de 635e 884a 878a 858ab 856ab 786b

Root 753ab 795a 765ab 748b 696cd 874a 875a 839ab 810ab 695c


Leaf 763a 782a 715ab 562bcd 134e 875a 809cd 776de 681f 496h

Stem 762a 435d 563bcd 515cd 000e 875a 833bc 764e 608g 526h

Root 779a 743ab 707abc 383d 190e 880a 851ab 813bcd 806cd 761e

LSD 1918 0400Radish

Leaf 724ab 777a 627bc 363d 040f 881a 766de 599f 409g 000h

Stem 703ab 760a 767a 733a 533c 864ab 871ab 863ab 842abc 804bcd

Root 706ab 758a 698ab 685ab 225e 883a 819abcd 804bcd 777cd 697e


Leaf 713ab 705abc 571c 306d 106e 887a 790ab 594c 362d 035e

Stem 614bc 774a 748ab 731ab 365d 848ab 849ab 824ab 769b 563c

Root 698abc 741ab 731ab 781a 755a 871ab 873ab 843ab 813ab 788ab


Leaf 720ab 739ab 729ab 716ab 728ab 867ab 804ef 773fg 672i 452j

Stem 733ab 601c 207d 000e 000e 862abc 840abcde 819de 762gh 734h

Root 707b 739ab 752ab 742ab 767a 868ab 875a 846abcd 833bcde 827cde


Leaf 494ab 396d 420cd 455bcd 176e 875a 741d 543f 266g 000h

Stem 520ab 531ab 533a 459abcd 486abc 860ab 846abc 797abcd 765cd 644e

Root 486abc 500ab 520ab 533a 520ab 873a 849abc 831abc 833abc 775bcd



rattlepod showy rattlebox red sunnhemp and hyacinthbean However results obtained in this study indicate thepotential of these moderately used cover crops as sources ofbio-herbicidal compounds for annual grass and broad leafcontrol e fact that the aqueous extracts of the aboveground parts of these cover crops were phytotoxic to weedsmakes them promising candidates for use as allelopathiccover crops due to the ability of the plants to produce a lot offoliage within a short period of time

Tephrosia leaves were highly inhibitory to the germinationand seedling growth of both weeds e allelopathic activity oftephrosia leaf extracts and volatiles as well as soil incorporatedbiomass on several weeds was reported previously [44] esefindings corroborate the work of Purohit and Pandya [31] whoreported reduced weed germination of IndianNettle (Acalyphaindica L) spiny amaranth (Amaranthus spinosus L) swollenfingergrass (Chloris barbata Sw) and marvel grass(Dichanthium annulatum Forssk) seeds that had been treatedwith tephrosia extracts ey attributed the inhibitory activityof tephrosia leaves to the presence of coumarins flavonoidscarotenoids and quercetin [31]

Furthermore results obtained in this study proved thehypothesis that soil incorporated biomass of cover cropscould suppress the emergence and growth of weeds ebiomass of all the cover crop plant tissues used differentiallysuppressed the emergence of both weeds It is suggested thatthe suppression of emergence and seedling growth observedwas due to the presence of allelochemicals that were releasedby the cover crop residues into the soil ese findingsconfirm the allelopathic suppression of the weeds that wasobserved in the laboratory bioassays in this study esuppression of weed emergence by soil-incorporated bio-mass of allelopathic plants was also reported by several otherauthors [3 13 42] e inhibition of weed emergence couldbe due to the disruption of mitosis which results in a re-duction in root elongation and a concomitant reduction inroot volume [45] As a result the roots fail to absorb enoughmoisture to support the emergence of the germinatedseedling For example Soares et al [46] reported thatL-DOPA an allelochemical found in velvet bean and manyspecies of the Fabaceae family caused deformed and mal-functioning roots is interference with root growth could

Table 7 Effect of soil incorporated residues of different plant parts of cover crops on emergence () dry weight and seedling vigor index(SVII) of blackjack

Cover crop Leaf inhibition Stem inhibition Root inhibition Mean inhibition RankEmergence ()

Control 670a 670a 670a

Hyacinth bean 80c 88 140dc 79 120de 82 83 2lowastlowastlowastRed sunnhemp 80c 88 290bc 57 190cde 72 72 4lowastlowastShowy rattlebox 390b 42 240bc 64 50e 93 66 8lowastlowastCommon bean 40c 94 290bc 57 310bc 54 68 7lowastlowastCommonrattlepod 340b 49 340b 49 350b 48 49 10lowast

