184 – II.3. BRASSICA CROPS (BRASSICA SPP.)
and potassium, significant areas are deficient in the micronutrients zinc and boron, while
shortages of manganese, copper and iron also occur.
Herbicide resistant B. napus
B. napus is not considered a significant weed in managed ecosystems (AAFC, 1994).
However, due to the high level of seed lost during harvest it can be an abundant weed in
subsequent crops. Légère et al. (2001) ranked B. napus as 18th^ in relative abundance
among Canadian weed species in western Canada, and Leeson et al. (2005) found
B. napus plants in 10.5% of the fields surveyed. Studies in both Canada and Europe have
shown that the incorporation of genes for resistance to specific herbicides imparts no
altered weediness or invasive potential for glyphosate, including different events (AAFC,
1995b, 1996a; Norris et al., 1999: Crawley et al., 2001); glufosinate-ammonium,
including its combination with the hybrid system (AFFC, 1995a, 1995d, 1996b; Rasche
and Gadsby, 1997; Norris et al., 1999; MacDonald and Kuntz, 2000); bromoxynil (PBO,
1998) and non-GM imidazolinone (AAFC, 1995c). Experience in western Canada from
1995 through 2011, with all HR systems, have confirmed the validity of these earlier
assessments (Beckie, 2011; Warwick, Beckie and Hall, 2009; Beckie et al., 2006).
However, GM-HR volunteers can occur in subsequent B. napus crops. The level will
depend on the interval between oilseed rape crops in the rotation and how well the
producer has controlled volunteer B. napus in the intervening years. The shorter the
rotation and the less volunteer control, the greater the contamination level in the second
planting. The presence of one GM-HR canola plant per square metre throughout a field of
conventional oilseed rape calculates to a GM content of 2.5% in the harvested
conventional crop (planted at 40 plants/m^2 ). This calculation assumes that the number of
seeds produced by a volunteer plant is the same as that produced by the conventional
plants (CETIOM, 2000). However, Gruber and Claupein (2007) report that volunteer
winter B. napus plants, growing in a sown rapeseed crop only yield 45% of the seed
produced by corresponding sown plants.
Off-type volunteer plants can come from multiple sources, including the seed bank
from previous crops, movement of farm equipment and animals, pollen flow and
contaminated seed stocks. In Australia, Stanton, Pratley and Hudson (2002) found sheep
can excrete viable or germinable B. napus seed up to five days after ingestion. Similarly,
Martens (2001) claimed that manure from oilseed rape-fed chickens resulted in volunteer
plants when the manure was spread on a field 12 months later. In Canada, Downey and
Beckie (2002) and Friesen, Nelson and Van Acker (2003) found certified pedigreed seed
lots of conventional varieties contained unacceptable levels of GM seeds, apparently
resulting from pollen flow in breeding nurseries. The seed industry quickly purified their
breeding stocks but absolute exclusion cannot be guaranteed. Feral populations may
disseminate genes to nearby oilseed rape crops but the incidence would be very small and
far less than several of the sources noted above (CETIOM, 2000; Wilkinson et al., 1995).
In all oilseed rape growing regions, leaving the soil untilled for a period after harvest
and using non-inversion tillage is an effective strategy for minimising the size of the seed
bank (Gruber and Claupein, 2007; Gulden, Shirtliffe and Thomas, 2003a). Ploughing, as
done in Europe, will bury the seeds below germination depth but when the field is again
ploughed the dormant seeds will be brought to the surface. Pre-emergence and in-crop
post-emergence herbicide applications are effective in controlling volunteers even if they
contain one, two or three different herbicide-resistance genes (Table 3.5; Downey and
Buth, 2003). In western Canada, where herbicide tolerant oilseed rape has been grown
