Available online at www.sciencedirect.com
Domestic Animal Endocrinology 38 (2010) 86–94
Metabolic adaptations to heat stress in growing cattle
M.D. O’Brien, R.P. Rhoads, S.R. Sanders, G.C. Duff, L.H. Baumgard ∗,1
Department of Animal Sciences, The University of Arizona, Tucson 85721
Received 1 June 2009; received in revised form 4 August 2009; accepted 14 August 2009
Abstract
To differentiate between the effects of heat stress (HS) and decreased dry matter intake (DMI) on physiological and metabolic
variables in growing beef cattle, we conducted an experiment in which a thermoneutral (TN) control group (n = 6) was pair fed (PF)
to match nutrient intake with heat-stressed Holstein bull calves (n = 6). Bulls (4 to 5 mo old, 135 kg body weight [BW]) housed in
climate-controlled chambers were subjected to 2 experimental periods (P): (1) TN (18 ◦ C to 20 ◦ C) and ad libitum intake for 9 d,
and (2) HS (cyclical daily temperatures ranging from 29.4 ◦ C to 40.0 ◦ C) and ad libitum intake or PF (in TN conditions) for 9 d.
During each period, blood was collected daily and all calves were subjected to an intravenous insulin tolerance test (ITT) on day 7
and a glucose tolerance test (GTT) on day 8. Heat stress reduced (12%) DMI and by design, PF calves had similar nutrient intake
reductions. During P1, BW gain was similar between environments and averaged 1.25 kg/d, and both HS and PF reduced (P < 0.01)
average daily gain (-0.09 kg/d) during P2. Compared to PF, HS decreased (P < 0.05) basal circulating glucose concentrations (7%)
and tended (P < 0.07) to increase (30%) plasma insulin concentrations, but neither HS nor PF altered plasma nonesterified fatty
acid concentrations. Although there were no treatment differences in P2, both HS and PF increased (P < 0.05) plasma urea nitrogen
concentrations (75%) compared with P1. In contrast to P1, both HS and PF had increased (16%) glucose disposal, but compared
with PF, HS calves had a greater (67%; P < 0.05) insulin response to the GTT. Neither period nor environment acutely affected
insulin action, but during P2, calves in both environments tended (P = 0.11) to have a blunted overall glucose response to the ITT.
Independent of reduced nutrient intake, HS alters post-absorptive carbohydrate (basal and stimulated) metabolism, characterized
primarily by increased basal insulin concentrations and insulin response to a GTT. However, HS-induced reduction in feed intake
appears to fully explain decreased average daily gain in Holstein bull calves.
Published by Elsevier Inc.
Keywords: Heat stress; Metabolism; Insulin; Glucose
1. Introduction
Environmentally induced hyperthermia decreases
efficiency and production, detrimentally affects reproduction, and can compromise end-product quality in
agriculturally important animals. One of the first noticeable production signs of heat stress (HS) is reduced
∗ Corresponding author. Iowa State University, Department of Animal Science, Ames, IA 50011. Tel.: +1 515 294 3615.
E-mail address: [email protected] (L.H. Baumgard).
1 L.H. Baumgard is now affiliated with the Department of Animal
Science, Iowa State University, Ames, IA.
0739-7240/$ – see front matter. Published by Elsevier Inc.
doi:10.1016/j.domaniend.2009.08.005
nutrient intake, which is presumably an evolutionary
strategy to reduce the “heat increment” of feeding. The
dystrophia is thought to be responsible for reduced production in growing cattle [1] and lactating cows [2].
However, most traditional thermal stress experiments
have not controlled for reduced intake, which makes it
difficult to distinguish between the direct and indirect
(ie, mediated by reduced feed intake) effects of environmentally induced hyperthermia. Recently, using a
pair-feeding thermoneutral (TN) model, we have demonstrated that reduced nutrient intake accounts for about
35% to 50% of the decrease in milk synthesis from heatstressed cows, and the remaining portion is a direct result
M.D. O’Brien et al. / Domestic Animal Endocrinology 38 (2010) 86–94
of heat [3,4], but the mechanism(s) by which a thermal
load Zreduces body weight gain in growing ruminants
have not been established.
