II.3. BRASSICA CROPS (BRASSICA SPP.) – 209
determining the presence and amount of volunteer plants and populations in subsequent
crops. Harvest losses can be substantial and the survival and persistence of this seed is
greatly influenced by environment, seed dormancy as well as crop and field management.
Contribution of B. napus harvest losses to persistence
Harvest losses in the United Kingdom, when the winter B. napus crop is straight
combined under ideal conditions, ranged from 2% to 5%, but under unfavourable harvest
environments could amount to 50% (Price et al., 1996). Pekrun et al. (1998) placed these
losses between 200-300 kg/ha or about 5 000-7 000 seeds/m^2. Lutman et al. (2005) in the
United Kingdom and Gruber, Pekrun and Claupein (2004) in Germany recorded average
harvest losses of 3 000-3 500 seeds/m^2. Similarly, French studies estimated harvest losses
to be between 1.5% and 8.5% of the average yield. This calculates to 50-300 kg/ha of
seed remaining on the field after harvest or 1 100-6 700 seeds/m^2 (CETIOM, 2000;
Messéan et al., 2007). In Canada, Gulden, Shirtliffe and Thomas (2003a) reported that
spring B. napus, harvest losses averaged 5.5%, or about 3 590 seeds/m^2 , while Légère et
al. (2001) estimated the losses at 2 000/m^2. Similarly, Warwick et al. (2003) reported
spring B. napus harvest losses averaging 5.5%, or about 3 590 seeds/m^2. Salisbury (2002)
estimated Australian losses would be similar to those found in Canada. However, a vast
majority of the seed remaining in the field after harvest will not survive the first year.
The Brassica oilseed density of the seed bank in western Canada is reported to drop
ten fold in the first year and to decline slowly thereafter, due to replenishment of the seed
bank by uncontrolled volunteer plants. However, where post-harvest tillage is shallow
and delayed and volunteers in subsequent crops are controlled, very few plants are found
four years after a spring B. napus crop (Gulden, Shirtliffe and Thomas, 2003b).
Seed dormancy
Seed dormancy can play an important part in determining the amount and persistence
of volunteer Brassica plants in subsequent crops. There are two main types of seed
dormancy: primary and secondary. Primary dormancy is when seed germination is
prevented during the seed maturation process and for some time after the seed has been
removed from its parent (Karssen, 1980/81; Hilhorst and Toorop, 1997). To overcome
primary dormancy, a period of after-ripening is usually required. Secondary dormancy is
a reduction in seed germinability that develops after the seed is separated from the parent
plant and may, in some cases, be induced prior to the complete alleviation of primary
dormancy. Primary dormancy does not occur in ripe seeds of any of the cultivated
Brassica oilseed, vegetable or condiment crops. For seed certification status, these crops
require a minimum germination of at least 90%. However, during seed maturation,
germination percentages may be low in spring and winter B. napus but increase with
maturity (Finkelstein et al., 1985) to where at harvest no primary dormancy occurs
(Schlink, 1995). However, secondary dormancy can be induced in B. napus and
cultivated B. rapa under certain conditions (Hails et al., 1997; Pekrun, Lutman and
Baeumer, 1998; Adler et al., 1993). An exception to the rule occurs in the weedy forms of
B. rapa, where primary dormancy is present as a recessive trait in weedy B. rapa. Thus,
crossing between weedy and cultivated B. rapa, as well as between weedy B. rapa and
B. napus, will produce seed that does not exhibit primary dormancy (Linder, 1998;
Landbo and Jorgensen, 1997; Adler et al., 1993).
The main factors contributing to secondary dormancy of B. napus seed are elevated
temperatures, darkness, osmotic stress and limited oxygen (Gulden, Thomas and
Shirtliffe, 2004; Pekrun et al., 1997). Studies in Europe (Pekrun, Potter and Lutman,