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Effect of high temperature on feeding behaviour and heat production in
group-housed young pigs
Anne Collin, Jacob van Milgen, Serge Dubois and Jean Noblet*
Unite
Â
Mixte de Recherches sur le Veau et le Porc, Institut National de la Recherche Agronomique, 35590 Saint-Gilles,
France
(Received 31 May 2000 ± Revised 18 December 2000 ± Accepted 11 January 2001)
To assess the acclimation of pigs to heat stress, the effects of high (338C) or thermoneutral
(238C) constant temperatures on feeding behaviour and components of energy balance were
studied in group-housed young pigs. Three groups of five pigs were used at each temperature.
After 1 week of adaptation, voluntary feed intake (VFI) and heat production (HP) were recorded
for thirteen consecutive days. Animals were fed ad libitum. Fasting HP was measured on the last
day. Average initial body weights (BW) were 21´4 and 20´9 kg at 23 and 338C respectively.
Feeding behaviour was measured individually and rate of feed intake and characteristics of
feeding behaviour were calculated. The O
2
consumption, CO
2
production and physical activity
of the group were used to calculate total HP (HP
tot
) and its components, i.e. fasting HP (HP
fas
),
HP due to physical activity (HP
act
) and thermic effect of feed (TEF). The BW gain and VFI were
reduced by 37 and 30 % respectively at 338C. The decrease in VFI corresponded to reduced
consumption time (234 %) and size of the meals (232 %). Feeding behaviour was mostly
diurnal (66 % of the VFI), and the rate of feed intake (28 g/min) was not affected by
temperature. Daily HP
tot
,HP
fas
and TEF, expressed per kg metabolic weight (BW
0´60
), were
significantly decreased at 338C by 22, 18 and 35 % respectively, whereas HP
act
was not
affected; TEF expressed per g feed was not affected (2 kJ/g). The decrease in HP
tot
at 338C was
caused by a reduction in TEF and HP
fas
(kJ/d per/kg BW
0´60
), which are both related to
reduction in VFI.
Pig: High temperature: Heat production: Feeding behaviour
Within the thermoneutral zone, dietary energy is used for
growth, maintenance and physical activity. Below thermo-
neutrality, additional energy may need to be diverted from
productive processes in order to maintain homeothermy.
Under warm conditions, all heat produced has to be
evacuated. Pigs exposed to warm environments acclimate
by increasing (evaporative) heat loss and by reducing heat
production, to maintain body temperature within narrow
limits. However, they have a limited capacity to lose heat
by water evaporation (Ingram, 1965) and acclimation to
warm climatic conditions mainly occurs by reducing heat
production (Nienaber & Hahn, 1982; Quiniou et al. 2001).
Reduction in voluntary feed intake (VFI) and the associated
thermic effect of feeding (or heat increment; TEF) is an
efficient mechanism to reduce heat production (Quiniou
et al. 2001). Heat production may also be reduced by a
decrease in physical activity or BMR. Although there is
information about the effect of temperature on the total heat
production (HP
tot
) in pigs (Nienaber & Hahn, 1982;
Nienaber et al. 1987; Quiniou et al. 2001), little is known
about the change in the components of heat production at
high temperature. Moreover, these phenomena have been
studied mainly in growing pigs and sows. A previous study
showed that VFI was at a maximum between 19 and 258C
in young pigs weighing about 20 kg, decreased regularly
between 25 and 338C and was severely depressed above
338C, suggesting that young pigs are heat stressed above
258C (Collin et al. 2001). Based on these results, two
temperatures were chosen, representing temperatures
within the thermoneutral zone (238C) or causing severe
heat stress (338C). The present study is part of a larger
programme in which the effects of high v. thermoneutral
temperatures are studied in young pigs. The programme
includes studies on the physiological (fractional distribu-
tion of blood flow) and metabolic and nutritional
(mitochondrial metabolism, heat production, feeding
DOI: 10.1079/BJN2001356British Journal of Nutrition (2001), 86,63±70
q The Authors 2001
Abbreviations: BW, body weight; BW
0´60
, metabolic body weight; HP
act
, heat production due to physical activity; HP
fas
, fasting heat production;
HP
tot
, total heat production; TEF, thermic effect of feed; VFI, voluntary feed intake.