Radish 60c 91 280bc 58 270bcd 60 70 5lowastlowastTephrosia 110c 84 40d 94 100e 85 88 1lowastlowastlowastBlack sunnhemp 90c 87 290bc 57 360b 46 63 9lowastlowast

Dry weight (g)Control 0074a 0074a 0074a

Hyacinth bean 0009c 88 001dc 86 0008c 89 88 1lowastlowastlowastRed sunnhemp 0020bc 73 0046b 38 0018bc 76 62 8lowastlowastShowy rattlebox 0047b 36 0029c 61 0044b 41 46 10lowastlowastCommon bean 0014c 81 0028c 62 0037bc 50 64 6lowastlowastCommonrattlepod 0018c 76 0030c 59 0028bc 62 66 7lowastlowast

Radish 0021bc 72 0041b 45 0035bc 53 56 9lowastlowastTephrosia 0017c 77 0001d 99 0022bc 70 82 2lowastlowastlowastBlack sunnhemp 0023bc 69 0023dc 69 0023bc 69 69 4lowastlowast

Seedling vigor index (SVII)Control 230c (5007) 230c (5007) 230c (5007)Hyacinth bean 077a (0087) 98 083ab (0190) 96 079a (0132) 97 97 1lowastlowastlowastRed sunnhemp 083a (0200) 96 136c (0401) 92 092abcd (0359) 93 94 7lowastlowastlowastShowy rattlebox 150b (0914) 82 115abc (0031) 99 083ab (0197) 96 92 10lowastlowastCommon bean 076a (0082) 98 115abc (0873) 83 129de (1207) 76 86 6lowastlowastlowastCommonrattlepod 099a (0560) 89 119bc (0980) 80 112abcde (0820) 84 84 8lowastlowastlowast

Radish 082a (0192) 96 127bc (1224) 76 121cde (1016) 80 84 9lowastlowastlowastTephrosia 085a (0225) 96 072a (0016) 100 085abc (0242) 95 97 2lowastlowastlowastBlack sunnhemp 087a (0280) 94 107abc (0660) 87 116bcde (0960) 81 87 4lowastlowastlowast

Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowastlowast represent species with mean inhibition over controlge80 ge50 and ge20 respectively Data were radic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages


be responsible for reduced emergence and seedling dryweight that was observed in this study due to the poorgrowth of the nutrient and moisture starved plants [47]

e study revealed that the ability of the soil-incorpo-rated biomass of different plant parts varied significantlyamong cover crops with leaves showing more inhibitoryactivity than the stems and roots on blackjack but all thetissues exhibited the same level of germination and growthsuppression on goose grass e presence of more putativeallelochemicals in leaf tissues of other plants compared tostems and roots has previously been reported [36] and hasbeen attributed to greater metabolic activity in the foliagethan other plant parts except in radish where roots are theprincipal storage organ of the plant [13 41] e other covercrops showed variable efficacy in inhibiting weed emergenceVariable inhibition was observed on goose grass with leafand stem biomass of showy rattlebox exhibiting lack ofinhibitory activity on this monocotyledonous weed

Morphological examination showed that the goosegrass plants that had emerged turned yellow and slowlybecame necrotic a symptom which is characteristic ofphotosystem 2 inhibiting herbicides e fact that chlorosiswas only observed in pots where soil was mixed withGMCC biomass but not in the control where the soil wasnot mixed with GMCC biomass suggests the presence ofphotosynthesis inhibiting allelochemicals Alternativelychlorosis could be a result of the reduced nutrient uptakewhich can be caused by a reduction in root volume andfunction triggered by the phytotoxic activity of alle-lochemicals on root cells [48]ere is also a possibility thatthe allelochemicals produced could interact with the soil byincreasing cation exchange capacity (CEC) which led to thereduction of nutrient uptake It is also possible that theallelochemicals could have reacted with nutrients to forminsoluble complexes that are not available for plant uptakeresulting in chlorosis

Table 8 Effect of soil-incorporated biomass of different plant parts of cover crops on the emergence of goose grass

Cover crop Leaf inhibition Stem inhibition Root inhibition Mean inhibition Rank

Emergence ()Control 983e 983e 983e

Hyacinth bean 444a 55 381ab 61 325ab 67 61 1lowastlowastRed sunnhemp 457a 54 569cd 42 425bc 57 51 6lowastlowastShowy rattlebox 880de 10 890e 9 860e 13 11 10Common bean 750cd 24 685d 30 635d 35 30 9lowastCommonrattlepod 435a 56 435abc 56 315ab 68 60 3lowastlowast

