Summary:
Perennial weeds such as Cirsium arvense (L.) Scop. and Sonchus arvensis L. pose major challenges in weed management for both conventional and organic agricultural systems. Due to the negative impacts on the environment and human health, reducing the use of synthetic herbicides has become essential. Consequently, regulatory restrictions are implemented by governments in many countries, including those within the European Union. Considering the central role of herbicides like glyphosate in controlling perennial weeds, identifying and developing ecologically sustainable alternatives, such as bio-herbicides derived from natural compounds, has become a priority in weed management strategies. Pelargonic acid (PA), a naturally occurring fatty acid, has gained attention as a potential bio-herbicide for controlling weeds in arable cropping systems. The primary aim of this thesis was to evaluate strategies to enhance PA efficacy and explore its potential in managing both perennial weeds and cover crops. Given the limited information available on bio-herbicides for cover crop termination, another objective of this study was to fill this knowledge gap by evaluating the herbicidal efficacy of PA for cover crop desiccation. In this context, the study focused on leveraging PA's benefits, addressing its limitations, and assessing its effectiveness in suppressing perennial weeds and serving as a desiccation tool for terminating cover crops.
Furthermore, the variability in herbicide efficacy, short-term efficacy, weed regrowth, and requirements for multiple applications or higher dosages were considered the main challenges in PA application as a bio-herbicide. Therefore, the following sub-objectives were defined. The first one was to investigate whether considering the following technical aspects: (1) carrier volume increase, (2) adding adjuvant, and (3) application at a specific growth stage C. arvense, affects PA efficacy. The second aim was to evaluate the effectiveness of twice PA application over two consecutive years to the same pots—where each large pot contained a patch of C. arvense and S. arvensis, grown from a single ramet, considering their initial ramet sizes. Therefore, the study aimed to assess if the initial ramet size affects the PA efficacy and the regrowth of these two species following PA application and whether there are differences between these species concerning PA efficacy and regrowth patterns.
The experimental setups of the current thesis include greenhouse, semi-field, and field trials. To address the first sub-objective, PA efficacy on C. arvense was assessed under semi-field and greenhouse conditions in 2020 and 2021. The semi-field experiment evaluated various PA application volumes—specifically 16 L/ha in 200 L and 400 L carrier volume—across five phenological growth stages of C. arvense. The greenhouse experiment examined the effect of paraffin oil as an adjuvant to enhance PA’s herbicidal efficacy. Results indicated that applying 16 L/ha of PA in 400 L of carrier volume significantly improved its efficacy, and adding paraffin oil enhanced this effect further. The highest control was achieved during the early elongation and seven-to-ten leaf stages of C. arvense.
To evaluate the long-term efficacy of PA on perennial weeds, a semi-field experiment was conducted from spring 2020 to autumn 2021. PA was tested on C. arvense and S. arvensis with initial ramet sizes of 5 cm, 10 cm, and 15 cm, and its effects were compared to those of glyphosate. Results showed that PA applied twice over consecutive growing seasons reduced plant coverage, above and belowground biomass, and flower production, especially for plants with smaller initial ramet sizes (5 cm). PA was more effective on C. arvense than S. arvensis but did not prevent regrowth, while glyphosate demonstrated superior efficacy.
To address the cover crop management, the study compared PA with two other synthetic herbicides for cover crop desiccation. This experiment was conducted in 2019 and repeated in 2021 in northeast Germany, employing a completely randomized block design. Various herbicide treatments were tested, including PA at 16, 8, and 5 L/ha, glyphosate, and pyraflufen. Assessment methods included visual estimates of crop vitality and drone-based vegetation indices (RGB and NIR). PA, especially at 16 L/ha or as a double application of 8 L/ha, effectively desiccated cover crops within a week, though its efficacy decreased over time due to cover crop regrowth. Glyphosate showed the most consistent effectiveness, resulting in the lowest crop vitality by the end of the experiment. Drone-based vegetation indices,
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particularly RGB indices like EXG, provided detailed and accurate assessments, demonstrating their potential for cost-effective crop and weed infestation monitoring.
In summary, the efficacy of PA as a contact herbicide was improved when all relevant technical factors, such as carrier volume, weed growth stage, and adjuvant, were considered simultaneously. This means that when these factors were combined, efficacy increased significantly. This indicates that optimizing the PA application requires considering coverage, water solubility, timing of application relative to weather and weed growth stages, and the biological characteristics of the target weed species. PA can be used alongside other tools to lower perennial weed infestations. It also has shown potential for cover crop desiccation. Although, it is still not registered for broader management in arable crops. Currently, PA is approved in Europe as a plant desiccant for potatoes, for killing suckers and weed control in non-cultivated areas. Given the urge to reduce the use of synthetic herbicides in farming, further research into the technical aspects of the PA application as well as economic features are crucial to support its use in integrated weed and crop management.