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Heat Stress in Dairy Cows - Reproductive Problems and Control Measures

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  • Odisha University of Agriculture & Technology (OUAT)

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All animals have a range of ambient environmental temperatures termed as thermo-neutral zone. This is the range of temperatures that are conducive to health and performance. The upper critical temperature is the point at which heat stress begins to affect the animal adversely. There are a number of environmental factors like high temperature, high humidity and radiant energy (sunlight) that contribute to heat stress. Heat stress has long been recognized as reducing both the productivity and reproductive efficiency of dairy cattle. Although heat stress has a direct effect on reproduction, it depends on the magnitude and duration of thermal stress, milk yield, lactation status, breed, composition of diet, dry mater intake and physical activity of animals. Further studies are needed to better understand the associations between climatic conditions and reproductive physiology and to evaluate the efficacy of various nutritional, environmental and reproductive strategies in a particular region for combating heat stress.
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International Journal of Livestock Research ISSN 2277-1964 ONLINE www.ijlr.org
Vol 3(3) Sept’13
Page14
Heat Stress in Dairy Cows - Reproductive Problems and Control Measures
Samal, L.
Odisha University of Agriculture & Technology, Bhubaneswar -India
Corresponding Author: lipismitasamal@gmail.com
Rec. Date: May 17, 2013 01:45; Accept Date: Sep 20, 2013 20:54
Abstract
All animals have a range of ambient environmental temperatures termed as thermo-neutral zone. This is
the range of temperatures that are conducive to health and performance. The upper critical temperature
is the point at which heat stress begins to affect the animal adversely. There are a number of
environmental factors like high temperature, high humidity and radiant energy (sunlight) that contribute
to heat stress. Heat stress has long been recognized as reducing both the productivity and reproductive
efficiency of dairy cattle. Although heat stress has a direct effect on reproduction, it depends on the
magnitude and duration of thermal stress, milk yield, lactation status, breed, composition of diet, dry
mater intake and physical activity of animals. Further studies are needed to better understand the
associations between climatic conditions and reproductive physiology and to evaluate the efficacy of
various nutritional, environmental and reproductive strategies in a particular region for combating heat
stress.
Keywords: Heat stress, reproductive problems, dairy cows
Introduction
Heat stress can be defined as the point where the animal cannot dissipate adequate quantity of
heat to maintain body thermal balance. Climatic factors that may influence the degree of heat
stress include: temperature, humidity, radiation and wind. Figure 1 illustrates the challenges that
heat stress poses for dairy cows.
The environmental conditions that induce heat stress can be calculated using the temperature
humidity index (THI).
THI = (Dry bulb temperature 0C) + (0.36 * dew point temperature 0C) + 41.2
When the THI is >72°F (22.2°C), heat stress begins to occur in dairy cattle. Table 1 contains
some of the signs that cows exhibit as the THI increases.
International Journal of Livestock Research ISSN 2277-1964 ONLINE www.ijlr.org
Vol 3(3) Sept’13
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Fig 1 - Depiction of heat stress in dairy cows
Table 1 - Effect of heat stress on dairy cattle
THI
Stress
level
Effects
<72
None
72-79
Mild
Dairy cows will adjust by seeking shade, increasing respiration rate and
dilation of the blood vessels. The effect on milk production will be
minimal.
80-89
Moderate
Both saliva production and respiration rate will increase. Feed intake
may be depressed and water consumption will increase. There will be an
increase in body temperature. Milk production and reproduction will be
decreased.
90-98
Severe
Cows will become very much uncomfortable due to high body
temperature, rapid respiration (panting) and excessive saliva production.
Milk production and reproduction will be markedly decreased.
>98
Danger
Potential cow deaths can occur
Effect of Heat Stress on Female Reproductive Functions
The negative effects of heat stress on dairy cows are multifaceted. Summer heat stress has long
been recognized as reducing the reproductive efficiency of dairy cattle. Here are a few ways by
which reproductive function is impaired by summer heat.