Radish 480a 51 420abc 57 260a 74 61 1lowastlowastTephrosia 575ab 42 355ab 64 445bc 55 53 5lowastlowastBlack sunnhemp 710bc 28 290a 70 445bc 55 51 6lowastlowast

Dry weight (g)Control 114ef 114b 114c

Hyacinth bean 113ef 1 009a 92 006a 95 63 8lowastlowastRed sunnhemp 024ab 79 004a 96 003a 97 91 3lowastlowastlowastShowy rattlebox 087de 24 128b minus12 032b 72 28 10Common bean 042abc 63 005a 96 003a 97 85 4lowastlowastlowastCommonrattlepod 025a 78 002a 98 002a 98 92 2lowastlowastlowast

Radish 072cd 37 005a 96 004a 96 76 6lowastlowastTephrosia 081de 29 002a 98 004a 96 75 7lowastlowastBlack sunnhemp 058bcd 49 001a 99 003a 97 82 5lowastlowast

Seedling vigor index (SVII)

Control 975de

(11178) 975de (11178) 975de

(11178)Hyacinth bean 686bc (4991) 55 186ab (374) 97 134a (163) 99 84 8lowastlowastlowastRed sunnhemp 306a (1066) 90 163ab (246) 98 137a (151) 99 96 3lowastlowastlowast

Showy rattlebox 862cd (7662) 31 1064cd

(11416) minus2 496b (2583) 77 35 10

Common bean 559b (35) 68 184ab (329) 97 142a (168) 98 88 4lowastlowastlowastCommonrattlepod 330a (1102) 90 122a (1148) 99 100a (056) 99 96 2lowastlowastlowast

Radish 578b (3638) 67 155ab (218) 98 118a (095) 99 88 6lowastlowastlowastTephrosia 681bc (4822) 57 111a (077) 99 141a (2163) 99 85 7lowastlowastlowastBlack sunnhemp 638b (4220) 62 155a (053) 100 144a (2176) 98 87 5lowastlowastlowast

Means followed by the same letter in the same column are not significantly different at plt 005 lowastlowastlowast lowastlowast and lowast represent species with mean inhibition overcontrol ge80 ge50 and ge20 respectively Data wereradic (x+ 05) transformed Untransformed data given in brackets were used to calculate the inhibitionpercentages


e different cover crop tissues differentially affected thedry weight accumulation and vigor indices of goose grasse fact that some of the cover crop treatments reducedemergence but the stimulated dry weight of weeds showsthat phytotoxicity lasted for only a short time [25] Alter-natively the high biomass in pots that had low emergencecould be a result of reduced intraspecific competition forresources amongst the plants in the same pot is couldprobably be the reason why the goosegrass dry weight in thecontrol and velvet bean was similar to that of hyacinth beanalthough both had twice as many plants that emerged thanthe hyacinth bean Whilst it is possible to ascribe the dif-ferences in emergence percentages to the presence of pos-sible allelochemicals in the cover crop tissues it is probablethat many other factors could also be responsible for dif-ferences in dry matter accumulation observed ese find-ings are in agreement with those of Yang et al [49] whoreported that 0002 g of Eucalyptus spp leaf extracts in-creased the biomass of broadleaved weeds in potted ex-periments Growth stimulation was not observed inblackjack which demonstrates that this dicotyledonousweed is more susceptible to allelochemicals than themonocotyledonous goose grass ese findings contradictthose of Runzika et al [35] who reported the suppression ofthe dry weight of these two weeds by whole plant extracts ofthe cover crops used in this study e differences could bebecause they used whole plant extracts where allelochemicalsfrom different plant parts could have acted synergisticallyMoreover they used higher residue concentrations of 10compared to 1 used in this study e growth stimulationobserved can be attributed to high levels of nitrogen in theleaves of these leguminous cover crops [50] or hormeticeffects of allelochemicals at low concentrations [3] Althoughmany studies have demonstrated the allelopathic potential ofsome cover crops weed suppression was observed withartificially high concentrations yet allelochemicals exist invery low concentrations under field conditions [51] As suchthese present results are of practical significance since theyrepresent conditions that are most likely to occur undernatural conditions In such cases where hormesis is mostlikely to occur it may be necessary to combine the use ofallelopathic mulches or extracts with herbicides or otherweed control options in order to achieve a satisfactorycontrol of weeds [45]

