Published November 20, 2014
Effect of elevated ambient temperature at parturition on duration of gestation,
ruminal temperature, and endocrine function of fall-calving beef cows1
E. C. Wright,2 B. H. Boehmer, M. J. Cooper-Prado, C. L. Bailey,3 and R. P. Wettemann4
Department of Animal Science, Oklahoma Agricultural Experiment Station, Stillwater 74078-0425
ABSTRACT: Fall-calving Angus cows were used to
evaluate the effect of ambient temperature on duration of gestation. In Exp. 1, cows were AI and calved
in August (n = 14) or October (n = 10). Cows grazed
native prairie pasture in Oklahoma and had a BCS of
6.0 ± 0.5 (1 = emaciated, and 9 = obese) at parturition.
Commencing 2 wk before the expected calving date,
blood samples were taken from the coccygeal vein
every 2 to 3 d until calving. Cows that calved in August
tended to have shorter gestations (P = 0.07) compared
with cows that calved in October. Maximum daily
ambient temperature during the last 14 d of gestation
was greater for August-calving cows (P < 0.001) compared with October cows. Concentrations of cortisol in
plasma during the last 4 d of gestation were greater in
cows that calved in August (P < 0.04) compared with
cows that calved in October. In Exp. 2, cows were AI
and calved in either mid-August (n = 7), late-August
(n = 6), September (n = 6), or October (n = 8) to evaluate the effects of elevated ambient temperature on
duration of gestation, ruminal temperature at parturition, and plasma cortisol, progesterone, and estradiol.
Temperature boluses (SmartStock, LLC, Pawnee, OK)
programmed to transmit temperature every hour were
place in the rumen at 255 d of gestation. Cows grazed
native prairie pasture in Oklahoma and had a BCS of
6.5 ± 0.4 at calving. Maximum ambient temperatures
during d 263 to 273 of gestation were influenced by
month of calving × day (P < 0.001). Duration of gestation was shorter for mid-August cows (P < 0.05) compared with October cows, but did not differ compared
with late-August (P = 0.29) and September (P = 0.50)
cows. Ruminal temperature during the 4 d before calving was not influenced by month of calving (P = 0.76).
Ruminal temperature was decreased during the 24 h
before parturition for cows in all months (P < 0.01)
compared with 2 to 4 d before parturition. Concentrations of cortisol in plasma during d 271 to 276 of gestation were less (P < 0.05) for late-August compared
with cows that calved during the other months. Concentrations of progesterone were greater during 7 d
before parturition in October compared with cows that
calved in September. Estradiol in plasma of cows during late gestation was not affected by month of calving
(P = 0.76). Exposure of beef cows to elevated ambient temperature resulted in shorter gestations. Ruminal
temperature in cows decreased ³ 0.3°C the day before
parturition
Key words: beef cows, gestation, heat stress, parturition
© 2014 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2014.92:4449–4456
doi:10.2527/jas2014-8055
INTRODUCTION
1Approved by the director of the Oklahoma Agric. Exp. Sta. This
research was supported under project H-2331.
2Present address: U.S. Meat Animal Research Center, USDA,
ARS, Clay Center, NE 68933–0166
3Present address: Department of Animal Science, Louisiana State
University, Baton Rouge 70803
4Corresponding author: bob.wettemann@okstate.edu
Received May 14, 2014.
Accepted July 11, 2014.
Exposure of cattle to elevated ambient temperature
reduces performance (Gwazdauskas et al., 1975; Collier
et al., 1982a). Duration of gestation can be influenced by
breed, genetics, number of calves, sex of calf, and environment (Cundiff et al., 1998). Exposure of beef cows to
heat stress decreased the duration of gestation compared
with cows exposed to a cooler environment (Kastner et
al., 2004). Parturition is initiated by the calf; the fetal hypothalamus secretes a corticotropin releasing hormone,
which causes the pituitary to release ACTH, and cortisol
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Wright et al.