In lactating ruminants, HS induces a variety of postabsorptive metabolic adaptations that would not be
predicted based on their reduced nutrient intake and calculated energy balance. For example, despite reduced
feed intake and body weight loss, heat-stressed cows
have increased basal and stimulated insulin concentrations and increased glucose clearance in response
to a glucose tolerance test (GTT) [4]. Furthermore,
heat-stressed cows do not mobilize adipose tissue,
even though they are in a negative energetic state and
markedly losing body weight [3,5], likely the outcome
of insulin’s potent antilipolytic action. Consequently,
the lactating heat-stressed cow fails to enlist glucosesparing mechanisms in the face of reduced feed intake,
and therefore production decreases substantially more
than would be expected from the inadequate nutrient
status [3,4].
It is currently unknown how much (or if) reduced
feed intake explains decreased body weight gain in thermally stressed growing cattle. In addition, the effects
of HS (without the confounding effects of dissimilar
feed intake) on post-absorptive metabolism and nutrient partitioning in a ruminant growth model have not
been thoroughly evaluated. Study objectives were to
evaluate the effects of thermal stress on production variables and post-absorptive energetics in growing Holstein
calves.
2. Materials and methods
2.1. Animals and research design
Study protocol and procedures involving animals
were approved and conducted in accordance with the
University of Arizona Institutional Animal Care and
Use Committee. Growing male beef cattle (4-5 mo
old, n = 12) were assigned to individual tie stalls based
on body weight (BW) (135 ± 13 kg [BW]) in one of
2 environmental chambers in the University of Arizona’s William J. Parker Agriculture Research Complex.
Throughout the experiment, calves were fed an 86% concentrate diet composed primarily of steam-flaked corn
and alfalfa hay (Table 1), formulated to meet or exceed
National Research Council recommendations [6], at 6:00
AM and 5:00 PM daily, and orts were recorded daily prior
to the morning feeding.
After adjusting to the environmental chambers (9 d),
calves in both treatment groups were exposed to constant
TN conditions (20 ◦ C, 20% humidity, with a 13 h/11 h
87
Table 1
Ingredients (DM basis) and chemical composition of diets fed to growing Holstein beef bulls.
Ingredient (% of DMI)
Steam-flaked corn
Alfalfa
Soybean meal
Molasses
Mineral premixa
61.6
28.3
1.9
6.3
1.9
Chemical analysis
Diet DM %
CP, % DM
ADF, % DM
NDF, % DM
Crude fat, % DM
TDN, % DM
88.8
11.2
13.5
20.1
4.8
74.8
Abbreviations: ADF, acid detergent fiber; CP, crude protein; DM, dry
matter; NDF, neutral detergent fiber; TDN, total digestible nutrients.
a Mineral pre-mix composition: limestone, 46.03%; dicalcium phosphate, 1.05%; potassium chloride, 7.81%; magnesium oxide, 3.44%;
ammonium sulfate, 6.52%; salt, 11.75%; cobalt carbonate, 0.0017%;
copper sulfate, 0.1534%; iron sulfate, 0.1328%; calcium iodate,
0.0031%; manganese sulfate, 0.4881%; selenium premix, 0.1220%;
zinc sulfate, 0.8334%; vitamin A (30,000 IU/g), 0.2739%; vitamin E
(500 IU/g), 0.5435%; Rumensin-80, 0.7063%; ground corn, 20.15%.
light/dark cycle) and fed ad libitum for 9 d (experimental
period [P] 1). During P2, the heat-stressed (HS; group 2)
calves were exposed to a cyclic HS environment (29.4 ◦ C
to 40.0 ◦ C, 20% humidity) for 9 d, and group 1 calves
remained in TN conditions, but they were pair fed (PF) so
that nutrient intake mirrored that of HS cattle. During P2,
each HS calf’s reduced feed intake was calculated daily
on the percentage decrease from the average dry matter
intake (DMI) during P1, and the amount offered to the
PF TN calves was reduced by that amount. This experimental approach was adapted to eliminate confounding
effects of dissimilar planes of nutrition.
During each period, daily heat parameters were
obtained from all animals at 7:00 AM, 12:00 PM, 4:00
PM, and 6:00 PM. Heat parameters included rectal temperature (RT; GLA M700 Digital Thermometer, San
Luis Obispo, CA), respiration rate (RR), heart rate (HR),
and sweating rate (SR; VapoMeter, Delfin Technologies
Ltd., Kuopio, Finland), as previously described [5]. Body
weights were recorded daily prior to the AM feeding.