* Corresponding author: J. Noblet, fax +33 223 48 50 80, email noblet@st-gilles.rennes.inra.fr
behaviour) consequences of exposure to high temperatures.
The present paper focuses on the effects of a high constant
temperature on VFI, heat production and its components in
20±30 kg pigs.
Materials and methods
Animals and husbandry
Crossbred (Large WhiteLandrace)Pie
Â
train male pigs,
castrated at about 14 d of age and weaned at 28 ^ 1d of
age were used for the experiment. After weaning, pigs were
reared in groups of twelve animals in a conventional
nursery room and offered ad libitum a starter diet providing
213 g crude protein N 6´25=kg; 14´0 g lysine/kg and
13´6 MJ metabolisable energy (ME)/kg. At 2 weeks after
weaning a group of six pigs of similar body weight (BW)
and age, but from different litters, was constituted and
placed for adaptation on a 2´3 1´6 m flat-deck in a room
controlled for temperature and relative humidity. The
temperature was initially set at 258C, and was then
progressively decreased or increased within 4 d to either
23 or 338C. At the beginning of the adaptation period, the
pigs, BW was 15´2 (
SD 1´6) kg and their age was 46 (SD 1)
d. Three groups of pigs were used successively at each
temperature.
Rectal temperature and feed intake attain a new constant
value within 96 h after exposure to high temperatures
(Rafaõ
È
, 1974; Giles, 1992). Consequently, 1 week was
considered to be sufficient to adapt the pig to their new
environment (at least for the criterion of interest). Five pigs
were selected (day 0) from the group on the basis of a
similar BW and placed in a pen 2´3 1´6 m in a
respiration chamber (12 m
3
) equipped with a metal slatted
floor and a slurry pit allowing storage of urine, faeces and
water spillage. The slurry pit was emptied on days 6 and 13.
A feed dispenser and a drinking station allowed measure-
ment of water and feed intakes during the experiment. Each
pig had an ear tag in order to be detected when close to the
trough. A fence prevented two pigs from being present at
the same time near the trough. Pigs were weighed in the
morning of days 1, 6, 13 and 14. At the start of the
measurement period (day 1), the average BW was 21´2 (
SD
1´4) kg. Animals were offered ad libitum a standard
pelleted diet (Table 1) over the adaptation period and the
first 12 d of the measurement period and fasted over day
13. The photoperiod was set at 12 h light (08.00±20.00
hours). Water was available from a nipple drinker located
at a distance of 2 m opposite the trough.
The experiment consisted of measuring the feeding
behaviour and heat production at one of the two temperatures
over twelve and thirteen consecutive days respectively.
Values for day 1 and day 6 (weighing days) were excluded
from the data set. Three periods (days 2±5, 7±9 and 10±12)
were distinguished, to account for the rapidly changing BW
of the pigs during the experimental period.
Behaviour
Measurements. The equipment was the same as that
described by Quiniou et al. (2000). Brief, variables
concerning feed intake behaviour were recorded daily and
included pig number, times at the initiation and the end of
the instability of weight of the trough (i.e. visit), and feed
consumption during each visit. Physical activity of the
group was recorded using force sensors (type 9104A;
Kistler, Winterthur, Switzerland) on which the metabolism
cage was mounted.
Calculations. Several visits can occur within the same
meal, with short pauses detected by the computer as a
`steady weight' of the trough. To allow comparisons to be
made between studies carried out under various conditions,
successive visits were combined according to a meal
criterion (Bigelow & Houpt, 1988; Labroue et al. 1994).
Two successive visits, separated by an interval longer than
the meal criterion, were considered as belonging to two
different meals. Based on data of the present experiment,
91 % of the individual estimated meal criteria was below
2 min. This value, also reported by Quiniou et al. (2000) in
heavier pigs, was used as the meal criterion in further
calculations.
The feeding behaviour was described for each pig by the
daily number of meals, the duration and amount of feed
consumed and the feeding rate. Ingestion time was
calculated as the cumulative duration of visits, and
consumption time as the cumulative duration of meals.
These calculations were carried out for the day and the
night periods.