is secreted by the adrenal glands (Wagner et al., 1974). Increased concentrations of fetal cortisol initiate a cascade
of endocrine events that culminate with parturition (Bazer
and First, 1983). Heat stress during late gestation alters
maternal and placental endocrine functions (Collier et al.,
1982b). Concentrations of cortisol in plasma of cows increased with exposure to elevated ambient temperatures
and decreased as animals became acclimated (Christison
and Johnson, 1972). Exposure of cows to elevated temperatures may reduce the decrease in progesterone in plasma
that occurs at parturition (Collier et al., 1982b). A decrease
in body temperature of cows occurs before parturition and
may be used to predict day of parturition in cows (Wrenn
et al., 1958; Lammoglia et al., 1997; Cooper-Prado et al.,
2011). We hypothesized that exposure of beef cows to elevated ambient temperatures during late gestation would
stimulate the fetus to prematurely initiate endocrine events
that cause parturition. Objectives of these experiments
were to evaluate the influence of exposure of cows to elevated ambient temperatures on duration of gestation, ruminal temperature (RuT), and maternal plasma concentrations of progesterone, cortisol, and estradiol.
MATERIALS AND METHODS
Institutional Animal Care and Use Committee of
Oklahoma State University approved all animal related
experimental procedures used in this study (AG061).
Experiment 1
Animals and Management. Angus cows (4 to 8 yr of
age) were stratified by age and BCS (Wagner et al., 1988),
AI, and calved in either August (n = 14) or October (n =
10). Ovulation was synchronized by treatment of cows
on d 0 with GnRH (86 µg, intramuscularly [i.m.]; Fertagyl, Intervet, Millsboro, DE) and a controlled intravaginal drug-releasing insert (CIDR; 1.38 g of progesterone;
Pfizer Animal Health, New York). On d 7 the CIDR was
removed and cows were treated with PGF2α (25 mg, i.m.;
Lutalyse, Pfizer Animal Health). An injection of GnRH
was given on d 9 and cows were AI to one of two Angus
bulls. An equal number of cows in each calving group was
AI to each bull. August-calving cows were AI on November 11 and October-calving cows were AI on January 5.
Cows grazed native prairie pasture (Andropogon scoparius, Andropogon gerardii) and were supplemented with
38% CP when needed to maintain a BCS of ³ 4.5. Weight
of calves was recorded within 20 h of birth. Hourly ambient temperature was obtained from the mesonet for the
Marena site (www.mesonet.org) located 2 km from the
pasture where cows were maintained.
Blood Collection and RIA. Two weeks before the
expected calving date, blood samples were obtained
by puncture of the coccygeal vein every 2 to 3 d until
calving. Blood samples were obtained in 10-mL tubes
containing EDTA. Samples were maintained at 4°C and
centrifuged (1500 × g for 20 min) within 2 h. Plasma
was decanted and samples were stored at -20°C until
analysis. Each RIA included equal numbers of cows
from each month with all samples for a cow in the same
assay. Concentrations of progesterone in plasma were
quantified by RIA (Vizcarra et al., 1997). Interassay
and intra-assay coefficients of variation were 4.7% and
7.5%, respectively. Concentrations of cortisol in plasma
were quantified using a solid-phase RIA (Coat-a-count
cortisol kit; www.siemens.com/diagnostics). When 25,
50, 100, and 200 μL of bovine plasma were analyzed,
concentrations of cortisol were parallel to the standard
curve. Interassay and intra-assay coefficients of variation were 15.9% and 14.1%, respectively.
Statistical Analyses. The effects of treatment (month)
on duration of gestation and birth weight of calves were
analyzed as a randomized design using PROC MIXED
(SAS Inst. Inc., Cary, NC) with assay as a random effect
and treatment, sire of calf, and sex of calf as fixed effects.
Effects with a P > 0.10 were eliminated from the final
model. Concentrations of progesterone and cortisol in
plasma were analyzed using repeated measurements over
time using PROC MIXED with assay as a random effect
and treatment (month) and day as fixed effects. At least
3 covariance structures were evaluated (compound symmetric, unstructured, and autoregressive), and the autoregressive structure, with spatial power, had the best modelfit criteria. Cow within treatment was the error term to
test main effects, whereas the pooled residual was the error term to test treatment × day. Degrees of freedom for
the pooled error term were calculated using the Satterthwaite approximation. When main effects were significant,
means were compared using LSD (pdiff option of SAS).
