Biomedical Engineering Reference
In-Depth Information
The acute phase in the regulatory inhibition of
milk secretion in cows subjected to heat stress is
related to upregulation of the local PA-PG-PL-
b-CN f (1-28) peptide in milk and that this
peptide in turn downregulates the activity of K
+
channels on apical membranes-derived vesicles.
The putative apical K
+
channels belong to the
family of voltage-gated channels and that b -CN f
(1-28) causes membrane depolarisation, explain-
ing its milk downregulatory effect (Silanikove
et al.
2009
). To understand the exact mechanism
on milk synthesis, further research is needed to
determine the nature of the interaction of b -CN f
(1-28) with regulatory elements in the apical
membrane of mammary gland epithelial cells and
to identify these channels and the components of
the inward signal transduction. Decrease in Na
+
concentration, increase in K
+
concentration and
the consequent decrease in the Na/K ratio are
sensitive indicators of the disruption of the tight
junction of the mammary gland epithelial cells,
which relates to differing ion contents in milk
and blood plasma (Stelwagen et al.
1998
) .
environmental chambers with constant temperatures
of 10 or 26.7°C, Holstein heifers raised in the
26.7°C environment were lighter than heifers in
the cool environment by 8.2 kg at 3 months and
30.4 kg at 11 months of age. It took Holsteins in
the warm environment 11/2 months longer to
reach 299-kg body weight (Johnson and Ragsdale
1959
). Although the temperature was constant
with no diurnal variation, 26.7°C is not extremely
hot. In Australia, Friesians, Brahman × Friesian
F
1
crosses and Brahmans were exposed to 17.2
and 37.8°C temperatures (Colditz and Kellaway
1972
). Comparing the hot versus the cool
temperature environments, rectal temperature
and respiration rates increased more for Friesians
as compared to Brahmans. Intake declined about
17% for Friesians, 1.4% for F
1
crosses and 12%
for Brahmans, but initial intake was greater for
Friesians, and thus a greater decline would be
expected. Gains for Friesians were greatest during
cool temperatures but were the least of the three
groups when exposed to high temperatures.
Because heifers generate less body heat and
can dissipate heat more readily than lactating
cows, do heifers require additional cooling? In
Egypt, heifers were exposed to winter conditions
(17.3°C, 54.5% RH), summer conditions (36°C,
47% RH) and summer conditions with water
spraying and an oral diaphoretic (Marai et al.
1995
). A diaphoretic (e.g. ammonium acetate) is
a compound fed orally to cattle to increase per-
spiration. Heifers were sprayed with water seven
times daily during the hottest period of the day.
Heifers that received cooling had lower rectal
temperature and respiratory rate, and gain was
improved by 26.1% with cooling during summer,
even though heifers were only sprayed during
the hottest part of the day without the benefit
of fans.
10
Heat Stress Effects on Heifers
Heifers have been observed to generate far less
metabolic heat than cows and have greater surface
area relative to internal body mass and, therefore,
are expected to suffer less from heat stress.
However, it has been observed that Holstein
females raised at latitudes less than 34 °N weighed
6-10% less at birth and average approximately
16% lower body weight at maturity than those in
more northern latitudes, even when sired by the
same bulls (NRC
1981
). Thus, there are several
factors contributing to slower growth and smaller
body size, including greater maintenance require-
ments during hot weather, poor appetite and
lower quality forages that are influenced by the
same environmental conditions that lead to slow
growth of cattle.
During hot weather, reduced feed intake is
common, but increased maintenance costs reduce
efficiency of feed conversion. In a study from the
1950s where Holsteins, Brown Swiss and Jersey
heifers were raised from 1 to 13 months of age in
11
Genetic Factors Regulating
Response to Heat Stress
Development of genomics tools has permitted a
much better evaluation of the genotype × environ-
ment interactions (G×E). Estimation of G×E
effects in dairy cattle has indicated that these effects
