Biomedical Engineering Reference
In-Depth Information
during the life of the second corpus luteum after
calving were lower during summer than the win-
ter and that THI during the first 14 days after
calving was negatively correlated with progester-
one production. Younas et al. (
1993
) measured
plasma progesterone concentrations in cooled
and non-cooled cows during summer and found
that plasma progesterone concentrations were
lower in non-cooled cows compared to cooled
cows. Ronchi et al. (
2001
) also reported that
plasma progesterone concentrations were lower
in heat-stressed Holstein heifers as compared to
thermoneutral Holstein heifers. In contrast, expo-
sure of heifers to heat stress for two successive
cycles has been reported to result in increased
plasma progesterone concentrations on day 2-19
of the first cycle and on day 2-8 of the second
cycle (Abilay et al.
1975a
) , and Trout et al. (
1998
)
found higher progesterone concentrations in
heat-stressed cows until day 19 of oestrus cycle.
Wise et al. (
1988a
) found that plasma proges-
terone in non-cooled cows tended to be higher
on day 3-5 of the oestrus cycle when compared
with cooled cows. However, plasma progester-
one concentrations depend on its rate of pro-
duction by the corpus luteum, possible adrenal
release of progesterone, the degree of haemo-
dilution and haemoconcentration, metabolic
clearance rate, hepatic blood flow and feed intake
(Vasconcelos et al.
2003
) . An increase in hepatic
blood flow occurs following feed intake
(Sangsritavong et al.
2002
) and more than 90%
of progesterone in hepatic portal blood is meta-
bolised during the first pass through the liver
(Parr et al.
1993
) . Insuf fi cient progesterone
secretion by the corpus luteum is a possible
cause of low fertility of cows during summer.
Studies also report higher progesterone concen-
trations in summer or lower or similar to that in
winter (Wolfenson et al.
1997
) . These variations
among the findings on plasma progesterone
concentrations have been attributed to several
factors associated with low luteal cell blood
perfusion, progesterone metabolism in the liver,
blood volume changes, adrenal release of proges-
terone, level or degrees of hyperthermia, extent
and duration of heat exposure and differences in
physiology of the animal, nutritional level and
stage of lactation of a cow and other factors that
affect plasma progesterone (Wolfenson et al.
2000
; De Rensis and Scaramuzzi
2003
) .
However, heat stress may also directly alter
progesterone production by the corpus luteum
(Wolfenson et al.
1997
) . Chronic heat stress
possibly impairs follicle and corpus luteum,
and luteinised theca cells are more susceptible
to heat stress than luteinised granulosa cells
(Wolfenson et al.
2002
) . Plasma progesterone
levels are significantly higher in winter than in
summer (Wolfenson et al.
2002
) . Low proges-
terone prior to AI is related to enhanced uterine
PGF
2a
secretion, to alterations in the growth
pattern of ovarian follicles and to their steroi-
dogenic capacity (Mann and Lamming
2001
;
Santos et al.
2004
) .
Low progesterone concentrations in the cir-
culation of cows have been associated with
compromised reproductive function and reduced
pregnancy rates (Butler et al.
1996
; Lamming
et al.
1989
; Mann et al.
1995,
2001
) . Wolfenson
et al. (
2002
) analysed progesterone production
in vitro by theca and granulosa cells obtained
from cows in cool and hot seasons as well as
progesterone concentrations in general circula-
tion. Under chronic summer stress conditions,
progesterone production was markedly low and
heat stress-induced damage to follicular func-
tions may be carried over to the subsequently
formed corpus luteum. Low plasma progester-
one concentrations during the luteal phase of
the pre-conception oestrus cycle can compro-
mise follicular development leading to abnor-
mal oocyte maturation and early embryonic
death (Ahmad et al.
1995
). At the time of con-
ception, low progesterone concentrations may
lead to the failure of implantation (Mann et al.
1999
; Lamming and Royal
2001
) . The in fl uence
on conception occurs most probably due to the
need for synchronous development of the
embryo and corpus luteum as the delayed or
advanced development of the corpus luteum
may lead to implantation failure (Lamming and
Royal
2001
). The pattern of the postovulatory
progesterone may alter fertility (Darwash
et al.
1999
). However, the use of exogenous
progesterone post-insemination to supplement