If treatment × week was significant, polynomial response
curves of appropriate order were fitted and tested for heterogeneity of regression (Snedecor and Cochran, 1968) to
evaluate effect of treatment.
Experiment 2
Animals and Management. Angus cows (4 to 8 yr
of age) were stratified by age and BCS and randomly assigned to four calving groups. Estrus was synchronized
by 2 treatments with PGF2α (25 mg, i.m.; Lutalyse) at an
11-d interval, and estrus was detected with Heatwatch
(CowChips LLC, Manalapan, NJ). Cows were AI at 12
to 24 h after the onset of estrus to one of 2 Angus bulls
and calved in mid-August (expected calving August 22,
n = 7), late-August (expected calving Aug 29, n = 6),
September (expected calving September 9, n = 6) or October (expected calving October 17, n = 8). Cows calved
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Ambient temperature and gestation of cows
with a BCS of 6.0 ± 0.5, and calves were weighed within
20 h of birth. Cows grazed native prairie pasture and
were supplemented with CP as described for Exp. 1.
Temperature boluses (www.smartstock-usa.com;
SmartStock, LLC) were placed in the rumen of cows
at 255 ± 5 d of gestation with a balling gun, and temperature was recorded. The Rut data collection system
consisted of radio frequency RuT sensor boluses (8.25
cm × 3.17 cm; 114 g), 3 antennas at the perimeter of the
calving pasture (3.2 ha) for data collection from boluses,
a receiver antenna for transmitted data, and a personal
computer with software for data storage. Data collection
and the receiver antennas were within 100 m. Date, time,
cow identification, and RuT (every hour) were transmitted by radiotelemetry and stored for analyses. Ambient
temperature was recorded as described for Exp.1.
Blood Collection and RIA. Blood plasma samples
were obtained, as described for Exp. 1, every second day
from 255 to 265 d of gestation, and then daily until parturition. Samples were assigned to RIA and concentrations of progesterone and cortisol in plasma were quantified as described for Exp. 1.
Estradiol MAIA assay (RADIM S.p.A., Pomezia, Italy) was used to determine concentrations of estradiol in
plasma as described by Evans et al. (1994) and modified
by Vizcarra et al. (1997). Interassay and intra-assay coefficients of variation were 9.1 and 29.2%, respectively.
Statistical Analyses. The effects of treatment on maximum ambient temperature, duration of gestation, RuT,
and birth weight of calves were analyzed as a randomized design using PROC MIXED as described for Exp.1.
Concentrations of progesterone, estradiol, and cortisol in
plasma were analyzed as described for Exp. 1. Concentrations of estradiol in plasma were transformed to the
natural log (x +1) before analyses because of heterogeneous variances (Steel et al., 1997). Partial correlations,
corrected for day, using CORR (SAS Inst. Inc.) were used
to evaluate the relationship between progesterone and estradiol in plasma. Ruminal temperature before parturition
was analyzed using PROC MIXED as described for Exp.
1. The 96 h before to 23 h after parturition were divided
into five 24 h periods to compare changes in average ruminal temperature among -1 to -24 h, -25 to -48 h,
-49 to -72 h, -73 to -96 h before parturition, and 0
(h of birth) to 23 h after parturition. The model included
treatment and period as fixed effects and cow was repeated within treatment. Simple correlation (CORR) was used
to determine relationships between RuT and average ambient temperature on the same period before parturition,
with cow repeated in the model.
Figure 1. Least squares regression lines for concentration of progesterone
(ng/mL) in plasma of beef cows during 10 d before parturition (d 0) in August
(n = 14) and October (n = 10). Day effect, P < 0.001. SE = 0.32.
RESULTS
Experiment 1
Average maximum ambient temperature during the
last 14 d of gestation was greater for August cows (36.6
± 4.5; P < 0.001) compared with October-calving cows
(25.2 ± 7.0°C). Sire and sex of calf did not influence duration of gestation or birth weight of calves (P > 0.57).
Cows that calved in August tended to have a shorter gestation (n = 14; 275.2 ± 1.3 d, P = 0.07) compared with
cows that calved in October (n = 10; 278.8 ± 1.4 d).
Birth weights of August calves were similar (36.7 ± 1.1;
P = 0.87) compared with October calves (37.0 ± 1.3 kg).
