II.3. BRASSICA CROPS (BRASSICA SPP.) – 211
Persistence
Very few seeds of oilseed rape survive in the seed bank compared with their wild
relatives (Chadoeuf, Darmency and Maillet, 1998). Most seeds of the cultivated Brassica
crops, if left on or near the soil surface, will germinate and be killed by frost or
cultivation or be eaten by rodents, birds and insects. Nevertheless, a small proportion may
not germinate and secondary dormancy may be induced, particularly if the seed is buried.
Studies in Europe with winter B. napus found that when seeds were buried immediately
after seed shed, 30% of the seed bank survived one winter compared to only 0.1% when
seeds were left on the undisturbed soil surface (Pekrun and Lutman, 1998). Similarly for
spring B. napus in western Canada, Gulden, Shirtliffe and Thomas (2003a) found spring
seedlings, from fall-sown seeds buried at a 1 cm depth, to be only 0.1-1.5% of the
original seed bank. In Canada, oilseed rape is typically grown on the same land once in
four years with most of the volunteers occurring in the year following oilseed rape
production. However, volunteers can occur four to five years after production
(Légère et al., 2001; Simard et al., 2002; Beckie and Owen, 2007). Harker et al. (2006)
found that if first-year volunteers were prevented from producing seed, the densities of
volunteers in subsequent years were reduced to levels that would not require herbicidal
intervention. Surveys in southern Australia by Baker and Preston (2008), where zero and
minimum till are practiced, found zero germination of seed sampled from fields 3.5 years
after the last B. napus harvest. But in Germany, Förster and Diepenbrock (2002) reported
more than 0.5 plants/m^2 of winter B. napus three years after the last oilseed rape harvest.
However, no information on timing or type of post-harvest cultivation was provided. In
France, two conventional oilseed rape varieties, one of which was dwarf, were planted on
fields that had grown three different HR varieties three to eight years before (Messéan et
al., 2007). The percentage of GM HR seed occurring in the harvest of the conventional
varieties was determined. HR seed from two of the GM varieties never exceeded 0.9% of
the conventional harvested seed. However, one GM variety that was grown five years
previous made up 4-18% of the conventional harvest, with the highest values occurring in
the seed harvested from the dwarf variety. Since all oilseed rape volunteers were removed
from the rotation crops in the intervening years, the volunteers must have arisen from
dormant seed in the seed bank. The results illustrate the importance of breeding varieties
without the secondary dormancy trait, not only for GM varieties, but more generally for
the production of pure seed stocks and segregation of specialty oil types.
In the United Kingdom, Lutman et al. (2005) recorded a large average harvest seed
loss (3 575 seeds/m^2 ) from four B. napus winter varieties grown in multiple-site,
multi-year trials. Within six months, the number of seeds present declined by an average
of 63%, with a slower decline recorded at 18 and 30 months. Appreciably more seeds
were found on sites that were ploughed immediately after harvest compared to sites
where cultivation was delayed by about four weeks. These data support the
recommendations of Pekrun et al. (1998) and Gulden, Shirtliffe and Thomas (2003a) that
cultivation of B. napus stubble should be avoided for several weeks after harvest.
Regression models applied to the Lutman et al. (2005) data predicted that it would take an
average of nine years to reduce the seed in the soil bank by 95%. However, other studies
(Lutman, Freeman and Pekrun, 2003) indicate that the 95% reduction would occur in
three to four years. Indeed, Beismann and Roller (2003) in Germany reported that no
viable B. napus seeds could be found in soil sample cores taken from sites where
transgenic plots were sown five and six years before.
Studies in the United Kingdom and Canada with winter and spring forms of B. napus
indicate that seed bank persistence is less in lighter than heavier, clay containing, soils