Feed intake and water intake were recorded daily, and
jugular catheters were inserted into all animals 1 d prior
to P1 and maintained for the duration of the trial.
2.2. Metabolic tests
Insulin tolerance tests (ITT; 3.0 ␮g/kg BW) were
conducted on day 7 of each period at 1:00 PM, as previ-
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M.D. O’Brien et al. / Domestic Animal Endocrinology 38 (2010) 86–94
ously described [7]. Bovine insulin (Sigma Chemical, St.
Louis, MO) was initially dissolved to 1 mg/mL in 0.1 M
HCl, then diluted in sterile saline and kept at -80 ◦ C until
the challenges. The insulin solution was administered via
a jugular catheter and immediately flushed with 10 mL
of sterile saline (0.15 M NaCl). Blood sampling occurred
at -30, -20, -10, 0, 2.5, 5, 7.5, 10, 15, 20, 30, 45, 60, 90,
and 120 min relative to insulin administration.
A glucose tolerance test (GTT; 0.3 g/kg BW) was
conducted on day 8 of each period at 1:00 PM, as previously described [4]. Glucose (50% dextrose; AgriLabs,
St. Joseph, MO) was infused as a 50% solution in sterile saline and flushed with 10 mL of pure sterile saline.
Blood samples were collected at -30,-20, -10, 0, 5, 7.5,
10, 15, 20, 30, 45, 60, and 90 min relative to glucose
infusion.
2.3. Plasma assays
Plasma glucose, insulin, plasma urea nitrogen (PUN),
and nonesterified fatty acid (NEFA) concentrations were
determined enzymatically using commercially available kits validated for use in our laboratory (Autokit
Glucose C2, Wako Chemicals USA, Richmond, VA;
bovine insulin EIA, Alpco Diagnostics, Salem, NH;
PUN assay kit, Advanced Bio-screen, Fullerton, CA;
NEFA-HR(2) assay kit, Wako Chemicals USA, Richmond, VA). These procedures were conducted in 96-well
microplates (Rainin Instrument, LLC; Oakland, CA) and
read using a microplate photometer (Multiskan Ascent;
Thermo Electron Corporation, Vantaa, Finland). The
inter- and intra-assay coefficients for plasma glucose,
insulin, PUN, and NEFA assays were 3.1%, 4.4%; 4.7%,
5.3%; 5.0%, 6.2%; and 5.5%, 5.9%, respectively.
2.4. Calculations and statistical analysis
Blood metabolite responses to the ITT and GTT
were calculated as area under the curve (AUC). The
ITT AUC was calculated as a linear trapezoidal summation between successive pairs of glucose concentrations
and time coordinates after correcting for the mean baseline concentrations, as previously described [7]. Baseline
glucose concentrations were defined as a mean of the 3
samples prior to insulin administration. Plasma glucose
concentrations reached nadir 30 to 50 min after insulin
injection. To minimize the contribution of clearance and
counter-regulatory effects, the response area of plasma
glucose to insulin administration was calculated over 5
to 120 min of the insulin challenge. The rate of glucose
clearance in response to insulin was determined using
glucose concentrations over the initial declining phase
of the response (2.5 to 30 min post-challenge). This
result was expressed as the fractional rate of clearance
(FROC) and was determined from the slope of the natural logarithm of glucose concentration plotted vs time
[7]. As a result of the GTT, plasma glucose concentrations increased in a similar temporal pattern and peaked
at 5 min post-glucose infusion. Again, to minimize the
contribution of clearance and counter-regulatory effects,
the glucose AUC was calculated for the entire challenge
(0 to 90 min). Furthermore, the slope of glucose disposal
(SOGD) was calculated from 5 to 60 min post-glucose
infusion.
All data were statistically analyzed using the PROC
MIXED procedure of SAS, version 9.1 (SAS Institute,
Inc., Cary, NC; 2005). Analysis was conducted to test
differences between environments and periods, and the
model included group, period, and group × period interaction. Period 2 data were also tested against each other
(HS [group 2] vs PF [group 1]) using the P1 value
as a covariate. For daily measurements (DMI, average
daily gain [ADG], metabolites, rectal temperature, etc.),
each animal’s respective parameter was analyzed using
repeated measures with an autoregressive covariance
structure and day (1 to 9) as the repeated effect. The
model contained covariate, environment, time (day, if a
repeated measure), and an environment × time interaction (if a repeated measure). Data are reported as least
square means and were considered significant if P < 0.05
and interpreted as a trend if P < 0.10.
