A Danish decision support system (DSS) named Crop Protection Online (CPO) for integrated management of weeds in cereals and beet has been developed during the past 20 years. CPO is based on a model that runs in three main steps: model step 1 quantifies the level of weed control needed on a field level, model step 2 selects candidate herbicides and calculates dose rates to meet the need, and model step 3 calculates tank mixtures of herbicides with two to four mixing components, if advantageous. CPO has been developed in cereals and beet, and various prototype versions have been validated in 1679 field tests. CPO secured yield potentials, and the level of residual weeds was not increased when compared with reference treatments. The potential of CPO to reduce herbicide use has been observed in all model crops, but the potential was greatest in cereals. In spring cereal field trials highly infested with weeds, the present version of CPO suggested 35% of one full herbicide dose on average and in winter cereals CPO suggested 44% on average of one full dose. The results from validation trials demonstrate that CPO is capable of suggesting robust treatment options with a low input of herbicides. The system architecture of CPO has been exported to Poland and the Baltic countries, and the system is expected to be suitable for export to other countries too.

The highly complex knowledge of scientific disciplines makes nuanced analysis and modelling possible. However, the information produced often does not reach farmers because it is presented in a way that does not correspond to the way their work is carried out in practice. The decision support system Crop Protection Online is widely used by advisors and as a learning tool for students. Although the system has been validated in many field trials over the years and has shown reliable results, the number of end‐users among farmers has been relatively low during the last 10 years (approximately 1000 farmers). A sociological investigation of farmers’ decision‐making styles in the area of crop protection has shown that arable farmers can be divided into three major groups: (a) system‐orientated farmers, (b) experience‐based farmers and (c) advisory‐orientated farmers. The information required by these three groups to make their decisions varies and therefore different ways of using decision support systems need to be provided. Decision support systems need to be developed in close dialogue and collaboration with user groups.

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A Danish decision support system (DSS) on integrated pest management (including pathogens and weeds) has been developed during the past 20 years. This DSS is distributed as an integrated part of the ‘Danish Integrated Farm Management System’, which is a PC program, of which the plant protection module presently has about 2500 subscribers in Denmark. This program has been designed to propose relevant options for treatment with plant protection products according to observations from a field inspection. The new object‐oriented web‐based system architecture of this DSS was developed in 2001. This allows local (national) adaptation of the DSS in terms of language, crops, pests, products and features. Model complexity and DSS algorithms can also be adjusted to local conditions. The system has been implemented in a relational database structure (Microsoft SQL Server) and a dynamic web application (Java and Jscript in Active Server Pages on an Internet Information Server). The system architecture has been designed with emphasis on a high level of flexibility for future adjustments due to agronomic and legal requirements. The DSS models have been linked to a pest identification module and to a comprehensive database on label information on plant protection products.

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