5 Conclusion

In conclusion the study indicated that all the GMCCs usedin this study contain possible allelochemicals that could beresponsible for the inhibition exhibited on goose grass andblackjack germination as well as seedling growth Leafextracts of all the GMCCs were more efficient in inhibitinggermination and seedling growth of both weeds exceptradish roots extracts that exhibited greater phytotoxic ac-tivity than the other tissue extracts ese results provide areasonable basis for suggesting the use of these eight covercrop aqueous extracts andor mulches for broad spectrumweed control in maize Whilst these findings complementthe results from previous research studies where these cover

crops where used there is still a knowledge gap on thepossible mode of action of these chemicals in susceptibleplants Future studies should therefore focus on identifyingand quantifying the putative allelochemicals in differentplant parts of the cover crops as well as evaluating theirefficacy on weeds and crops when applied postemergence Itis further recommended that the allelopathic potential ofthese cover crops can be studied under field conditions todetermine their efficacy in reducing seed viability andconcomitantly weed emergence in arable fields eseGMCCs are also known to be resistant to common pests anddiseases as well as reduces weeds by their smothering effectsince they produce a lot of biomass rapidly Farmers aretherefore likely to experience remarkable weed germinationand growth suppression in the early season in addition to theother known benefits of these cover crops if these cover croptissues are used at concentrations that are inhibitory to weedgrowth [52ndash54]

Data Availability

e data used to support the findings of this study areavailable from the corresponding author upon request

Conflicts of Interest

e authors declare that they have no conflicts of interest

Acknowledgments

is research was supported through a capacity buildingcompetitive grant training of the next generation of scien-tists provided by Carnegie Corporation of New Yorkthrough the Regional Universities Forum for CapacityBuilding in Agriculture (RUFORUM)

References

[1] Y Lou A S Davis and A C Yannarell ldquoInteractions betweenallelochemicals and the microbial community affect weedsuppression following cover crop residue incorporation intosoilrdquo Plant and Soil vol 399 no 1-2 pp 357ndash371 2016

[2] O A Abdin X M Zhou D Cloutier D C CoulmanM A Faris and D L Smith ldquoCover crops and interrow tillagefor weed control in short season maize (Zea mays)rdquo EuropeanJournal of Agronomy vol 12 no 2 pp 93ndash102 2000

[3] V Rueda-Ayala O Jaeck and R Gerhards ldquoInvestigation ofbiochemical and competitive effects of cover crops on cropsand weedsrdquo Crop Protection vol 71 pp 79ndash87 2015

[4] B Mhlanga S Cheesman B Maasdorp W Mupangwa andC ierfelder ldquoContribution of cover crops to the produc-tivity of maize-based conservation agriculture systems inZimbabwerdquo Crop Science vol 55 no 4 pp 1791ndash1805 2015

[5] V Nichols N Verhulst R Cox and B Govaerts ldquoWeeddynamics and conservation agriculture principles a reviewrdquoField Crops Research vol 183 pp 56ndash68 2015

[6] J K Norsworthy M McClelland G Griffith S K Bangarwaand J Still ldquoEvaluation of cereal and brassicaceae cover cropsin conservation-tillage enhanced glyphosate-resistant cot-tonrdquo Weed Technology vol 25 no 1 pp 6ndash13 2011

[7] Z R Khan C A O Midega T J A Bruce A M Hooper andJ A Pickett ldquoExploiting phytochemicals for developing a


lsquopush-pullrsquo crop protection strategy for cereal farmers inAfricardquo Journal of Experimental Botany vol 61 no 15pp 4185ndash4196 2010

[8] J B Morris C Chase D Treadwell et al ldquoEffect of sunn hemp(Crotalaria juncea L) cutting date and planting density onweed suppression in Georgia USArdquo Journal of EnvironmentalScience and Health Part B vol 50 no 8 pp 614ndash621 2015

[9] J R Teasdale ldquoCover crops smother plants and weedmanagementrdquo in Integrated Weed and Soil ManagementJ L Hatfield D D Buhler and Stewart Eds Ann ArborPress Chelsea MI USA 1998

[10] T Muoni and B Mhlanga ldquoWeed management in Zim-babwean smallholder conservation agriculture farming sec-torrdquo Asian Journal of Agriculture and Rural Developmentvol 4 pp 267ndash276 2014

[11] S R Bezuidenhout C F Reinhardt and M I WhitwellldquoCover crops of oats stooling rye and three annual ryegrasscultivars influence maize and Cyperus esculentus growthrdquoWeed Research vol 52 no 2 pp 153ndash160 2012

[12] C Halde R H Gulden and M H Entz ldquoSelecting cover cropmulches for organic rotational No-till systems in ManitobaCanadardquo Agronomy Journal vol 106 no 4 pp 1193ndash12042014