Oestrus expression: Cows in heat stress conditions show fewer, less intense heats than in cooler
temperatures. Studies have shown that undetected oestrous events were between 76 and 82%
from June to September compared to 44 to 65% from October to May. Heat stress also decreases
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the length and intensity of oestrus. Heat stress decreased follicular estradiol which might
decrease the oestrus intensity. Another reason of reduced oestrus expression might be the
physical inactivity caused by heat stress. Cows are less active and therefore less likely to ride
other cows during oestrus. So, in summer, dairy cows had approximately one-half the number of
mounts per oestrus compared to dairy cows in winter. Oestrus activity is also lowered due to the
cows' reduced motor activity, a means of trying to decrease her endogenous heat output.
Endocrine status: Females raised under high temperatures have low estradiol. This decrease in
estradiol synthesis could influence expression of oestrus, ovulation and corpus luteum. Thermal
stress also alters goandotrophin, inhibin and PGF2 secretion. The length of the luteal phase in
heat-stressed cows is longer than in females kept in thermo-neutral environment. It seems that
the uterus secretes less PGF2 because of the reduction in estradiol synthesis and/or because high
temperatures can interfere with the release of PGF2 by endometrial cells. It’s well known that
the uterine endometrium must be primed by estradiol to produce enough prostaglandin and
trigger luteolysis. Thermal stress also alters the concentrations of FSH and inhibin and corpus
luteum function, as well as decreases the fluid content of follicles. High temperatures also reduce
the number of granulosa cells, aromatase activity and secretion of androstenedione by theca cells
(Wolfenson et al 1993).
Follicular selection and development: The first reproductive challenge facing the heat stressed
cow is altered follicular development. Heat stressed cows decrease feed intake causing less
frequent pulses of the luteinizing hormone (LH) resulting in longer follicular waves. This
lengthening of the follicular wave leads to the selection and ovulation of multiple, smaller
dominant follicles (Sartori 2002). Follicles are responsible for producing estrogen, a hormone
that causes cows to show signs of heat. Smaller follicles will produce less estrogen than larger
ones; therefore, resulting in less oestrus activity. Ovarian follicles contain oocytes as well as
somatic cells that synthesize estradiol. Estradiol has a variety of actions that include causing
oestrus and the LH surge. Heat stress impairs follicle selection and increases the length of
follicular waves, which reduces the quality of oocytes. It also allows for more than one dominant
follicle to develop, explaining the increased twinning seen from cows conceiving in summer time
heat. The somatic cells within the follicles (theca and granulosa cells) are also damaged by heat
stress.
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Corpus luteum: In addition to influencing the ovarian follicles, heat stress can affect the corpus
luteum. Progesterone from the corpus luteum is required for pregnancy and there is an
association between low progesterone and infertility. Estradiol from the follicle initiates
luteolysis in cattle. The cells of the corpus luteum differentiate from the cells of the follicle.
Therefore, if heat stress decreases blood progesterone then the decrease could arise from the
effects of heat stress on the follicle which ultimately carries over to the corpus luteum.
Alternatively, changes in metabolic rate associated with heat stress may alter the metabolism of
progesterone.
Embryo development: Embryo quality and growth is often reduced during heat stress. Thermal
stress also alters the ability of embryos to develop into blastocysts. It causes early embryonic
development, increased risk of early embryonic deaths and decreased foetal growth. There are
effects of heat stress on the ovary and these effects may influence the ability of cows to become
pregnant. The period of greatest susceptibility is immediately after the onset of oestrus and early
during the post-breeding period. Putney et al (1989) demonstrated that embryonic development
was impaired in heifers subjected to heat stress for 10 hours after the onset of oestrus. This is an
interesting period of development because it represents a time after the LH surge but before
ovulation. The period of embryonic sensitivity to heat stress begins early during the development
of the follicle and continues until about 1 day after breeding. The high uterine temperature of the
heat stressed cow can impair embryonic development, resulting in poor embryo implantation and
increased embryo mortality.
