Biomedical Engineering Reference
In-Depth Information
with lactation (Brody
1945
). Cooling the cows by
mister system results in improvement in concep-
tion rate in cows and buffaloes (Aggarwal
2009
) .
Conception rate was observed to be higher for
cooled cows than control cows (83.3 vs. 66.6%)
when THI ranged between 79.44 and 87.72.
Therefore, evaporative cooling increases concep-
tion and pregnancy rates during heat stress. In
control group cows, during hot-humid season, the
number of inseminations required ranged from
2 to 9 with an average of 4.8 inseminations in
comparison to 2-4 (average 3.1 inseminations).
Conception rate to all services was higher for
cooled cows than control cows (83.3 vs. 66.6%).
The change in vaccination schedule to avoid
peak summer days may help in avoiding stress
imposed due to vaccine reactions. Possibly, dur-
ing severe heat waves, it would prove beneficial
to delay vaccinations at dry-off if the dry pen
does not contain adequate cooling.
Embryo transfer may become a more effective
strategy to increase pregnancy rates as compared
to AI in lactating cows during periods of heat
stress, and the magnitude of the increased tem-
perature does not seem to influence overall suc-
cess following transfer (Hansen and Arechiga
1999
) . As embryos advance in their develop-
ment, the effects of elevated temperatures become
less significant because embryos become more
resistant to the deleterious effects of elevated
temperatures (Ealy et al.
1992,
1995
; Ealy and
Hansen
1994
; Edwards and Hansen
1997
; Rivera
and Hansen
2001
). As a result, pregnancy rates
following embryo transfer during heat stress are
higher than pregnancy rates to AI (Putney et al.
1989b
; Ambrose et al.
1999
; Al-Katanani et al.
2002a
) although not in the absence of heat stress.
One potential constraint for embryo transfer in
lactating cows is the short duration of oestrus and
lack of intense mounting activity seen in dairy
cows (Dransfield et al.
1998
) . This phenomenon
is exacerbated by heat stress (Nebel et al.
1997
)
and will limit the number of embryos transferred
in lactating cows in a programme that is depen-
dent upon oestrus detection. The first report of a
timed embryo transfer (TET) protocol, where
ovulation was synchronised using an Ovsynch
protocol, was by Ambrose et al. (
1999
) who
evaluated the efficiency of TET using either
fresh or frozen-thawed in vitro produced (IVP)
embryos and TAI under heat stress conditions.
Pregnancy rates in cows that received a fresh
IVP embryo were higher compared to cows in
the TAI group.
Embryo transfer can significantly improve
pregnancy rates during the summer months.
Embryo transfers can bypass the period (i.e. before
day 7) in which the embryo is more susceptible to
heat stress and to bypass the harmful effects of
heat stress on oocyte quality that limit embryonic
development (Al-Katanami et al.
2002
; Zeron
et al.
2001
). A study (Al-Katanami et al.
2002
)
used timed embryo transfer to study the effect of
heat stress on fertility in lactating dairy cows and
showed that timed embryo transfer improved preg-
nancy rates under heat stress conditions but only
when fresh embryos were transferred (Al-Katanami
et al.
2002
). Nevertheless, embryo transfer is not a
widely adopted technique. Improvements need to
be made in the in vitro embryo production tech-
niques, embryo freezing and timed embryo trans-
fer, and lowering the cost of commercially available
embryos before this becomes a feasible solution.
11
Conclusions
Heat stress has a wide range of effects on the
reproductive axis. Some of these effects directly
affect individual reproductive organs such as the
hypothalamus, the anterior pituitary gland, the
uterus, the follicle and its oocyte and the embryo
itself, while the other effects of heat stress are
indirect and probably mediated by changes in the
metabolic axis in response to low feed intake or
reduced dry matter intake. There is no single
mechanism by which heat stress can reduce post-
partum fertility in dairy cows, and this problem is
due to the accumulation of the effects associated
with several biophysical factors. Thus, it is likely
that the direct impact of global warming (i.e. con-
sequences for body temperature regulation) on
mammalian reproduction will be severe for
domestic animals. Buffaloes are suited to the hot
and humid climates although they exhibit signs
of great distress when exposed to direct solar
