To evaluate silage from transgenic inulin synthesizing potatoes as compared to that from the parental cultivar, nutrients and undesirable substances were analysed and their digestibility and production potential were studied. The content of starch decreased (60 vs 67% in DM), and the total alkaloids in fresh potatoes increased (904 vs 728 mg mg/kg DM). The digestibility of the fibre fraction decreased (73 vs 81%), which was almost compensated by an improved NfE-digestibility. The energetic feeding value was 14.3 and 14.6 MJ ME/kg DM for the silage of transgenic or isogenic potatoes, respectively. These results were confirmed in a feeding test over 42 days. Average daily liveweight gain of the pigs fed the transgenic silage was 43 g lower than the controls.