The recorded physical activity was based on the average
signal of the force sensors during a 10 s interval. Data were
analysed to distinguish different types of activity such as
long periods of relative calm (e.g. resting) from short,
intense movements (e.g. fighting or locomotion). Five
arbitrary classes of activity (based on the signal of the force
sensors) were defined, allowing quantification of the
intensity of physical activity for each class of activity.
Table 1. Ingredients and chemical composition of the diet offered to
group-housed pigs
Ingredients (g/kg)
Maize 280´5
Soyabean meal 255´0
Wheat 232´5
Barley 145´0
Fish meal 50´0
Dicalcium phosphate 18´0
Calcium carbonate 9´0
Mineral and vitamin mixture 5´0
Salt 4´0
Lysine hydrochloride 0´5
Fungicide 0´5
DM (g/kg) 884
Analysed levels (g/kg DM)
Ash 78
Crude protein N 6´25 248
Crude fibre 31
Starch 437
Fat 29
Energy values (MJ/kg DM)
Gross energy 18´3
Metabolisable energy* 16´1
Net energy² 11´8
* Determined in a 6 d balanced trial with six pigs averaging 17´2 (SD 1´3) kg
and kept at an environmental temperature of 258C.
² Calculated from equations of Noblet et al. (1994).
64 A. Collin et al.
Heat production
Measurements. An open-circuit respiration chamber
(12 m
3
) based on a design similar to that of Vermorel
et al. (1973) was used. Both temperature and relative
humidity were maintained constant (23 or 338C and 70 or
60 % relative humidity respectively) during the experi-
ment. Gas (CO
2
and O
2
) contents of ingoing and outgoing
air and ventilation rate (and weight of the trough and
physical activity, see earlier) were continuously and
simultaneously recorded over 10 s intervals during the
13 d experimental period.
Calculations. Calculations were carried out according to
the model of van Milgen et al. (1997). This relates observed
changes in O
2
and CO
2
concentrations in the respiration
chamber to physical aspects of gas exchange and to O
2
consumption and CO
2
production by the animals; the
ACSL/Optimize software (version 2.4; AEgis Simulation,
Inc., Hurtsuille, AL, USA) was used. Heat production was
calculated from gas exchanges according to the formula of
Brouwer (1965). In the present experiment a large number
of visits to the trough were recorded (on average 240/d and
group) and they were strongly correlated with peaks of
physical activity. It was therefore impossible to estimate
simultaneously heat production associated with physical
activity (HP
act
) and the short term TEF. To circumvent this
problem, the heat production per unit force was estimated
only during fasting and it was assumed that this value was
independent of BW and feeding status. The product of this
variable and the actual daily force measured in fed pigs
provided an estimate of daily HP
act
. Measurements of heat
production in fasting animals allowed the calculation of
fasting heat production (HP
fas
). It was assumed that the
HP
fas
value obtained on the last measurement day was
applicable to fed animals during the previous 12 d (based
on the estimated BW for each day and assuming a constant
HP
fas
on a per kg metabolic BW (BW
0´60
) basis). TEF was
the difference between HP
tot
and the sum of HP
act
and
HP
fas
. Energy balance data were expressed as MJ/d or per
kg BW
0´60
(according to Noblet et al. 1999).
Statistics
Some data were measured in individual animals (e.g.
feeding behaviour, feed intake), whereas other data were
measured for a group of animals (e.g. heat production). In
the analysis all results are reported on a `per pig' basis.
Performance data (BW, BW gain, VFI, voluntary water
intake and feed conversion ratio), as measured between
days 1 and 12, and individual feeding behaviour were
analysed using the general linear models procedure of
Statistical Analysis Systems (release 6´07; SAS Institute
Inc., Cary, NC, USA). To account for the rapidly-changing
BW of the animals, the experimental period was divided in
to three sub-periods (period 1, days 2±5, period 2, days 7±
9; period 3; days 10±12). The individual components of
feeding behaviour (per period) were subjected to ANOVA
with temperature, period, their interaction, the interaction
of temperature and the group, and the interaction of
temperature, group and animal as main effects, and using
temperaturegroup as the error term for testing tempera-
ture, and temperaturegroupanimal as the error term for
testing temperaturegroup. The diurnal percentage of total
activity was calculated from the measurements per group
and analysed with temperature, period, temperatureperiod
and temperature group as main effects, and tempera-
turegroup as the error term for testing temperature.
