The paper discusses an ex ante evaluation of the derogation on the Nitrates Directive for Flanders, Belgium, which is a case of intensive but highly productive livestock areas. The aim is to develop an accurate simulation model to detect small differences in manure surpluses caused by changes of manure production and/or utilization. The system of models consists of various modules to fine tune the calculations of manure production, fertilizing behaviour and manure allocation and disposal on and off farm. The results show that derogation may cause the existing manure surpluses to expire, if only nitrogen limits are considered and no transactions costs are taken into account. When also phosphate fertilization limits are considered, the increase in manuring possibilities is much lower than expected. Ongoing research focuses on the marginal shifts in manure surplus at farm level and possible effects of transactions costs.