Biomedical Engineering Reference
In-Depth Information
Table 5
Progesterone concentrations in peripheral plasma of dairy cows exposed to various types of heat stress
Type of heat exposure
Response
References
Short term, acute
(hot chamber)
Increased
Wilson et al. (
1998b
) , Trout et al. (
1998
), and Gwazdauskas et al. (
1981
)
No change
Short term, acute
(solar radiation)
No change
Roman-Ponce et al. (
1981
) and Roth et al. (
2000
)
Increased
Long term, chronic
(summer heat stress)
Decreased
Howell et al. (
1994
) , Jonsson et al. (
1997
) , Wolfenson et al. (
1988,
2002
) ,
Younas et al. (
1993
) , and Wise et al. (
1988a
)
No change
compared to the cows under cooling. Conflicting
results have been reported regarding the preovu-
latory LH surge in heat-stressed cows. Madan
and Johnson (
1973
) reported a reduction in the
endogenous LH surge caused by heat stress in
heifers, and some authors reported that it was
unchanged in cows (Gwazdauskas et al.
1981
;
Rosenberg et al.
1982
; Gauthier
1986
) . Gilad
et al. (
1993
) have suggested that these differences
are related to preovulatory oestradiol levels
because heat stress had no effect on tonic LH
secretion or GnRH-induced LH release in cows
with high concentrations of plasma oestradiol
and heat stress depressed LH concentrations in
cows with low concentrations of plasma oestra-
diol. Because most studies report that LH levels
are decreased by heat stress, it can be concluded
that in summer, the dominant follicle develops in
a low LH environment and these results in
reduced oestradiol secretion from the dominant
follicle leading to poor expression of oestrus and
low fertility (De Rensis and Scaramuzzi
2003
) .
Plasma inhibin concentrations in summer are
low in heat-stressed cows (Wolfenson et al.
1993
)
and in cyclic buffaloes (Palta et al.
1997
) , per-
haps reflecting reduced folliculogenesis since a
significant proportion of plasma inhibin comes
from small and medium size follicles.
Gilad et al. (
1993
) reported low concentra-
tions of FSH in acute and chronic heat-stressed
cows which also had lower concentrations of
oestradiol while no alterations in concentrations
of FSH were observed in cows which had normal
concentrations of oestradiol. Conversely, Ronchi
et al. (
2001
) reported no differences in frequency,
amplitude of FSH pulses and baseline concentra-
tions of FSH between cows exposed and unex-
posed to high ambient temperatures. However,
Roth et al. (
2000
) observed high plasma
concentrations of FSH in heat-stressed cows
than in cooled cows. Increased concentration of
FSH in heat-stressed cows has been attributed to
the concentration of inhibin (Roth et al.
2000
;
De Rensis and Scaramuzzi
2003
) . Increased
concentrations of FSH in heat may be due to
decreased plasma inhibin production by compro-
mised follicles as inhibin is an important factor
in the regulation of FSH secretion. A negative
relationship between plasma FSH and immunore-
active inhibin concentrations has been observed
(Findlay
1993
; Kaneko et al.
1995,
1997
) .
9.2
Progesterone
The studies on the effect of heat stress on plasma
progesterone concentrations report variable
results (Table
5
). Wilson et al. (
1998a,
b
) observed
that heat stress had no effect on plasma proges-
terone levels in lactating cows and dairy heifers
during the second half of the oestrus cycle
exposed to heat in a climatic chamber and luteo-
lysis was delayed. In another study, Roth et al.
(
2000
) observed that plasma progesterone during
the oestrus cycle in cows were almost similar in
heat-stressed cows and cooled cows and did not
differ between groups and during the subsequent
cycle. However, Wolfenson et al. (
1988
) and
Wise et al. (
1988b
) found that plasma proges-
terone concentrations were decreased in heat-
stressed cows. Rosenberg et al. (
1982
) found that
plasma progesterone concentrations measured
during the oestrus cycle before the first insemina-
tion were higher during winter than the summer
in multiparous cows. Jonsson et al. (
1997
) also
reported that plasma progesterone concentrations