The overall effect of temperature (for the whole
experimental period) on mean VFI, heat production and
its components, retained energy and RQ was analysed by
ANOVA. To account for the changing BW during the
experiment, data (per period) were analysed using tem-
perature, period, their interaction and the temperature
group interaction as main effects. The latter term was used
as the error term to test the effect of temperature.
Results
Performance
As expected, VFI was reduced at 338C (Table 2), being 30 %
lower than at 238C with subsequent lower BW gain at 338C
than at 238C (621 g/d v. 987 g/d). In addition, the feed
conversion ratio was significantly lower at 238C than at 338C
(1´50 v. 1´68; P , 0´05). Despite the lower feed intake, there
was a tendency for increased water consumption at 338C.
Table 2. Effect of temperature (T) on performance of group-housed young pigs over the experimental period²
(values are expressed per pig)
T(8C)
Statistical significance
of effect of T23 33 Residual
SD
No. of observations³ 3 3
Initial body weight (kg)§ 21´4 20´9 0´4
Final body weight (kg)§ 33´2 28´4 0´8 **
Fasting body weight (kg)§ 30´8 26´9 1´0 **
Body weight gain (g/d) 987 621 49 **
Voluntary feed intake (g/d) 1483 1045 74 **
Voluntary water intake (g/d) 4408 5863 2595
Feed: gain 1´50 1´68 0´07 *
*P , 0´05; **P , 0´01:
² Performance from day 1 to day 12; for details of animals and procedures, see p. 64.
³ Group of five pigs per observation.
§ Initial and final body weights were measured in fed pig (morning of days 1 and 13) and fasting body weight on the morning of day 14 (after
1 d without feed).
65High temperature and energy utilisation in pigs
Behaviour
The effects of temperature, period and their interaction on
components of feeding behaviour are given in Table 3. The
decrease of 30 % in feed intake at 338C was associated with
shorter daily ingestion time (228 %) and consumption
time (234 %) at 338C. However, the daily number of meals
and the rate of feed intake were not affected by
temperature, so that meal size was significantly lower at
338C P , 0´05: The difference between consumption
time and ingestion time was smaller at 338C, which
suggests that pigs spent more time near the trough at
thermoneutrality than at high temperature. The duration of
the meals (consumption time) was numerically decreased at
338C (3´9 min v. 5´9 min, on average).
The partitioning of feeding activity between day (67 %)
and night (33 %) was not affected by temperature. Two-
thirds of physical activity (force) was recorded during the
day at 238C. At 338C there was a more uniform partitioning
between diurnal and nocturnal activity. Even though heat
production associated with physical activity was not
affected by ambient temperature, the characteristics of
physical activity differed between the two temperatures
(Fig. 1). The physical activity in pigs kept at 238C appeared
to be more extreme (either very calm or very active)
compared with that of pigs kept at 338C. It can be assumed
that the highest levels corresponded to the standing position
and locomotion, while the lower levels corresponded
mainly to respiration in the lying position.
As anticipated, period affected VFI (g/d), but there was a
significant interaction between period and temperature
(Table 3). In other words, VFI increased regularly over
successive periods at 238C, whereas the increase was quite
small at 338C. These changes in daily VFI over the
experiment were achieved through an increased meal size.
The rate of feed intake increased from 23 to 29 g/min on
average between period 1 and period 3, irrespective of the
temperature.
Meal size and VFI, when expressed as g/kg BW
0´60
,
increased slightly over the experiment. However, the
interaction previously observed between temperature and
period for VFI and meal size was not significant.
