Optimizing the use of pre-emergent herbicides in wheat production, under conservation agriculture practices in the South-Western Cape Region

Ntombela, Bongani Nkosinathi (2019-04)

Thesis (MScAgric)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: The interception of herbicide by residue cover can result into poor weed management, leads to more weed seeds reaching the soil seed bank, which results in higher weed pressure in the following seasons and, in the long-term, these sub-lethal doses can lead to the development of non-target site herbicide resistance. Therefore, the aim of this study was to investigate possible dosage rate/application rate combinations that can result in the required amount of selected pre-emergent herbicides to reach the soil surface at different levels of residue cover. The efficacy of pyroxasulfone herbicide when applied on increased amounts of residue cover was executed in a field trial, discussed in Chapter 3 and a glasshouse trial, discussed in Chapter 4. For field trials at Langgewens residue cover were arranged to 0, 6 and 12 t ha-1 and to 0, 4.5 and 9 t ha-1 at Tygerhoek. Pyroxasulfone’s treatment were as follows; recommended and 1.5 times the recommended dosage rates and recommended and double the recommended application rates. Weed and crop counts were made at seven weeks after planting and at crop anthesis. Vegetative growth parameters were determined at anthesis. Yield components were determined just before harvesting. After harvesting; yield, thousand kernel mass and hectolitre mass were determined. For the glasshouse trial, 50 commercial ryegrass seeds and five wheat seeds were planted per pot. Wheat residue cover was arranged as 0, 1.5, 3, 4.5 and 6 t ha-1. Pyroxasulfone was applied at similar dosage rates and application rates as in Chapter 3. After the herbicide was applied, 5 mm of clean water was used to wash off the herbicide. Ryegrass counts was made at 7 weeks after planting (WAP). After harvesting; vegetative growth parameters and yield were determined. Doubling of recommended application rate had a little impact compared to increasing the dosage rate. A field trial comparing the effectiveness of weed control of pyroxasulfone, prosulfocarb plus triasulfuron and triallate when applied on increased amounts of residue cover is discussed in Chapter 5. The study was executed at Tygerhoek and Langgewens with residue cover arranged to result in 0, 5.5 and 11 t ha-1 and 0, 4.8 and 9.6 t ha-1 respectively. At both sites, herbicide dosage rates and application rates were arranged as in Chapter 3. Data was collected as in Chapter 3. Pyroxasulfone at increased dosage rates, controlled weeds better than other herbicides followed by prosulfocarb plus triasulfuron. Triallate treatment performed poorly across residue cover levels, even on an increased dosage rate. The results showed similar trends to the previous two chapters indicating that an increase in dosage rate was more effective than an increase in application rate to improve the efficacy of pre-emergence herbicides under high residue conditions.

AFRIKAANSE OPSOMMING: Die onderskepping van vooropkoms onkruiddoders deur oesreste kan lei tot swak onkruidbeheer wat kan lei tot meer onkruidsaad wat die saadbank bereik en ‘n hoër onkruiddruk in die volgende seisoen en op die lang duur kan dit lei tot die ontwikkeling van nie-teikensetel onkruiddoderweerstand. Die doel van die studie was dus om verskillende dosis- en watertoedieningshoeveelhede te ondersoek wat kan bydra om die korrekte hoeveelheid vooropkoms onkruiddoder te laat kontak maak met die grondoppervlakte by verskillende hoeveelhede oesreste. Die effektiwiteit van pyroxasulfone as dit op verhoogde oesresvlakke toegedien word is in ‘n veldproef wat in Hoofstuk 3 beskryf is en ‘n glashuisproef, wat in Hoofstuk 4 beskryf word, getoets. Vir die veldproewe op Langgewens is die oesreste gemanipuleer tot 0, 6 en 12 t ha-1 en op Tygerhoek tot 0, 4.5 en 9 t ha-1. Pyroxasulfone behandelings was as volg: die onkruiddoderdosis was die geregistreerde dosis en 1.5 keer die geregistreerde dosis terwyl die watertoedingsvlak die aanbevole vlak was en dubbel die aanbevole vlak. Onkruid en gewastellings is gedoen sewe weke na plant en op antesestadium van die koring. Vegetatiewe groeiparameters is bepaal met antese. Opbrengskomponente is net voor oes bepaal en na oes is opbrengs, duisendkorrelmassa en hektolitermassa bepaal. In die glashuisproef is 50 raaigrassade van ‘n kommersiële kultivar asook vyf koringsade per pot gesaai. Oesreste is gemanipuleer tot 0, 1.5, 3, 4.5 en 6 t ha-1. Pyroxasulfone is toegedien teen dieselfde dosisse en watertoedieningshoeveelhede as in Hoofstuk 3. Na toediening is 5 mm skoon water op elke pot toegedien om die onkruiddoder in te was. Raaigrastellings is gedoen sewe weke na plant. Na oes is die vegetatiewe groeiparameters en die oesopbrengs bepaal. Verdubbeling van die watertoedieningshoeveelheid het min impak gehad vergeleke met ‘n hoë onkruiddoderdosis wat effektiwiteit oor alle oesresvlakke in beide die veld- en glashuisproef veroorsaak het. ‘n Veldproef wat die effektiwiteit van pyroxasulfone met prosulfocarb plus triasulfuron en ook triallate vergelyk het, word in Hoofstuk 5 bespreek. Die veldproef is uitgevoer op Tygerhoek met oesresvlakke van 0, 5.5 en 11 t ha-1 en op Langgewens met oesresvlakke van 0, 4.8 9.6 t ha-1. Die onkruiddoders is toegedien teen dieselfde dosisse en watertoedieningshoeveelhede soos beskryf in Hoofstuk 3. Data is versamel soos beskryf in Hoofstuk 3. Pyroxasulfone teen verhoogde dosisse het die beste onkruidbeheer tot gevolg gehad, gevolg deur prosulfocarb plus triasulfuron. Triallate het oor alle behandelings swak resultate gelewer. Resultate was oor die algemeen (met uitsondering van triallate) dieselfde as in die vorige hoofstukke met verhoogde onkruiddoderdosisse wat beter beheer by hoë oesresvlakke tot gevolg gehad het as verhoogde watertoedieningshoeveelhede.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/106226
This item appears in the following collections: