Germination and competition studies on selected weed species in cereal cropping systems in the Western Cape

Manoto, Martha Mmamontsheng (2002-12)

Thesis (MSc)--Stellenbosch University, 2002.

Thesis

ENGLISH ABSTRACT: The high cost of herbicides for weed control in crop fields in the Western Cape is a major cause of reducing farmers' net income. As chemical weed control became more difficult and expensive, it became necessary to focus on the technique of reducing weed impact, which does not only involve herbicide usage. Aspects such as tillage method, sowing date of crops, crop rotation, weed ecology and germination requirements, amongst other, may play a role in reducing weed impact. The first experiment was done to determine whether temperature and light had an effect on the germination of six selected weed species, namely Arctotheca calendula, Avena fatua, Bromus diandrus, Emex australis, Lolium temulentum and Raphanus raphanistrum. Seeds of the aforementioned weeds were collected from Langgewens during 2000 and stored at room temperature before being used in this study. The seeds were germinated in a germination cabinet at three temperature regimes namely 5 "C /15 "C, 10°C / 15°C and 10°C / 20°C. Most seeds showed a positive germination response at the 10°C / 15°C treatment, except for Emex australis and Lolium temulentum, which was believed to germinate throughout the year under favourable conditions. Among the germinated weed species, Avena fatua germinated best with a cumulative value of 90% as compared to Raphanus raphanistrum, which germinated least with a cumulative value of 12%. The second experiment was done to evaluate the effect of three growth regulators, namely gibberellic acid, hydroxylamine (auxin), and kinetin (cytokinin) to break dormancy and enable simultaneous germination of the six weed species mentioned above. Weed seeds were germinated in a germination cabinet at 20 °C using the test solutions of the aforementioned growth regulators. The germination was assessed after 3, 7, 10 and 14 days of incubation and the tetrazolium test for the viability of seeds was done for ungerminated seeds. The result obtained showed that no chemical/concentration proved to be successful in stimulating the germination of all species tested. As for example a high concentration of hydroxylamine increased germination of Emex australis to nearly 100% and inhibited germination of Raphanus raphanistrum to less than 12.5% at all hydroxylamine concentrations. The third experiment was conducted with the aim of determining the competitiveness of the six weed species mentioned above when grown together with wheat in order to decide when weed control will be cost-effective. An additive series experiment was conducted in a glasshouse. The influence of weed species on wheat plant height, tiller number at different growth stages, vegetative dry mass and gram mass was determined by using different varying weed densities. Weed densities, through reduction in number of tillers, reduced wheat grain yield. By increasing the density of wheat this competitive effect could be reduced. Different weed densities caused a significant difference in wheat plant height, tiller number, dry mass and grain mass. The results showed that species had a significant effect (p < 0.05) on wheat plant height during tillering, stem elongation and heading growth stages. Total wheat plant above-ground dry mass was significantly (p < 0.05) reduced by an increase in weed density.

AFRIKAANSE OPSOMMING: Die hoë koste van onkruiddoders vir onkruidbeheer in graangebiede van die Wes- Kaap is een van die hooffaktore wat graanprodusente se netto inkomste verlaag. Weens moeiliker en duurder chemiese onkruidbeheer en om die impak van onkruide te verlaag, het dit noodsaaklik geword om op die tegnieke te fokus wat nie die gebruik van chemikalieë insluit nie. Faktore wat in rol kan speel sluit bewerkingspraktyke, saaidatum van gewasse, wisselbou, ekologie en ontkiemingsvereistes van onkruide in. Die eerste eksperiment is gedoen om te bepaal of temperatuur en lig 'n effek het op die ontkieming van ses gekose onkruide, naamlik Arctotheca calendula, Avenajatua, Bromus diandrus, Emex australis, Lo/ium temulentum en Raphanus raphanistrum. Sade van die voorgenoemde onkruide is in 2000 op Langgewens versamel en by kamertemperatuur geberg voordat dit in hierdie studie gebruik is. Die sade is in 'n ontkiemingskabinet geïnkubeer by drie temperatuurreekse naamlik 5 oe / 15 oe, 10 oe / 15 oe en 10 oe / 20 oe. Die meeste sade het 'n positiwe ontkiemingsreaksie getoon na die 10 oe / 15 oe behandeling, met die uitsondering van Emex australis en Lolium temulentum, wat oënskynlik heeljaar salontkiem onder gunstige omstandighede. Avena fatua het die hoogste ontkiemingspersentasie oor alle spesies getoon met 'n kumulatiewe waarde van 90% en Raphanus raphanistrum die minste met 'n kumulatiewe waarde van 12%. Die tweede eksperiment is gedoen om die invloed van drie groeireguleerders, naamlik gibberelliensuur, hidroksielamien (ouksien) en kinetin (sitokinien), op die opheffmg van saadrus te ondersoek en om ook die gelyktydige ontkieming van bogenoemde ses onkruidspesies moontlik te maak. Onkruidsade is in 'n ontkiemingskabinet by 20 oe ontkiem deur gebruik te maak van toetsoplossings van bogenoemde groei-reguleerders. Die ontkieming van die sade is na inkubasie periodes van 3, 7, 10 en 14 dae geëvalueer en die tetrazoliumtoets VIr saadkiemkragtigheid is toegepas vir onontkiemde sade. Die verkreë resultate het getoon dat geen chemiekalie / konsentrasie die ontkieming van alle spesies suksesvol kon stimuleer nie. Hoë konsentrasies hidroksielamien het die ontkieming van Emex australis tot byna 100% verhoog en het die ontkieming van Raphanus raphanistrum geïnhibeer tot minder as 12.5% by alle hidroksielamien konsentrasies. Die derde eksperiment is uitgevoer met die doelom die kompetisievermoë van dieselfde ses onkruidspesies te bepaal as dit saam met koring groei en om te besluit wanneer onkruidbeheer koste effektief sal wees. 'n Additiewe reeks eksperiment wat uit vier herhalings bestaan het, is in plastiekpotte in 'n glashuis uitgevoer. Die invloed van die onkruidspesies op koring planthoogte, halmgetal by verskillende groeistadiums, droë massa en graanmassa is bepaal deur onkruiddigthede te gebruik. Onkruide het koring opbrengs verlaag deur die vermindering van halmgetal en hierdie kompeterende effek kon verminder word deur 'n toename in die plantestand van koring. Verskillende onkruiddigthede het 'n betekenisvolle invloed op koring planthoogte, halmgetal, droë massa en graanmassa gehad. Die resultate het getoon dat spesies 'n betekenisvolle effek (p< 0.05) op koring planthoogte, stem verlenging en aarverskyning gehad het. Die totale bogrondse droë massa van koring is betekenisvol (p< 0.05) verminder deur 'n verhoging in onkruiddigtheid.

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