Heat production and its components
Data on heat production in the fed and fasting states over
the experimental period are given in Table 4. Pigs exposed
at 338C had a lower fasting heat production expressed per
kg BW
0´60
(214 %) than those kept at 238C. In fed pigs the
Table 3. Effects of temperature (T) and period (P) on individual components of the feeding behaviour of group-housed young pigs²
T(8C)
23 33 Statistical significance of effect of²²²
P³ 1 2 312 3s
1
kk s
2
¶¶ T P T PT G
No. of observations§ 15 15 15 15 15 15
Duration of P (d) 433433
Mean body weight (kg) 23´6 28´0 31´5 22´7 25´5 27´4 0´4 3´0 * ** **
Mean components of daily feeding behaviour
No. of visitsk 55 58 57 38 40 39 9 53
No. of meals¶ 14´8 14´0 14´2 14´7 15´8 13´8 2´2 9´9
Feed intake (g) 1290 1576 1710 967 1129 1095 174 250 ** ** **
Feed intake (g/kg BW
0´60
) 194 214 215 148 161 150 24 30 ** *
Ingestion time²²(min) 56 57 59 43 43 40 6 5 **
Consumption time³³ (min) 77 81 80 53 55 51 8 38 *
Rate of feed intake (g/min) 23 28 30 23 27 28 3 9 **
Rate of feed intake (g/min per kg BW
0´60
) 3´5 3´8 3´7 3´6 3´9 3´8 0´4 1´2 **
Characteristics of the meal
Meal size (g) 93 123 132 69 78 86 16 60 * ** *
Meal size (g/kg BW
0´60
) 13´9 16´6 16´6 10´7 11´2 11´8 2´1 8´9 **
Ingestion time (min) 4´1 4´5 4´6 3´1 3´0 3´1 0´5 2´8
Consumption time (min) 5´5 6´3 6´1 3´9 3´7 4´0 0´7 5´2 **
Diurnal feeding behaviour (% total)
No. of meals 67 66 65 64 66 67 7 9
Feed intake 67 66 66 64 68 70 6 13
Ingestion time 67 66 65 63 69 69 7 13
Diurnal physical activity (% total)§§ 68 66 65 61 55 49 8 5 *
BW
0´60
, metabolic body weight; G, group.
*P , 0´05; **P , 0´01:
² For details of animals and procedures, see p. 64.
³ P 1 included days 2±5, P 2 included days 7±9, P 3 included days 10±12. Days 1 and 6 were not taken into account (weighing days).
§ Each observation was the mean over 3 or 4 d for one pig (five pigs per group).
k Duration of instability of the trough.
¶ One meal corresponded to a group of successive visits separated by ,2 min (i.e. meal criterion).
²² Cumulative duration of the visits.
³³ Cumulative duration of the eating and non-eating sessions within meals.
§§ Observation was physical activity of the group and the model included the effects of T, P, their interaction T P and the effect of the group within temperature
(G(T)); the error for testing T effect was G(T).
kk Residual standard error for testing the effect of P.
¶¶ Residual standard error for testing the effect of T.
²²² The model included the effects of T, P, their interaction T P; the interaction of T with G T G and the interaction of G with T and animal (A; T G A); the
errors for testing T and T G effects were T G and T G A; respectively. The effect of T G A was significant whatever the component of feeding behaviour
considered P , 0´01; except for the percentage of diurnal ingestion time P . 0´05:
66 A. Collin et al.
increase in ambient temperature from 23 to 338C resulted
in a 22 % decrease in HP
tot
. The contributions of HP
act
,
TEF and HP
fas
variations to the decrease in HP
tot
with
temperature rise were 10, 45 and 45 % respectively; the
reductions were significant for TEF and HP
fas
only
(P , 0´05 in both cases). The reduction in TEF was not
due to a decrease in TEF per g feed (2 kJ/g at both
temperatures), but essentially to the effect of VFI reduction.
The RQ was significantly lower at 338C (1´08 v. 1´12;
P , 0´05).
As indicated in Table 5, total metabolisable energy
intake, HP
tot
,HP
act
and retained energy increased over the
experiment, but the variation was smaller at 338C than at
238C for total metabolisable energy intake and retained
energy. When expressed per kg BW
0´60
, total metabolisable
energy intake, HP
tot
, its components (except HP
act
) and
retained energy were reduced at 338C (Table 5).
Discussion
Performance
The results of the present experiment demonstrate a clear
decrease in VFI and, subsequently, in BW gain at 338C.
The comparison of results obtained at 23 and 338C
indicates that VFI declined by 45 g/d per 8C, which is
similar to the 39 g/d per 8C calculated from a previous
study (Collin et al. 2001), and to results of Sugahara et al.