Concentrations of progesterone in plasma of cows
during the 10 d before parturition were not influenced
by month of calving (P = 0.11; Fig. 1) or by month ×
day (P = 0.54). Concentration of progesterone in plasma
of cows decreased (P < 0.001) from 10 d (5.8 ± 0.6 ng/
mL) before parturition to 1 d (1.9 ± 0.4 ng/mL) before
parturition. Concentrations of progesterone in plasma of
August and October cows did not differ (P = 0.66) during d 265 to 280 of gestation and were influenced by day
(P = 0.02). Concentrations of progesterone were greater
on d 265 compared with d 274 to 280 (P < 0.05).
Concentrations of cortisol in plasma during the 4
d before parturition were greater (P = 0.04) for August
(12.5 ± 0.9 ng/mL) compared with October cows (9.5 ±
1.0 ng/mL; Fig. 2); cortisol was influenced by day (P =
0.008) but not by month × day (P = 0.78). Concentrations of cortisol in plasma of August cows tended to be
greater (P = 0.10) compared with October cows during
265 to 280 d of gestation.
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Wright et al.
Figure 2. Least squares regression lines for concentration of cortisol
(ng/mL) in plasma of beef cows during 4 d before parturition (d 0) in August
(n = 14) and October (n = 10). Month of calving (P < 0.04) and day (P <
0.008) effects. SE = 0.67.
Experiment 2
Maximum daily ambient temperatures during 263 to
273 d of gestation were influenced by month of calving ×
day (P < 0.001) and were described by linear regression
equations. Mid-August cows were exposed to different
ambient temperatures (P = 0.06) compared with lateAugust cows (Fig. 3a). Daily maximum ambient temperature during the 9 d before parturition was greater for
mid-August and late-August cows (P < 0.001) compared
with September and October cows (Table 1). Duration of
gestation was less for cows that calved in mid-August (P
< 0.05) compared with cows that calved in October, but
did not differ (P > 0.10) from late-August or September
cows (Table 1). Sire and sex of calf did not influence duration of gestation (P > 0.23) or birth weight (P > 0.19).
Birth weights did not differ (P = 0.57) for calves born in
mid-August (37.9 ± 2.2), late-August (37.4 ± 1.7), September (36.0 ± 1.8), and October (39.3 ± 1.4 kg).
Daily maximal Rut during d 263 to 273 of gestation
were best fit by linear regressions and were different for
mid-August and late-August compared with September
and October cows (P < 0.001; Fig. 3b). Maximal Rut did
not differ between mid-August and late-August (P = 0.65)
or September and August cows (P = 0.89) during gestation. Calving month (P = 0.76) and calving month × day
before parturition (P = 0.90) did not influence RuT during
4 d before and on the day of parturition. Ruminal temperature was less (P < 0.001) the day before parturition compared with 2 to 4 d before and the day of parturition (Fig.
4). Mean daily maximum ambient temperature the week
before parturition was not correlated (P = 0.13) with RuT.
Concentrations of progesterone in plasma during the
7 d before parturition were effected by month of calving
× day (P = 0.03; Fig. 5). Concentrations of progesterone
Figure 3. Least squares regression lines for (a) maximum ambient
temperature and (b) maximum ruminal temperature during 263 to 273 d of
gestation for mid-August (n = 7), late-August (n = 6), September (n = 6), and
October calving (n = 8) beef cows. Month of calving × day effect for ambient
temperature (P < 0.001; SE = 0.04). Regression lines for ruminal temperature
differed for mid-August and late-August compared with September and
October cows (P < 0.001). SE = 0.09.
were best fit by a cubic regression equation, and cows
that calved in October had greater (P < 0.04) concentrations of progesterone compared with cows that calved in
September. Concentration of progesterone did not differ
between cows that calved in mid-August and late-August. Concentrations of progesterone in plasma of cows
during 271 to 276 d of gestation were influenced by day
Table 1. Relationship between ambient temperature during the 9 d before parturition and duration of gestation in
fall calving beef cows
Calving month
Mid-August
Late-August
September
October
aMean
Cows
Gestation
Max temperaturea
No.