[13] E M Skinner J C Dıaz-Perez S C PhatakH H Schomberg and W Vencill ldquoAllelopathic effects ofsunnhemp (Crotalaria juncea L) on germination of vegeta-bles and weedsrdquoHortScience vol 47 no 1 pp 138ndash142 2012

[14] M I Ferreira and C F Reinhardt ldquoField assessment of cropresidues for allelopathic effects on both crops and weedsrdquoAgronomy Journal vol 102 no 6 pp 1593ndash1600 2010

[15] M Farooq K Jabran Z A Cheema A Wahid andK H Siddique ldquoe role of allelopathy in agricultural pestmanagementrdquo Pest Management Science vol 67 no 5pp 493ndash506 2011

[16] T d G Baratelli A C Candido Gomes L A WessjohannR M Kuster and N K Simas ldquoPhytochemical and allelo-pathic studies of Terminalia catappa L (Combretaceae)rdquoBiochemical Systematics and Ecology vol 41 pp 119ndash1252012

[17] E L Rice Allelopathy Academic Press Orlando FL USA2nd edition 1984

[18] M J Ayeni ldquoBio-herbicidal potential of the aqueous extractsof the leaves and barks of Gliricidia sepium (Jacq) Kunth ExWalp on the germination and seedling growth of Bidens pilosaLrdquo Donnish Journals of Agricultural Research vol 3 pp 17ndash21 2016

[19] P Barberi and B Lo Cascio ldquoLong-term tillage and croprotation effects on weed seedbank size and compositionrdquoWeed Research vol 41 no 4 pp 325ndash340 2001

[20] M Liebman and A Davis ldquoIntegration of soil crop and weedmanagement in low-external-input farming systemsrdquo WeedResearch vol 40 no 1 pp 27ndash48 2000

[21] J T Rugare P J Pieterse and S Mabasa ldquoEvaluation of thepotential of jack bean [Canavalia ensiformis (L) DC] andvelvet bean [Mucuna pruriens (L) DC] aqueous extracts aspost-emergence bio-herbicides for weed control in maize (Zeamays L)rdquo Asian Journal of Agriculture and Rural Develop-ment vol 10 no 1 pp 420ndash439 2020a

[22] J T Rugare P J Pieterse and S Mabasa ldquoEffects of greenmanure cover crops (Canavalia ensiformis L and Mucunapruriens L) on seed germination and seedling growth of maizeand Eleusine indica L and Bidens pilosa L weedsrdquo AllelopathyJournal vol 50 no 1 pp 121ndash139 2020b

[23] USDA-NRCS Carbon to Nitrogen Ratios in Cropping SystemsSoils httpsusdagovsqi 2011

[24] N H Hong T D Xuan T Eiji T Hiroyuki M Mitsuhiroand T D Khanh ldquoScreening for allelopathic potential ofhigher plants from Southeast Asiardquo Crop Protection vol 22no 6 pp 829ndash836 2003

[25] T D Xuan E Tsuzuki H Terao et al ldquoAlfalfa rice by-products and their incorporation for weed control in ricerdquoWeed Biology and Management vol 3 no 2 pp 137ndash1442003

[26] J T Rugare P J Pieterse and S Mabasa ldquoEffect of short-termmaize-cover crop rotations on weed emergence biomass andspecies composition under conservation agriculturerdquo SouthAfrican Journal of Plant and Soil pp 1ndash9 2019

[27] S Sisodia and S Siddiqui ldquoAllelopathic effect by aqueousextracts of different parts of Croton bonplandianum Baill onsome crop and weed plantsrdquo Journal of Agricultural Extensionand Rural Development vol 2 pp 22ndash28 2010

[28] A Abdul-Baki and J D Anderson ldquoVigor determination insoybean seed by multiple criteriardquo Crop Science vol 13pp 630ndash633 1973

[29] J A Caamal-Maldonado J J Jimenez-Osornio H Torres-Barragan and A L Anaya ldquoe use of allelopathic legumecover and mulch species for weed control in cropping sys-temsrdquo Agronomy Journal vol 93 pp 27ndash36 2001

[30] Y Fujii ldquoAllelopathy in the natural and agricultural eco-systems and isolation of potent allelochemicals from Velvetbean (Mucuna pruriens) and Hairy vetch (Vicia villosa)rdquoBiological Sciences in Space vol 17 no 1 pp 6ndash13 2003

[31] S Purohit and N Pandya ldquoAllelopathic activity of Ocimumsanctum L and Tephrosia purpurea (L) Pers leaf extracts onfew common legumes and weedsrdquo International Journal ofResearch in Plant Science vol 3 pp 5ndash9 2013