(1970; 42 g/d per 8C) in young growing pigs. However, this
decrease is greater than that reported by Rinaldo & Le
Table 4. Effect of temperature on voluntary feed intake, heat production and its components in group-housed young pigs²
T(8C)
23 33 Residual
SD Statistical significance of effect of temperature²²
No. of observations³ 3 3
Body weight (BW; kg) 27´3 24´9 0´8 *
Fed state
Voluntary feed intake (g/d) 1502 1054 69 **
Metabolisable energy intake (MEI; MJ/d) 21´69 15´13 0´99 **
Heat production (MJ/d)
Activity 2´01 1´76 0´19
Thermic effect of feed 3´10 2´00 0´34 *
Fasting heat production§ 6´12 5´01 0´34 *
Total 11´23 8´77 0´43 **
Retained energy (MJ/d) 10´45 6´36 0´59 **
RQ 1´12 1´08 0´01 *
Thermic effect of feed (kJ)
/kg feed 2´05 1´87 0´27
/kJ ME 0´14 0´13 0´02
/kJ MEI
0
k 0´17 0´15 0´03
Fasting heat production¶, (kJ/d per kg BW
0´60
) 849 728 49 *
*P , 0´05; **P , 0´01:
² For details of animals and procedures, see p. 64.
³ One observation corresponds to data measured in a group of five pigs; the data is expressed per pig.
§ Estimated from fasting heat production measured on day 14.
k MEI 2 heat production for activity.
¶ Calculated with BW of fed pigs (morning of day 13) as reference.
²² Days 1 and 6 were not taken into account (weighing days).
Fig. 1. Repartitioning of physical activity (signal of force sensor) at 238C( )or
338C(A) in group-housed young pigs.
a,b,c,d,e,f,g
Mean values with unlike
superscript letters were significantly different P , 0´05: For details of animals
and procedures, see p. 64.
67High temperature and energy utilisation in pigs
Dividich (1991a; 28 g/d per 8C) between 25 and 31´58Cin
young pigs between 9 and 30 kg. The reduction in BW gain
due to temperature (or feed intake) corresponded to 37 g/d
per 8C, which is greater than that reported in other studies
in young pigs (20 g/d per 8C according to Sugahara et al.
(1970) and Rinaldo & Le Dividich (1991a)). The reasons
for this discrepancy could be the higher initial BW (21 kg
v. 9 kg) in the present experiment. Also, in the present
experiment pigs were housed in groups, which may
stimulate feed intake, at least at thermoneutrality. In
addition, the relatively high BW gain and VFI recorded
at 238C could also have induced accentuated effects of heat
exposure.
Feeding behaviour
Feed intake and feeding patterns in growing pigs are
affected not only by physiological, genetic and social
factors (Bigelow & Houpt, 1988; Labroue et al. 1994), but
also by environmental factors (Nienaber & Hahn, 1982; Le
Dividich et al. 1998; Quiniou et al. 2000). In the present
study, pigs acclimatised to the hot environment by reducing
their feed intake (23´0 %/8C), which is consistent with the
decrease described by Quiniou et al. (2000) in heavier pigs
(23´4 %/8C). Chronic exposure to 338C also affected other
components of feeding behaviour in pigs. Ingestion time
per d and occupation time of the feeding station were
reduced, and the duration of non-eating sessions within
meals at 238C (22 min) was twice that at 338C (11 min).
Temperature did not seem to affect the daily number of
meals, which is consistent with results of Nienaber et al.
(1993) and Quiniou et al. (2000), also obtained in group-
housed animals.
The reduction in VFI at 338C can be partly explained by
a reduction in BW, which is the combined result of the hot
environment and the decrease in VFI. Consistent with
results reported by Bigelow & Houpt (1988) in young
growing pigs, and by Nienaber et al. (1990), Labroue et al.
(1994) and Quiniou et al. (2000) in heavier pigs, ingestion
time and consumption time remained relatively constant
during the trial, whereas the rate of feed intake increased.
Meal size increased with BW (period), whereas the number
of meals remained constant, so that the total VFI increased.
