Research paperTiming and depth of post-harvest soil disturbance can reduce seedbank and volunteers of oilseed rape

- Time period between harvest and tillage determines soil seed bank size of oilseed rape.
- Soil seed bank and volunteer density decrease with delayed timings of stubble tillage.
- Very shallow stubble tillage can lead to soil seed banks and volunteers.
- Seed set of volunteer oilseed rape depends on timing and depth of post-harvest tillage.

Appropriate timing of post-harvest tillage to oilseed rape (OSR, Brassica napus L.) is crucial for the creation of a soil seed bank and unwanted OSR volunteers. Existing recommendations, however, can contrast with needs for early post-harvest tillage e.g. for pest control. To specify and to systematically further improve preventive volunteer management through post-harvest tillage of imidazolinone-tolerant oilseed rape (Clearfield®; CL OSR), field experiments were set up at two locations in two separate trial periods at each location (2013-2014 and 2014-2015) in south-west Germany. After OSR harvest, and average harvest losses of 1500 m−2 seeds, or artificially broadcasting of 20 000 OSR seeds m−2, stubble tillage was performed at weekly intervals (immediate (0), 1, 2, 3, and 4 weeks) at each of three depths (1-2, 6-8, and 15-17 cm). To depict a worst case scenario in which herbicides were ineffective, no herbicides were applied. There were significantly smaller soil seed banks found in spring the longer the period lasted between seed rain and stubble tillage; 265 vs 145 seeds m−2 referred to 0 week vs. 4 weeks on average. Depth of stubble tillage had no significant effects on the seed number in the soil. In the first spring after seed loss, 0-2.6 CL OSR volunteers m−2 were found in the winter wheat, and the number decreased with delayed timing depth of stubble tillage. These volunteers mainly emerged in the treatment of shallow stubble tillage (1-2 cm), with 0.7-0.9 flowering volunteer m−2, while volunteers averaged less than 0.04 plants m−2 in the treatment “15-17 cm”. OSR volunteers in winter wheat got ripened only in one out of the four fields, and produced 1.0 seed m−2 on average. The delay between harvest and stubble tillage can be limited to approximately three weeks if stubble tillage is performed at a depth of about 6-8 cm; further delay did not bring additional advantage. We recommend to retain the stubble for three weeks as further delay does not bring additional advantage, and then to apply stubble tillage deeper than 6 cm. Very superficial or shallow stubble tillage should be avoided. This procedure seems appropriate for avoiding oilseed rape volunteers without use of herbicides.