[32] W C Conway L M Smith and J F Bergan ldquoPotentialallelopathic interference by the exotic Chinese tallow tree(Sapium sebiferum)rdquo e American Midland Naturalistvol 148 no 1 pp 43ndash53 2002

[33] C T A da Cruiz-Silva and E B Matiazo ldquoAllelopathy ofCrotalaria juncea aqueous extracts on germination and initialdevelopment of maizerdquo IDESIA vol 33 pp 27ndash32 2015

[34] S Anese P U Grisi L d J Jatoba V d C Pereira andS C J Gualtieri ldquoPhytotoxic activity of differents plant parts ofDrimys brasiliensis miers on germination and seedling devel-opmentrdquo Bioscience Journal vol 31 no 3 pp 923ndash933 2015

[35] M Runzika J T Rugare and S Mabasa ldquoScreening greenmanure cover crops for their allelopathic effects on someimportant weeds found in Zimbabwerdquo Asian Journal of Ruraland Agriculture Development vol 3 pp 554ndash565 2013

[36] A Gulzar and M B Siddiqui ldquoAllelopathic effect of aqueousextracts of different part of Eclipta alba (L) Hassk on somecrop and weed plantsrdquo Journal of Agricultural Extension andRural Development vol 6 pp 55ndash60 2010

[37] K A Ali ldquoAllelopathic potential of radish on germination andgrowth of some crop and weed plantsrdquo International Journalof Biosciences vol 9 pp 394ndash403 2016

[38] P Jing L-H Song S-Q Shen S-J Zhao J Pang andB-J Qian ldquoCharacterization of phytochemicals and antiox-idant activities of red radish brines during lactic acid fer-mentationrdquo Molecules vol 19 no 7 pp 9675ndash9688 2014

[39] Y H Zhou and Q J Yu Allelochemicals and PhotosynthesisAllelopathy A Physiological Process with Ecological Implica-tions Springer Dordrecht Netherlands 2006


[40] J Petersen R Belz F Walker and K Hurle ldquoWeed sup-pression by release of isothiocyanates from Turnip-Rapemulchrdquo Agronomy Journal vol 93 no 1 pp 37ndash43 2001

[41] A Uludag I Uremis M Arslan and D Gozcu ldquoAllelopathystudies in weed science in Turkey-a reviewrdquo Journal of PlantDiseases and Protection vol 20 pp 419ndash426 2006

[42] M J Adler and C A Chase ldquoComparison of the allelopathicpotential of leguminous summer cover crops cowpea sunnhemp and velvetbeanrdquo HortScience vol 42 no 2pp 289ndash293 2007

[43] D J Pilbeam and E A Bell ldquoA reappraisal of the free aminoacids in seeds of Crotalaria juncea (Leguminosae)rdquo Phyto-chemistry vol 18 no 2 pp 320-321 1979

[44] R L Wang X Y Yang Y Y Song et al ldquoAllelopathic po-tential of Tephrosia vogelii Hook f laboratory and fieldevaluationrdquo Allelopathy Journal vol 28 pp 53ndash62 2011

[45] K Jabran Z A Cheema M Farooq and M Hussain ldquoLowerdoses of pendimethalin mixed with allelopathic crop waterextracts for weed management in canola (Brassica napus)rdquoInternational Journal of Agricultural Biology vol 12pp 335ndash340 2010

[46] A R Soares R Marchiosi R dC Siqueira-Soares R Barbosade Lima W Dantas dos Santos and O Ferrarese-Filho ldquoerole of L-Dopa in plantsrdquo Plant Behavior and Signaling vol 9pp 1ndash7 2014

[47] M Niakan and K Saberi ldquoEffects of Eucalyptus allelopathy ongrowth characters and antioxidant enzymes activity in Pha-laris weedrdquo Asian Journal of Plant Sciences vol 8 no 6pp 440ndash446 2009

[48] M Shahid B Ahmed R A Khatak G Hassan and H KhanldquoResponse of wheat and its weeds to different allelopathicplant water extractsrdquo Pakistan Weed Science Journal vol 12pp 61ndash68 2006

[49] I Yang Y Chen Y Huang J Wang and M Wen ldquoMixedallelopathic effect of Eucalyptus leaf litter and understoreyfern in South Chinardquo Journal of Tropical Forest Sciencevol 28 pp 436ndash455 2016

[50] M d M Gomes D J Bertoncelli Jr G A C Alves et alldquoAllelopathic potential of the aqueous extract of Raphanussativus L on the germination of beans and corn seedsrdquoOALib vol 4 no 5 pp 1ndash10 2017

