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

c. Integrat;on of populat;on ;mprovement and hybr;d research
The population improvement and hybrid research are in fact two sides of a COln and one
cannot be divorced from the other. There is enough complementary in both types of germplasm
products and approaches that a breeder should try to integrate both aspects to increase resource
efficiency and effectiveness of both programs. Both programs have enough information and
germplasm that can be shared and used. Several aspects of the population improvement program can
be easily integrated with hybrid research. By affecting sensible modifications in important
breeding schemes, one can easily identify early generation lines with good combining ability.
some of the recurrent selection schemes such as recurrent selection for GCA and recurrent
selection for SCA do have built-in procedure to.identify early generation products for the hybrid
program. Interpopulation schemes such as reciprocal recurrent selection and reciprocal full-sib
selection allow integration between the two approaches in a more natural way. Population
improvement programs involving testers greatly facilitate identifying potential hybrid combinations
which could be single crosses, three-way, crosses and/or topcrosses depending upon the
tester parent used.
Inbreeding process is often used to develop inbred lines. It is erroneous to think solely
of this objective alone. During inbreeding process one can in fact perform several other
important activities including improvement of source populations, development of narrow genetic
base synthetics for further inbreeding work, and/or to serve as non-inbred tester or as a parent
of a non-conventional hybrid. Combined with GCA evaluation, the inbred line program provides
enormous opportunities for improving source germplasm and to develop hybrid-oriented source F2
populations for further inbreeding work.
At CIMMYT, we.have used modified full-sib (FS) recurrent selection for a long time. More
recently we have made some modifications in this scheme to obtain more information for hybridoriented
work. This scheme is currently known as Modified FS1. Modified FS2 was introduced a
few years ago, where FS progenies are generated from S1 x S1 or S2 x S2 crosses. This scheme
permits identification of potentially useful intrapopulation inter-line hybrids. Identification
of early generation lines and combining ability information can be obtained by arranging preplanned
systematic mating among systematic progenies. Two new schemes-Modified HS1 and Modified
HS2 have been recently introduced. Both of these schemes involve use of a tester which is a bulk
of selected S1 lines in the former and S2 selected lines bulk for the latter. The fifth scheme
involves two heterotic populations 21 and 32 currently under improvement using modified reciprocal
recurrent selection.
Introducing inbreeding and combining ability evaluation will make populations more geared
for hybrid work in the long run. Cooperators testing progenies can request seed of early
generation lines for inbreeding purposes and to develop synthetics to meet needs of OPVs.
Regular topcross trials as part of the hybrid research program will perform essentially the same
role.
d. Hybrid developnent and performance
Hybrids are placed under two broad classes namely conventional and non-conventional
hybrids. Conventional hybrids involve inbred parents. The non-conventional hybrids on the other
hand involve at least one non-inbred parent. The conventional hybrid mostly consists of single,
3-way, double, modified singles, and double modified singles. Similarly, the non-conventional
category includes intervarietal, family, topcross, and double topcross hybrids. Advantages and
disadvantages of each class of hybrids were presented in earlier publications (Vasal and Srinivasan
1991). Rather than presenting voluminous information from such trials, a summary of performance
of different kinds of hybrids are presenteq in Tables 6 and 7. Conventional hybrid trials
data agree with information already known from past experience. The best double crosses showed a
yield superiority of 20-26%, three-ways 26-33% and single crosses 40-53% over the best experimental
variety (EV). The percent superiority over EV was 15-18% for varietal, 20-30% for family
hybrids, 25-30% for double topcrosses and 33-40% for topcross hybrids. It is obvious that in
both types, the two-parent hybrids gave the highest performance. Data on the performance of
modified singles involving one highly inbred line and the other early generation 'FRIP' line is
quite interesting. The 'FRIP' lines refer to early generation lines developed through a forward
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