3. Results
Compared to the PF calves (group 1) in TN conditions, animals in the cyclical HS climate (group 2) had
marked (P < 0.01) increases in body temperature variables at 7:00 AM, 12:00 PM, 4:00 PM, and 6:00 PM
(Table 2). At 4:00 PM, HS calves had a 1.15 ◦ C increase
in RT, and >2.5-fold increase in RR. Climatic conditions
had no effect on heart rate at any time point (Table 2).
Heat stress reduced (P < 0.01) DMI by approximately
12%, and by design, PF calves in TN conditions had a
similar decrease in nutrient intake (Table 3). Although
not different from each other, both HS and PF calves had
reduced (P < 0.01) ADG compared to that of P1 (1.24 vs
-0.09 kg/d; Table 3). Gain-to-feed ratio (G:F) was similar between groups, but it was reduced (P < 0.01) in P2
compared to P1 (0.30 vs -0.03). During P2, HS calves
had increased (P < 0.05) water intake (5.56 vs 2.71 L/d)
compared to PF controls.
Calves in the HS climate had reduced (7%; P < 0.04;
Table 4) circulating glucose concentrations compared to
M.D. O’Brien et al. / Domestic Animal Endocrinology 38 (2010) 86–94
89
Table 2
Effects of heat stress and pair feeding on heat stress variables in growing Holstein bull calves.
Period 2†
Period 1*
Parameter
Group 1 (TN)
Group 2 (TN)
Group 1 (PF)
Rectal temperature (◦ C)
7:00 AM
38.81a
12:00 PM
38.88a
4:00 PM
38.95a
6:00 PM
38.96a
38.70a
38.84a
39.03a
38.98a
38.84a
39.01a
39.39b
39.32b
Respiration rate (breaths/min)
7:00 AM
39a
12:00 PM
38a
4:00 PM
44a
6:00 PM
45a
39a
40ab
44a
43a
43a
45b
49a
47a
Heart rate (beats/min)
7:00 AM
108
12:00 PM
101
4:00 PM
120
6:00 PM
118
108
105
109
111
99
105
119
113
P
SEM
Group
PER
Group × PER
0.05
0.04
0.06
0.07
0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
80b
108c
126b
117b
2
2
2
2
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
102
103
111
107
2
3
3
3
0.40
0.61
<0.01
0.02
<0.01
0.75
0.98
0.07
0.61
0.23
0.50
0.90
Group 2 (HS)
39.23b
39.60b
40.57c
40.10c
Abbreviations: HS, heat stress; PER, period; PF, pair feeding; SEM, standard error of the mean; TN, thermoneutral.
Note: a,b,c Values within row of each variable with differing superscripts indicate statistical difference.
* During period 1, calves in both groups were treated identically (housed under TN conditions and allowed to eat ad libitum).
† During period 2, calves were either heat stressed and allowed to eat ad libitum or pair-fed and kept under TN conditions.
PF controls, and this difference was most pronounced
(14%) during the middle of P2 (temporal pattern not
shown). Basal NEFA concentrations were unaffected by
environment and period (Table 4). Daily insulin concentrations tended (P < 0.06) to increase (33%) during HS
compared to PF controls (Table 4; Fig. 1). Although similar to each other during P2 (Table 4), both HS and PF
had increased (P < 0.01; 77%) circulating PUN concentrations compared to P1 (Fig. 2).
Neither environment nor period affected the glucose
response (AUC or SOGD) to the GTT (Table 4). However, there was a tendency (P < 0.07) for HS calves to
have a greater (59%) insulin response to the GTT during
P2 (Table 4). Both HS and PF tended (P < 0.10) to ameliorate the overall glucose response (AUC) to the ITT,
but there were no differences between environments in
the AUC or the FROC (Table 4).
4. Discussion
Animal productivity is maximized in a narrow
thermal range as energy, and nutrients are diverted
away from growth/milk/reproduction toward maintaining euthermia when environmental conditions are not
ideal. This change in nutrient partitioning priority
decreases animal performance and is therefore a sig-
Table 3
Effects of pair feeding and heat stress on production variables in growing Holstein bull calves.
