Since 1991 a Danish Decision Support System (DDSS) named PC-Plant Protection has been distributed in Denmark. This system offers decision support to identify and control 75 weed species in 11 crops. DSS models have also been implemented for pests and diseases. Presently, there are about 2,500 subscribers to the system. The weed-model used in the DDSS until March 1999 is briefly discussed. A submodel based on expert knowledge is used to quantify the need for weed control at a field level at the time of herbicide application. A logistic dose-response function quantifying about 500,000 combinations for different herbicide susceptibility due to: herbicide name, crop name, time of application, weed species, growth stages of weeds, temperatures, r.h. and water stress is used to calculate rates of single herbicides and tank-mixtures approved by Danish Institute of Agricultural Sciences. In March 1999 the Additive Dose Model (ADM) was integrated in the decision models of 8 cereal crops. Implications are discussed. An ongoing political pressure in Denmark to reduce the total mass of pesticides used and the 'treatment frequency index' (TFI). TFI can be interpreted as a sum of the proportions of standard (maximum) rates used in a crop in one season. In 1996 a simple prototype integrating ADM into the DDSS for optimisation of treatment cost or TFI was constructed. This prototype was tested in 28 field experiments in cereals in 1996-98 with heavy weed infestations. Satisfactory weed control was achieved in all experiments. In spring barley the average rate was 35% of the normal rate and in winter wheat the average rate was 44% of the normal rate. Compared to the latest version of the DDSS without ADM this is a 27-29% reduction on TFI and 18-24% reduction on cost of treatment.