7
6
6
8
d
274.7 ± 1.4b
277.0 ± 1.5bc
276.2 ± 1.5bc
278.8 ± 1.3c
°C
35.7 ± 0.6d
34.2 ± 0.6d
29.4 ± 0.6e
27.6 ± 0.6e
maximum ambient temperature during the 9 d before parturition.
in a column without a common superscript differ (P < 0.05).
d,eMeans in a column without a common superscript differ (P < 0.001).
b,cMeans
Ambient temperature and gestation of cows
4453
Figure 4. Maximal ruminal temperature during the 4 d before, and on
the day of parturition (h 0), for cows (n = 27) calving in mid-August, lateAugust, September, and October. a–cMeans without common letters differ (P
< 0.001). SE = 0.14.
Figure 6. Least squares regression lines for concentration of cortisol
(ng/mL) in plasma during 7 d before parturition (d 0) for cows calving in
mid-August (n = 7), late-August (n = 6), September (n = 6), and October (n
= 8). SE = 1.56.
(P < 0.02; Fig. 5), but were not influenced by month of
calving (P = 0.22) or month × day (P = 0.56). Concentrations of progesterone were greater on d 271 of gestation compared with d 274 to 276 (P < 0.03).
Month of calving, day, and month × day did not influence concentrations of cortisol in plasma during 1 to 7 d
before parturition (P > 0.15; Fig. 6). Month of calving (P =
0.02), but not day (P > 0.28) or month × day (P > 0.27), influenced concentrations of cortisol in plasma during d 271
to 276 of gestation. Concentrations of cortisol in plasma
were less (7.6 ± 1.1 ng/mL; P < 0.05) in late-August cows
during 271 to 276 d of gestation compared with cows that
calved during the other months (11.4 ± 1.2 ng/mL).
Concentrations of estradiol in plasma during d 1 to
7 before parturition were influenced by day (P < 0.001;
Fig. 7), but not calving month (P = 0.80) or by month
× day (P = 0.51). Estradiol increased from 7 d before
to 1 d before parturition and was greater the day before
parturition compared with d 3 to 7 before parturition (P
< 0.01). Concentration of estradiol in plasma during 271
to 276 of gestation were not influenced by calving month
(P > 0.76) or month × day of gestation (P > 0.93); however, day influenced plasma estradiol (P = 0.002). Concentrations of estradiol were less on d 271 (P < 0.003)
compared with d 273 to 276.
There was a partial correlation (r = -0.41; P <
0.001; corrected for calving month) between concentrations of progesterone and estradiol in plasma. There
were partial correlations between day of gestation with
concentrations of progesterone (r = -0.31; P < 0.001)
and estradiol (r = 0.23; P = 0.006) in plasma.
Figure 5. Least squares regression lines for concentration of
progesterone (ng/mL) in plasma during 7 d before parturition (d 0) in beef
cows that calved in mid-August (n = 7), late-August (n = 6), September (n
= 6), and October (n = 8). Regression lines for September and October cows
differed (P = 0.04). SE = 0.5.
Figure 7. Least squares regression lines for concentration of estradiol
(pg/mL) in plasma during 7 d before parturition (d 0) for cows calving in midAugust (n = 7), late-August (n = 6), September (n = 6), and October (n = 8).
Day effect (P = 0.001). SE = 0.28.
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DISCUSSION
Cows that calved in mid-August were exposed to
elevated ambient temperatures compared with cows that
calved in September or October. Exposure to greater
maximum daily temperatures in mid-August resulted in
shorter gestations compared with cows that calved in October. Length of gestation for the late-August cows did
not differ compared with October cows. Late-August
cows were exposed to cooler maximal ambient temperatures compared with mid-August cows during the last
week of gestation, and this may be associated with the
lack of effect of ambient temperature on length of gestation. Kastner et al. (2004) determined that beef cows
exposed to heat stress in August had a 5.2 d shorter gestation compared with cows that calved when exposed to
a cooler environment in October. Duration of gestation
was 4 d less in heat stress compared with cooled dairy
cows (Tao et al., 2012b). The time during late gestation
when elevated ambient temperature can shorten gestation
is probably after 265 d of gestation. The duration of exposure to elevated temperatures that causes a decrease in
length of gestation could be 10 d or less and may be influenced by the amount that ambient temperature is elevated
in late gestation. Studies are needed to determine the stage
of gestation when heat stress causes shorter gestation.
