Agriculture Reference
In-Depth Information
continual check of what advances selection is making
in producing more desirable lines.
Bar charts or histograms of data can be helpful in
understanding the variation and distribution of data
for individual traits. Inspection of distributions along
with trait means and variances can help to determine
possible culling levels (i.e. target values which must be
met to be retained).
After each character has been analysed individually,
it is very important to consider the relationships which
exist between different traits. This can be achieved by
simple correlation analysis.
If two characters of interest are positively correlated
then there may not be any difficulty in selection (except
that there will tend to be greater emphasis in either
independent culling or index selection with positively
correlated traits). However, if the expression of two
characters is negatively correlated it may be impossible
to select for high expression in both traits simultane-
ously. Lower culling levels or index selection will be
necessary to identify lines which may be intermediate
performance for both characters, as the more desirable
recombinants are not present in the sample of materials
evaluated.
Correlation analysis can be easily carried out using
a variety of different computer software packages. The
use of computers in all aspects of plant breeding will
be discussed later. It is sufficient to say at this time that
selection is one area where statistical analysis is helpful
in understanding the vast data sets which are likely to
arise and can also act as useful tools to select the better
lines based on the data collected.
previously been assessed for all (or the majority of ) char-
acters of interest to the breeding objectives. At this stage
there are relatively few (under 100, and no more usu-
ally than between 10 and 50 lines) selections that have
survived the previous selection stages. Selections would
therefore be expected to have shown some value for
yield, quality, disease resistance, pest resistance and be
relatively free from obvious defects.
At this stage it is also assumed that there are relatively
large amounts of planting material which allows evalu-
ation at a number of different locations throughout the
target region.
The major aims of advanced selection are:
To confirm the past performance of selected lines over
a wide range of different environments (locations and
years)
Identify the superior lines based on either specificor
general adaptability
There is usually little response to selection at the
advanced stages because most of the genetic variation
has been reduced; the few remaining genotypes rep-
resent a highly selected (see response to selection in
Figure 7.2) group.
Choice of advanced trial locations
The choice of land suitable for trials is discussed in the
field plot techniques section. It is sufficient to emphasise
some points and state a few additional factors here.
Locations for advanced testing must be representative
of the environments in which the potential new cultivars
will be grown. If the target region has several diverse
locations (i.e. large differences in rainfall, temperature,
soil type), or if different agronomic practices are applied
in different regions (i.e. irrigation versus rain fed) then
attempts should be made to ensure that at lease one
location is chosen to represent each environment type.
If trial sites are a long way from the central research
offices then it may be necessary to find good collab-
orators (farmers, county agricultural agents, extension
personnel) who will take care of the research plots.
In some instances trials may be grown in other states
or different countries. In this case it may be necessary
to arrange appropriate phytosanitary inspections of the
previous seed crop or the seed that will be planted.
Selection
Most effective selection will result from most accurate
data collection and highest heritability. This in turn is
related to good experimentation.
A good understanding of the relationship between
different characters and genetic variation within char-
acters can be of tremendous help in deciding whether
to apply independent culling (along with the cull levels)
or whether a selection index is more appropriate.
Advanced selection
At the advanced selection stage it is assumed that
all remaining genotypes in the selection scheme have
 
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