Proceedings of the Fifth Asian Regional Maize Workshop - Search ...

Modern maize breeding includes two separate, but equally important, components: 1)
germp1asm improvement and 2) development of inbred lines for use in hybrids. Genetic advance
depends on the systematic improvement of germp1asm, and all breeding programs should include both
components. In areas where hybrids do not have widespread use, the systematic improvement of
germp1asm resources will provide genetically improved cu1tivars to the growers. In areas that
emphasize the use of hybrids, systematic improvement of germp1asm resources provides the source
breeding materials on a regular basis. Improvement of genetic resources and development of lines
for hybrids will be discussed to illustrate their relative importance in successful breeding
programs.
Germp1asm improvement
Genetic variability is the essential raw material (or ore) that is necessary for successful
plant breeding. In maize, genetic variability has not been a restraint in making genetic
progress. Extensive quantitative genetic studies during the 1950s and 1960s provided evidence
that adequate genetic variability was available in maize populations to expect response to
selection. Generally, it has been found that if effective screens are either available or
developed, response to selection will be realized. Effective screens are those techniques that
separate genetic and environmental effects so that selection is based primarily on genetic
differences and not confounded by the effects of the environments, such as escapes, nonuniform
infection, genotype environment interactions, etc. Effective screens include artificial
infestation with eggs, larvae, or adults of insects to reduce the occurrence of escapes, artificial
infection with spores of diseases to ensure uniform coverage of all plants, standardized
chemical analyses for quality traits, and replicated trials replicated in different environments
to separate and estimate components of variance caused by genetic effects, environmental effects,
and genotype by environment interaction effects.
Careful consideration, therefore, should be given in the choice of germp1asm included in
breeding programs. But genetic variability, for the sake of genetic variability, can be misleading.
Because most maize populations seem to have adequate genetic variability to expect response
to selection, other parameters should be used in choice,of germp1asm. Other parameters include
the overall productivity of the population, general resistance to important pests in your area,
general tolerance to acid soils if needed for your area, grain type and color for acceptance by
growers and users, proper maturity, drought tolerance, and other traits that may be important for
maize production and use. Definitive information on maize populations for these traits, however,
often has been lacking. It is necessary that you contact colleagues and organizations to secure
all information available before you make your choice of germplasm. contact individuals within
your country and other countries who have experience in maize breeding, including regional and
international centers conducting maize research, and individuals in other areas of the world who
have similar maize growing conditions. Read carefully the information available in reports of
local and regional workshops, conferences, and symposia. It has been realized recently that
breeding information (combining ability, inbreeding depression, heterosis, and performance per
se) is essential to make wise choices in germp1asm. CIMMYT (International Maize and Wheat
Improvement Center) and the Latin American Maize Project (LAMP) have been evaluating maize
populations, and very useful information can be gathered from their reports to assist you in the
choice of tropical germp1asm that can be of potential value for your respective areas.
After you carefully choose the maize population(s) that you judge have the necessary
traits for your area, the choice of selection method(s) to genetically improve the population(s)
needs to be determined. These selection methods for the cyclical improvement of populations are
collectively designated as methods of recurrent selection. Paradoxically, the choice of
method(s) seems to be of less importance than the choice of populations. Evidence suggests that
if effective screens are available all selection methods, with few exceptions, are effective
(Hallauer and Miranda, 1988). Because maize is a cross-fertilizing species, several different
methods of recurrent selection have been applied to maize populations (Table 1). The choice of
method, therefore, should be the one that best meets your specific needs and conditions. The
choice of method also can be either modified or changed if your situation warrants a change,
i.e., changes in disease and insect pressures. A useful equation to assist you in choice of
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