Kastner et al. (2004) found that beef calves that
were born during heat stress in August weighed 3.0 kg
less than calves born in October. Birth weight was less
for fall-born calves that were exposed to elevated ambient temperatures during late gestation compared with
spring-born calves (Selk and Buchanan, 1990). The
lack of effect of exposure of cows to elevated ambient
temperatures on weight of calves at birth in the current
experiments may be related to the limited number of
animals per treatment to evaluate this variable trait. Duration of gestation was less when sheep were exposed
to elevated ambient temperature in late gestation (Shelton and Huston, 1968). Heat stress of ewes during late
gestation can cause development of dwarf lambs (Shelton and Huston, 1968; Brown et al., 1977). Calves born
to Holstein cows that were heat stressed without shade
weighed less compared with cows provided with shade
during gestation (Collier et al., 1982b). Cooling dairy
cows during the last 45 d of gestation by sprinklers (Tao
et al., 2012a), or soakers and fans over feed bunks (Tao
et al., 2014), increased birth weight of calves compared
with heat-stressed cows. Heat stress causes a decrease
in fetal muscle protein which could be caused by decreased uterine blood flow and altered fetal metabolism
(Dreiling et al., 1991). Feed intake and feed efficiency
decrease during heat stress of cattle (Mader and Davis,
2004). Breed of cows, duration of exposure, maximum
ambient temperature, humidity, or other factors may in-
fluence the effect of exposure to elevated ambient temperature on fetal growth in late gestation.
Concentrations of progesterone in plasma during
late gestation were not influenced by month of calving in
Exp. 1. The tendency for a month of calving × day effect
on progesterone during the 7 d before parturition in Exp.
2 was associated with greater concentrations of progesterone in October compared with September-calving
cows. However, in agreement with Smith et al. (1973)
and First (1979), progesterone in plasma decreased in all
cows during the 7 d before parturition and without a difference between cows that calved in mid-August, lateAugust, and October. Decreased progesterone at parturition is due to decreased progesterone production by
the ovary and placenta (Smith et al., 1973; First, 1979).
The bovine ovary is not required for the maintenance
of pregnancy after the seventh month of gestation (Tanabe, 1970). Treatment of heifers with ACTH increased
concentrations of progesterone in plasma during gestation (Willard et al., 2005). However, a physiological role
for altered progesterone in plasma during stress has not
been determined. Collier et al. (1982b) observed a tendency for plasma concentrations of progestins in dairy
cows to be increased during gestation when exposed to
elevated ambient temperature, although plasma concentrations of progesterone were not altered by heat stress
during the estrous cycle of lactating cows (Wolfenson et
al., 1995). Progesterone in plasma of gilts that were exposed to elevated ambient temperatures after AI, and did
not become pregnant, was less compared with pregnant
and nonpregnant cool gilts (Hoagland and Wettemann,
1984). Although these studies indicate that heat stress
may influence plasma concentrations of progesterone,
the effect of heat stress on progesterone in plasma of
gestating animals is not established.
Concentrations of cortisol in plasma during late gestation were greater in August compared with Octobercalving cows in Exp. 1. There was a tendency for late-August cows to have less cortisol in plasma compared with
the other groups in Exp. 2. Cortisol in plasma of dairy
cows during late gestation was not influenced by heat
stress (Tao et al., 2012a). There is an inconsistent effect
on cortisol in plasma when gestating cows are exposed to
elevated ambient temperatures. This may occur because
cortisol secretion in cows is variable and is influenced by
environmental conditions and exposure to unique situations or stress. Estimates of experimental error are large
when evaluating cortisol in plasma of cows (Adkinson
et al., 1976). The effect of exposure of gestating cows to
elevated ambient temperatures on plasma concentrations
of cortisol is not established (Wise et al., 1988; Elvinger
et al., 1992; Dikmen et al., 2008) and will require special
experimental designs to minimize experimental error.
