208 – II.3. BRASSICA CROPS (BRASSICA SPP.)
seeds attached to the placenta are soon dislodged by wind action. Threshing operations
easily separate the seed from the intact siliques.
All commercially grown Brassica crops, as well as weedy species, tend to shatter
their seed when ripe. However, the ease or degree of shattering varies among species.
Within the oilseed crops, B. napus has the greatest tendency to shatter its seed, with
B. rapa intermediate and B. juncea the least. Breeding work is developed to transfer the
shatter-resistant characteristic from B. juncea to B. napus (Wang, Ripley and Rakow,
2007). The vegetable Brassica species follow a similar pattern. However, with the
high-value F 1 seed of B. oleracea hybrids and the relatively small fields used for seed
production, every precaution, sometimes including hand harvesting, is taken to ensure
little or no seed is lost. Pod shatter is rare in the closely related S. alba (yellow or white
mustard) species, but some loss of intact ripe pods, due to wind or mechanical action,
does occur at harvest.
Seed that falls to the ground can be dispersed by wind and water as well as by birds
and other animals. Because the seed is small and round it is difficult to prevent some loss
during transportation of farm equipment from field to field, or from field to bin and from
bin to its ultimate destination. Significant losses can occur from truck containers of
uncovered oilseed rape due to the wind vortex caused by the movement of the truck.
The faster the truck goes, the greater the loss. The distribution of seed from the truck
vortex will depend on seed size and the direction and velocity of the wind prevailing at
the time of loss. For spring B. napus, the distance such seed will travel at various wind
speeds has been calculated (Table 3.11), although for average spring and winter B. napus
seed, which is larger and heavier than that used to calculate the table, the wind-borne
dispersal distance would be reduced.
Table 3.11. Estimated dispersal distances of spring B. napus seed released from transport
vehicles at various heights above adjacent fields
Height (metres)
Wind speed in km/h
10 20 30 40 50 60
Horizontal dispersal in m
1.0 1.4 2.7 4.1 5.5 6.8 8.2
2.0 2.1 4.1 6.2 8.3 10.3 12.4
3.0 2.6 5.1 7.7 10.2 12.8 15.3
4.0 3.1 6.2 9.3 12.3 15.4 18.5
5.0 3.6 7.2 10.8 14.4 18.0 21.6
6.0 4.1 8.2 12.4 16.5 20.6 24.7
7.0 4.6 9.3 13.9 18.5 23.2 27.8
8.0 5.1 10.3 15.4 20.6 25.7 30.9
Note: 1.^ Estimates based on small seeds of spring B. napus, calculated to weigh 2.2 mg with a diameter of
1.8 mm, that are the most likely to become air borne and travel the farthest.
Source: Hertz (1999).
Seed viability, longevity and dormancy, germination, seedling establishment
Well-developed, fully mature Brassica oilseeds may remain viable for at least
25 years if dry seed is refrigerated in sealed containers (Ellis et al., 1994). As of 2009,
seed of oilseed Brassica, harvested in 1977 and stored in manila envelopes at -20°C in the
Saskatoon AAFC Seed Bank, had retained its high germination (Downey, personal
communication). Viability of seed lost during harvest is an important factor in
