Principles of Plant Genetics and Breeding

430 CHAPTER 23
Advantages
1 Reduces the effects of soil heterogeneity.
2 No precise arrangements are needed, hence conventional
plot planters can be used.
3 Suitable also for selecting on the basis of plant traits
with low heritability.
4 Easier and more effective to compare plants within a
small group than in a large group.
5 Selection intensity can be varied by selecting more
than one plant per block.
Honeycomb (hexagonal) design
Proposed by A. Fasoulas, a key feature of this design is a
planting arrangement in which each plant is equidistant
from others in a hexagonal pattern. Furthermore, the
spacing is determined to remove interplant competition.
Plants are selected only if they are superior to all in
their hexagonal units. The selection intensity can be
varied by widening the hexagon. When a plant in the
immediate hexagon outyields its surrounding six plants,
it is selected by 14.3% selection intensity. If it outyields
the 18 plants within the second concentric hexagon,
it is selected by 5.3% selection intensity, and so on.
When using this method in breeding, the unit of selection
at all stages in the breeding program are single
plants, not plots.
Disadvantages
1 Conventional equipment can not be used for planting.
2 It is more complex to conduct.
Evaluating multiple plants
Unreplicated tests
When conducted, unreplicated tests often entail planting
single rows of genotypes with check cultivars strategically
located for easy comparison. A breeder may
use this design to evaluate a large number of genotypes
quickly, in order to eliminate inferior ones prior to more
comprehensive field trials.
Advantages
1 They save space.
2 They are less expensive to conduct.
3 Large number of genotypes can be quickly evaluated.
Disadvantages
1 Such tests are susceptible to the effects of field heterogeneity.
There is no other plot of the genotype in the
test for confirming performance. Poor soil may mask
the genotypic potential of a superior genotype by
causing it to perform poorly.
2 Experimental error estimation is problematic.
Replicated tests
Replication in field plot technique entails the representation
of a particular entry (genotype) multiple times
(usually 2–4) in a test. With multiples of each entry,
the important and critical design consideration is how
to arrange duplicates of genotypes in the field. The
statistical concept of randomization requires treatment
allocation to be by chance, such that each genotype
has an equal chance of being allocated to each available
plot. Even though randomization may not be imposed
on certain occasions for practical reasons, plant breeders
normally use randomization in advanced trials.
Different types of experimental design are used to
conduct replicated trials. Designs impose varying degrees
of restriction on randomization. A major consideration
in plant breeding research is the number of entries to
include in an evaluation. As stated elsewhere, plant
breeding is a numbers game. The numbers are larger in
the early part of the program. Three categories of experimental
designs are used in plant breeding.
1 Complete block designs. These designs are suited to
evaluating a small number of entries. Each block contains
at least one complete set of entries (genotypes).
That is, the number of replications and the number of
blocks are the same.
2 Incomplete block designs. These designs are suited
to evaluating a very large number of entries. Under
such conditions, complete blocking is impractical
because of the large numbers. Instead, each block
contains only part of the complete set of entries
being evaluated in the study. Hence, the number of
replications and the number of blocks are not the
same.
3 Partially balanced designs. These designs are generally
complex to use. Some pairs of treatments occur
in the same block an equal number of times and
hence comparisons among treatments are not equally
precise.
Complete block designs: completely randomized
design (CRD)
This design assumes that the entire experimental area
is homogeneous, hence there is no need for local
control.