Doctoral Degrees (Agronomy)

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Browsing Doctoral Degrees (Agronomy) by browse.metadata.advisor "Mabasa, Stanford"

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    Allelopathic effects of green manure cover crops on the germination and growth of blackjack (Bidens pilosa L.) and rapoko grass [Eleusine indica (L.) Gaertn]
    (Stellenbosch : Stellenbosch University, 2018-03) Rugare, Joyful Tatenda; Pieterse, P. J.; Mabasa, Stanford; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
    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.