Dry matter intake: One of the first reactions cows show to heat stress is less feed intake,
supplying less energy for use which may interfere with their reproductive performance. The
reduction in feed intake depends on several factors, including the proportion of concentrate and
forage of the diet and milk yield. Since cows have been bred to produce high volumes of milk,
cows use available energy for daily maintenance and milk production first, with fewer nutrients
available for reproductive health.
Carryover effects: Even after the heat of summer is over, the carryover effects of heat stress can
cause fertility to be delayed. This can reduce conception rates for a longer period of time and
result in poor heat detection, more services per pregnancy and longer days open. For all these
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reasons, pregnancy losses increase dramatically as the temperature-humidity index (THI) rises.
So, there is decrease in conception and pregnancy rates in hot seasons.
Nutritional Strategies to Beat the Heat Stress
Feeding of high quality feedstuffs/rations: Early lactation cows exposed to heat stress may go
even further into negative energy balance because they aren’t consuming as much feed as
needed. Consequently, they are more likely to have lower reproductive performance due to
altered follicle development and lower oestrus activity. Feeding high quality forages and
balanced rations will decrease some of the effects of heat stress. Potassium levels should also be
increased in the diet as it is the primary sweat gland regulator in cattle.
Feeding of bypass fat: Cows in heat stress conditions are prone to rumen acidosis, so fibre
quality should be enhanced to maximize rumen buffering and saliva production. Feeding a high-
quality bypass fat provides an energy-dense diet at a time when cows are consuming little feed.
The use of fat in diets could also lower the heat load because of high energy density and lower
metabolic heat when compared with other ingredients such as fibre and carbohydrate. Inclusion
of fat may therefore increase milk yield but it depends on the environment where cows are
raised. Moreover, fats have significant effects on concentrations of cholesterol, progesterone,
rate of synthesis and metabolism of prostaglandin F2 (PGF2), follicle growth and pregnancy
rates in dairy herds (Oldick et al 1997; Staples et al 1998).
Environmental Strategies to Beat the Heat Stress
To reduce the negative reproductive effects of heat stress, following recommendations should be
followed.
Cooling: Infertility during heat stress is primarily caused by elevated body temperature. Cooling
dairy cows during heat stress should, therefore, improve conception rates. A variety of cooling
systems are available for heat-stressed dairy cows. Perhaps the most widely used system is a
combination of water sprinklers, shades, soakers and fans. Sprinkling cows with water and
subsequently blowing air over the cow with a fan causes evaporative cooling. The evaporative
cooling decreases body temperature.
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Putney et al (1989) have shown that d-17 conceptuses incubated at high temperatures synthesize
heat shock proteins, which may enhance their resistance to heat stress whereas in vitro
endometrial tissue subjected to elevated temperatures increased release of prostaglandins into
culture medium. If such an effect occurred in vivo, it might initiate premature luteal regression or
compromise the function of the CL. In addition, hyperthermia on d-17 of pregnancy increased
uterine production of prostaglandin F2α in response to oxytocin (Wolfenson et al 1993). Thus, it
appears that cooling is needed from at least 42 d before ovulation to over 40 d post-insemination.
Reproductive Tools to Beat the Heat Stress
While the changes to environment and rations will help alleviate the negative effects of heat
stress on reproduction, other changes to reproductive protocols can help many folds.
Improving rates of oestrus detection: Some of the effects of heat stress are caused by reduced
intensity of oestrus expression. Therefore, it may be possible to improve reproduction in dairy
herds by improving oestrus detection methods. Some of the oestrus detection aids like
pedometers, tail chalk and pressure activated patches or electronic devices placed on the tail head
can improve reproductive performance of dairy cows.
Timed artificial insemination (Timed AI): If oestrus detection is a problem in heat stressed
dairy cows then it may be possible to improve reproduction by using timed AI. A timed-A.I.
(TAI) protocol can help improve fertility during summer months. TAI is done by administering a
series of gonadotropin releasing hormone (GnRH) and PGF2 injections. Insemination is
performed at a predetermined time following the last GnRH injection. Timed AI shortened the
interval to first service and increased pregnancy rates in heat stressed cows when compared to
insemination at observed oestrus.