[51] R G Belz K Hurle and S O Duke ldquoDose response-achallenge for allelopathyrdquoNonlinearity in Biology Toxicologyand Medicine vol 3 pp 173ndash211 2005

[52] O A Chivinge ldquoA weed survey of arable small lands of thesmall-scale farming sectorrdquo Zambezia vol 15 pp 167ndash1791988

[53] M Liebman and D N Sundberg ldquoSeed mass affects thesusceptibility of weed and crop species to phytotoxinsextracted from red clover shootsrdquoWeed Science vol 54 no 2pp 340ndash345 2006

[54] R Naderi and E Bijanzadeh ldquoAllelopathic potential of leafstem and root extracts of some Iranian rice (Oryza sativa L)cultivars on barnyard grass (Echinochloa crusgalli) growthrdquoPlant Knowledge Journal vol 1 pp 37ndash40 2012


nitrogen ratio (C N) of the decomposing cover crop ma-terial soil type and the environment [5] In addition covercrops can suppress weeds by competing for resources [6]disruption of life cycles of crop bound and crop associatedweeds [7] through resource and light competition [8 9]creation of soil conditions that promote seed decay andpredation [10] and prevention of weed seed set and dispersal[5] Furthermore some cover crops such as stooling rye(Secale cereal L) barley (Hordeum vulgare L) and blacksunnhemp (Crotalaria juncea L) have demonstrated alle-lopathic activity [11ndash13] Ferreira and Reinhardt [14] re-ported the possibility of using cover crops to manageherbicide-resistant weeds

Allelopathy a phenomenon occurring in natural oragricultural communities of plants is defined as the in-hibitory or stimulatory effects of one plant species onanother through the release of chemicals called alle-lochemicals [15] Allelochemicals are released by liveplants directly into the environment as volatiles leachatesandor root exudates [16] e allelochemicals are alsoreleased when the residues of the plants decompose [17]According to Ayeni [18] allelopathy can be exploited toachieve reduced application of synthetic herbicides sincemost allelochemicals do not have residual effects and cantherefore be exploited for early season weed control inarable fields without affecting successive crops in croprotations Barberi and Lo Cascio [19] reported that themaize-cover crop rotations reduced weed density underfield conditions demonstrating the possibility ofexploiting allelopathic cover crops for effective weedcontrol in IWM

e fact that allelopathic effects from decomposingcover crop residues were reported to be more pronouncedon small seeds and early emerging species compared tolarge-seeded crops which offers an opportunity forexploiting them for selective weed control in arable cropproduction [20] Aqueous extracts of velvet bean (Mucunapruriens L) and Jack bean (Canavalia ensiformis L) re-duced weed germination and early seedling development[21 22]

Most of the cover crops being proposed for adoption insmallholder CA are legumes with a low carbon to nitrogen(C N) ratio resulting in a faster decomposition than cerealresidues that have higher C N ratios [23] As such whenallelopathic materials of leguminous cover crops are appliedin the field a considerable amount of nutrients are addedwhich may cause stimulation of weed growth due to min-eralisation [23 24] and hormesis e need for finding al-lelopathic cover crops whose phytotoxicity outweighs theirweed growth stimulation activity is therefore indispensable[25] is work aimed at the assessment of the allelopathicpotential of leaf stem and root aqueous extracts and in-corporated biomass of eight cover crops that are beingpromoted for the adoption in smallholder CA in Zimbabwee hypothesis tested in the present study was that aqueousextracts and soil-incorporated cover crop biomass suppressblackjack (Bidens pilosa L) and goose grass [Eleusine indica(L) Gaertn] germination emergence and early seedlinggrowth

2 Materials and Methods

21 Experimental Site e study was carried out at theUniversity of Zimbabwe (UZ) situated at 1778degS 3105degEwith an altitude of 1523m above sea level Laboratorybioassays were carried out in Petri dishes in the WeedScience laboratory and pot experiments were carried out ina greenhouse at the Department of Plant Production Sci-ences and Technologies between January 2015 and June2016 Average day and night temperatures in the glasshousewere 281degC and 152degC No artificial light was used in theglasshouse

22 Biomass Preparation e GMCCs used in the studywere grown under irrigation in a field at UZrsquos Department ofPlant Production Sciences and Technologies in September2014 e GMCCs were grown in soils with 18 clay 16silt and 66 sand with a pH (CaCl2) of 52 Exchangeablecations in milliequivalents percent (me ) of soil were 761417 and 032 for calcium magnesium and potassiumrespectively Green manure cover crops were grown in thefield at the University of Zimbabwe as described in Rugareet al [26] e preparation of dry biomass powder was donefollowing the method described by Rugare et al [21] withoutany amendments

