Biomedical Engineering Reference
In-Depth Information
are larger, and genetic differences in heat tolerance
between animals appear to be exacerbated under
high temperature conditions. Preimplantation
embryos from
B. indicus
cattle are better able to
withstand thermal stress as compared to embryos
from
B. taurus
cattle. Thus, identifying genetic
causes of differences between animals in their
response to the environment has potential for
improving productivity of animals in adverse
environments such as heat stress. The ability to
use powerful new tools in genomics, proteomics
and metabolomics to evaluate genetic differences
between animals in their response to thermal
stress will yield important new information in the
next quarter-century and will permit the selection
of cattle for resistance to thermal stress.
There is genetic variation in heat loss via tis-
sue conductance, non-evaporative heat loss and
evaporative heat loss, but more efficient heat loss
occurred for Brahman and Brahman cross cattle
than with Shorthorn cattle (Finch
1985
) . Using
Brahman, Friesian and Brahman × Friesian F
1
cross heifers, the Brahman × Friesian crosses had
superior gains at 38°C but gains for Friesians were
greater at 17°C (Colditz and Kellaway
1972
) .
Brahmans gained more slowly at 38°C thereby
indicating to be benefits from hybrid vigour under
heat stress conditions. Hair colour influences the
susceptibility of the cow to heat stress because coat
colour is related to the amount of heat absorbed
from solar radiation. In
B. Indicus
cattle, the
inward flow of heat at the skin of black steers
was 16% greater than for brown steers and 58%
greater than for white steers (Finch
1986
) .
B. Taurus
cattle with dark coats exhibited greater
heat transfer to the skin, higher body tempera-
ture and sharply reduced weight gains than those
with white coats, with increasing woolliness of
the coat accentuating the effect (Finch
1986
) .
When dairy cows from an Arizona herd were
categorised into white (less than 40% black),
mixed (40-60% black) or greater than 60%
black, no production traits were different (per-
haps because cows were cooled for the first 130
days of lactation), but white cows calving in
February and March required fewer services per
conception and had fewer open days than mixed
and black cows (King et al.
1988
) . Heritability
of coat colour was 0.22. In a study using cows
characterised as greater than 70% white or
greater than 70% black, white cows had slightly
lower body temperatures and greater milk yield,
regardless of whether they were in shade or no
shade conditions (Hansen
1990
) .
Because genetic variation exists for traits
important to thermoregulation in livestock spe-
cies, the potential to select sires that can transmit
important traits must be considered. However,
when bulls were evaluated for genotype environ-
ment interactions using daughters in California,
New York and Wisconsin, there was no sire
region interaction for milk or fat yield (Carabaño
et al.
1990
). Because the genetic correlation
between production and heat tolerance was
approximately −0.3, the continued selection for
production by ignoring heat tolerance results in
decrease in heat tolerance. The correlation being
small, a combined selection for production and
heat tolerance is likely possible. Functional
genomics establishes a verifiable link between
gene expression and phenotype. Gene expression
arrays in particular allow global analysis of gene
expression responses to environmental change.
Stress is defined as an external event or condition
that produces a 'strain' in a biological system
(Lee
1965
). When the stress is environmental, the
strain is measured as a change in body tempera-
ture, metabolic rate, productivity or heat conser-
vation and/or dissipation mechanisms. At the
cellular level, acute environmental change initi-
ates the 'heat-shock' or cellular stress response.
Changes in gene expression associated with a
reaction to an environmental stressor involve
acute responses at the cellular level (in most if
not all cells) as well as changes in gene expres-
sion across a variety of organs and tissues which
associated with the acclimation response.
Early work by Guerriero and Raynes (
1990
)
demonstrated elevated heat-shock proteins in
response to thermal stress in bovine blood leu-
cocytes. Moderate heat shock (41°C) causes
increased heat-shock protein synthesis, decreased
protein synthesis, mitochondrial swelling and
movement of organelles away from the plasma
membrane associated with cytoskeletal reorgani-
sation in the early bovine embryo (Edwards and
