Agriculture Reference
In-Depth Information
breeding programme is the key factor in determining
the success of the scheme, whether it is one based solely
on traditional techniques or it incorporates molecular
based technology.
It is common to evaluate breeding lines ( test entries )
in comparison with existing cultivars ( controls or
checks ) within the same trial. In some cases a single
cultivar is used, but more often several cultivars are
included in the evaluation trials. The choice and num-
ber of control entries is largely dependant on the range
and number of cultivars that are presently grown in the
target region for the new cultivars, the type of trial and
the number of evaluations that are to be made. For
example, an evaluation trial may contain the highest
yielding cultivar available to compare yielding perfor-
mance, the best quality cultivar to provide a baseline
for quality, a cultivar with disease resistance to evaluate
response under disease pressure, and so on. It is always
desirable to include as many control entries as possible
within reason for the extra effort involved. It should
be noted, however, that evaluation trials can be costly
and that the cost of an evaluation trial is often directly
related to the number of total entries that are included.
If several thousand breeding lines are to be evaluated
then it may be unwise to include only a few control
entries. If, however, only a few lines were to be consid-
ered then it would be unwise to include many hundreds
of control plots. A simple rule of thumb which is often
useful is that the number of control plots (not always
entries) should be about 1/10th of the total number of
trial entries if between 1 and 200 breeding lines are to
be tested, and up to 1/20th of the total number in the
trial if more than 200 lines are under evaluation.
A wide spectrum of possible designs is available but
only a limited number will be detailed here, namely:
replicate designs are used. These include:
Non-randomized designs without control entries,
where genotypes that are to be evaluated are arranged
in plots in a systematic order (e.g. numerical order,
alphabetical order etc.) (Figure 9.1 (a)). Only test
genotypes are evaluated and there are no control
(check) cultivars grown at the same time. It is not
possible to obtain any estimate of error from this type
of design or make any direct comparison with known
cultivars.
Non-randomized designs with control entries, where
the test entries are arranged in plots in a systematic
order (as above), but control cultivars are inter-spaced
amongst the test entry plots (Figure 9.1 (b)). The
control cultivars can be arranged in a systematic order
(e.g. every 20 plots), or they can be allocated to plot
positions at random. In most cases several control
cultivars are included. Each control entry may also
be replicated more than once in the whole design.
Multiple entry plots of control cultivars can often be
useful to determine an estimated error variance for
the overall field trial.
Randomized designs without control entries, where
the test entries are arranged within the trial at random
but no control entries are included (Figure 9.2 (a)).
(a)
No randomization, without controls
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(b)
No randomization, with controls
Unreplicated designs
Randomized complete block designs
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Factorial designs
Split-plot designs
Unreplicated designs
Unreplicated designs, as their name suggests, are exper-
imental designs where test entries are not replicated
and so appear only once. There are, however, several
(or indeed many) different options even when single
Figure 9.1 Non-randomized single replicate plot designs
without control entries (top) and with control entries
arranged systematically throughout (bottom).
 
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