Emergence and establishment of volunteer oilseed rape in spring and winter crops

Volunteer oilseed rape (Brassica napus L.) can contribute to gene dispersal by pollen flow or by seed admixtures. The study should analyse the competition capacity of different crops towards oilseed rape volunteers. A field with a large soil seed bank of oilseed rape was planted with three winter crops (winter turnip rape (replacing oilseed rape, similar performance of turnip and oilseed rape presumed), winter barley, winter wheat) and three spring crops (faba beans, spring barley, maize). The crops were managed according best management practice and compared with "fallow" plots where all operations were similar to the crop plots, except sowing. The fallows should allow assessing the competition of crops and volunteers. The crop with the lowest competition capacity and highest risk for gene dispersal by oilseed rape volunteers was turnip rape (30 volunteers m(-2)), most of which flowered and set seeds (475 seeds plant(-1)), and thus exceeded the labelling threshold for transgenic admixture. Half as much volunteers established in faba beans (14 plants m(-2)), flowered and set seed (13 plants m(-2)) Though flowering volunteers were observed in spring barley, no seeds were produced. Volunteers emerged in all other crops as well, but reached neither flowering nor seed maturity. The comparison of crop and fallow showed that turnip rape, winter wheat and spring barley had the highest competition capacity towards oilseed rape volunteers. If the management practise (use of herbicides) is additionally taken into account, the crops with the lowest risk of gene dispersal by volunteer oilseed rape were maize, winter wheat and winter barley. The fallow plots were useful to assess the potential of emerging volunteers from the soil seed bank, and of gene dispersal. Plots like these (10-20 m(2)) could be routinely established in commercial fields as control measurement for model calculations and predictions in terms of risk assessment.