Plant Breeding: Heterosis

Hybrid vigour, also called heterosis, is the superiority of the hybrid over both its parents in characters such as size, growth rate, yield and resistance.
Maize is a cross-pollinated crop. Repeated self-pollination (inbreeding) is first used to produce pure inbred lines, which are homozygous but show reduced vigour (inbreeding depression).
When two different inbred parental lines are crossed, the F1 hybrid is highly heterozygous and shows marked hybrid vigour, with a much higher yield.
Hybrid maize seed is produced commercially in this way (single cross and double cross hybrids).
The vigour declines in later generations because of segregation, so fresh hybrid seed has to be produced every year; in vegetatively propagated crops it can be maintained.
Selecting the most productive plants, inducing mutations and DNA bombardment are different techniques and do not exploit hybrid vigour.
So in maize hybrid vigour is exploited by crossing two inbred parental lines.

Hybrid vigour or heterosis is the superiority of the hybrid over both its parents in size, growth rate, yield and other characters.
Maize is a cross-pollinated crop. Pure inbred lines are first developed by repeated self-pollination; these are homozygous but weak (inbreeding depression).
When two different inbred parental lines are crossed, the F1 hybrid is highly heterozygous and shows marked hybrid vigour and a much higher yield.
Hybrid maize seed is produced commercially by this method.
Since the vigour is lost in later generations through segregation, fresh hybrid seed is produced every year.
Bombarding protoplasts with DNA, selecting productive plants and inducing mutations are other breeding techniques that do not exploit heterosis.
So in maize hybrid vigour is exploited by crossing two inbred parental lines.