The beneficial effects of the first service TAI during summer were maintained over the course of
a year with fewer cows being culled (12.9 vs. 22.0%) and more cows eventually conceiving
(87.0 vs. 77.9%) if TAI was used for first service compared with cows first inseminated at
detected oestrus. Thus, on-farm implementation of TAI programs help to inseminate cows
independent of expressed oestrus.
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Embryo transfer to enhance summer fertility: Embryos are sensitive to the effects of heat
stress. However, greatest sensitivity occurs during early embryonic development. During later
embryonic development (morula or blastocyst stage), embryos develop some tolerance for heat
stress. It should be possible, therefore to improve pregnancy rates in heat-stressed cattle by using
embryo transfer of frozen embryos collected from cows that are not heat stressed. Embryo
transfer nearly doubled the conception rates when compared to dairy cows inseminated
artificially. So, it is possible to by-pass early embryonic stages and improves conception rates
during heat stress.
Although embryo transfer of good quality embryos appears to provide a methodology to enhance
summer-time fertility, it is not without problems. During periods of heat stress, the number of
embryos produced following superovulation may be reduced as a result of fewer oocytes
released in response to superovulatory drug therapies, lower fertilization rates, or reduced
embryo quality. These adverse effects appear more pronounced in dairy cattle than beef cattle,
particularly when maximum air temperature exceeds 32°C (Hansen et al 2001). To mitigate the
negative effects of heat stress on superovulation, the strategies are namely: reduce heat stress
with cooling, alter the genetic make-up of the cattle involved, use lower producing or non-
lactating donors, and enhance detection of oestrus by using synchronization schemes or other
detection aids (Hansen et al 2001).
The factors which have prevented widespread commercial adoption of embryo transfer (Rutledge
2001) to bypass effects of heat stress are: 1) using dairy heifers as donors may delay their first
parturition, and hence productivity; 2) the negative relationship between embryo quality and
ambient temperature means the fewest good to excellent quality embryos are available when the
most are needed; 3) embryo recovery is highly technical and expensive; 4) for embryos that have
been frozen, so far only those produced in vivo produced frozen embryos have increased the
percentage of animals pregnant compared with AI; and 5) the cost of in vitro-produced embryos
is reduced compared to in vivo production of embryos; however, in vitro-produced embryos
currently only enhance results when transferred fresh and not frozen. Thus, the potential for
commercial adoption of embryo transfer to increase reproductive success depends on enhancing
the outcome while minimizing cost. By inducing heat shock proteins, the success rate of embryo
transfer under heat stress can be enhanced.
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Inducing accessory corpus luteum: Wolfensen et al (2000) suggested that chronic heat stress
reduces progesterone concentrations, although progesterone concentrations may be elevated after
acute heat stress. Injection of a GnRH agonist or hCG on d 5 of the estrous cycle results in
ovulation of the first wave dominant follicle and formation of an accessory corpus luteum (CL),
with subsequent elevation of plasma progesterone levels. However, lactating dairy cows
receiving injections of hCG on d 5 or 6 after insemination did not improve conception rate
during heat stress (Schmitt et al 1996). The mechanism by which hCG might enhance conception
rate is by minimizing estrogen during pregnancy recognition. When treatment occurred on d 5
after oestrus, hCG induced progesterone concentration as well as three-wave follicular cycles.
However, there are two drawbacks of commercial application of hCG on d 5 post-insemination.
First, hCG is a potential immunogen; therefore, repeated use should be avoided. The second
concern is the additional handling of animals. To circumvent both problems, one potential
method is incorporation of Deslorelin implants (long-acting GnRH agonist) into the TAI
protocol.
