Field history of imidazolinone-tolerant oilseed rape (Brassica napus) volunteers in following crops under six long-term tillage systems

• Confirmed for Clearfield Brassica napus: lower dormancy level leads to smaller seed bank.
• Confirmed: immediate post-harvest tillage leads to higher soil seed bank.
• Lesser volunteers in first following year with high dormancy variety.
• Rate of release from dormancy in seed banks seems important factor.
• Clearfield system with B. napus suppresses non-Clearfield B. napus.

Volunteer oilseed rape (OSR, Brassica napus L.) exhibits weedy behavior in crops, and can contribute to gene flow or unwanted seed admixture, particularly if its variety is tolerant to specific herbicides and if the proportion of OSR in a crop rotation is high. The aim of this study was to monitor the fate of seeds of imidazolinone-tolerant oilseed rape (Clearfield®; CL OSR) lost at harvest over the two years following its intentional sowing. A 5-yr experiment (2011–2015) with non-CL OSR and CL OSR in the same rotation was conducted on an existing long-term tillage experiment in south-west Germany to investigate OSR volunteer dynamics. The experiment included different modes of primary tillage (inversion tillage, non-inversion tillage, no-till, with or without additional stubble tillage prior to primary tillage). The crop sequence was non-CL OSR, winter wheat (Triticum aestivum L.), CL OSR (a medium and a high dormancy variety), winter wheat, and maize (Zea mays L.). High dormancy CL OSR resulted in a larger soil seed bank (147 vs. 58 seeds m−2), but in fewer volunteers (0.9 vs. 1.9 volunteers m−2) than the medium dormancy variety in the first year after CL OSR. Dormancy release likely resulted in different volunteer emergence rates of the two varieties. Immediate stubble tillage after CL OSR increased seed bank and volunteers by 0.93 and 12.7 times, respectively. Inversion tillage resulted in 30 times fewer volunteers in the first year after CL OSR, but in an equal volunteer number in the second year compared to non-inversion tillage. Slight segregation of imidazolinone-tolerant genes occurred in the offspring of CL OSR, likely leading to different CL herbicide-tolerance levels in volunteers though the number of these individuals was small.Sound strategies to control OSR volunteers should include (1) use of low dormancy varieties with low potential to establish a seed bank, (2) varieties with fast dormancy release to trigger more dormant seeds to germinate in a very short period, and (3) a period of time between harvest of OSR and first tillage operation to reduce the possibility of seeds entering soil seed bank.