Browsing by Subject "Cover crop establishment methods"

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    Advancing cover cropping in integrated weed management through novel modelling approaches, management strategies, and allelopathic investigations
    (2025) Merkle, Michael; Gerhards, Roland
    Climate change, herbicide-resistant weeds, and the lack of new herbicidal modes of action are increasing the pressure on farmers to reduce herbicide usage. Environmentally friendly and integrated weed control methods, such as cover cropping – where cover crops (CCs) provide various ecosystem services, including the suppression of weeds and volunteer crops – can be implemented in crop rotations to minimize herbicide use. The intensity of this suppression var-ies between cover crop (CC) species. Additionally, it is influenced by the competitiveness, allelopathic activity, and biomass production of the CCs, as well as by the CC establishment method, sowing time, soil type, and climatic conditions. The prediction of weed suppression by CCs in the field, as well as the enhancement of their abilities to suppress weeds and volun-teer crops, is challenging. Furthermore, there are unstudied allelochemicals in CCs that could serve as herbicidal active ingredients in bioherbicide formulations. Therefore, this dissertation focused on the following research objectives: • Identifying models to predict weed suppression by winter-kill fall-to-spring CC spe-cies in the field; • Examining the impact of different establishment methods for winter-kill fall-to-spring CC species, including various sowing dates, to enhance their weed and volunteer cere-al suppressive abilities in the field; • Enhancing the allelopathic potential of winter-kill fall-to-spring CC species at an early growth stage by applying artificial stresses to improve their ability to suppress weeds and volunteer cereals; • Investigating phytocannabinoids derived from Cannabis sativa L. and aqueous C. sati-va shoot tissue extracts regarding their allelopathic effects on Zea mays L. and weeds, as well as their potential use as herbicidal active ingredients in bioherbicide formula-tions. The objectives of the dissertation were addressed in four research articles. The first research article compared two non-linear models to predict the weed suppressive ef-fect of six different winter-kill fall-to-spring CC species in the field. The models considered the shoot dry matter of the CCs and weeds (including volunteer crops) approximately 12 weeks after CC sowing, based on 11 field studies conducted between 2010 and 2020. Addi-tionally, the allelopathic potential of the CCs was considered in the models by investigating the allelopathic effects of aqueous shoot extracts from the shoot biomass of field-grown win-ter-kill fall-to-spring CCs on the seed germination of four different annual weed species in laboratory Petri dish bioassays. The three-parameter Weibull model was more accurate for predicting weed and volunteer crop suppression by CCs in the field than the Cousens model. Furthermore, the study showed that CCs with high allelopathic potential, such as Avena stri-gosa Schreb., can suppress weeds just as effectively (>80%) as Brassicaceae or Polygonaceae species, while requiring only one-third of the shoot biomass. The second research article compared three different establishment methods as well as differ-ent sowing dates for five different in monoculture sown winter-kill fall-to-spring CC species, along with a CC mixture, in a three-year field trial (2020–2022). The aim of this study was to improve the development, biomass production, and weed and volunteer suppression ability of CCs by identifying the most suitable establishment method and sowing date for each CC spe-cies and the CC mixture. The results showed that, 10 weeks after the harvest of the main crop, the establishment methods of pre-harvest sowing into the maturing cereal crop 10 days before harvest and direct sowing after cereal harvest without tillage resulted in, on average, 30% and 42% higher CC shoot dry matter and 81% and 78% lower weed density, respectively, com-pared to the post-harvest establishment method with reduced tillage. In contrast, the volunteer cereal densities and the weed and volunteer cereal control efficacies were consistent across the three CC establishment methods. However, the results varied among the different CC species and the trial years. The CC mixture demonstrated greater stability than monocultures regard-ing shoot biomass production under varying climatic conditions and establishment methods. In addition, the sowing date and climatic conditions had a significant impact on the success of CC establishment. The third research article investigated the effect of artificially induced stress on plant physio-logical processes and the allelopathic potential of CCs with the aim to enhance their suppres-sive effects on weeds and volunteer cereals during an early growth stage. Three different types of stress (mechanical stress through harrowing (1), phytohormonal stress by applying methyl jasmonate (2), and insect stress by applying Hermetia illucens paste with leaf injuries (3)) and a stress combination (1 + 3) were applied in greenhouse trials (2021–2022) to the CCs A. strigosa, C. sativa, and Sinapis alba L. at the 3-4 leaf stage. All applied stress types resulted in no or minimal changes in shoot biomass production and photosystem II activity of the CCs a few days after the stress application. The aqueous shoot tissue extracts from the shoot bio-mass of C. sativa subjected to combined stress and S. alba plants subjected to insect stress showed 1.7 and 1.9 times significantly higher total phenolic content, respectively, five days after stress application compared to the extracts of the control plants. Moreover, Petri dish bioassays were carried out under laboratory conditions using aqueous extracts derived from the shoot biomass of both treated and untreated CCs to assess stress-induced alterations in the allelopathic potential of their shoot tissues. The bioassays demonstrated that aqueous shoot tissue extracts from the shoot biomass of phytohormonally, insect- and combined-treated C. sativa and S. alba, as well as mechanically treated A. strigosa, exhibited stronger germination-inhibiting effects on the seeds of the two different annual weed species, as well as on volun-teer cereal seeds, compared to the seeds of the control plants that were treated with the aque-ous shoot tissue extracts from the shoot biomass of untreated CCs. In the fourth research article, five different phytocannabinoids derived from C. sativa and aqueous C. sativa shoot tissue extracts were investigated for their allelopathic effects on the cereal crop Z. mays and four different annual weed species in laboratory Petri dish bioassays and pre-emergence trials in the greenhouse (2021–2022). The application of phytocanna-binoids and aqueous shoot tissue extracts at the highest concentration significantly reduced root length in Z. mays and the four weed species by up to 91% in the bioassays, 10 days after treatment, compared to the untreated control plants. The application of phytocannabinoids in high concentrations in the bioassays primarily decreased the germination rate of the weed seeds by up to 53% compared to the untreated control plants. Depending on the weed species, the type of phytocannabinoid, and the concentrations applied, stimulatory effects on weed seed germination were also observed. In contrast, the germination of Z. mays seeds was not affected by the phytocannabinoids in the bioassays. However, high concentrations of the aqueous C. sativa shoot tissue extracts suppressed the germination rate of Z. mays seeds by 34% and that of the four weed species by 76–96%. In pre-emergence greenhouse experiments, stimulatory effects on the germination of Z. mays and four weed species were observed 21 days after the application of bioherbicidal formulations containing phytocannabinoids such as cannabidiol, cannabidivarin, or aqueous C. sativa shoot tissue extracts as active ingredients. In summary, this dissertation demonstrates that the suppression of weeds by winter-kill fall-to-spring CCs can be predicted using non-linear models and can be enhanced, along with the suppression of volunteer cereals, through suitable establishment methods, sowing dates, and CC species. The allelopathic potential of CCs, particularly that of their shoot tissues, can be increased through stress induction, and phytocannabinoids can have both stimulating and sup-pressive effects on Z. mays and weeds. These findings enhance the understanding of cover cropping in integrated weed management, particularly highlighting management strategies to improve the weed and volunteer cereal suppressive abilities of CCs.