Allelopathic effects of green manure cover crops on the germination and growth of blackjack (Bidens pilosa L.) and rapoko grass [Eleusine indica (L.) Gaertn]

Rugare, Joyful Tatenda (2018-03)

Thesis (PhDAgric)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: The use of green manure cover crops to improve the nitrogen levels of soils and other physical as well as biological soil properties has been extensively researched and documented in several parts of the world. However, there is paucity of information on their use in weed management particularly through the exploitation of their allelopathic properties on some of the difficult to control weeds in arable crop production. Identification and integration of cover crops with weed suppressive ability is likely to reduce overreliance on herbicides and tillage-associated soil degradation. Moreover, the use of allelopathic mulches is likely to solve the problem of early season weed pressure commonly experienced in conservation agriculture where the use of pre-emergence herbicides is practically ineffective due to the presence of mulch at the time of planting. This study sought to establish the allelopathic potential of cover crops and evaluate their efficacy in suppressing weeds when their aqueous extracts and residues were used for weed control in maize. The study comprised different sets of experiments and was conducted between 2014 and 2017 at the University of Zimbabwe. The first study involved laboratory evaluation of 0, 1.25, 2.5, 3.75 and 5% wv-1 aqueous extracts of dry leaf, stem and root residues of ten cover crops on the germination, plumule and radicle growth of two test weed species namely, goosegrass (Eleusine indica (L.) Gaertn) and blackjack (Bidens pilosa L.) as well as of the crop species maize (Zea mays L.). The cover crops used included jack bean (Canavalia ensiformis (L.) DC), velvet bean (Mucuna pruriens (L.) DC var utilis), hyacinth bean (Lablab purpureus L), red sunnhemp (Crotalaria ochroleuca G. Don), showy rattlebox (Crotalaria grahamiana Wight & Arn.), common bean (Phaseolus vulgaris L.), common rattlepod (Crotalaria spectabilis Roth.), radish (Raphanus sativus L.), tephrosia (Tephrosia vogelii L.) and black sunnhemp (Crotalaria juncea L.). There was a significant interaction between extract concentration and extracted tissue with all cover crops and with all studied germination parameters of goosegrass and blackjack. Results showed that extracts of all cover crops exhibited an allelopathic potential on both test weeds that decreased in the order leaf > stem > root extract except with radish whose root extracts were more phytotoxic than extracts of the other tissues on goosegrass. In contrast, maize germination was not affected by aqueous extracts of the cover crops except for extracts of jack bean and common rattlepod. Most of the cover crops reduced seedling growth of maize except extracts of hyacinth bean and tephrosia. The second study involved an assessment of the allelopathic potential of soil incorporated biomass of the different cover crop tissues on the emergence and dry weight of goosegrass, blackjack and maize. The experiment which was laid out as a Complete Randomised Design (CRD) was carried out in the greenhouse. The soil incorporated leaf, stem and root residues of all cover crops significantly affected seedling emergence, dry weight and vigor indices of both weeds. Overall jack bean and hyacinth bean residues exhibited the highest phytotoxic activity on weed seedling emergence and growth. On the other hand, none of the cover crop residues exhibited deleterious effects on maize emergence, dry weight and vigor indices. Samples of the cover crop tissues were subjected to Liquid Chromatography Mass Spectrometry (LC-MS) at the Central Analytical Facilities (CAF) at Stellenbosch University in order to determine the phenolic composition of the extracts. The analyses showed the presence of kaempferol, naringenin, rutin, and genistein in jack bean. On the other hand genistein, atropine and kaempferol were detected in velvet bean tissues. A new compound was detected in the tissues of both cover crops and was tentatively identified as quercetin 3-O-glucoside 7-O-rhamnoside. Thereafter, standards of compounds detected in the samples were used to carry out germination bioassays using goosegrass and blackjack as test species. Highest inhibition of seedling growth of the two test weed species was obtained with standards of kaempferol and rutin. The effect of post emergence sprays of aqueous extracts of jack bean and velvet bean alone or in combination with reduced atrazine dosages on the test weed species and maize was evaluated in a greenhouse experiment that was laid out as a CRD. Application of the different treatments at the 3-4 leaf stage of test species significantly reduced chlorophyll content, plant height, and dry weight and resulted in irreversible weed damage at 6 and 10 days after spraying of blackjack and goosegrass, respectively. Maize was not affected by the repeated post emergence applications of the different herbicidal treatments. Field experiments were carried out during the summer cropping seasons 2014-15 to 2016-17 on two fields at the University of Zimbabwe and on one field at CIMMYT, Harare station in Zimbabwe. A Randomised Complete Block Design (RCBD) was used to evaluate the effect of different maize-cover crop rotations on the weed community composition. Soil samples were taken at 0-5, 5-10 and 10-15 cm depths in a bid to evaluate the influence of maize-cover crop rotations on weed seed bank size and species composition in an experiment that was laid out as a split plot design under greenhouse conditions. The results showed that there were no significant differences in weed density and species composition among maize-cover crop rotations in all seed banks. However, there was a significant decline in weed densities in the second maize phase of the rotations. Weed species richness, evenness and diversity was also significantly affected in the second year of the rotation although there were no significant variations among maize-cover crop rotations. Generally, cover crop rotations significantly reduced blackjack density in the second maize phase of the rotation but the density of goosegrass remained unchanged. The effect of maize-cover crop rotation type on weed emergence was significant. Maize-velvet bean and maize-red sunnhemp rotations consistently reduced total weed density and biomass across seasons. However, the different cover crop rotations did not exhibit deleterious effects on maize emergence and height. Based on the findings of this study it can be concluded that all the cover crops are allelopathic to goosegrass and blackjack and could be used to manage weeds either as surface mulches in maize-cover crop rotations or as post emergence sprays when weeds are still very small.