Period 2†
Period 1*
P
Parameter
Group 1 (TN)
Group 2 (TN)
Group 1 (PF)
Group 2 (HS)
SEM
Group
PER
Group × PER
DMI (kg/d)
DMI (%BW)
ADG (kg/d)
G:F
Water intake (L/d)
4.1
2.69
1.1
0.28
10.0a
4.2
2.80
1.3
0.34
10.5a
3.6
2.30
−0.3
−0.09
9.7a
3.8
2.38
0.1
0.02
21.0b
0.1
0.05
0.2
0.06
0.9
0.07
0.08
0.21
0.26
<0.01
<0.01
<0.01
<0.01
<0.01
<0.01
0.80
0.81
0.64
0.63
<0.01
Abbreviations: ADG, average daily gain; BW, body weight; DMI, dry matter intake; G:F, gain-to-feed ratio; HS, heat stress; PER, period; PF, pair
feeding; SEM, standard error of the mean; TN, thermoneutral.
Note: a,b,c Values within row of each variable with differing superscripts indicate statistical difference.
* During period 1, calves in both PF and HS were treated identically (housed under TN conditions and allowed to eat ad libitum).
† During period 2, calves were either heat-stressed and allowed to eat ad libitum or pair-fed and kept under TN conditions.
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M.D. O’Brien et al. / Domestic Animal Endocrinology 38 (2010) 86–94
Table 4
Effects of heat stress (HS) and pair feeding (PF) on basal energetic variables and stimulated whole body responses to metabolic challenges in
growing Holstein bull calves.
Period 1a
Parameter
Basal
Insulin (ng/mL)
Glucose (mg/dL)
NEFA (␮Eq/L)
PUN (mg/dL)
Period 2b
P
Group 1 (TN)
Group 2 (TN)
Group 1 (PF)
Group 2 (HS)
0.35
97.8
82.2
1.71
0.41
93.6
86.8
1.48
0.47
100.7
101.4
2.80
0.59
93.4
88.7
2.84
3227
−0.13
111
3071
−0.13
87
2987
−0.14
137
581
0.01
20
−0.11
−3854
−0.11
−2984
−0.10
−2908
0.02
380
Stimulated
GTT
AUC (mg/dL*min)
3924
SOGD (mg/dL*min)
−0.11
Insulin responsec (mg/dL*min)
122
ITT
FROC
−0.09
Glucose responsed (mg/dL*min) −3280
SEM
Group
0.05 0.07
1.4 <0.01
6.4
0.53
0.18 0.59
PER
Group × PER
<0.01
0.36
0.10
<0.01
0.49
0.27
0.18
0.46
0.36
0.18
0.35
0.51
0.18
0.83
0.60
0.94
0.14
0.54
0.48
0.96
0.11
0.30
0.44
Abbreviations: AUC, area under the glucose response curve; FROC, fractional rate of glucose clearance; GTT, glucose tolerance test; HS, heat
stress; ITT, insulin tolerance test; NEFA, nonesterified fatty acids; PER, period; PF, pair feeding; PUN, Plasma urea nitrogen; SEM, standard error
of the mean; SOGD, slope of glucose disappearance; TN, thermoneutral.
a During period 1, calves in both PF and HS were treated identically (housed under TN conditions and allowed to eat ad libitum).
b During period 2, calves were either heat-stressed and allowed to eat ad libitum or pair-fed and kept in TN conditions.
c Insulin response to the GTT.
d Area under the insulin-induced glucose response curve.
nificant financial burden in most countries and may
be the largest global contributor to reduced animal
agriculture income. The negative effects of HS will
become even more apparent in the future if climate
change continues, as some predict, and as the world’s
population and thus food supply continues to increase
in and migrate toward, respectively, the tropical and
subtropical regions. In addition, genetic improvement
programs that enhance production traits increase animals’ susceptibility to environmental HS because of
the positive relationship between metabolic heat gen-
eration and production level. Therefore, identifying the
mechanisms by which a heat load negatively influences
growing cattle may ultimately provide clues to the development of nutritional and pharmaceutical strategies to
ameliorate the negative effects of high ambient temperatures. Unfortunately, many of the negative effects
of HS on production and biology are shared with
a lower plane of nutrition. To differentiate between
the direct effects of environment-induced hyperthermia
versus the indirect effects of heat-induced decreased
nutrient intake, we used a pair-feeding TN model to
Fig. 1. Effects of heat stress and pair feeding on basal insulin concentrations in growing Holstein bull calves. The vertical line separates period
1 (thermoneutral conditions and ad libitum feed intake) from period 2 (either heat stress conditions and ad libitum feed intake or thermoneutral
conditions and pair-fed).
