Biomedical Engineering Reference
In-Depth Information
Heat stress
Appetite centre
Hypothalamus
Decreased appetite
Hyperthermia
Decrease in progesterone(?)
Decrease in DM intake
Body temperature increases
Negative energy balance
GH and NEFA increases
Insulin, leptin and IGF-1 decreased
Lethargy and compromised
Uterine environment
Reduced GnRH, LH and oestradiol
Poor oestrus detection
Less no. of cows inseminated
Implantation failure
Poor detection of oestrus
Less no. of cows inseminated
Poor quality oocyte
Reduced fertility
Fig. 1
Two pathways showing the effect of heat stress on reproductive performance
to preovulatory oestradiol levels because the
amplitude of tonic LH pulses and GnRH-induced
preovulatory plasma LH surges are decreased in
cows with low plasma concentrations of oestra-
diol but not in cows with high plasma concen-
trations of oestradiol (Gilad et al.
1993
) . Plasma
inhibin concentrations in summer are lower in
cows under heat stress (Wolfenson et al.
1995
) and
in cyclic buffaloes (Palta et al.
1997
) , re fl ecting
reduced folliculogenesis since a significant pro-
portion of plasma inhibin comes from small- and
medium-sized follicles. Concentrations of plasma
FSH are higher during the preovulatory period
in summer, and this was associated with lower
circulating concentrations of inhibin (Ingraham
et al.
1974
). However, a reduced FSH response
in heat-stressed compared to control cows was
observed after administration of a GnRH
analogue (Gilad et al.
1993
) .
Because decreased LH levels have been
reported in most studies under heat stress, the
dominant follicle develops in an environment of
low LH, and this results in reduced oestradiol
secretion from the dominant follicle leading to
poor expression of oestrus. FSH is increased
due to heat stress, and this in turn decreases
plasma inhibin production by follicles. FSH
increases during summer and appears insuffi-
cient to overcome the effect of low LH concen-
trations and therefore a reduced availability
of androgen precursors for oestradiol synthesis
(Roth et al.
2000
) .
