238 – II.3. BRASSICA CROPS (BRASSICA SPP.)
Figure 3.42. Breeding scheme combining the backcross and pedigree selection systems to
develop a low erucic, low glucosinolate variety with high seed and oil yield
Notes: This breeding scheme uses agronomically superior parents that contribute either high (●) or low (○)
erucic acid levels and high (■) or low (□) glucosinolate content.
Source: Downey and Rakow (1987).
Recurrent selection method
This method is standard procedure for improving populations of self-incompatible
species. Any type of segregating population may be a candidate for improvement.
Normally open-pollinated seed is harvested at random from individual plants within the
population, and a progeny row or rows sown from each plant. However, some seed from
each plant is held in reserve. The progeny rows are evaluated and the best performing
identified. An equal amount of the reserve seed from the best single plants, based on the
performance of their progeny, is bulked. The first cycle of recurrent selection is complete
when the new seed composite is sown in an isolation plot, and the second cycle begins
with the harvesting of random single plants within the new composite. A bulk sample
from the remaining plants can be harvested and planted in replicated trials to measure the
response to selection. Recurrent selection is continued as long as it is anticipated that
there will be a reasonable response to selection.
With every additional trait under selection, the intensity of selection increases
exponentially, thus it is difficult to improve a population for several traits simultaneously.
This constraint is overcome by having specialised composites for different traits that are
brought together after the original objectives for each have been met.