Crossbreeding: To introduce greater heat tolerance into the population of dairy cattle,
crossbreeding is done. Heterosis for production traits ranges from 0 to 10%, while fertility ranges
from 5 to 25%; so, there may be opportunities to improve fertility and production simultaneously
by crossbreeding. The crossbred cattle had a slightly higher rate of survival than the purebred
Holsteins, while the Guernseys had the lowest survival rate. Crossbreds had superior
reproduction with some indication of even more favourable heterosis in the warm season,
especially for Holstein crosses. However, Swan and Kinghorn (1992) attributed the lack of
widespread crossbreeding in dairy cattle in temperate climates to the notable merit of purebred
Holstein strains for milk production.
In a study assessing heat tolerance of temperate Bos taurus, tropical Bos taurus, and Bos indicus
beef cattle, Hammond et al (1996) reported that rectal temperatures in Senepol cattle were less
than those in Brahman cattle, but the converse was true of respiration rates. Angus heifers on the
other hand had the highest respiration rates and temperatures. Because the Hereford × Senepol
crosses had rectal temperatures similar to the purebred Senepol, there appears to be a high degree
of dominance associated with the genes responsible for controlling rectal temperature in this
tropical Bos taurus breed. Preliminary results provided evidence of a gene influencing hair length
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and heat tolerance in the Senepol breed. If this gene can be identified as the bovine genome is
mapped, perhaps it could be introduced into dairy breeds. Alternatively, a crossbreeding program
starting with Senepol, and selecting for high milk production and low rectal temperature during
heat stress, might develop a dairy cow with greater heat tolerance without compromising
productivity.
Conclusion
Heat stress reduces reproductive efficiency of dairy cattle through a variety of different
mechanisms. However, it depends on mainly milk production, housing conditions, nutrition,
disease control, inbreeding and heat stress tolerance. The decrease in fertility is caused by
elevated body temperature that influences ovarian function, oestrous expression, oocyte health
and embryonic development. Also, the use of milk production systems based on grazing reduces
the costs per kg of milk but also expose the animals to greater heat load. In response to these
limitations, environmental and reproductive management of cows should be increased during
heat stress. Methods that show promise in enhancing conception rate in summer are transfer of
embryos collected from superovulated donors and induction of accessory CL. Research is needed
to evaluate the potential of crossing traditional and non-traditional dairy breeds to enhance
reproductive capability, while maintaining productivity at acceptable levels.
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... The experimental animals calved during the interval from November to April and were followed up for two-week post-partum. Table (2) clarifies the classification of zones based on THI values in cattle with THI model according to Samal (2013). Mild Dairy cows will adjust by seeking shade, increasing respiration rate and dilation of the blood vessels. ...
... Classification of zones based on THI values in cattle with THI model according to(Samal, 2013) ...
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... Summer heat stress has long been recognized as reducing the reproductive efficiency of dairy cattle. Cows in heat stress conditions show fewer, less intense heats than in cooler temperatures (Samal, 2014). Heat stress also decreases the length and intensity of oestrus (Black et al., 2018). ...
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Climate change is a global threat to livestock sector to so many species and ecosystem in different parts of the world. Climate change, heat stress, and nutritional stress are the major intriguing factors responsible for reduced fertility in farm animals in tropical countries. Heat and nutritional stresses affect the reproductive performance by decreasing the expression of estrous behavior, altering ovarian follicular development and hormonal profiles, compromising oocyte competence, and inhibiting embryonic development in livestock. Climate is changed by greenhouse gases that released into atmosphere through man-made activities. Livestock contribute 18% of the production of greenhouse gases itself and causes climate change including heat stress, which has direct and indirect impact on fertility of the animals as well as reduce milk production. Adaptation to climate change and lowering its negative effect by alteration of animal micro-environment using different essential technologies are the main mitigation strategies to recover heat stress damage in this respect.