The number of meals calculated from the present experi-
ment (fifteen meals/d) is higher than the nine to eleven
daily meals calculated in heavier pigs (Xin & DeShazer,
1991; Nienaber et al. 1996; Quiniou et al. 2000). It is
consistent with results of Quiniou et al. (1999), who found
a decrease in the number of meals with increasing BW. The
period effect on feed intake (Table 5) was mainly due to the
increased BW over successive periods. The significant
interaction between temperature and period on feed intake
P , 0´05 illustrates the smaller increase at the highest
temperature, which may suggest a long-term acclimation
effect to high temperature.
Feeding behaviour was mainly diurnal, with two-thirds
of the feed consumed during the day, and was not affected
by ambient temperature. This value is similar to that
reported by Labroue et al. (1994) and Quiniou et al.
(2000a) in growing pigs kept under similar conditions.
Studies converge to show that the main factor determining
the partitioning of VFI between day and night on a given
light pattern is BW, feeding behaviour of pigs becoming
more diurnal with increasing BW (Bigelow & Houpt, 1988;
Labroue et al. 1994; Quiniou et al. 1999). Although high
temperature induced a decrease in the diurnal percentage of
number of meals in the study of Quiniou et al. (2000), it did
not change the partitioning between day and night in our
Table 5. Effect of temperature (T) and period (P) on heat production and its components in group-housed young pigs²
T(8C)
23 33 Statistical signifiance of effect of¶:
P³ 123123s
1
§ s
2
k TPT P G(T)
Mean body weight (kg) 23´6 28´0 31´5 22´7 25´5 27´4 0´3 1´3 * ** ** **
Voluntary feed intake (g/d) 1290 1576 1711 967 1128 1095 72 111 ** ** *
Metabolisable energy intake (MEI; MJ/d) 18´62 22´76 24´71 13´88 16´20 15´72 1´05 1´61 ** ** *
Heat production (MJ/d)
Activity 1´81 2´07 2´21 1´57 1´86 1´93 0´07 0´32 ** **
Thermic effect of feed 2´44 3´34 3´74 1´72 2´23 2´15 0´38 0´55 * **
Fasting heat production 5´62 6´23 6´69 4´73 5´07 5´30
Total 9´87 11´64 12´64 8´02 9´16 9´38 0´43 0´72 ** **
Retained energy (MJ/d) 8´75 11´12 12´07 5´86 7´04 6´34 0´66 0´94 ** ** *
Energy balance (kJ/kg BW
0´60
per d)
MEI 2744 3030 3103 2132 2328 2168 147 174 **
Heat production
Activity 271 281 279 241 266 265 11 38 * **
Thermic effect of feed 367 452 473 262 319 294 54 79 *
Total 1482 1577 1595 1231 1312 1286 59 68 **
Retained energy 1312 1507 1523 900 1009 869 95 114 **
RQ 1´10 1´11 1´14 1´08 1´09 1´08 0´01 0´02 *
*P , 0´05; **P , 0´01:
² For details of animals and procedures, see p. 64.
³ P 1 included days 2±5, 3±5, P 2 included days 7±9, P 3 included days 10±12. One observation corresponds to data measured in a group of five pigs; data is
expressed per pig.
§ Residual standard error for testing the P effect.
k Residual standard error for testing the T effect.
¶ The model included the effects of T, P, group within-T G(T), the error being G(T).
68 A. Collin et al.
study. It appears that, as a result of heat stress, heavier pigs
shift a part of their meals to the night, which has been
described in studies with cyclic temperatures mimicking
real daily temperatures (Feddes et al. 1989; Xin &
DeShazer, 1991).
Acclimation to high temperatures
Acclimation to high temperature results in both increased
evaporative heat loss and decreased heat production. The
first mechanism is limited in pigs, owing to the low
capacity for cutaneous evaporative heat loss. As expected,
results of the present experiment indicated a great reduction
in heat production when pigs were exposed to 338C, which
agrees with the findings of studies by Stombaugh & Grifo
(1977), Nienaber & Hahn (1982), Nienaber et al. (1987)
and Quiniou et al. (2001) in heavier pigs. Gray &
McCracken (1974) in 22 kg pigs obtained similar heat
production values at 22 and 298C, but their experiment was
carried out to ensure similar daily intakes.
