Biomedical Engineering Reference
In-Depth Information
observed to significantly influence their preg-
nancy rates. Studies on cows demonstrated that
pregnancy rates of cows and heifers were reduced
from 80 to 55% (after three inseminations) when
the daily maximum temperature increased above
27°C (Orr et al.
1993
). Both high and low tem-
perature affect oestrous expression, conception
rate and calving per cent. Both adapted and non-
adapted breeds experience depression in repro-
ductive traits due to thermal stress (Singh and
Mishra
1980
) .
4
Female Reproduction
Females raised at temperatures between 31 and
33.5°C and 60% of humidity had lower oestra-
diol in the follicular phase of the cycle and small
size of the growing and ovulatory follicles. The
length of the luteal phase in heat-stressed cows
has been observed to be longer than in females
kept in thermoneutral environment. The uterus
secreted less PGF2a because of the reduction in
oestradiol synthesis and/or because high temper-
atures can interfere with the release of PGF2a by
endometrial cells (Malayer et al.
1990
) . The uter-
ine endometrium must be 'primed' by oestradiol
to produce enough prostaglandin and trigger lute-
olysis (Silvia et al.
1991
) . Thermal stress alters
the concentrations of FSH and inhibin (Badinga
et al.
1994
; Roth et al.
2000
; Wolfenson et al.
1997
; Palta et al.
1997
) and corpus luteum func-
tion (Wilson et al.
1998
), as well as decreases the
fluid content of follicles (Badinga et al.
1993
) .
High temperatures reduce the number of granu-
losa cells and aromatase activity and secretion of
androstenedione by theca cells (Wolfenson et al.
1997
) . Ambient conditions in fl uence oocytes and
embryo quality. Oocytes exposed to high tem-
peratures in between the onset of oestrous and
insemination produce less number of viable
embryos (Putney et al.
1989a
) . Environmental
temperature and humidity 2 days before insemi-
nation is critical for conception rates than at any
other phase of the cycle, including the period
from breeding to 2 days after breeding (Ingraham
et al.
1974
). A rise in rectal temperature shifts
blood flow from the visceral organs to the periph-
eral circulation and this could reduce perfusion
of nutrients, and hormones to the early stages of
embryo development (8- to 16-cell stage) are
more susceptible to heat stress, but there is also a
high risk of embryonic loss at days 13 and 14 of
pregnancy (Biggers et al.
1987
; Ryan et al.
1993
) .
Also, conceptuses cultured at high temperatures
have been observed to reduce the secretion of
interferon which impairs implantation and also
maintenance of corpus luteum (Putney et al.
1988
). As a consequence of these effects, high
ambient temperatures on dairy cows have been
4.1
Ovarian Follicle
The low summer fertility of about 60% of the
world dairy cattle population is associated with
high ambient temperatures. However, during the
autumn, when air temperatures decrease and
cows are no longer exposed to thermal stress,
conception rates remain lower than in the winter
(Hansen
1997
) due to susceptibility of ovarian
follicles to heat stress (Badinga et al.
1993
;
Wolfenson et al.
1995
). Normally, it takes about
40-50 days for small antral follicles to develop
into large dominant follicles (Lussier et al.
1987
) .
Heat stress inhibits ovarian follicular development
leading to diminished reproductive efficiency of
domestic animals during summer.
Heat stress affects development of follicle
(Badinga et al.
1993
; Wilson et al.
1998
) . The
duration of dominance of the preovulatory folli-
cle is increased in summer, and in beef heifers,
duration of dominance has been observed to be
negatively correlated with fertility (Mihm et al.
1994
). More than one dominant follicle may
develop, and twinning may occur during summer
(Ryan and Boland
1991
) . Thus, both decrease in
follicular steroid secretion and increase twinning
rate have been observed under heat stress.
Oestradiol concentration in the follicular fluid
and androstenedione production by thecal cells
have been observed to be both lower in dominant
follicles collected in autumn than in those col-
lected in winter (Wolfenson et al.
1997
) . Effect of
heat stress on steroid production in bovine folli-
cles of medium size has been studied (Roth et al.
2001b
), and a delayed effect of heat stress on
