Key advantages and disadvantages of the bulk breeding method

There are key advantages and disadvantages of the bulk breeding method.

Advantages

_ It is simple and convenient to conduct.

_ It is less labor intensive and less expensive in early generations.

_ Natural selection may increase frequency of desirable genotypes by the end of the bulking period.

_ It is compatible with mass selection in self-pollinated species.

_ Bulk breeding allows large amounts of segregating materials to be handled. Consequently, the breeder can make and evaluate more crosses.

_ The cultivar developed would be adapted to the environment, having been derived from material that had gone through years of natural selection.

_ Single plant selections are made when plants are more homozygous, making it more effective to evaluate and compare plant performance.

Disadvantages

_ Superior genotypes may be lost to natural selection, while undesirable ones are promoted during the early generations.

_ It is not suited to species that are widely spaced in normal production.

_ Genetic characteristics of the populations are difficult to ascertain from one generation to the next.

_ Genotypes are not equally represented in each generation because all plants in one generation are not advanced to the next generation. Improper sampling may lead to genetic drift.

_ Selecting in off-season nurseries and the greenhouse may favor genotypes that are undesirable in the production region where the breeding is conducted, and hence is not a recommended practice.

_ The procedure is lengthy, but cannot take advantage of off-season planting.

Modifications

Modifications of the classic bulk breeding method include the following:

_ The breeder may impose artificial selection sooner to shift the population toward an agriculturally more desirable type.

_ Rouging may be conducted to remove undesirable genotypes prior to bulking.

_ The breeder may select the appropriate environment to favor desired genotypes in the population. For example, selecting under disease pressure would eliminate susceptible individuals from the population.

_ Preliminary yield trials may be started even while the lines are segregating in the F3 or F4.

_ The single seed descent method may be used at each generation to reduce the chance of genetic drift. Each generation, a single seed is harvested from each plant to grow the next bulk population. The dense planting makes this approach problematic in locating individual plants.

_ The composite cross bulk population breeding, also called the evolutionary method of breeding, was developed by C.A. Suneson and entails systematically crossing a large number of cultivars. First, the pars of parents are crossed, then pairs of F1s are crossed. This continues until a single hybrid stock containing all parents is produced. The method has potential for crop improvement but it takes a very long time to complete.

Single seed descent

The method of single seed descent was born out of a need to speed up the breeding program by rapidly inbreeding a population prior to beginning individual plant selection and evaluation, while reducing a loss of genotypes during the segregating generations. The concept was first proposed by C.H. Goulden in 1941 when he attained the F6 generation in two years by reducing the number of generations grown from a plant to one or two, while conducting multiple plantings per year, using the greenhouse and the off season. H.W. Johnson and R.L. Bernard described the

procedure of harvesting a single seed per plant for soybean in 1962. However, it was C.A. Brim who, in 1966, provided a formal description of the procedure of single seed descent, calling it a modified pedigree method.