During hybridization offsprings with hybrid vigour superior to both parents are self-pollinated for few successive generations to 

Reason:  A plant is called homozygous for a particular gene when both alleles are similar at that particular given locus (dominant or recessive). A homozygous plant maintains a high degree of consistency for particular characters determined by the gene throughout the subsequent generations. When both parents are getting self pollinated for few successive generations, the pure charecters are started to be conserved.