Impacts of high intensity crop rotation and N management on oilseed rape productivity in Germany

A rotational field experiment was established in the year 2002 at the experimental farm Etzdorf in the Hercynian dry region of central Germany. Since 2005 field measured datasets were used to determine the effect of different preceding crop combinations and different nitrogen (N) fertilisation treatments on the seed yield, oil content, oil yield and N-use efficiency of oilseed rape (Brassica napus L.). The preceding crop combinations compared were winter wheat (Triticum aestivum L.)-winter wheat (WW), WW-oilseed rape (OSR), OSR-OSR and an OSR monoculture. In addition to the preceding crop combination, N fertiliser treatments with either 120kg N ha(-1) or 180kg N ha(-1) were established in the year 2013. Overall the results demonstrated that seed yield, oil yield and N-use efficiency all declined with an increased cropping intensity for the period 2005-2012. Higher N rates in the 2013-2014 seasons increased seed yield and oil yield when OSR followed WW-WW pre-crops. OSR monoculture had lowest yield independent of applied N. Seed yield declined from 4.61tha(-1) (OSR following WW-WW) to 4.28tha(-1) in the OSR monoculture with 120kg N ha(-1), and from 4.81tha(-1) (following WW-WW) to 4.42tha(-1) in the OSR monoculture with 180kg N ha(-1). Higher N rates generally reduced N-use efficiency, with highest N-efficiency for WW-WW-OSR (38.4kgkg(-1)), and lowest for continuous OSR receiving 180kg N ha(-1) (24.5kgkg(-1)). These results emphasise the importance of crop rotation to maintain seed yield and oil yield of oilseed rape, and to maximise the response to applied N. A reduced N rate increased N-use efficiency and reduced the risk of high N surpluses without a significant/equivalent decrease of the seed yield when the rotation was optimised.