Principles of Plant Genetics and Breeding

420 CHAPTER 23
variety of data in addition to yield. Farmer-managed
trials should be designed to permit farmers to utilize
existing equipment and machinery already at their
farms. Also, data collection, if it is to be done by the
farmer, should be easy to collect and as minimal as
possible. Some breeders sometimes request a farmer
to make available land for the trial. Planting, management,
and data collection are then done by the
researcher.
5 Number of seasons/years to conduct the trial. For
effective evaluation of genotype × location (G × E)
interaction, at least 2 years of testing (more for
repeatability) are needed for annual crops.
6 How to analyze and interpret the analysis to
draw valid conclusions. The breeder may use more
efficient designs based on incomplete blocks. There
may be unbalanced data (e.g., missing plots). Analyzing
data over seasons, years, and locations is complex.
The breeder should have the software for these and
other analyses (e.g., stability analysis) and be familiar
with the statistical methods (or at least have assistance
to correctly analyze and interpret the results).
Before describing the steps involved in conducting
field trials, some key concepts that are critical to the
design and analysis of such experiments will be discussed.
Role of the environment in field trials
The terms site and location are used interchangeably to
indicate spatial variation. The term environment is used
to represent the conditions under which plants grow
and includes locations, years, and management practice
adopted. A location/year constitutes one environment.
The nature and effect of the environment has implications
in the design and conduct of field trials. Test
environments may be artificial (e.g., different levels of
fertilizer) or natural (e.g., seasons, location) or both.
Types of environmental variables
The environmental variables that plant breeders face
during genotype evaluation may be divided into two
general categories – predictable and unpredictable factors.
1 Predictable factors. Predictable environmental factors
are those that occur in a systematic fashion or can
be controlled and manipulated by the breeder. These
include natural variables such as soil type (e.g., clay,
sandy, organic), which are immutable over a short
range of time, and breeder-imposed variations (e.g.,
planting dates, intra- and inter-row spacing, rates
of fertilizer or irrigation application). Breeders may
design studies to evaluate each of the imposed factors
separately or several simultaneously. Variations in the
soil are managed through the way plots are oriented,
their shape, and sizes, as well as how the breederimposed
variations (called treatments) are allocated
to plots in the field.
2 Unpredictable factors. Unpredictable factors of the
environment are those that vary erratically over short
or long periods of time. The local weather (the shortterm
meteorological characteristics of a place) is more
fickle and relatively unpredictable than climate (the
long-term patterns in meteorological characteristics
of a region). As previously indicated, climate is the
basis of crop adaptation; weather is the basis for crop
production. Key unpredictable environmental factors
of interest in genotype evaluation include rainfall,
temperature, and relative humidity. To evaluate the
effect of these factors, breeders test their materials at
different locations (genotype × location) or in different
years (genotype × years) or a combination of
these factors (genotype × location × years).
Scale
Another way in which environments are categorized is
according to scale:
1 Microenvironment. This is the immediate environment
often pertaining to the organism (plant). This
includes soil and meteorological factors (e.g., light,
moisture, temperature), and biotic factors in a limited
space, intimately associated with the organism.
2 Macroenvironment. This refers to the abiotic and
biotic factors on a larger scale (location, region) at a
particular period of time.
Genotype × environment (G × E) interactions
Genotype × environment (G × E) interaction is said
to occur when two or more genotypes are compared
across different environments and their relative performance
(responses to the environment) are found to
differ. That is, one cultivar may have the highest performance
in one environment but perform poorly in
others. Another way of stating this is that, over different
environments, the relative performance of genotypes is
inconsistent. G × E is a differential genotypic expression
across multiple environments. The effect of this interaction
is that the association between phenotype and