23 Aqueous Extract Preparation Fifty grams of GMCCpowder were soaked in 1000ml distilled water to produce a5wvminus1 solution on a dry weight basis e solution wasthen stirred for 24 hours at room temperature (25degC) on anorbital shaker at 100 rpm e extracts were then strainedthrough four layers of cheese cloth before being centrifugedat 4000 rpm for 15 minutes e clear solution was pipettedto separate it from the supernatant is stock solution wasthen stored in parafilm sealed bottles at 4degC for 24 hoursbefore being used e stock solution was then diluted withdistilled water to produce 125 25 and 375 plantextracts prior to use Conductivity of the 5wvminus1 aqueousextract concentrations of the ten GMCCs was measuredusing a conductivity meter (Model SX713 version 20 2013-7-30) and the values obtained were used to calculate theosmotic potential using the following formula [27]

Osmotic potential (in MPa) conductivity (in mS)lowast minus0036Osmotic potential values for the different stock solu-tions (5wvminus1 concentrations) of the different GMCCtissue aqueous extract treatments ranging from minus026to minus009

In order to ascertain whether the inhibitory activityexhibited by different extract concentrations of the covercrop tissues was not due to differences in the osmoticpressure of the solutions germination bioassays wereconducted using polyethylene glycol (PEG) 6000 solutions at000 005 010 015 020 025 and 030MPa (ie the rangeof osmotic potentials of the 5wvminus1 of the cover croptissues)



G a

b1113874 1113875lowast 100 (1)










(3)




1113876 1113877 lowast 100 (4)




3 Results










Organiccarbon

pH(CaCl2)

Organicmatter





4 Discussion




Extracttissue









































Extracttissue


















LSD 7671 5167Radish





















Extracttissue


















LSD 5242 5039Radish






















Extracttissue


















LSD 1918 0400Radish













































Control 975de

(11178) 975de (11178) 975de



(11416) minus2 496b (2583) 77 35 10






5 Conclusion



Data Availability




Acknowledgments


References




























































G a

b1113874 1113875lowast 100 (1)










(3)




1113876 1113877 lowast 100 (4)




3 Results










Organiccarbon

pH(CaCl2)

Organicmatter





4 Discussion




Extracttissue









































Extracttissue


















LSD 7671 5167Radish





















Extracttissue


















LSD 5242 5039Radish






















Extracttissue


















LSD 1918 0400Radish













































Control 975de

(11178) 975de (11178) 975de



(11416) minus2 496b (2583) 77 35 10






5 Conclusion



Data Availability




Acknowledgments


References




























































3 Results










Organiccarbon

pH(CaCl2)

Organicmatter





4 Discussion




Extracttissue









































Extracttissue


















LSD 7671 5167Radish





















Extracttissue


















LSD 5242 5039Radish






















Extracttissue


















LSD 1918 0400Radish













































Control 975de

(11178) 975de (11178) 975de



(11416) minus2 496b (2583) 77 35 10






5 Conclusion



Data Availability




Acknowledgments


References




























































4 Discussion




Extracttissue









































Extracttissue


















LSD 7671 5167Radish





















Extracttissue


















LSD 5242 5039Radish






















Extracttissue


















LSD 1918 0400Radish













































Control 975de

(11178) 975de (11178) 975de



(11416) minus2 496b (2583) 77 35 10






5 Conclusion



Data Availability




Acknowledgments


References
































































Extracttissue


















LSD 7671 5167Radish





















Extracttissue


















LSD 5242 5039Radish






















Extracttissue


















LSD 1918 0400Radish













































Control 975de

(11178) 975de (11178) 975de



(11416) minus2 496b (2583) 77 35 10






5 Conclusion



Data Availability




Acknowledgments


References






























































Extracttissue


















LSD 5242 5039Radish






















Extracttissue


















LSD 1918 0400Radish













































Control 975de

(11178) 975de (11178) 975de



(11416) minus2 496b (2583) 77 35 10






5 Conclusion



Data Availability




Acknowledgments


References































































Extracttissue


















LSD 1918 0400Radish













































Control 975de

(11178) 975de (11178) 975de



(11416) minus2 496b (2583) 77 35 10






5 Conclusion



Data Availability




Acknowledgments


References






















































































Control 975de

(11178) 975de (11178) 975de



(11416) minus2 496b (2583) 77 35 10






5 Conclusion



Data Availability




Acknowledgments


References




























































5 Conclusion



Data Availability




Acknowledgments


References



























































allelopathic potential of green manure cover crops on

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