204 – II.3. BRASSICA CROPS (BRASSICA SPP.)
a level they equated to the outcrossing rates observed at 200 m by Scheffler, Parkinson
and Dale (1995) (Table 3.7). They concluded that under western Canadian conditions, the
current regulations, which require a 10 m wide continuous border surrounding the pollen
donor, would effectively contain the majority of pollen-mediated gene flow, but would
not completely eliminate gene escape.
Table 3.8. Mean outcross percentages of pollen donor to B. napus recipient populations,
for various isolation distances and two design classes
Distance from
pollen source (m)
Continuous design Discontinuous design
Mean Standard deviation^ Number of data points Mean Standard deviation^ Number of data points
0-10 1.78 2.48 26 0.94 0.51 10
10-20 0.33 0.45 7 0.40 0.47 8
20-50 0.05 0.05 10 0.14 0.11 11
50-100 0.04 0.04 3 0.11 0.11 11
>200 n.d. n.d. n.d. 0.05 0.05 6
Note: n.d. = insufficient data.
Source: Hüsken and Dietz-Pfeilstetter (2007).
Field size experiments by Reboud (2003), using 24 m borders, indicated that for short
isolation distances gaps of bare ground between the donor and recipient plots/fields
should be avoided. Outcrossing declined more rapidly when there were intervening
plants, e.g. when the pollen donor was separated from the recipient field by a 3-4 m gap
the level of outcrossing was similar to that found 1 m into the crop where the gap was
zero. The same effect was noted by Dietz-Pfeilstetter and Zwerger (2004) when a bare
gap between donor and recipient fields was increased from 0.5 m to 10 m.
In the large field studies, not all the factors contributing to gene flow have been
controlled. Weekes et al. (2005) found the level of outcrossing to be considerably higher
in winter than in spring oilseed rape (Table 3.9) while Ramsay, Thompson and Squire
(2003) found the opposite to be true. They attributed the low value in the winter rape trial
to poor pollinating weather in May.
However, Reboud (2003) and Dietz-Pfeilstetter and Zwerger (2009) observed that
varieties used as pollen donors differed significantly in their outcrossing potential. The
outcrossing values in some fields in the Rieger et al. (2002) study may have been
overestimated since seed sown in the recipient fields was not tested as to the possible
presence of imidazolinone-tolerant seeds (Salisbury, 2002). Such contaminant HR seed
could have been present in seed sown in the recipient fields as a result of outcrossing or
admixture during the breeding and multiplication of the donor and recipient varieties, as
was observed in Canada by Downey and Beckie (2002) and Friesen, Nelson and Van
Acker (2003).
Also, it has been suggested that outcrossing levels were underestimated due to the
segregation of the two genes required to provide full tolerance to the selective herbicide.
Hall et al. (2000) identified some herbicide-resistant seedlings from recipient plants
situated some 650 m from an HR field. However, Downey (1999b) suggested the seed
may have been transported by the farmer’s swathing and harvesting equipment as
observed in the Dietz-Pfeilstetter and Zwerger (2009) study.
The outcrossing percentages reported by Stringam and Downey (1982) are
substantially higher than recorded for other studies listed in Table 3.7. However, it should
be noted that in the Stringam-Downey trials the pollen donors were fields of >60 hectares
