Agriculture Reference
In-Depth Information
reused for plot experiments. A second source of soil
heterogeneity is related to unplanted alleys or road-
ways from previous experiments. If possible unplanted
alleys from previous research should be marked and
avoided.
Grading (ground levelling) usually removes soil from
elevated areas and redistributes it to the lower areas. This
operation, which is designed to reduce slopes, results in
uneven depths of top-soil and often exposes unfertile
subsoil. These differences can prevail for many years
and should be avoided unless soil heterogeneity trials
determine that the grading effect is minimal.
Large trees and other structures can cause shade,
which will affect plant performance, and also their roots
spread further than their canopies and so will influence
plant growth. Areas near buildings may be affected by
soil movement and heterogeneity caused by the building
operation. Plots adjacent to trees or wooded areas can
also carry a greater risk of damage by birds or mammals.
The evaluation of soil heterogeneity requires growing
uniformity trials
. These involve growing a single cul-
tivar (or a number of cultivars) in plots with very high
levels of replication. Uniformity trials result in deter-
mining soil fertility gradients and identify particularly
productive or non-productive areas in fields. Unifor-
mity trials can be used to produce contour maps of
productivity. Statistical routines such as serial correla-
tion studies or least mean squares between rows, column
and diagonals can be applied to determine significance
of soil heterogeneity.
Although uniformity trials have their place in field
experimentation they usually have little to offer a plant
breeder. Uniformity trials indicate the response of spe-
cific genotypes to a given area in a given season. When
these trials are repeated with different genotypes or in
different years then a different result is often obtained
(not surprisingly). In plant breeding evaluation trials,
the number and diversity of genotypes under test are
usually far greater than what can be considered in uni-
formity trials. Also it should be noted that often there
is little choice of what land can or cannot be used for
plot trials.
A plant breeding programme usually uses a number
of different locations. One main location may be iden-
tified where the majority of material is evaluated in the
early and intermediate selection stages or where seed
is increased. A number of different locations will be
used (dispersed throughout the region where the new
cultivars will be targeted) where advanced lines are tested
for adaptability. Where many locations are used it is
common to use farmers' fields for test plot evaluation.
Some of the distinct differences between a farmer's field
and the conditions, which would prevail at, say, an
experimental research station, would include:
•
Lack of experimental equipment or lack of small plot
machinery. This can often be easily overcome by tak-
ing planting, spraying and harvest machinery from
the research farm.
•
Lack of experimental facilities such as precise irri-
gation control and pest or disease control, weather
stations etc.
•
Lack of post-harvest storage or assessment facilities.
Therefore harvested produce needs to be transported
to a central testing laboratory for post-harvest quality
assessment.
•
Large variation between farms and fields within
farms. This is often not a major problem as the
majority of trials on these farms are to select for such
adaptability over a range of environments.
The farm sites are usually further away from the base
research laboratory and sometimes long trips are nec-
essary to visit the plots. Therefore visits are usually
limited and it can be difficult to identify potential
problems as they arise and hence avoid their worst
effects.
•
Despite all the potential difficulties with off-station
or farm trials it is possible to achieve very good results.
Best results are usually obtained when the 'better' farm-
ers are chosen for the tests and when these farmers
are specifically interested in the results from the trials.
Finally, when trials are to be carried out on farmers' land
it is always advisable to keep the experiments simple and
to have relatively large plot units, and this make them
'more' robust.
Plot size and replication
It is always assumed that larger plots are more efficient
and more representative than small plots in yield and
other assessment trials. Similarly there is no doubt that
greater replication levels are always more desirable than
fewer replicates. The difficulty of organizing efficient
field trials is often related to some compromise in plot
