Integration of cover crops into arable rotations over winter results in difficulty in determining the nitrogen (N) requirement for the following commercial crop. The region of Northern Ireland (NI) has had no previous field research on cover crops and how they may affect N supply to the following commercial crop. Stubble turnips (Brassica rapa oleifera L.) were sown as a cover crop, after the harvest of winter barley (Hordeum vulgare L.) and retained over winter. Prior to planting the stubble turnips, pig slurry was applied to maximise cover crop growth. The stubble turnips accumulated 111 and 150 kg N/ha in their biomass. This equates to 79 and 107% of the N requirement of a 5 t/ha spring barley crop, if this N is released sufficiently. In this experiment, the cover crop of stubble turnips was over-sown with spring barley and supplemented with different rates of organic manures (either applied at 50 m3/ha of pig slurry or not applied), and inorganic N fertiliser (0, 70 and 140 kg N/ha), at two different timings (early or late). In the two experimental years, additional N supplied in the form of inorganic N or organic manures, did not significantly enhance spring barley yields. No control area of fallow was included in this trial. However, this study demonstrates that in this region there may be a greater rate of N release from the cover crop of stubble turnips than estimated due to agronomic management practices applied and conducive climatic conditions. This means that in this study location, a reduced N rate programme supplemented to the spring barley is possible, which lead to considerable financial savings.