The reduced TEF contributed to 45 % of the decrease in
HP
tot
at 338C, resulting directly from the decreased VFI.
The second factor contributing to the decrease in HP
tot
was
the HP
fas
(45 %). This finding conflicts with data from
Holmes (1974), who did not observe such a decrease in
heavier pigs during a 48 h fast, despite different preceding
feeding levels. It can be hypothesised that, in our study,
HP
fas
was overestimated at 238C because pigs suffered
from cold. However, Bernier et al. (1996) showed that
HP
fas
was constant at and above 248C in individually-
housed 30±50 kg pigs fed at constant feeding levels before
HP
fas
measurements. An alternative explanation for
decreased HP
fas
at 338C is the decrease in the viscera
mass (Rinaldo & Le Dividich, 1991a) and the associated
decrease in heat production (Koong et al. 1983; van Milgen
et al. 1998) resulting from the lower feeding level. Viscera
make an important contribution to HP
fas
, and 24 h of
fasting may not be sufficient to attenuate differences in
visceral mass and heat production. Summarising, the
reduction in heat production at high ambient temperatures
seems to be the result of both direct (TEF) and indirect
(HP
fas
) effects of reduced feed intake.
The contribution of HP
act
to total metabolisable energy
intake (9 %) and to HP
tot
(18 %) at 238C was slightly
higher than results obtained by Quiniou et al. (2001) at
thermoneutrality (8 % of the total metabolisable energy
intake and 14 % of the HP
tot
) in 30±90 kg pigs. At 338C,
corresponding values were higher (12 and 20 % respec-
tively) partly in connection with the reduced total
metabolisable energy intake at this temperature. In the
present study HP
act
was lower at 338C and the variation
contributed to 10 % of the HP
tot
decrease. Results of
Quiniou et al. (2001) suggested a tendency for increased
HP
act
at high ambient temperature, in connection with
intense panting of animals under heat stress. The results of
the present study also support this hypothesis, as shown in
Fig. 1. As indicated earlier, the upper values of activity
were associated with walking or standing, whereas the
lower values were associated with resting or lying. Panting
can be considered an intermediate level of activity. Pigs
kept at 338C seem to reduce energy expenditure by
avoiding voluntary movements, as in having long intervals
between visits within a meal (Quiniou et al. 2000; the
present study). Standing appears to be very energy
expensive in pigs (Dauncey, 1990; Noblet et al. 1993;
van Milgen et al. 1998) and the reduction in non-essential
activity can be considered as an adaptation to the hot
environment. Although panting seems to cause an increase
in activity, the benefit (heat loss) probably outweighs the
cost (heat production).
The RQ is indicative of the proportional rates of
substrates used for catabolism and anabolism. Catabolism
of substrates (e.g. for ATP synthesis) results in an RQ #1,
whereas fatty acid synthesis results in an RQ .1. The
observed RQ is a combination of both catabolism and
anabolism, and the RQ in growing pigs is almost always
.1. The exposure to 338C resulted in a reduction in both
feed intake and energy retention (and thus reduced fatty
acid synthesis). Similar results were found by Rinaldo & Le
Dividich (1991b), and are probably the cause of the
reduced RQ at 338C.
In conclusion, the present study provides some evidence
that the reduction in heat production in heat-stressed young
pigs is essentially caused by a reduction in the TEF and
HP
fas
. Both effects are directly related to a marked
reduction in VFI at high ambient temperatures. This
finding implies that nutritional adjustments, such as a
reduction in the fibre or protein content of the feed, could
be efficient in attenuating the negative effects of hot
climatic conditions. The present results also suggest that,
although HP
act
was not different between 23 and 338C, its
partitioning between voluntary movements (standing and
locomotion) and breathing depends on ambient tempera-
ture.
Acknowledgements
The authors thank J. Le Dividich, N. Quiniou and D.
Renaudeau for helpful discussions and suggestions, L.
Delaby for statistical advice and A. Roger, H. Renoult, J.
Gauthier, F. Thomas and M.T. Gauthier for technical
assistance. This work was supported by grants from the
Institut National de la Recherche Agronomique, Tours and
Rennes.
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