Breeding Schemes - An Introduction to Plant Breeding - page 40

rows at F 4 . Simultaneously, the harvested F 3 bulk is
planted in a preliminary yield trial. The seed from the
bulk yield trial is used to plant a more extensive bulk
yield evaluation trial at the F 5 stage. Based on the results
from the F 4 bulk yield trial, the most productive popula-
tions are identified. Single plant selections are selected
from the corresponding plant progeny rows and the
remaining row is bulk harvested for a further yield trial
the following year at the F 5 generation. This process
is repeated at the F 5 to F 6 stage, by which time near
homozygosity is achieved in the remaining lines.
The advantage of the modified pedigree scheme is
that it attempts to utilize progeny bulk evaluation
for yield and other quantitatively inherited characters,
while single gene traits can be screened for on a sin-
gle plant/single row basis. In addition, this scheme
allows for evaluation of quantitative characters while
simultaneously inbreeding selections.

'cross prediction' techniques (see Chapter 7), it is pos-
sible to reduce the number of crosses screened on a
large scale and hence allowing more effort to be spent
on evaluating larger sized populations of the selected
crosses.

Seed increases for cultivar release

At the other end of the breeding programme, once
desired cultivars have been identified it is necessary to
produce a suitable quantity of seed that will be grown
and increased for varietal release. This seed lot is usu-
ally called Breeders' seed as in most cases producing this
seed is the responsibility of the breeder rather than of a
seedsman. It is vital that breeders produce breeders' seed
lots that are pure, free from variants, and the genotype
that is true-to-type to the cultivar that is to be released.
Breeders' seed is used to plant ' foundation ' seed, which,
in turn, is used to plant the various levels of ' registered '
or ' certified seed ', which is eventually sold to farmers.
Producing high quality breeders' seed is very similar
to the breeding schemes (described above). In general
there are two basic methods of producing breeders' seed
mass bulk increase and progeny test increase.

Number of segregating populations
and selections

There have been numerous debates amongst plant
breeders concerning the question of how many plants or
populations should be evaluated, and selected, at each
stage in a breeding scheme. Unfortunately, there is no
simple recipe to help new breeders, and the questions
can only be addressed from an empirical standpoint.
Plant breeding is a numbers game and greater suc-
cess is often associated with screening many thousands
of breeding lines. However, plant breeding programs
should only be as large as the specific breeding group
can handle. Therefore, it is not productive to assess
more lines at any stage in a breeding scheme than can
be effectively and accurately assessed .
It is often easier to work backwards and ask how many
lines you can handle at say, the advanced yield trial stage
in the breeding scheme, and then move backwards to the
previous stage and predict how many lines are required
at that stage to ensure that the required number are
selected, and so on.
Similarly, the number of initial cross combinations
that should be used, differs markedly in different breed-
ing programs. Often, a large number of crosses need
to be screened, as the breeder cannot identify the most
productive cross combinations. With experience of spe-
cific parents in cross combination and the benefitof

Mass bulk increase
In mass bulk increase schemes a uniform sample of
seed from the selected line is chosen and planted
once to result in the breeders' seed lot. The advan-
tage of this simple method is that it is inexpensive and
takes only a single year to obtain the required seed.
Disadvantages are mainly related to the purity, homozy-
gosity and homogeneity of the cultivar entering into
commercialization.

Progeny test increase
The progeny test increase method is more expensive
and takes longer to obtain the seed required. This
method is very similar to the bulk/pedigree breeding
scheme. A number of single plants are selected from
the homozygous/near homozygous advanced breeding
line. These are grown as plant progeny rows. Indi-
vidual plant rows are discarded according to off types
or non-uniformity. The remaining rows are harvested
individually and the seed from each row is used to
plant larger progeny plots the following season. These
plots are again inspected and those that do not have