Assessment of the effects of reduced herbicide applications on selected arable weeds by a simulation model

Between 1986 and 2003 the Danish government introduced three Pesticide Action Plans to reduce the pesticide usage. In order to simulate the expected effects of the third Pesticide Action Plan a large quantity of field data was collected to calibrate and validate the simulations of the growth of a number of weed species and crops in a simulation model. The model proved able to simulate the observed data of wheat, barley, oilseed rape and rye with different compositions of weeds. The scenarios for standard fields of winter wheat, spring barley and winter oilseed rape estimated that a reduction of the treatment intensity index from 2.04 to 1.7 would result in an 11–20% increase in weed dry weight when measured just before harvest. However, the weed levels were still low in barley and oilseed rape fields (4.20 and 3.94 g/m2) due to the extremely low weed amounts in the monitored fields. A higher amount of weeds was seen in winter wheat fields (47.03 g/m2) primarily due to Tripleurospermum inodorum (46.0 g/m2). A further reduction to 1.5 was simulated and predicted an increase of between 10% and 214% weed biomass. The dry weight of weed in barley and oilseed rape was in this case predicted to, respectively, 10.38 g and 3.99 g/m2. Again a much higher weed biomass was estimated in the wheat fields (123.47 g/m2) due to T. inodorum (120.57 g/m2).