Browsing by Person "Ritter, Carina"

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Evaluation of weed populations under the influence of site-specific weed control to derive decision rules for a sustainable weed management
(2008) Ritter, Carina; Gerhards, Roland
In context of reduction programs for chemical plant protection, herbicide use needs to be strictly controlled and reduced to the absolute necessary extent in order to minimise negative side effects for the environment and pesticide residues in the food chain. The site specific weed management is a promising way to reduce herbicide use. It aims at managing weeds with respects to their spatial and temporal variability. Post-emergence herbicides are only applied at highly infested locations in the fields. Several studies on site-specific weed control have shown that this practice is reasonable, and it has been successfully implemented in various crops, resulting in a considerable reduction of herbicide use, treatment costs, and consequently benefits to the environment. However, there is still lack of knowledge on the population dynamics of weeds and the interactions between crop and weeds under the site-specific weed management. Long term effects of the site-specific weed control have not been studied in detail yet. Additionally, an experimental approach was needed to create precise decision algorithms for site-specific weed management. Therefore the applied scientific objective of this research was: - to analyse the spatial and temporal distribution of weeds, - to provide information on weed population dynamics under the influence of the site-specific weed control, - to detect if site-specific weed management leads to an increase in weed density, and if weed patches remain stable in density and location over time, - to determine herbicide savings and efficacy of the site specific weed management, - to design an experimental on-farm approach to explain yield variation caused by within-field heterogeneity of weed density, soil quality and herbicide application, in order to derive decision rules for site-specific weed control. During the course of this work site specific weed management tested in field trails, long term effects were examined, population dynamics were analysed and a model approach to derive management decision was approved. It was proved that weed distribution was heterogeneous in all experimental fields. The average weed density remained stable when economical weed thresholds were applied. The application of effective herbicides in every year did not reduce density in high density weed patches. The patches were persistent over eight years, with slight variations in density from year to year. It is suggested that a combination of chemical, mechanical, and cultural weed management strategies would be necessary to effectively control weeds in high density locations. However, the knowledge about the spatial stability of weed patches of individual species offers possibilities to use this information for weed management strategies. Population dynamic parameters such as weed seedling emergence, crop-weed competition, seedlings mortality, herbicide efficacy, seed production and viability were found to be weed density dependent. With increasing weed density weed biomass and fecundity increased. These findings support that weed density has to be considered in weed management strategies. Site-specific weed management was effective over time. The amount of herbicides used could be decreased significantly due to site specific herbicide application, without loosing performance. Only 26-35 % of herbicides were sprayed compared to uniform application of herbicides that is still the standard method of weed control. Additionally, a new experimental design based on an anisotropic exponential model with nugget effect was established. The influences of the co-variables weed and soil on yield and the side-effects of herbicides were quantified separately with this model, by overlaying and spatially joining all data. Out of this information, yield losses due to weed and herbicide injury could be defined, and valid decision rules for site-specific weed management could be ascertained. For the first time the injury to the crop due to herbicide application could be numeralised with this experimental design. This large loss of yield can be avoided and considerable reductions in herbicide rates can be achieved by site-specific weed management based on weed thresholds. This experimental approach enables to explain the variation of yield within agricultural fields, and an understanding of the effects on yield of the factors and their causal interactions. This work is seen as a mayor step forward in order to precisely manage weeds with respect to their spatial and temporal dynamics.