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Hyperthermia affects most aspects of reproductive performance in mammals by compromising the physiology of reproductive tract, through hormonal imbalance, disrupting the development and maturation of oocyte, causing embryonic mortality, abortion, growth retardation, and major developmental defects. Heat stress reduces the steroidogenic capacity of its theca and granulosa cells by altering the efficiency of follicular selection resulting in drop of luteinizing hormone and estradiol secretions from the dominant follicle in the plasma, reduced intensity, and duration of estrus expression. The mechanism for the developmental stage-dependent change in heat tolerance is considered to be the accumulation of antioxidants in embryos in response to heat-inducible production of reactive oxygen species. Morula or blastocysts can repair heat-induced misfolded or unfolded proteins or facilitate DNA damage induced apoptosis. Therefore, embryo transfer (ET) that can bypass the heat-sensitive stage could be a good solution to improve the conception rate under heat stress. However, further research is required to improve the reduction in pregnancy rates due to summer heat stress.
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Holstein heifers (n = 16) were used to determine whether heat stress prior to ovulation increases the incidence of embryonic abnormalities. Heifers were superovulated with Follicle Stimulating Hormone (FSH-P; 32 mg total), beginning on Days 10 or 11 of the estrous cycle. Prostaglandin F2α (Lutalyse; 60 mg total) was administered on Day 3 of FSH-P treatment. Heifers were maintained at either thermoneutrality (24°C) or under hyperthermic conditions (exposure to 42°C for 10 h) beginning at the onset of estrus. Following artificial inseminations at 15 and 20 h after the onset of estrus, heifers were continuously maintained under environmental conditions of thermoneutrality for 7 days as provided by environmental shade structures. On Day 7 post estrus, embryos were recovered nonsurgically and evaluated morphologically for stage of development and quality. The distribution of embryos classified as normal, retarded and/or abnormal, or as unfertilized ova differed (P < 0.001) between heat stress and thermoneutral treatments. Only 12.0% of 25 embryos recovered from heat-stressed heifers were normal compared with 68.4% of 19 embryos from thermoneutral heifers. Stressed heifers had a higher (P < 0.001) incidence of retarded and/or abnormal embryos with degenerated blastomeres. These data indicate that thermal stress during the periovulatory period increases the incidence of retarded and/or abnormal embryos in superovulated heifers.
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The effects of acute heat stress (HS) and oxytocin (OT) injection on plasma concentrations of PGF2α and OT were examined in cyclic (C; n = 15) and pregnant (P; n = 11) dairy heifers. On Day 17 of synchronized estrous cycles, animals were randomly assigned to either thermoneutral (TN; 20°C, 20% RH) or HS (42°C, 60% RH) chambers. The jugular vein of each heifer was cannulated and blood samples collected hourly for 4 h, then every 15 min for an additional 3 h. Oxytocin (100 IU) was injected (IV) 5 h after the start of blood collection. Plasma samples were assayed subsequently for concentrations of 13,14-dihydro-15-keto PGF2α (PGFM) and OT. During the 7-h experiment, body temperature of HS heifers reached 41.2°C as compared to 38.5°C in control heifers. Plasma concentrations of PGFM increased (P<0.05) and peaked 30 min after OT injection in C (890 pg/ml) and P (540 pg/ml) heifers. In C heifers, heat stress failed to alter PGFM concentrations either before or after OT injection. In the P group, PGFM concentrations following OT injection tended to be higher in HS heifers were further TN heifers (peak values of 690 vs. 410 pg/ml). Pregnant TN and HS heifers were further classified as responders or non-responders to OT challenge according to a cutoff value for PGFM of 193 pg/ml (overall mean of C heifers minus 1 SD). Five of six HS and one of five TN pregnant heifers were classified as responders (P<0.06). Oxytocin concentrations in plasma prior to injection of exogenous OT were not affected by HS or pregnancy status. It is concluded that in C heifers, acute HS in vivo does not cause any further rise in PGF2α secretion. However, in P heifers, HS appears to antagonize suppressive effects of the embryo on uterine secretion of PGF2α, as indicated by the larger proportion of P heifers responding to OT challenge.