Ambient temperature and gestation of cows
Concentrations of estradiol in plasma during late
gestation were not influenced by month of calving. Similarly, Collier et al. (1982b) found that heat stress did
not alter estradiol in plasma of dairy cows in late gestation and concentrations increased during the last 14 d
of gestation. The negative correlation between plasma
concentrations of progesterone and estradiol preceding
parturition in the current experiment is consistent with
others (First, 1979; Eley et al., 1981).
Plasma concentrations of cortisol are usually increased at parturition (Smith et al., 1973; Hudson et al.,
1976). An increase in concentrations of cortisol in plasma
of cows at parturition was probably not detected in the
present experiments because cows were only sampled
once a day or stress of sampling influenced secretion of
cortisol. The decrease in concentrations of progesterone
and increase in estradiol in plasma before parturition in
cows exposed to either elevated ambient temperatures
or cooler temperatures indicate that maternal endocrine
functions at parturition were not altered, although gestation was shortened when cows were exposed to elevated
ambient temperatures. Parturition of cattle is initiated by
the fetus with activation of the hypothalamo-pituitaryadrenal axis (First, 1979; Liggins, 1994; Lye, 1996).
Cortisol is secreted by the fetal adrenal gland and acts on
the placenta to initiate a cascade of endocrine events that
culminates with parturition (Smith et al., 1973; Wagner
et al., 1974; Lye, 1996). Stress increases concentrations
of cortisol in plasma of cows (Christison and Johnson,
1972; Thun et al., 1998) and heifers (Szenci et al., 2011).
Exposure to elevated ambient temperatures could potentially cause an acute increase in plasma cortisol in the
fetus in addition to an increase in cortisol in plasma of
cows. In Exp. 1, cows that calved in August had greater
concentrations of cortisol in plasma compared with October cows; however, this did not occur in Exp. 2. Christison and Johnson (1972) observed that cows exposed to
elevated ambient temperature for a short time have greater plasma concentrations of cortisol, and long-term exposure to elevated ambient temperatures allowed animals to
become acclimated to the environment and have reduced
concentrations of cortisol. The inconsistent responses in
plasma concentrations of cortisol in the 2 experiments
when cows were exposed to elevated ambient temperatures and the constant effect of heat stress on duration of
gestation indicate that maternal concentrations of cortisol
may not be the major initiator of premature parturition.
Ruminal temperature was greater for mid-August and
late-August cows compared with September and October
cows; this indicates a physiological response to exposure
to elevated ambient temperatures. Mid-August cows were
exposed to ambient temperatures greater than 35°C from
269 to 275 d after AI, and parturition occurred earlier
compared with October cows. Boehmer et al. (2011) de-
4455
termined that exposure of beef cows to ambient temperatures greater than 32°C caused an increase in RuT.
Ruminal temperature was less the day before parturition compared with 2 to 4 d before parturition, which
indicates that RuT could be used to predict day of parturition. A decrease in body temperature at calving has
been observed (Wrenn et al., 1958; Sawada et al., 1988;
Cooper-Prado et al., 2011) and may be related to reduced plasma concentrations of progesterone preceding
parturition (Wrenn et al., 1958). Lammoglia et al. (1997)
did not find a relationship between sex hormones and the
decrease in body temperature at parturition when temperature was measured in the flank of cows.
Maternal stress probably does not directly initiate
parturition, but elevated body temperature of the cow
could stimulate the fetal brain, hypothalamus, pituitary,
and/or adrenal and cause early maturation resulting in a
shorter gestation. An increase in fetal temperature, associated with heat stress of the dam, could cause premature
release of fetal cortisol. Determination of temperature
of fetal tissues and fetal concentrations of corticotropin
releasing hormone, ACTH, and cortisol would be necessary to test this hypothesis. Duration and magnitude
of heat stress, and the time during gestation when heat
stress occurs, probably influence the onset of premature
gestations. Mechanisms by which exposure to elevated
ambient temperature may cause a shorter gestation have
not been determined in beef cows.
We conclude that exposure of beef cows to elevated
ambient temperature during late gestation shortens gestation. Reproductive endocrine function at premature
parturition induced by exposure of dams to elevated
ambient temperature is normal. We hypothesize that increased body temperature of the cow during heat stress
results in increased temperature of the fetus and early
initiation of the signal to the hypothalamo-pituitary-adrenal axis that results in fetal secretion of cortisol and
initiation of parturition.
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