The influence of manganese on glyphosate efficacy in ryegrass (Lolium spp.)

Verster, Enrike (2018-12)

Thesis (MSc)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: In the Western Cape of South Africa, ryegrass (Lolium spp.) developed resistance to the herbicide glyphosate within wheat fields, orchards and vineyards. The influence that plant available Mn might have on glyphosate resistance in ryegrass were investigated by measuring percentage survival and dry matter production in response to Mn treatments and glyphosate dosage rates. Four different trials were conducted in this study. In the first trial, glyphosate dosage response was tested against the effect of added Mn treatment levels on glyphosate susceptible and resistant ryegrass populations. In the second trial, glyphosate dosage response was tested against the effect of soil types of differing Mn contents. In the third trial, glyphosate dosage response was tested against ryegrass species found in agricultural setting in soils of different Mn content from the same farm where it naturally grew. Glyphosate dosage response in ryegrass were lastly also tested under varying temperature ranges and Mn treatment levels. Glyphosate efficacy was found to exhibit trends of decline at higher Mn treatment levels, especially at sublethal dosage rates or soils of naturally higher Mn content. Ryegrass originating from a soil of higher Mn content (biotype R3) as opposed to their counterparts seemed to be naturally more inclined to a magnified resistance, regardless of the soil into which it was transplanted. When testing glyphosate dosage response against Mn treatment level and temperature conditions, trends showed that regardless of the temperature conditions ryegrass were subjected to, Mn levels played a distinct role in glyphosate efficacy. Added Mn exhibited trends of better survival rates than control treatments throughout the temperature conditions. At 25-30°C, glyphosate efficacy was reduced compared to the lower temperature ranges. Generally, it was found that Mn treatments seemed to exert some effect of decreased glyphosate efficacy in ryegrass albeit only at low, sub-lethal dosage rates. Trends indicated that some levels of added plant available Mn seemed to curb glyphosate efficacy at low dosage rates. It was concluded that some effect must exist where plant available Mn influences the herbicidal activity of glyphosate in a negative manner. Reduced translocation is the most likely explanation for this trend. This reduced efficacy probably occurs as a result of either complexation of glyphosate with Mn and subsequent reduced uptake or vacuolar sequestration. However, Mn did not affect glyphosate efficacy at recommended dosage rates. Results of this study emphasize the importance of applying glyphosate at recommended dosage rates and never to apply below recommended dosage rates.

AFRIKAANSE OPSOMMING: Raaigras (Lolium spp.) het in die Wes-Kaap van Suid-Afrika oor die jare heen weerstandigheid ontwikkel in koringlande, wingerde en boorde teen die onkruiddoder glifosaat. Die invloed wat plant beskikbare Mn mag hê op glifosaat weerstandigheid in raaigras, is ondersoek deur oorlewingspersentasie en droëmassaproduksie te meet teenoor Mn behandelings en glifosaat dosisse. Vier verskillende proewe is in hierdie studie afgehandel. In die eerste proef is glifosaat se reaksie getoets teen die effek van toegediende Mn vlakke op glifosaat weerstandige en -vatbare raaigras populasies. In die tweede proef is glifosaat se werking getoets teen die effek van verskillende grondtipes wat verskillende natuurlike Mn vlakke bevat. In die derde proef is die werking van glifosaat getoets teen raaigras populasies wat in landbougrond van verskillende Mn vlakke gevind is op die plaas waar dit natuurlik gegroei het. Laastens was die werking van glifosaat ook getoets in verskeie temperature in kombinasie met Mn behandelings. Daar is bevind dat die effektiwiteit van glifosaat afneem by hoër vlakke van Mn behandelings of grondtipes van natuurlike hoër Mn inhoud – veral by sub-letale dosisse. Raaigras afkomstig van grondtipes met ‘n hoër Mn inhoud (biotipe R3), blyk meer geneigd te wees tot ‘n verhoogde weerstandigheid in vergelyking met ander raaigras populasies; ongeag van die grond tipe waarin dit oorgeplant is. Tendense het getoon dat Mn by alle temperatuurvlakke ‘n wesenlike rol speel in die effektiwiteit van glifosaat. Behandelings van toegevoegde Mn het tendense getoon van beter oorlewing by al die temperatuurvlakke teenoor die kontrolebehandelings. By 25-30°C was die werking van glifosaat verminder vergeleke met die laer temperatuurvlakke. Oor die algemeen is daar bevind dat Mn behandelings tog ‘n effek van verlaagde effektiwiteit uitoefen op die werking van glifosaat op raaigras alhoewel slegs by lae, subletale dosisse. Tendense het getoon dat veral sekere vlakke van toegevoegde plant beskikbare Mn die werking van glifosaat stu. Die gevolgtrekking is gemaak dat plant beskikbare Mn heel moontlik die onkruiddodende aktiwiteit van glifosaat negatief beïnvloed. Verminderde translokasie deur die plant is heel moontlik die mees voor die hand liggende verduideliking van hierdie tendens. Die verlaagde effektiwiteit is moontlik as gevolg van kompleksasie van die glifosaat molekule met Mn en die daaropvolgende verlaagde opname of vakuolêre sekwestrasie. Mangaan het egter nie die effektiwiteit van glifosaat benadeel wanneer dit teen geregistreerde dosisse gespuit is nie. Die resultate van hierdie studie beklemtoon weereens die belangrikheid om glifosaat teen aanbevole dosisse toe te dien en nie teen laer as geregistreerde dosisse nie.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105022
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