A crop rotation representative of a typical Danish dairy farm was established at the Taastrup field station in 1988. Detailed information of the manuring, nitrogen fixation, harvested dry matter and harvested nitrogen was available for the period 1988–1992, indicating that the harvested nitrogen almost exactly countered the input, including the biologically fixed nitrogen. As no measurements of N-losses were available a deterministic model (DAISY) was applied in order to obtain an estimate. Dry matter harvest (R = 0.9) and N harvest (R = 0.93) were simulated adequately, and only in the case of spring barley the model slightly overpredicted in a consistent manner, indicating that the use of N as delimiter of crop production in this ecological crop rotation was realistic. Considerable leaching losses of N (as high as 100–180 kg ha−1yr−1) were predicted in connection with the application of 50 tonnes of solid manure to bare soil in winter time preceeding the sowing of fodderbeet. Thus the model interpretation of the available data indicated that the apparent balance between harvested nitrogen and input nitrogen was non- sustainable and achieved by a depletion of soil organic nitrogen in the order of 40 kg N ha−1yr1. Strategies for achieving sustainability are discussed in the light of the model results.