AFRIKAANSE OPSOMMING: Die gebruik van groenbemestingsdekgewasse om stikstofvlakke en ander fisiese en biologiese grondeienskappe te verbeter is deeglik ondersoek en gedokumenteer in verskeie dele van die wêreld. Daar is egter min inligting beskikbaar oor hulle gebruik in onkruidbestuur, veral die gebruik van hulle allelopatiese eienskappe vir moeilik beheerbare onlruide in eenjarige gewasproduksie. Identifisering en integrasie van dekgewasse met onkruidonderdrukkende vermoëns sal waarskynlik die oorafhanklikheid van onkruiddoders en grondagteruitgang as gevolg van grondbewerking verminder. Die gebruik van allelopatiese deklae sal waarskynlik die probleem van vroeë seisoen onkruiddruk wat algemeen voorkom in bewaringsbewerkingstelsels as gevolg van oesreste wat die werking van vooropkoms middels benadeel, oplos. Die studie poog om die allelopatiese potensiaal van dekgewasse te bepaal asook hulle effektiwiteit om onkruide te onderdruk wanneer waterekstraksies en reste gebruik word vir onkruidbeheer in mielies. Die studie het bestaan uit verskillende eksperimente en is tussen 2014 en 2017 uitgevoer by die Universiteit van Zimbabwe. Die eerste studie is in die laboratorium uitgevoer en het die evaluering van 0, 1.25, 2.5, 3.75 and 5% mv-1 konsentrasies van droë stam-, blaar- en wortelekstrakte van tien dekgewasse op die ontkieming, kiemplantjie- en kiemwortellengte van twee onkruidspesies nl. jongosgras (Eleusine indica (L.) Gaertn) en knapsekêrel (Bidens pilosa L.), asook die toetsgewas, mielies (Zea mays L.), behels. Die dekgewasse het ingesluit included “jack bean” (Canavalia ensiformis (L.) DC), “velvet bean” (Mucuna pruriens (L.) DC var utilis), “hyacinth bean” (Lablab purpureus L), “red sunnhemp” (Crotalaria ochroleuca G. Don), “showy rattlebox” (Crotalaria grahamiana Wight & Arn.), “tuinboon” (Phaseolus vulgaris L.), “common rattlepod” (Crotalaria spectabilis Roth.), radys (Raphanus sativus L.), “tephrosia” (Tephrosia vogelii L.) en “black sunnhemp” (Crotalaria juncea L.). Daar was betekenisvolle interaksie tussen die ekstrakweefsel en ekstrakkonsentrasie van al die dekgewasse ten opsigte van al die ontkiemingsparameters van jongosgras en knapsekêrel. Resultate het getoon dat alle dekgewasse se ekstrakte allelopatiese potensiaal getoon het op albei onkruidspesies en die intensiteit van die werking het afgeneem in die volgorde van blaar>stam>wortel behalwe in die geval van radys waar die wortelekstrakte meer fitotoksies was as ander ekstrakte op jongosgras. In teenstelling daarmee is geen effek van die ekstrakte op die ontkieming van mielies waargeneem nie behalwe vir ekstrakte van “jack bean” en “common rattlepod”. Die meeste van die dekgewasse het die saailinggroei van mielies onderdruk behalwe ekstrakte van “hyacinth bean” en “tephrosia”. Die tweede studie het die allelopatiese potensiaal van droëmateriaal van verskillende dele van die verskillende dekgewasse wat in die grond ingewerk is, op die vestiging en droëmassa van jongosgras, knapsekêrel en mielies bepaal. Die eksperiment wat as ‘n Volkome Ewekansige Ontwerp uitgelê is, is in ‘n glashuis uitgevoer. Die ingewerkte blaar-, stam- en wortelmateriaal van al die dekgewasse het saailingvestiging, droëmateriaal en lewenskragtigheidsindekse van beide onkruide betekenisvol beïnvloed. Oor die algemeen het “jack bean” en “hyacinth bean” se reste die grootste fitotoksiese aktiwiteit op onkruidsaailing vestiging en groei gehad. Aan die ander kant het nie een van die dekgewasse enige nadelige uitwerking op