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

Recycling of good lines also can be practiced to develop new lines. Recycling is often
done between lines from popu1ation(s) within the same heterotic group. There are no set guidelines
for the recycling of lines. Much depends also on how a breeder is handling his germplasm
and what kinds of difficulty(ies) he is encountering in developing good lines. From ongoing
population improvement programs involving S1 or S2 as a procedure or as a step in the breeding
process, one can always resort to recycling of early generation lines. Also, for
germp1asm/materia1s known to be notably poor for line development, it would be advisable to reinitiate
recycling of early generations to increase the possibility of developing good lines.
This will be a good strategy at least in the early maturing germp1asm where inbreds with high
degree of inbreeding are much more difficult to.obtain. Information on combining ability of
early generation lines is often obtained from the ongoing population improvement programs.
Systematic com~ining ability tests are often conducted at S4 or past S4 stage. By this time one
will have a fairly good idea as to whether or not the line is usable. Promising lines are also
evaluated in line evaluation trials (LETs). Lines in these trials are characterized more
thoroughly and sometimes one or two replications are planted under high plant density and
artificial disease stress conditions. Data collected in these trials is of great help in
planning crossing program to develop single and three-way crosses.
8. Development of non-inbred progenitors.
Non-inbred progenitors or materials of an OPV can be used in developing non-conventional
maize hybrids. Most breeders in the past have used bulks of populations and varieties as
parent(s) of a non-conventional hybrid. As one would expect, if the parents are variable, the
hybrids would also be veriable. To obtain uniformity and appeal in such hybrids, it is therefore
quite important that such non-inbred progenitors should be created specifically for the purpose
of using parents of a non-conventional hybrid. It will be preferable to use experimental
varieties, elite line synthetics, and narrow genetic base synthetics which have been reconstituted
using only a few relatively uniform families and lines. There are great opportunities for
developing such progenitors from the ongoing intrapopu1ation and interpopulation improvement
programs. Interpopulation improvement programs permit development of cross-performance synthetics
using test cross data to maximize heterosis manifestation in intersynthetic hybrids.
9. Identification of testers.
Testers, inbreds as well as non-inbreds, serve exceedingly useful functions during various
phases of population and hybrid breeding programs. Non-inbred testers for population improvement
programs, evaluation of combining ability, and when used as a parent of a hybrid, should be
developed with some resource effort. These testers could be of narrow genetic base involving a
few lines or families and can be reconstituted using data from population improvement programs,
topcross tests, and from any other data which permits forming such progenitors. Perhaps, it may
also be more appropriate to develop such non-inbred testers using various groups/pairs of
heterotic population undergoing improvement or use in hybrid breeding program.
Identification of inbred testers will be of immense help in any hybrid breeding program.
In fact it is a priority item to identify a set of testers which will serve as a reference point
for meeting various objectives in a hybrid breeding program. Unfortunately, such tester lines
are not freely available, if at all they exist, to maize scientists of the developing countries
for the lowland tropical and subtropical germp1asm. Such tester lines could be identified using
special mating designs such as dia11e1s and design-2. Once such first-set of testers are
identified, they can be continuously replaced by new and superior lines resulting from the same
heterotic group. Though CIMMYT's effort in. hybrid development was initiated with a modest
effort, we made special effort to identify tester lines for different classes of germplasm.
These lines have been successfully used in developing hybrid combinations and to sort out
heterotic patterns of lines. Separation of lines by heterotic pattern(s) facilitates breeder's
job in developing hybrid combinations and to form and/or to broaden the already known heterotic
groups. Inbreds from the same heterotic group could also be used to develop and identify single
cross testers. In the near future, CIMMYT will announce lines that can be used as tester by the
NARS and the private sector breeders.
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