Application

As a modern method of plant breeding, mass selection has several applications:

_ It may be used to maintain the purity of an existing cultivar that has become contaminated, or is segregating. The off-types are simply rogued out of the population and the rest of the material bulked. Existing cultivars become contaminated over the years by natural processes or by human error.

_ It can also be used to develop a cultivar from a base population created by hybridization, using the procedure described in Section 16.5.3.

_ It may be used to preserve the identity of an established cultivar or soon to be released new cultivar. The breeder selects several hundred Plants and plants them in individual rows for comparison. Rows showing significant phenotypic differences from the other rows are discarded, while the remainder is bulked as breeder seed. Prior to bulking, sample plants or heads are taken from each row and kept for future use in reproducing the original cultivar.

_ When a new crop is introduced into a new production region, the breeder may adapt it to the new region by selecting for key factors needed for successful

production.This, hence, becomes a way of improving the new cultivar for the new production region.

_ Mass selection can be used to breed horizontal disease resistance into a cultivar. The breeder applies low densities of disease inoculum so that quantitative genetic effects can be assessed. This way, the cultivar is race-non-specific and moderately tolerant of disease. Further, crop yield is stable and the disease resistance is durable.

_ Some breeders use mass selection as part of their breeding program to rogue out undesirable plants, thereby reducing the materials advanced and saving

time and reducing cost of breeding.

Procedure

Overview

The general procedure in mass selection is to rogue out off-types or plants with undesirable traits. This is called by some researchers negative mass selection. The specific strategies for retaining representative individuals for the population vary according to species, traits of interest, or creativity of the breeder to find ways to facilitate the breeding program. Whereas rouging out and bulking appears to be the basic strategy of mass selection, some breeders may rather select and advance a large number of plants that are desirable and uniform for the trait of interest. Where applicable, single pods from each plant may be picked and bulked for planting. For cereal species, the heads may be picked and bulked.

Steps

The breeder plants the heterogeneous population in the field, looks for off-types to remove and discard. In this way the original genetic structure is retained as much as possible. A mechanical device may be used, or selection may be purely on visual basis according to the breeder’s visual evaluation. Further, selection may be based on targeted traits or indirectly by selecting a trait correlated with the trait to be improved.

_ Year 1. If the objective is to purify an established cultivar, seed of selected plants may be progenyrowed to confirm the purity of the selected plants prior to bulking. This would make a cycle of mass selection have a two-year duration instead of one year. The original cultivar needs to be planted alongside for comparison.

_ Year 2. Evaluate composite seed in replicated trial, using original cultivar as check. This test may be conducted at different locations and over several years. The seed is bulk-harvested.