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

use as OPVs and broad based synthetic as sources of new improved source germplasm for further
improvement. Inbreds from the same heterotic group could be put to other uses in the recycling
process to develop new superior lines. We have made use of good performing lines to develop four
inbred-based populations designated as IBP-1 (late white dent), and IBP-2 (late white flint),
IBP-3 (late yellow dent), and IBP-4 (late yellow flint). These four populations will serve as
new sources of hybrid-oriented germp1asm. Even though these populations were not made based on
heterotic patterns, their genetic constitutions and grain texture (dent vs. flint) suggest their
use as heterotic populations. The two white populations and two yellow populations can be
combined as heterotic partners within each grain color. Sufficient genetic mixing within each
population has already been achieved. In the near future, these materials will be subjected to
appropriate breeding schemes for further improvement.
6. Special trait germplasm..
Inbreeding exposes traits which are rarely encountered at population or family level in
maize germplasm. This applies to a whole array of traits most sought after by breeders such as
lodging resistance, disease resistance, low ear placement, erect leaves, prolifically, long ears,
ear shape, deep kernels, ear row number, and several others. It is important that whenever such
lines appear during the inbreeding process, one should make every attempt to make the least use
of such lines in an efficient and effective manner. A case in point is a lodging resistant line
(CML-9) which has withstood high velocity winds and storms of moderate nature at best in three
different cycles. Using this and other lines exhibiting the same characteristic, a lodging
resistant maize population has been formed. Every such character does not necessarily have to
end up in a population. Depending upon the simplicity or complexity of inheritance, such
character can be introduced in other lines through perhaps recycling involving a minimal of
effort.
7. Development of inbred pnogenitors.
Inbred line development constitutes an important part of an overall hybrid development
effort. Inbred lines serve as parents of conventional hybrids which could either be two-parent
or multiparent hybrids. Development of good inbred lines is a difficult task. Many maize
breeders in the past have experienced difficulty in developing agronomically desirable lines from
the germplasm that they had been working for varying periods of time. In several instances it
had been difficult to proceed with higher levels of inbreeding thus resorting then to use of
early generation lines. Even today some of the commercial hybrids developed by the public sector
use lines that have only a few generations of inbreeding. It may be pointed out that inbreeding
or selfing alone is not enough to develop good lines. Much will depend on the source germp1asm
and its ability to withstand inbreeding depression. It should be remembered that maize species
in general is moderately tolerant to inbreeding. There are, however, variations among materials.
As a general rule, unimproved materials from the germplasm bank succumb to inbreeding pressure
more readily than improved germplasm. Further, materials that have been improved using schemes
that involve inbreeding are more likely to be tolerant to inbreeding pressure.
Though inbred lines can be developed by several different ways, the pedigree method is
used more commonly by the maize breeders. CIMMYT maize program, in general, uses this method for
developing inbred lines. In this method, the inbred lines are developed by selfing in each
successive generation of inbreeding and a proper record of the pedigree of each line is kept in
each generation. Most CIMMYT inbreds are developed from ongoing population improvement programs,
recycling of advanced and early generation lines, and very rarely, if necessary, inbreeding is
practiced in populations, experimental varieties and synthetic bulks. Some important features in
inbred line development include high population density, keeping records on pollen-shedding and
si1king, and reserving one-half of the row for observation and the other half for inbreeding
purposes. The observation part is harvested and rated for yield, ear aspect, and ear rots. To
reduce volume of material, at times, lines are carried forward as bulk. In other words, ears
selected at harvest time from each line are shelled as a bulk. Resamp1ing of exceptionally good
lines is also occasionally done.
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