M.D. O’Brien et al. / Domestic Animal Endocrinology 38 (2010) 86–94
91
Fig. 2. Effects of heat stress and pair feeding on basal plasma urea nitrogen concentrations in growing Holstein bull calves. The vertical line separates
period 1 (thermoneutral conditions and ad libitum feed intake) from period 2 (either heat stress conditions and ad libitum feed intake or thermoneutral
conditions and pair-fed).
eliminate the confounding effects of dissimilar nutrient
intake.
To mimic a natural circadian pattern, we used an
HS protocol with temperatures slowly increasing from
29.4 ◦ C at 6:00 AM to a maximum of 40.0 ◦ C at 4:00 PM
before slowly returning to 29.4 ◦ C at 11:00 PM (20%
humidity). Calves in the HS climate had increased body
temperature variables at every measured time point, with
the maximum RR and RT indicating severe heat stress.
The marked hyperthermia and distinct body temperature
differences between the 2 environments created an ideal
model to study HS in growing ruminants.
The HS environment caused an immediate (by day
2) decrease (approximately 12%) in nutrient intake, and
feed intake remained stable thereafter (temporal pattern
not shown). By design, the PF calves had a similar pattern of reduced feed intake. The environmental protocol
actually provided a slightly higher heat load than our previous lactation trials [3,5], but the reduced feed intake
was much less than in lactating dairy cows (12% vs
40%). Reasons for the discrepancy are not clear, but it
may be explained in large part by the amount of time
animals experienced extensive hyperthermia. In our previous lactation trials, cows remained warmer throughout
the cooler night and were warmer (>1.0 ◦ C) than PF
controls at 6:00 AM. Calves in the current study were
only 0.39 ◦ C warmer than their PF controls at 7:00 AM.
Other contributing differences between studies include
(1) diet (concentrate vs forage); (2) surface-to-mass
ratio (heat dissipation is proportional to body surface
area); (3) physiological state; (4) the extent to which
a growing animal and lactating animal consume nutrients above their maintenance requirements; and (5) sex.
Breed genetics can probably be ruled out, as Holsteins
were used in both the lactation trial and the current
trial.
Both HS and PF climates eliminated body weight
gain (Table 3), which indicates heat-induced DMI reduction essentially explains all of the reduced weight gain;
this finding agrees with HS pig data [8]. However, this
result is in stark contrast to poultry data that indicated
that reduced DMI accounts for only about 50% of HSinduced decreased growth [9]. In addition, heat-induced
reductions in nutrient intake account for only 35%-50%
of the decreased milk yield in lactating cows [3,4]. Reasons for the differences between models and species are
not clear, but they nonetheless illustrate the importance
of strategies that maximize feed intake in heat-stressed
growing cattle.
Compared to PF controls, heat-stressed calves had
reduced circulating plasma glucose concentrations. The
reduction in glucose concentrations resulting from HS
agrees with studies using heat-stressed rats [10], chickens [11], sheep [12], and heifers and cows [6,13–15].
However, other studies indicate a heat-load–induced
increase in blood glucose in chickens [16], rabbits
[17], sows [18], and exercising men [19]. In addition, in our previous lactation trials, both heat-stressed
and PF cows had reduced basal blood glucose concentrations [3,4]. Reasons for the discrepancies are not
clear, but species differences, altered physiological states
(lactation vs growth), magnitude and duration of hyperthermia, and diet composition may all contribute to the
variation observed in circulating glucose concentrations.
Nonetheless, the decreased glucose in the present study
is probably a result of the heat-induced increase in circulating basal insulin concentrations (Fig. 1), which are
consistent with findings in our previous lactation trial [4].
