In bulk breeding, selection is practiced mainly in:
Natural selection predominates during early generations of bulk method, making artificial selection ineffective because heterozygosity buffers phenotypic expression and heritability of yield is low. Plants in F2-F4 still segregate, genotype-by-environment interaction confounds single plant performance, and competitive ability may not correlate with grain yield in pure stands. Breeders therefore postpone intensive selection until F5-F7, when inbreeding coefficient approaches 0.9375 and plants become largely homozygous. At this stage environmental fitness differences have already filtered malada
Ref: Acquaah G. Chapter 11 Bulk breeding; Poehlman & Sleper Breeding Field Crops, 5th ed.