Agriculture Reference
In-Depth Information
size and replication which will allow large numbers of
test lines to be evaluated at low cost and in as small an
area of land as may be available.
Land availability may not be the limiting factor in
determining plot size or replication level. It would be
pointless to organize more field plots than could be
effectively managed by the staff available. Similarly, data
needs to be collected from effective field trials and if too
many unit plots are grown than it may not be possible
to effectively evaluate either plants or the produce from
the trials. Finally, some crop species produce products
that are bulky or perishable. It may be necessary to store
the produce (or at least a sample of produce) from each
plot and the storage space available would then be a
major determining factor.
In the early selection stages the amount of planting
material available is often limited and this puts practical
constraints on the field trialling that is possible. For
example, if only2gofseed are available for evaluations,
and commercial seeding rates are 4 kg per acre then only
small plots with limited replication will be possible.
Increasing replication will always be more efficient
than increasing plot size. Therefore if 200 plants were
to be grown for evaluation purposes, then the most sta-
tistically efficient design would involve 200 replicates of
randomized single plants. From a practical standpoint
this may not, however, be the most effective or practical
or provide the most representative outcome. For exam-
ple, there may not be the necessary machinery available
that would allow for mechanized planting of completely
randomized single plants. Therefore the dimensions of
machinery available can be a determining factor when
setting plot dimensions. If the only plot seeder avail-
able plants six rows, then all plots are likely to be a
factor of six rows wide. Similarly if a small combine
harvester is available that has a cut of 1.5 metres then
plots are likely to match this harvesting capability. In
addition, single plant evaluation may take greater land
areas that would not be available. Finally, single plants,
if completely randomized, need to be spaced distinctly
apart to differentiate one from another. The phenotypic
performance of some crop species is markedly different
when grown at wide spacing (wider than would be nor-
mal for commercial production) than if grown at narrow
spacing.
A breeding plot can consist of a single plant, a single
row or multiple rows. The plot dimensions are often
determined by the availability of planting material.
Different plots in field trials invariably contain dif-
ferent genotypes. The performance of these genotypes
can, in some cases, be affected by competition from
the adjacent plots. For example if a short genotype
is grown next to a tall vigorous genotype then the
performance of the short type may be reduced com-
pared to a single stand of the short stature plants.
To a large extent these effects can be reduced by
good experimental design and replication where the
probability that adverse or advantageous competition
occurring in all replicates is reduced with increasing
replication.
Some researchers suggest growing larger plots and
harvesting or evaluating only the centre rows (i.e. that
portion that is completely surrounded by plants of like
type). It should be noted, however, that this would
require greater amounts of planting material and larger
land areas. It should also be noted that genotypes can
suffer as much (or greater) competition by being grown
by itself and
ripple effects
can occur. To examine ripple
effects consider a five row plot (rows A, B, C, D and E)
where row A is grown adjacent to a different tall and
very competitive genotype. In this case then the A row
may contain small stunted plants due to the compe-
tition from the tall genotype and hence will result in
lower yield. Row B, however, is likely to be affected
by competition because although grown next to a like
genotype, the like genotype (A row) is stunted and low
yielding. Therefore row B will be taller and more pro-
ductive due to the lack of competition from row A. In
a similar manner, row C will have to compete with the
larger more competitive B row plants and have reduced
yield. The competition effects will be reduced, however,
with increased distance from the tall different genotype
and hence the name ripple effect.
It should also be remembered that by harvesting only
a portion of the total plot the error variance will be
increased as the error variance of the mean (average of
all plants in the plot) is the error variance of a single
plant divided by the number of plants.
It should be remembered that the value of field plot
trials is to make comparisons and not to estimate defini-
tive yield performance. Therefore field trials are used to
compare the
relative performance
of different test lines
in comparison to control entries. In this case increased
or decreased yield as a result of competition will only
become a factor if there is interaction between edge
effects and genotypes.
