Impact of cultivar on survival of volunteer oilseed rape populations in fields is more important than field management

A joint assessment of two separate approaches investigated the occurrence of volunteer oilseed rape (Brassica napus L.; OSR) as affected by cultivar, field history and environment. Approach I comprised surveys for volunteers on >100 farmers’ fields in Germany in the years 2009 and 2010. Volunteers were assigned to the cultivars grown in previous years by inter-simple sequence repeat-PCR and cluster analysis. High-dormancy cultivars resulted in 0–7 and low-dormancy cultivars in 0–1.3 volunteers m−2. Highest numbers originated from the most recent harvests 2007 and 2006. Approach II was a meta-analysis based on 116 data sets from field trials and farmers’ fields in Germany to evaluate and to rank the impact of management factors on the soil seedbank and volunteers in following crops. Varietal disposition to seed dormancy turned out to be the significantly most relevant factor. The contribution of varietal dormancy to variation in the soil seedbank and of volunteers in the 1st and 2nd following crop was 1.2–2.3 times as great as the contribution of post-harvest tillage. Up to 45% of the variation in the observations originated from factors that can be controlled by human actions on a given location in a given year. The overall analysis confirmed the results from independent short-term trials and showed that both agronomists and breeders need to contribute to reducing OSR volunteers.