The effect of sowing date and nitrogen on the dry-matter yield and nitrogen content of forage rape (Brassica napus L.) and stubble turnips (Brassica rapa L.) in Ireland

Two field experiments were conducted at Teagasc, Moorepark, Ireland, to determine the effect of sowing date and nitrogen application on the dry-matter (DM) yield and crude protein (CP) content of forage rape and stubble turnips. The first experiment consisted of three sowing dates (1 August, 15 August and 31 August) with four rates of fertilizer N (0, 40, 80 and 120 kg N ha(-1)) on forage rape DM yields. The second experiment consisted of three sowing dates (1 August, 15 August and 31 August) with four rates of fertilizer N (0, 40, 80 and 120 kg N ha-1) over two soil sites (fertile or nitrogen depleted) on forage rape and stubble turnip DM yields. A delay in sowing from 1 to 31 August characterized a 74.5% decrease in forage rape DM yield, while stubble turnip DM yield decreased by 55.5%. Forage rape DM yields increased positively up to 120 kg N ha(-1) at the first two sowing dates over both sites. In contrast, stubble turnips showed less response beyond 40 kg N ha(-1) on site 1 in the first two sowing dates, while DM yield increased positively up to 120 kg N ha(-1) on the less fertile site. The results indicate that the optimal sowing time for forage rape and a stubble turnip in Ireland was early August.