Bulk population breeding

Bulk population breeding is a strategy of crop improvement in which natural selection effect is solicited more directly in the early generations of the procedure by delaying stringent artificial selection until later generations. The Swede, H. Nilsson-Ehle, developed the procedure. H.V. Harlan and colleagues provided additional theoretical foundation for this method through their work in barley breeding in 1940s. As proposed by Harlan and colleagues, the bulk method entails yield testing of the F2 bulk progenies from crosses and discarding whole crosses based on yield performance. In other words, the primary objective is to stratify crosses for selection of parents based on yield values. The current application of the bulk method has a different objective.

The rationale for delaying artificial selection is to allow natural selection pressure to eliminate or reduce the  productivity of less fit genotypes in the population. Just like the pedigree method, the bulk method also applies pure line theory to segregating populations to develop pure line cultivars. Genetic recombination in the heterozygous state cannot be used in selfpollinated species because self-pollination progressively increases homozygosity. By F6 the homozygosity is about 98.9%. The strategy in plant breeding is to delay selection until at the high level of

homozygosity.

It is a procedure used primarily for breeding selfpollinated species, but can be adapted to produce inbred populations for cross-pollinated species. It is most suitable for breeding species that are normally closely spaced in production.It is used for field beans and soybeans. However, it is not suitable for improving fruit crops

and many vegetables in which competitive ability is not desirable.

After making a cross, several hundreds to several thousands of F2 selections are planted at a predetermined close spacing. The whole plot is bulk harvested. A sample of seed is used to plant another field block for the next selection, subjecting it to natural selection pressure through the next 2–3 generations. In the F5, the plants are space-planted to allow individual plant evaluation for effective selection. Preliminary yield trials may start in the F7 followed by advanced yield trials, leading to cultivar release.