Biomedical Engineering Reference
In-Depth Information
Table 4
Effects of environmental heat on T
4
and T
3
in plasma and excretion in milk
Thermoneutral conditions
(17.6°C)
Heat stress conditions
(31.2°C)
D change
Plasma T
4
(ng/ml)
79.11 ± 2.11
66.07 ± 3.90
13.04
Plasma T
3
(ng/ml)
1.46 ± 0.20
0.62 ± 0.01
0.84
30.17 ± 1.40
16.77 ± 1.44
13.40
Milk T
4
excretion (m g/day)
Milk T
3
excretion (m g/day)
21.00 ± 0.82
9.00 ± 0.79
12.00
Milk yield (kg/day)
19.28 ± 0.42
11.92 ± 1.00
7.36
Rectal temp. (°C)
38.70 ± 0.03
40.20 ± 0.07
1.5
Source: Magdub et al. (
1982
)
male Friesian calves (Habeeb et al.
2001
) . Plasma
T
3
concentration declined from 151 to 126 ng/dl
in Friesian calves which were exposed to direct
solar radiation in summer (Yousef et al.
1997
) . In
lactating buffaloes, plasma T
3
concentration was
observed to decrease significantly (
P
< 0.01) with
the increase of ambient temperature from 17.5 to
37.1°C (Habeeb et al.
2000
) . T
4
exhibited, in gen-
eral, a similar trend to that of T
3
in buffaloes
(Dwaraknath et al.
1984
) . Thyroxine levels in
buffalo calves were 5.5, 5.3 and 5.6 mg/dl at 7, 8
and 9 months of age, respectively (Yousef
1992
) .
Thyroid hormones' changes in response to heat
stress are probably some of the attempts to reduce
metabolic heat production besides other endo-
crine and metabolic changes induced (El-Nouty
and Hassan
1983
; Abdalla et al.
1989,
1991
;
Johnson et al.
1989
) . Magdub et al. (
1982
)
reported significant reduction in concentrations
of triiodothyronine (T
3
) and thyroxine (T
4
) in
plasma and in milk of lactating cows during heat
stress (Table
4
).
Blood T
4
and T
3
levels have been negatively
correlated with RH, THI, rectal temperature and
respiration rate in Friesian heifers during hot-
humid and dry winter season (Perez et al.
1997
) .
In the hot-humid season, Perez and Fernandez
(
1988
) classify two groups of animals, with
signi fi cant differences in T
3
and T
4
levels between
the two groups. One of the two groups showed
serious impairment of all parameters and was
considered non-heat tolerant and the other was
less affected and was considered heat tolerant.
The results of Chaiyabutr et al. (
2008
) showed
that levels of arterial plasma T
4
were lower in
cooled animals than in non-cooled animals at all
stages of lactation, but this was not apparent for
arterial plasma T
3
concentrations. It is probable
that animals in both groups are able to restore
thermal balance without restriction of feed DMI,
resulting in unchanged plasma levels of thyroid
hormones at all stages of lactation.
A higher level of T
4
due to acute heat stress
has previously been observed in lactating cows
(Johnson and Van Jonack
1976
) . The inverse
relationship between milk yield and the plasma
T
4
level of cooled animals could be due to greater
utilisation of plasma T
4
and mammary drain on
iodine during high milk production (Johnson and
Van Jonack
1976
) .
3
Cortisol
The hypothalamic-pituitary-adrenal axis gets
activated in response to heat or climatic stress
that consequently increases plasma glucocorti-
coid concentrations in blood. Adrenal corticoids,
mainly cortisol, elicit physiological adjustments
that enable animals to tolerate stress (Christison
and Johnson
1972
). Blood cortisol levels increase
significantly due to increase in ambient tempe-
rature in cattle and buffaloes to different levels.
Exposure of non-pregnant female buffaloes for
2-3 h to solar radiation at 42.1°C increased
plasma cortisol concentration rapidly for 30 min,
followed by a gradual fall (Zhengkang et al.
1994
). The cooled animals have low plasma
cortisol concentration than that of non-cooled
animals (Aggarwal
2004
; Aggarwal and Singh
2010
). The elevated plasma cortisol concen-
trations of non-cooled animals may reflect
the stress due to high temperatures (Chaiyabutr
et al.
2008
) .
