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

can vary by the complexity of inheritance of the trait under selection. It seems, however, that
180 to 240 progenies are a minimum number for most traits considered for recurrent selection.
Phase 2: Determination of the correct breeding values of the sampled progenies is essential for
effective selection. Progenies are evaluated in replicated trials to separate the genetic and
environmental effects and to determine the relative importance of each (Table 2). Effective
screens are essential. It seems that a minimum of 6 to 10 replications are required to differentiate
among progenies for grain yield (Lamkey and Hallauer, 1984). The replications should be
distributed among environments to reduce bias due to genotype by environment interactions.
Phase 3: Intermating restores genetic variability to permit continued selection. To maintain
genetic variability for future selection, it is imperative that adequate effective population
sizes are used to reduce loss of genetic variability due to either inbreeding or genetic drift,
or both. Rawlings (1970) studied the effects of effective population size relative to expected
short-term and long-term responses to selection and concluded that 25 to 35 individuals should be
intermated for each cycle of selection. Numbers of progenies selected for intermating also
affects the methodes) used for intermating selected progenies. It is essential that selected
progenies are equally represented in the intermatings to ensure that the planned effective
population size is realized. If 10 or fewer progenies are intermated, which is not recommended,
the dia11e1 crossing scheme can be used to make all possible n(n-1)/2 crosses. If 20 or more
progenies are intermated this may be done either by use of an isolation planting or by the bulkentry
method. In all instances, it is necessary that each selection is equally represented in
all crosses, and an equal number of seeds is composited to form the selected population for the
next cycle of selection.
Recurrent selection methods are flexible and are useful for different aspects of maize
breeding. Recurrent selection can be used to increase the level of resistance to important
diseases and insects, change chemical composition of grain, change plant architecture, change
grain type, improve stress tolerance, increase productivity, and adapt exotic germp1asm.
Materials derived from recurrent selection programs that are integrated with other aspects of a
breeding program can have several useful end products (Eberhart et aT., 1967). Some possible
uses of materials derived from recurrent selection programs include the following:
Cultivars -- In those areas that are not committed to the use of hybrids, the improved cycles of
selection can be made available to the growers as improved cultivars. It is not to be expected
that each cycle of selection will be released for use by the growers: only when the improved
cycles are significantly better than cultivars currently being used by growers should they be
released. Otherwise, it will be difficult to convince growers that the "new" cultivars are
better than those currently used.
Cultivar cnosses -- If two populations are being improved in a parallel manner (e.g., reciprocal
recurrent selection), the cross of the improved populations can be released to the growers, using
the same criteria as for cu1tivar releases.
Genetic stocks -- Assume a particular pest (e.g., Asian downy mi 1dew) or problem (e.g., aluminum
tOXicity) is a major concern for a particular area. A recurrent selection program may be
initiated to develop a population with greater level of tolerance to downy mildew and/or aluminum
toxicity. Greater levels of tolerance are developed, but the level of productivity of the
population may not be acceptable in pest-free environments. The selected population for pest
tolerance could be crossed with the local cu1tivars to upgrade the level of tolerance. This
requires, however, careful planning. selection should be emphasized, if possible in local
materials, but frequency of alleles may be too low to increase levels of tolerance, and outside
source materials are used. Attention should be given in choice of populations used to select for
greater levels of tolerance as well as for prod~ctivity and maturity.
Recurrent selection methods also are flexible for the number of traits selected (mu1tipletrait
selection) and for generations within each cycle of selection (multiple-stage selection);
e.g., selection for different traits in S1 and S2 generations. Single-trait selection has been
shown to be very effective, but the correlated effects for other traits may not be desirable
(Hallauer et aT., 1988). The direct effects of selection are positive, but the indirect (or
correlated) effects of selection may not be acceptable. One, therefore, has to closely monitor
the selection methods to ensure useful germplasm sources are developed. Recurrent selection
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