1.14 Common breeding methods in maize
Maize improvement can be achieved by various breeding methods, depending on the end product, initial type and amount of materials available, in addition to the heritability of the required trait (Bänziger et al., 2006). Two major breeding strategies are normally used for maize improvement. Firstly, selection breeding, which is used to generate open pollinated varieties and mainly applies to traits with high heritability. This method also includes recurrent selection as one of the breeding procedures. The second breeding strategy is inbreeding and subsequent hybrid development.
1.14.1 Recurrent selection
Recurrent selection is a cyclic breeding procedure designed to improve mean performance of populations under selection. This can be accomplished by a gradual increase in the frequency of favorable alleles with the simultaneous maintenance of genetic variability (Bänziger et al., 2006).
Recurrent selection has been effective in gradually improving population performance as well as maintaining the performance of varieties (Tollenaar and Lee, 2002). Like any other method, recurrent selection requires critical selection of appropriate germplasm with accurate recording of pedigree information (Tollenaar and Lee, 2002). Under recurrent selection, populations are improved for specific quantitative traits before they can become popular sources of inbred lines (Sleper and Poehlman, 2006). In addition to being highly effective in improving traits with high heritability such as ear height, lodging resistance, resistance to diseases and days to maturity, recurrent selection is a simpler method (Troyer and Brown, 1972). Recurrent selection methods may be on an individual plant, family or progeny basis (Hallauer, 1992). The original population for improvement can be a landrace or one that is constructed by inter-mating inbred lines superior for the quantitative character that is to be improved. Recurrent selection operates by increasing the frequency of favorable alleles within a population (Hallauer and Miranda, 1988).
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Different recurrent selection procedures have been developed for maize; however, their effectiveness depends on the stage of the breeding program, the population being improved, the breeding objective, and the trait of interest (Pratt et al., 2003). Pandey and Gardner (1992) reported that intra-population improvement methods have proven to be more effective than inter-population methods for improving population means per se for all traits. In related studies, Duvick and Cassman (1999) also reported that intra- population selection methods have been effective in improving drought tolerance in source populations. Furthermore, Tollenaar and Lee (2002) noted an increased probability of developing superior drought tolerant inbred lines from such populations.
Similarly, Hallauer and Miranda (1988) noted that family based recurrent selection methods result in greater gains when the traits under selection are complex and of low heritability.
1.14.2 Mass selection
Mass selection is the oldest and simplest form of recurrent selection. Its simplicity and the completion of a cycle in the course of one year are its greatest advantages over other methods. Moreover, mass selection is most efficient for traits with high heritability (Hallauer and Miranda, 1988).
Mass selection has been shown to be highly effective in modifying highly heritable traits in maize (Smith, 1999). Selection effectiveness for yield improvement in a maize population is dependent upon the presence of additive genetic variation for yield (Levy, 1991). Mass selection has gained even greater importance, due to the introduction of the top cross system. The top cross system may minimize the yield disadvantage associated with conventional high oil corn hybrids. The top cross system uses the sterile version of a hybrid (90-95%) as a means to obtain a high yield, and high-oil population (5-10%) as a pollinator. Due to the effect of xenia, half of the oil content of the oil population is transferred to the sterile (female) version of the hybrid. In this way, it is possible to gain both high yield and high oil content (Smith, 1999).
31 1.14.3 Inbred line development
Inbred lines are pure lines developed through a series of selfing of selected heterozygous plants until homozygosity is reached. They ought to have desired traits and well defined heterotic groups. The first step in inbred line development is the selection of germplasm with the desired traits. According to Sleper and Poehlman (2006), superior lines for a particular quantitative trait can be extracted from recurrent selection populations designed to increase gene frequency of that trait, by repeated cycles of selection and inter-mating. Use of materials in which an increase in gene frequency for the character to be improved has been demonstrated in enhancing the development of superior inbred lines (Duvick and Cassman, 1999). Therefore, the success of inbred development is embedded in the characteristics and manipulation of the original germplasm selected for this purpose (Sleper and Poehlman, 2006).
1.14.4 Inbreeding and hybrid development
Inbreeding and hybrid development is another procedure through which the genetic constitution of plants can be manuiplated for the purpose of improving a particular population. The main purpose of inbreeding is the production of homozygous lines for subsequent crosses to develop hybrids to exploit hybrid vigor (heterosis). Falconer and Mackay (1996) noted that a crossing of inbred lines to produce hybrids plays a major role in crop improvement, most notably maize. Furthermore, they indicated that, in order to attain heterosis, the candidate lines for crosses need to be derived from different base populations; a cross between two unrelated base populations provides heterosis.
The degree of heterosis depends on the relative performance of the inbred lines and their crosses, as well as on the differential effect of the environment (Pandey and Gardener, 1992). Studies have shown that heterosis is greater in stress environments than under favorable conditions, due to the higher sensitivity of inbreds to stress than their hybrids (Ullustrup, 1970). This implies that it is more meaningful to characterize a