Biomedical Engineering Reference
In-Depth Information
associated with lower circulating concentrations
of inhibin. The neuroendocrine mechanisms
controlling gonadotrophin secretion are more
sensitive to heat stress particularly in animals
with low concentrations of plasma oestradiol.
Environmental temperature and humidity 2 days
prior to insemination is critical for conception
than at any other phase of the reproductive
cycle. A rise in rectal temperature diverts
blood from the visceral organs to the peripheral
circulation due to redistribution of blood to
alleviate heat, which could reduce perfusion of
nutrients and hormones to the endometrial
and oviductal tissues affecting reproductive
functions. In terms of steroid production, the
thecal cells are more susceptible than granu-
losa cells to heat stress and express a delayed
effect of heat stress in both medium-sized
and preovulatory follicles. A rise in testicular
temperature in bulls similar to other mammals
with external testes leads to reduced sperm
output, decreased sperm motility and an
increased proportion of morphologically
abnormal spermatozoa in the ejaculate. X and
Y spermatozoa are affected differentially by
high temperature. The plasma concentrations
of insulin, IGF-I and glucose are low in summer
months compared to winter months () probably
because of low dry matter intake and increased
negative energy balance. Insulin is required for
the development of follicles and has beneficial
effects on oocyte quality. Genetic selection for
heat adaptability, both natural and arti fi cial, is
likely to modulate the impact of heat stress on
reproductive functions, and therefore, genetic
selection for thermal tolerance may be a neces-
sity under climate change conditions.
blood flow occurs from the body core to the
periphery in order to increase sensible heat loss;
other homeokinetic homeothermic or thermo-
regulatory control mechanism leads to reduced
voluntary feed intake during heat stress.
A low feed intake that occurs in order to
reduce metabolic heat production leads to changes
in energy balance and nutrient availability affec-
ting reproductive cyclicity, pregnancy and fetal
development. Other reproduction process that
may be disrupted in heat-stressed livestock is
homeokinetic related to gonadal functions due to
impact on hormone synthesis.
Heat stress induces several physiological and
biochemical changes in body functions and nega-
tively affects milk production and reproductive
efficiency of both male and female animals. High
ambient temperature during summer has been
observed to negatively impact breeding efficiency
and drastically reduce conception rate and
increase embryonic loss (Gwazdauskas et al.
1981
; Hansen
2005
) . Berman et al. (
1985
) has
concluded that thermoregulatory ability in the
face of heat stress as a result of selection for milk
production in high-producing cows magnifies the
seasonal depression in fertility due to heat stress.
2
The Effect of Heat Stress
on Reproductive Functions
Under intensive and extensive livestock manage-
ment system, only productive or likely to be pro-
ductive in near future gets preference. Lactating
and pregnant animals get adequate protection and
care, and other animals like dry pregnant cows
are neglected or provided little protection from
heat stress. In fact, during early pregnancy,
neglect or limited attention predisposes them to
additional stressors due to abrupt physiological
changes, and any nutritional insufficiency and
environmental changes make them vulnerable.
These factors also increase the susceptibility of
pregnant cows to heat stress and have a critical
influence on fetal and dam's postpartum health and
productive performance. Therefore, protection
from heat stress during dry period is particularly
1
Introduction
Heat stress
in livestock drives their body tem-
perature above set-point temperature leading to
disruptions in reproductive functions. Two
homeokinetic mechanisms are involved in body
temperature homeostasis compromising repro-
ductive functions. First, the redistribution of
