At the beginning of breeding a new corn hybrid (and of any other crop), the breeder has to be clear about the requirements of the target environment to the new breed. He has to define which trait or multiple traits he wants to improve.
In general, the main breeding goals for corn are as follows:
- Yield + yield stability
- Resistance to pests and diseases
- Drought stress tolerance
- Cold tolerance of seedlings
- Resistance to lodging
- Early and late maturing cultivars
- Silage quality
Corn is a hybrid cultivar, which is obtained by crossing two inbred lines that are completely homozygous. The actual breeding process of a new corn hybrid starts with making crosses between two lines that show predefined traits. Afterwards, the offspring of the heterozygous parents are being transformed into homozygous individuals.
There are two ways to achieve pure inbred lines that are completely homozygous: First, lines are being selfed for several generations until a sufficient level of homozygosity is reached. Second, the doubled haploid technology is applied. Doubled haploid lines are routinely applied in many commercial hybrid maize breeding programs. Major advantages of DH lines compared to selfed lines include reduced breeding cycle length complete homozygosity from the very first generation and simplified logistics.The inbred lines or double haploid lines constitute a new population or heterotic group. In hybrid breeding it is important to keep heterotic groups separated until the final cross is made. New crosses are being made within each heterotic group.
In the next step the inbred lines or doubled haploid lines are being crossed to a so-called tester, which is an inbred line or double haploid line from the opposite heterotic group that has shown to have good combining ability. The process of cross-pollination of two inbred lines is called hybridization and results in heterosis (also called hybrid vigour). Heterosis is the increase in characteristics such as size, growth, fertility and yield of a hybrid over those of its parents. The resulting hybrids or testcrosses are being evaluated in several field locations and the best testcrosses are selected based on phenotypic data. Out of this crosses the best hybrid is selected for official registration and further commercialization.