The increased basal insulin concentration in response
to heat is consistent with findings in rodents [20], a
malignant hyperthermic porcine model [21], and lactating cows [4,14]. The elevated basal insulin concentration
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M.D. O’Brien et al. / Domestic Animal Endocrinology 38 (2010) 86–94
is surprising, as animals on a lower plane of nutrition typically have decreased circulating insulin. The increased
insulin may be an essential part of the adaptation mechanism. For example, human diabetics are more susceptible
to heat [22]. In addition, diabetic rats have an increased
mortality rate when exposed to heat, and exogenous
insulin increases their survival time [23]. Consequently,
it appears insulin and maintenance of insulin action play
a critical role in the ability of an animal to respond and
ultimately survive a heat load.
Plasma NEFA concentrations were unaffected by
either HS or PF, which illustrates that feed restriction
in the PF group was not severe enough to cause adipose mobilization. In our lactation trials [3,4], PF cows
exhibited the expected increase in plasma NEFA concentrations, but HS cows did not, despite both groups being
in a reduced energetic state and losing body weight.
However, the lack of NEFA response in the HS calves
agrees with the results of studies demonstrating reduced
NEFA concentrations in heat-stressed pigs [21], sheep
[24], heifers [15], and lactating cows [25]. The lack
of change or decrease in NEFA concentrations is consistent with increased basal insulin concentrations, as
insulin is a potent antilipolytic signal [26]. This finding is especially impressive, as HS (especially acute HS)
causes the release of catecholamines and glucocorticoids
[2], hormones that typically promote adipocyte lipolysis and NEFA mobilization. Presumably the prevention
of adipose lipid mobilization and subsequent substrate
competition maximize glucose utilization in skeletal
muscle, as elevated NEFA concentrations contribute
to insulin resistance and reduced glucose oxidation
[27].
Both HS and PF calves had overall increased PUN
concentrations compared to P1, but there were no differences between climatic conditions. This finding is
primarily a result of the rapid rise within the first 2-3
d, but then PUN concentrations gradually decreased and
reached basal concentrations by 5-6 d (Fig. 2). This finding differs from results of our lactation trial [4], which
indicated higher PUN concentrations in HS compared to
PF controls. The heat-induced increase in PUN (which
agrees with other HS ruminant trials [15,28]) may result
from inefficient rumen microbial nitrogen incorporation,
as heat stress is thought to alter rumen fermentation
patterns [29]. In addition, elevated PUN could be a downstream by-product of skeletal muscle proteolysis. A more
appropriate plasma indicator of muscle break down is 3methylhistidine or creatine, both of which increase in
heat-stressed rabbits [17], lactating cows [28,29], and
humans [19]. Additional evidence demonstrates that HS
decreases milk ␣ and ␤ casein concentrations [30] and
Fig. 3. Effects of heat stress (ad libitum feed intake) and pair feeding (thermoneutral conditions) on the insulin response to a glucose
tolerance test (GTT) in growing Holstein bull calves during period 2.
HS cows consistently have lower milk protein content
compared to PF controls [3,4], suggesting hyperthermia has direct effects on protein synthesizing machinery.
The effects of HS on muscle and mammary protein
metabolism is perplexing, as insulin stimulates protein
synthesis in both tissues [31,32]. Thus, there appear
to be differences between muscle and adipose tissue
responsiveness (resistant and sensitive, respectively) to
HS-induced increased basal insulin concentrations.
To gain a better appreciation for the changes in postabsorptive carbohydrate metabolism, we performed a
GTT and an ITT and observed that plasma glucose clearance in response to the GTT did not differ between HS
and PF calves. This finding contradicts those of our
lactation trial [4] and a diabetic rodent study [33], in
which a more rapid glucose clearance during HS was
observed. Despite a lack of difference in plasma glucose clearance to the GTT in the current study, HS
calves displayed a greater insulin response to the GTT
(Fig. 3) consistent with data from our lactation trial [4]
and data from heat-stressed sheep [12]. Moreover, both
HS and PF calves responded similarly with a blunted
(compared to P1) overall glucose response to the ITT.
The total AUC response to the ITT can be difficult to
interpret because of counter-regulatory systems that are
initiated (ie, glucagon and epinephrine) in response to
the insulin-induced hypoglycemia. The rate of glucose
disappearance immediately following insulin administration is a more sensitive measure of acute insulin
action, and neither period nor environment influenced
this parameter. Therefore, the HS-induced decreased
basal glucose in the current study and increased glucose disposal in our previous trial [4] may result from
increased blood insulin concentrations and/or enhanced
non-insulin–mediated glucose transport, rather than an
increase in insulin sensitivity.
