236 – II.3. BRASSICA CROPS (BRASSICA SPP.)
Analytical advances in the 1960s allowed breeders to identify plants with only
10-12 μmoles of aliphatic glucosinolate per gram oil free meal. These plants were crossed
with low erucic acid varieties to produce “double low” or “canola quality” varieties of
B. napus (Stefansson, 1983), B. rapa (Downey and Rakow, 1987) and B. juncea
(Love et al., 1990). The reduction in glucosinolate levels allowed canola meal to be fed at
maximum economic levels to non-ruminants and canola meal became the preferred
protein supplement for dairy cattle. Canola is defined as seeds of the genus Brassica
(Brassica napus, Brassica rapa or Brassica juncea) from which the oil shall contain less
than 2% erucic acid in its fatty acid profile and the solid component shall contain less
than 30.0 micromoles of any one or any mixture of 3-butenyl glucosinolate, 4-pentenyl
glucosinolate, 2-hydroxy-3-butenyl, or 2-hydroxy-4-pentenyl glucosinolate, per gram of
air-dried, oil free solid (Canola Council of Canada).
Breeding methods
The amphidiploids, B. napus, B. juncea and B. carinata, are largely self-pollinating
with the self-pollinated progeny exhibiting very little, if any, loss in vigour.
Thus, methods developed for highly inbred crops, such as the cereal grains, have been
adapted for these partially outcrossing species. In the oilseed forms of these species,
complete homozygosity is normally not the objective, although varietal distinctness,
uniformity and stability are still a requirement. However, with cole crops and hybrids,
high levels of homozygosity are required.
Regardless of the breeding technique employed, success is dependent upon the
identification of suitable parents that, when crossed, will yield progenies that express the
desirable traits of both parents.
Mass selection
This early plant-breeding technique relied on the identification and harvesting of seed
from the most productive or desirable plants within a population for sowing in the
following year. The system is one of population improvement based on plant phenotype
and is best suited to self-fertilised crops and where gene action is additive. It lacks
the efficiency of present-day techniques, but a variation is used today to preserve the
identity of established varieties whereby off-types are removed from elite lines and
breeder seed plots.
Pedigree method
In the past, most B. napus and B. juncea commercial varieties were developed using
the pedigree method. Crosses are made between parents exhibiting the traits to be
combined and the F 1 s are selfed or intercrossed. The progeny are selfed or allowed to
interpollinate and selection of the best F 4 rows is done within the best F 3 families.
By the F5-6, the vast majority of loci will be homozygous and the characteristics of the
breeding line are fixed.
The pedigree method may be modified in various ways depending on the inheritance
of the trait or traits being introduced or combined. The method is well suited to the
mainly self-pollinating species B. napus and B. juncea, because the seed multiplication
rate, unlike cereal grains, is high (ca. 1 000:1). In the self-incompatible Brassica
vegetables and oilseed B. rapa, inbreeding leads to a rapid loss in vigour and reduced
fertility. However, it is sometimes used to produce inbred lines destined for the
