Principles of Plant Genetics and Breeding

494 CHAPTER 28
Evaluation of hybrids in late stages emphasizes wide area evaluation with multiple environments. This extensive evaluation
permits the selection of hybrids for adaptation and stability. Managed environments may include cultural practices such as different
densities, planting dates, drought stress, fertilization levels, tillage, crop rotation, etc. Environments are representative of the
most common farmer field conditions. More weight in decision-making is applied to those environments more correlated with
the target environment. Traits that are emphasized during hybrid screening depend on the target environments. In the US, desired
traits include high grain yield, low moisture content at harvest, good standability, ear retention, stability over years, drought
tolerance, stay-green, disease and insect resistance, early vigor, and proper maturity (Troyer 2001). The hybrid seed industry has
concentrated on extensive and efficient testing, improved mechanization, reliable data collection and analysis, and high selection
intensities (Coors 1999). After all the processes described, only a very small percentage of the experimental hybrids tested
make it to commercial hybrids.
References
Bauman, L.F. 1981. Review of methods used by breeders to develop superior corn inbreds. Proc. Annu. Corn Sorghum Ind. Res.
Conf. 36:199–208.
Bernardo, R. 1999. Prediction of single-cross performance. In: The genetics and exploitation of heterosis in crops (Coors, J.G., and
S. Pandey, eds), pp. 269–276. American Society of Agronomy, Madison, WI.
Betrán, F.J., M. Bänziger, and M. Menz. 2003. Corn breeding. In: Corn: Origin, history, technology, and production (Smith, W.,
F.J. Betrán, and E. Runge, eds). John Wiley & Sons, New York.
Birchler, J.A. 1994. Practical aspects of haploid production. In: The maize handbook (Freeling, M., and V. Walbot, eds). Springer-
Verlag, New York.
Coors, J.G. 1999. Selection methodologies and heterosis. In: The genetics and exploitation of heterosis in crops (Coors, J.G., and
S. Pandey, eds), pp. 225–246. American Society of Agronomy, Madison, WI.
Duvick, D.N., and K.G. Cassman. 1999. Post-green revolution trends in yield potential of temperate maize in the north-central
United States. Crop Sci. 39:1622–1630.
Goodman, M.M., J. Moreno, F. Castillo, R.N. Holley, and M.L. Carson. 2000. Using tropical maize germplasm for temperate
breeding. Maydica 45:221–234.
Hallauer, A.R. 1990. Methods used in developing maize inbreds. Maydica 35:1–16.
Hallauer, A.R., and J.B. Miranda, Fo. 1988. Quantitative genetics in maize breeding, 2nd edn. Iowa State University Press, Ames,
IA.
Lee, M. 1995. DNA markers and plant breeding programs. Adv. Agron. 55:265–344.
Melchinger, A.E. 1999. Genetic diversity and heterosis. In: The genetics and exploitation of heterosis in crops (Coors, J.G., and
S. Pandey, eds), pp. 99–118. American Society of Agronomy, Madison, WI.
Shull, G.H. 1909. A pure line method of corn breeding. Rep. Am. Breeders Assoc. 5:51–59.
Smith, O.S., K. Hoard, F. Shaw, and R. Shaw. 1999. Prediction of single-cross performance. In: The genetics and exploitation of
heterosis in crops (Coors, J.G., and S. Pandey, eds). American Society of Agronomy, Madison, WI.
Troyer, A.F. 2001. Temperate corn – Background, behavior, and breeding. In: Specialty corn (Hallauer, A.R., ed), pp. 393–466.
CRC Press, New York.
Consequently, it is critical that the inbred lines from
which the crop will be produced, are high yielding.
Selection at each generation should also be based on
superior performance regarding disease and insect resistance,
grain quality, lodging resistance, and other traits
that support the breeding objectives.
Establishing a breeding nursery
Field nursery
A proper layout of the breeding nursery facilitates a
breeding program. A comprehensive corn breeding
program will have selfing nurseries for the maintenance
of inbred lines, increasing inbred lines, and inbred line
selection and breeding. There will also be crossing
blocks (for single, three-way, and double crosses, and
topcrosses) and recurrent nurseries.
The plants should be planted in rows with alleys for
easy movement of the breeder. Parents for crossing
should be arranged in paired rows. Where several crosses
have one row, the common parent may be used as the
male parent and planted adjacent to a block of the
female parent.
Other nurseries
As previously indicated, corn breeders use field nurseries
exclusively for breeding. Greenhouses and growth