Effect of Different Preceding Crops and Crop Rotations on Yield of Winter Oil‐seed Rape (Brassica napus L.)

To determine the effect of different preceding crops and crop rotations on the grain yield of oil‐seed rape, a long‐term rotation experiment was conducted at the Hohenschulen experimental station in Kiel, NW Germany. Additional factors included the nitrogen fertilization and the fungicide application. The results reported herein are based upon the harvest years 1988 to 1993. Averaged over the different rotations and husbandry treatments, the grain yields in the 6 experimental years varied between 2.71 t ha−1 and 3.99 t ha−1. In contrast, the effect of the different husbandry treatments was smaller and non significant. Averaged over 6 years, only the fungicide application caused small yield increase of 0.2 t ha−1. The highest grain yields of 3.77 t ha−1 or 3.65 t ha−1 occurred when oil‐seed rape was directly following peas. Low yields between 3.15 tha−1 and 3.33 tha−1 were obtained when oil‐seed rape was grown after oilseed rape. The lowest grain yield of 3.13 t ha−1 was produced with oil‐seed rape grown in monoculture only. In rotations with oil‐seed rape following a preceding cereal crop (wheat or barley), the grain yields averaged between 3.22 tha−1 in a two course rotation and up to 3.44 tha−1 in a four course rotation. In general, the yields of oil‐seed rape increase with the length of the rotation and the length of the break between two oilseed rape crops. The yield component number of seeds per m2 was affected by the previous cropping accordingly, whereas the thousand seed weight did not respond to the cropping history. Based upon disease assessments in the first years of this experiment, we argue that an increase in the incidence of fungal diseases has considerably contributed to the yield decrease of oil‐seed rape in short rotations.