- Title
- The testing of natural and synthetic adjuvants to reduce herbicide-use and/or improve efficacy for the control of difficult-to-kill forest weeds
- Creator
- Letaoana, Josias Tshukudu
- Subject
- Weeds -- Control
- Subject
- Invasive plants -- South Africa Invasive plants -- Ecology -- South Africa Forest ecology -- South Africa Forests and forestry -- South Africa
- Date Issued
- 2019
- Date
- 2019
- Type
- Thesis
- Type
- Masters
- Type
- MSc
- Identifier
- http://hdl.handle.net/10948/40371
- Identifier
- vital:36153
- Description
- Commercial plantations within South Africa occur across diverse sites in terms of climate and physiography. Intensive silvicultural practices (site preparation, fertilisation, weed management etc.) are carried out during the establishment phase to ensure that the actual production of timber approximates the potential capacity for a site. Of these silvicultural practices, weed management is regarded as one of the most important, as if not carried out, competition from weeds may negatively influence seedling survival, growth and uniformity. During the establishment phase (planting to canopy closure), weed management operations are generally effective, as most competing species are still in a juvenile growth phase, and easily controlled. Due to limited weed-tree competition following canopy closure, weed management operations are reduced to a single event per annum (if necessary). This reduction in weed management often allows semi-shade/shade tolerant species to become established, together with an increase in the effort required for their control. The South African forest industry makes use of independent international auditing bodies to ensure that plantations are managed in a responsible, yet sustainable manner. Integrated management practices are considered the most effective as they reduce reliance on a single control method, such as the use of herbicides alone. In conjunction with integrated management, would be the need to reduce the overall quantities of herbicides used, yet at the same time ensuring improved efficacy of those herbicides that are used. The testing of these two aspects (reduced herbicide use and/or improved efficacy) was conducted on three diverse weeds that occur during the post-establishment phase within South African plantations, namely: Setaria megaphylla (setaria); Hedychium gardnerianum (wild ginger); and Solanum mauritianum (bugweed). The management of these species is considered problematic, as current herbicides and rates of application are not always completely effective. Four trials were implemented in KwaZulu-Natal (Midlands). Setaria megaphylla (setaria): Two trials were initiated (spring and autumn), to test the seasonal application of glyphosate + adjuvant combinations for improved efficacy (and hence reduced herbicide-use). A control (no herbicide applied), Select® (clethodim cyclohexanedione 120 g a.i. ℓ-1), Roundup® (glyphosate isopropylamine salt 360 g a.i. ℓ-1), and five Roundup® + adjuvant combinations were tested in both trials. iii The five adjuvants included three natural and two synthetic products that exhibited different properties and modes of action. At both sites, the eight treatments were replicated three times and laid out in a randomised complete blocks design. Setaria cover was assessed prior to spraying (0 days relative to spraying), and then again after two months (64 days relative spraying), with absolute changes in regrowth analysed. Although the application of Roundup® on its own provided good control within both trials, complete setaria control could not be obtained. Select® and the tested adjuvants did not improve Roundup® efficacy at the rates at which they were applied. The application of Roundup® in autumn (x̄ = 5.2% live cover) was more effective than the spring application (x̄ = 8.5% live cover), indicating that future studies should focus more on a combination of a season of application and subsequent follow-up operations, rather than the role of the adjuvants tested. Hedychium gardnerianum (wild ginger): Currently no herbicides are registered for the control of wild ginger in South Africa. One trial was implemented to test the potential for the control of wild ginger with herbicides applied to the foliage, cut-stems or rhizomes. In addition to an untreated control (no herbicide applied), Roundup® (glyphosate isopropylamine salt 360 g a.i. ℓ-1 at 4%) was applied as a foliar spray, either alone or with five adjuvants (three natural and two synthetic adjuvants). Brush- Off® (metsulfuron-methyl 6 g a.i. ℓ-1) or Roundup® was also applied to the cut-stems and rhizomes. Herbicide phytotoxicity was assessed up till 229 days after treatment application in terms of mortality, reduced and/or abnormal growth (of the resprouts) and foliar necrosis and/or discolouration. Although the foliar application of Roundup® (alone or with adjuvants) resulted in limited phytotoxicity, these treatments were ineffective in terms of killing wild ginger. The application of Roundup® or Brush-Off® to either the cut rhizome or cut-stems were equally effective, however, the cut-stem application would be preferred due to the reduced effort required. Future studies should further optimize the application of Roundup® (lowered rates + adjuvants) to the cut-stems/rhizome. Solanum mauritianum (bugweed): One trial was implemented on a stand of mature bugweed (located within a pine stand). The potential to reduce rates and/or improve the efficacy of Garlon® (triclopyr butoxyethyl ester 480 g a.i. ℓ-1®) was tested through the use of four adjuvants (two natural and two synthetic products) when applied as either a cut-surface or basal bark application. Two untreated controls (cut-surface or iv basal bark) were included for comparative purposes. The trial consisted of a 2 x 5 factorial arrangement of 10 treatments replicated three times and laid out as a randomized complete block design (RCBD). Although the cut-surface treatments were more effective than the basal bark treatments, no treatment provided complete control of bugweed. Of the cut-surface treatments, Garlon® + Break-Thru® (polyether-polymethylsiloxane-copolymer 1 000 g ℓ-1), Garlon® + Orosorb® (orange oil 50 g ℓ-1), and Garlon® + Actipron Super® (mineral oil 820 g ℓ-1) were equally effective, with Garlon® + Nu-Film P® (poly-1-menthene 975 g ℓ-1) the least effective. As the adjuvants tested did not improve the efficacy of Garlon® at the rates applied, the use of the most cost-effective and environmentally acceptable product would be preferable. For effective long-term control of mature bugweed, a cutsurface application is advisable, together with follow-up operations required. The results from these trials indicate that for bugweed and setaria, the currently registered herbicides remain the products of choice, albeit not 100% effective on wellestablished plants. In addition, the use of the adjuvants tested did not improve herbicide efficacy (and hence the opportunity for reducing rates of application) for all three species tested. However, Roundup® or Brush-Off® applied to either the cut rhizome or cut-stems were equally effective in terms of wild ginger control, indicating an opportunity for further testing to optimise rates of application.
- Format
- xii, 85 leaves
- Format
- Publisher
- Nelson Mandela University
- Publisher
- Faculty of Science
- Language
- English
- Rights
- Nelson Mandela University
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NMU School of Natural Resource Management
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