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Four ruminally cannulated lactating dairy cows, arranged in a 4 x 4 Latin square design, were infused abomasally with 1) water (control), 2) 1 kg/d of glucose, 3) 0.45 kg/d of tallow, and 4) 0.45 kg/d of yellow grease. Cows were synchronized for estrus within each 35-d period by injection of a GnRH agonist followed 7 d later by an injection of PGF2 alpha. Dry matter intake was not affected by infusates. Apparent digestibility of total fatty acids was greater for cows receiving the fat infusions relative to those receiving the glucose infusion and tended to increase for cows receiving the yellow grease infusion compared with those receiving the tallow infusion. Energy infusions decreased apparent acid detergent fiber digestibility compared with effects of the control infusion. Fat infusions tended to increase milk fat percentage and decrease the energy status of cows relative to the glucose infusion. The feed efficiency was greater for cows receiving fat infusions than for those receiving the glucose infusion and was greater for cows receiving the yellow grease infusion than for those receiving the tallow infusion. Plasma progesterone concentration peaked higher during the estrous cycle for cows infused with fat than for those infused with glucose. Mean growth rate and maximum size of the first wave dominant follicle were greater with tallow than with yellow grease. During the period of infusion of yellow grease and afterward, release of 13,14-dihydro-15-keto-PGF2 alpha in response to an injection of oxytocin on d 15 of the estrous cycle was attenuated.
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Summer heat stress (HS) is a major contributing factor in low fertility in lactating dairy cows in hot environments. Although modern cooling systems are used in dairy farms, fertility remains low. This review summarizes the ways in which the functioning of various parts of the reproductive system of cows exposed to HS is impaired. The dominance of the large follicle is suppressed during HS, and the steroidogenic capacity of theca and granulosa cells is compromised. Progesterone secretion by luteal cells is lowered during summer, and in cows subjected to chronic HS, this is also reflected in lower plasma progesterone concentration. HS has been reported to lower plasma concentration of LH and to increase that of FSH; the latter was associated with a drastic reduction in plasma concentration of inhibin. HS impairs oocyte quality and embryo development, and increases embryo mortality. High temperatures compromise endometrial function and alter its secretory activity, which may lead to termination of pregnancy. In addition to the immediate effects, delayed effects of HS have been detected as well. Among them, altered follicular dynamics, suppressed production of follicular steroids, and low quality of oocytes and developed embryos. These may explain the low fertility of cattle during the cool autumn months. Hormonal treatments improve low summer fertility to some extent but not sufficiently for it to equal winter fertility. A limiting factor is the inability of the high-yielding dairy cow to maintain normothermia. A hormonal manipulation protocol, which induces timed insemination, has been found to improve pregnancy rate and to reduce the number of days open during the summer.
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The production of embryos by superovulation is often reduced in periods of heat stress. The associated reduction in the number of transferable embryos is due to reduced superovulatory response, lower fertilization rate, and reduced embryo quality. There are also reports that success of in vitro fertilization procedures is reduced during warm periods of the year. Heat stress can compromise the reproductive events required for embryo production by decreasing expression of estrus behavior, altering follicular development, compromising oocyte competence, and inhibiting embryonic development. While preventing effects of heat stress can be difficult, several strategies exist to improve embryo production during heat stress. Among these strategies are changing animal housing to reduce the magnitude of heat stress, utilization of cows with increased resistance to heat stress (i.e., cows with lower milk yield or from thermally-adapted breeds), and manipulation of physiological and cellular function to overcome deleterious consequences of heat stress. Effects of heat stress on estrus behavior can be mitigated by use of estrus detection aids or utilization of ovulation synchronization treatments to allow timed embryo transfer. There is some evidence that embryonic survival can be improved by antioxidant administration and that pharmacological treatments can be developed that reduce the degree of hyperthermia experienced by cows exposed to heat stress.
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Although heat stress has multiple effects to lower pregnancy rate in lactating dairy cows, a major pathway is in its effects on the early cleavage stage embryo. Conceptually, and in practice, higher pregnancy rates can be obtained with transfer of late cleavage stage embryos. The literature is reviewed, and conclusion is made that application of these technologies may be in part, a solution to this long-standing problem.