Agriculture Reference
In-Depth Information
tubers, which are bulky, and usually require refrigerated
storage.
These examples are, however, perhaps exceptions and
it should be noted that the majority of breeding pro-
grammes are concerned with increasing yield potential
within an already well established growing region.
Crop profitability is based on net profit rather than
gross product. By reducing input costs and at the same
time maintaining high yield per unit area, breeders can
increase crop profits. Input costs in crop production
include herbicides, insecticides and fungicides. There-
fore developing cultivars, that are more competitive
with weeds, resistant to damage by insect pest, or have
disease resistance, reduce inputs and alleviates the need
to purchase and apply chemicals. Other inputs will
include nutrients (mainly nitrogen) and water (which
can have a high price in irrigated farming regions).
It follows, of course, that developing cultivars that
require less nitrogen or are more tolerant to drought and
other stress factors will result in greater profitability to
growers.
This strategy has been adopted in breeding objectives
of many crops such as wheat, barley, oat, sunflower,
several legumes, along with fruit trees like apple,
orange, peach and cherry.
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Reproductive performance is suppressed in favour
of a vegetative product. This has been applied to
a number of vegetatively reproduced crops where
sexual reproduction has been selected against in
plant breeding programmes in favour of vegetative
growth (e.g. potato, sugarcane, sugarbeet and various
vegetables).
Vegetative production to different vegetative parts can
be used to increase yield of root and vegetable crops
like potato, rudabaga (swede) and carrot. In this case,
the breeder's task is to maximize the partition towards
one type of vegetative yield (e.g. tubers in the case of
potato) while maintaining the minimum biomass of
unused plant parts (e.g. the haulm of potato).
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Selection for yield increase
It is perhaps ironic that harvestable yield is arguably the
most important factor in all plant breeding schemes and
yet it is possibly the most difficult to select for. Increas-
ing yield is complex and involves multiple modifications
to the plants' morphology, physiology and biochem-
istry. Yield is, not surprisingly, quantitatively inherited,
and highly modifiable by a wide range of environmen-
tal factors. Evaluating accurately the genotypic response
to differing environments and genotype-environment
interactions are the major limiting factors to maxi-
mizing selection response in plant breeding. Despite
advances in molecular marker selection (mainly quanti-
tative trait loci), increased yield is achieved by evaluating
the phenotype of breeding lines under a wide range of
rather atypical environments.
Yield potential will be one of few characters that is
evaluated (or at least considered) at all stages of plant
breeding programmes. Plant breeding schemes begin
with many (often many thousands) genotypes on which
selection is carried out over years and seasons until
the 'best' cultivar is identified, stabilized and increased.
Usually the size of plots used for field evaluation trials
increases with increasing rounds of selection. On com-
pletion of the selection process, surviving breeding lines
must have produced phenotypically high yield in small
unreplicated plots (often a single plant), and a variety
Increasing harvestable yield
Yield, in the eyes of breeders, is considered to have
two main components:
biomass
, the ability to pro-
duce and maintain an adequate quantity of vegetative
material, and
partition
, the capacity to divert biomass
to the desired product (seeds, fruits, or tubers etc.).
Therefore, the partitioning of assimilate is very impor-
tant in obtaining maximum yielding ability. Partition,
in general, takes the form of enhancement of yield
of desired parts of the plant product at the expense
of unwanted plant parts (sometimes referred to as
increasing the harvest index). This can take three main
forms:
Vegetative growth is reduced to a minimum com-
pared to reproductive growth. In many crop species
plant breeders have selected plants that have short
stature, or indeed are dwarf mutants. This was mainly
driven by the need to reduce lodging (plants falling
over before harvest) under increasing levels of applied
nitrogen. However, short plant stature also allows
more convenient harvest and sometimes (e.g. in
fruit trees) allows for mechanical harvest and hence
avoiding more expensive harvest by hand-picking.
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