die vestiging, droëmassa en lewenskragtigheidsindeks van mielies gehad nie. Monsters van die dekgewasmateriaal is onderwerp aan vloeistofchromatografie massaspektrometrie by die Sentrale Analitiese Fasiliteit (SAF) by die Universiteit van Stellenbosch om die fenoliese samestelling van die ekstrakte te bepaal. Die ontledings het die teenwoordigheid van kaempferol, naringenin, rutin en genistein in “jack bean” aangedui. Genistein, atropine en kaempferol is waargeneem in materiaal van “velvet bean”. ‘n Nuwe verbinding is waargeneem in weefsels van beide dekgewasse en is voorlopig geïdentifiseer as quercetin 3-O-glucoside 7-O-rhamnoside. Daarna is standaardoplossings van die verbindings wat in die materiaal van die twee spesies opgemerk is gebruik om ontkiemingsbiotoetse op jongosgras en knapsekêrel te doen. Die grootste inhibering van saailinggroei van die twee toetsonkruidspesies is verkry met standaardoplossings van kaempferol en rutin. Die effek van na-opkoms bespuitings van waterekstrakte van “jack bean” en “velvet bean” alleen of in kombinasie met verlaagde atrasien dosisse op die onkruidtoetsspesies en mielies is uitgevoer in ‘n glashuiseksperiment wat uitgelê is as ‘n Volkome Ewekansige Ontwerp. Toediening van die verskillende behandelings op die 3-4 blaarstadium van die onkruidspesies het die chlorofilinhoud, planthoogte en droëmassa van die spesies betekenisvol verlaag en het gelei tot onomkeerbare skade na 6 en 10 dae op knapsekêrel en jongosgras onderskeidelik. Mielies is nie deur herhaaldelike na-opkoms toediening van die verskillende onkruiddoderbehandelings beïnvloed nie. Veldeksperimente is uitgevoer tydens die somersaaiseisoene van 2014-15 tot 2016-17 op twee lande by die Universiteit van Zimbabwe en een land by CIMMYT se Harare stasie in Zimbabwe. ‘n Volledig Ewekansige Blokontwerp is gebruik om die invloed van verskillende mielie-dekgewas rotasies op die onkruidgemeenskapsamestelling te ondersoek. Grondmonsters is geneem by 0-5, 5-10 en 10-15 cm dieptes in ‘n poging om die effek van verskillende mielie-dekgewas rotasies op onkruidsaadbankgrootte en –samestelling te bepaal. Die eksperiment is in ‘n glashuis met ‘n gesplete perseel ontwerp uitgevoer. Die resultate het getoon dat daar nie betekenisvolle verskille ten opsigte van onkruiddigtheid en –spesiesamestelling was tussen die verskillende mielie-dekgewas rotasies nie. Daar was egter ‘n betekenisvolle afname in die onkruiddigtheid tydens die tweede mieliefase van die rotasies. Onkruidspesierykheid, -gelykheid en –diversiteit is ook betekenisvol beïnvloed in die tweede jaar van die rotasie alhoewel daar nie betekenisvolle verskille tussen die verskillende dekgewasrotasies was nie. Oor die algemeen het die dekgewasse die digtheid van knapsekêrel in die tweede mieliefase betekenisvol verlaag maar daar was geen effek op die digtheid van jongosgras nie. Die effek van mielie-dekgewas rotasie op die vestiging van onkruide was betekenisvol. Mielie-“velvet bean” en mielie-“red sunnhemp” het deurlopend die totale onkruidbiomassa en –digtheid oor seisoene verlaag. Die verskillende dekgewasse het egter nie ‘n nadelige effek op mielievestiging en -hoogte gehad nie. Na aanleiding van die bevindings van hierdie studie kan die gevolgtrekking gemaak word dat al die dekgewasse allelopaties is jeens jongosgras en knapsekêrel en dat dit gebruik kan word om onkruid te beheer in mielie-dekgewas rotasies of as na-opkoms middel terwyl die onkruid nog klein is.

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