Estimating energy balance (EBAL) during heat stress
(for both lactating and growing animals) introduces
M.D. O’Brien et al. / Domestic Animal Endocrinology 38 (2010) 86–94
problems independent of those that are inherent to
normal EBAL estimations. Considerable evidence suggests that a heat load increases maintenance costs in
rodents [34], poultry [35], sheep [36], and cattle (≥ 25%;
[1,37,38], however, because of complexities involved
in predicting upper critical temperatures, no universal
equation is available to adjust for this increase in maintenance [38]. Maintenance requirements are thought to
increase, as there is presumably a large energetic cost of
dissipating stored heat, and the Q10 effect (Van’t Hoff’s
Law) predicts the increase in maintenance costs [39]. Not
incorporating an HS correction factor results in overestimating EBAL and thus inaccurately predicting energy
status.
In the current study, the PF TN controls did not gain or
lose body weight, suggesting nutrient and energy intake
satisfied maintenance requirements. The HS calves consumed similar quantities of the same diet and also had
static body weight. This latter observation may indicate,
at least during growth, that HS does not increase overall maintenance requirements. If HS were to increase
maintenance costs as is frequently reported, then the
energy requirements of the HS calves should have
exceeded those of their PF TN counterparts. Consequently, the HS calves would have been consuming
inadequate energy/nutrients and should have (by definition) lost body weight. However, this was not the case, as
HS calves did not lose body mass, suggesting that maintenance costs may not have increased. Further research
is necessary to evaluate the effects of heat on maintenance requirements and to determine if physiological
state (growth vs lactation) influences energy partitioning
during thermal challenges.
Reasons for the changes in HS-induced postabsorptive metabolism are not clear. Presumably they
are adaptive mechanisms employed in an attempt to
maintain a safe body temperature. The increased basal
and stimulated insulin response likely prevents fatty
acid mobilization while simultaneously ensuring glucose uptake, and we hypothesize this is one strategy to
minimize metabolic heat production. Glucose oxidation
appears most efficient [40], as in vivo glucose oxidation yields 38 ATP or 472.3 kcal of energy (assuming
-12.3 kcal/mole as the G for ATP hydrolysis under
cellular conditions [41]) compared to the 637.1 kcal of
energy released from glucose oxidation in vitro (74.1%
efficiency). In contrast, in vivo fatty acid (ie, stearic acid)
oxidation yields 146 ATP or 1,814 kcal of energy compared to 2,697 kcal from complete oxidation in vitro
(67.3% efficiency). This 10% improvement in capturing
energy may be the reason heat-stressed animals initiate
the aforementioned metabolic adaptations.
93
5. Conclusion
Environmental conditions that prevent adequate
heat dissipation increase body temperature, and this
hyperthermia reduces production (growth, milk yield,
reproduction, etc.) in agriculturally important animals.
In fact, HS-associated decrease in productivity may be
the most economically detrimental factor facing global
animal agriculture. Herein we demonstrate that reduced
feed intake appears to fully explain why HS stunts
growth in Holstein calves. However, the mechanism(s)
by which inadequate nutrient intake and environmentally induced hyperthermia reduce body weight
gain may differ. Independent of reduced feed intake,
heat-stressed calves have altered post-absorptive carbohydrate metabolism that would not have been predicted
based on their energetic state. This altered metabolism
is characterized primarily by an increase in basal and
glucose-stimulated insulin concentrations. It will be of
interest to evaluate how changes in the insulin/glucose
axis acutely and chronically affect the immune system.
Why (and how) environmentally induced hyperthermia alters post-absorptive metabolism is ill-defined, but
identifying these mechanisms may lead to preventative
strategies (nutritional, pharmaceutical, etc.) to maximize human food production during the warm summer
months.
Acknowledgments
This work was partially funded by The University
of Arizona Experiment Station, #ARZT-136339-H-24130, and the National Research Initiative Competitive
Grant no. 2008-35206-18817 from the USDA Cooperative State Research, Education, and Extension Service.
The authors express their appreciation to J. Wheelock, R.
Burgos-Zimbleman, S. Pearce, J. English, and K. Cannon for assistance at the Agriculture Research Complex.
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