Durability of Induced Heat Tolerance by Short Term Heat Challenge at Broilers Marketing Age
A trial was conducted to determine the influence of
short term exposure to high ambient temperature at 28 and 35 days of age
on deep body temperatures (Tb) and subsequent growth of birds until 42
days of age. A total of 90 day old chicks were reared in stainless steel
battery cages and were assigned at random into 18 pens of 5 birds each,
with 9 pens containing males and another 9 pens containing females. Three
treatment groups, each represented by 3 male and 3 female pens, were represented
by T1 without any heat exposure, T2 with heat exposure
starting at day 28 and T3 with heat exposure starting at day
35. Heat stress was defined as 180 min exposure to 35±1°C.
Tb and body weights were measured at 35, 37 and 39 days of age immediately
following heat exposure. Heat stress resulted in higher Tb and Onset of
heat stress at 28 days resulted in significantly lower Tb than onset of
heat stress at 35 days. Lower Tb in T2 than T3 permitted
recovery in body weight at 42 days. Sexes responded similarly to heat
Practical means to alleviate heat stress include improving ventilation,
pelleting diet, reducing stock density, fogging, evaporative cooling,
usage of feed additives and feed restriction (Liew et al., 2003;
Dozier et al., 2005). The thermoregulatory system and behavioral
thermoregulation matures during first 2-3 weeks post hatch in chicken
(Dawson and Whittow, 2000). Chickens are homoeothermic animals and able
to maintain constant deep body temperature within a thermoneutral zone
through various mechanisms. The Tb of unstressed domestic fowls normally
varies between 41.0 and 41.5°C and is essentially constant to an ambient
temperature of 27°C (Van Kampen, 1981), but rises with higher ambient
Tb is a useful and reliable indicator of stress in poultry (Teeter et
al., 1992; De Basilio et al., 2003). Heat challenges at 3-5
days post hatch has two positive effects at 6th week of age indicating
acclimation: increased growth rate and increased thermotolerance (Yahav
and Hurwitz, 1996; Yahav and McMurtry, 2001). Similar results from various
heat challenges were reported at older ages (May et al., 1987;
Lott, 1991; Teeter et al., 1992). But, there is no information
regarding the persistency of acclimation after thermoregulatory system
maturation. Chickens that have become acclimated exhibit larger combs
and wattles, less fat and feather cover than controls (Van Kampen, 1981;
Meltzer, 1987). Generally, acclimation is associated with increased heat
loss and decreased heat production.
To date, the persistency of acclimation of male and female broilers has
not been reported. The objective of this study was to measure the persistency
of heat acclimation on deep body temperature and growth performance of
male and female broilers after exposure to high environmental temperature.
MATERIALS AND METHODS
Ninety day-old commercial broilers (Arbor Acres Plus) were obtained from
a government accredited commercial hatchery and housed at poultry unit
of Department of Animal Science. The breeder age was 45 weeks old. Males
and females were reared separately in stainless steel wire meshed floor
battery cages in an environmentally controlled chamber where temperature
and ventilation were controlled. Ambient temperature of the chamber was
initially set at 32±1°C, reduced to 29±1°C on day
4 and then gradually decreased to 24±1°C by day 21 and thereafter.
On day 27, birds were individually weighed and sorted into 4 weight classes
between 850-1050 g for females and between 950-1150 g for males (each
class with a 50 g interval). Then one chick from each class was assigned
to each pen. The three treatment groups consist of T1 as control
without heat stress exposure, T2 with heat stress from day
28 onwards and T3 with heat stress from day 35 onwards. Each
treatment consisted of 30 birds. Provided diets (Gold Coin Sdn. Bhd.)
were 22% CP, 2950 kcal ME kg-1 as crumble from day 1 to day
21 and 20.6% CP, 3100 kcal ME kg-1 as pellet form thereafter.
Water and feed were available ad libitum and continuous fluorescent
illumination was provided.
From day 28 to 42, birds from T2 were removed from their pens
daily, placed in plastic crates and transferred to another environmentally
controlled chamber with ambient temperature of 35±1°C for 3
h and 65-75% relative humidity. From day 35-42, birds from T3
were also subjected to the same physical movement and heat challenge.
T1 (control) broilers were not subjected to heat challenge.
However, they catch and placed in plastic crates to avoid the confounding
effect of being removed and placed in plastic crates from day 28-42. Feed
and water were not available during the heat challenge.
Tb was recorded for 8 males and 8 females from each treatment groups
using digital thermometer (RS stock No. 612849) on day 35, 37 and 39.
Temperature was recorded after 1 min probe insertion (3 cm depth) into
the bird`s rectum. Tb was recorded within the last 15 min of heat stress
period. After the treatment period, birds were transferred from the crates
back to the battery cages. Mortality after commencement of treatment (28
days onward) was recorded.
Statistical analysis: Body weight and relative weight gain data
were subjected to analyses of variance in a 3x2 factorial arrangement.
The three heat challenge treatments and sex of birds were considered as
factors. Tb data were analyzed by GLM procedure under split plot design
and trend analysis to investigate the effect of time (at 35, 37 and 39
days) on Tb and to find any possible linear or quadratic regression between
time and Tb. All data were analysed by computational package (SAS®
Institute, 1996). Means were separated by Duncan multiple range test when
appropriate by using SAS. The significance levels were reported at p<0.05.
Mortality data were analysed by Chi square test.
The effect of sex and onset of heat challenge on body weights and relative
weight gains of broiler chickens at 35 to 42 days of age are presented
in Table 1. Birds that were exposed to heat challenge
weighed less (p<0.01) and had lower relative weight gains (p<0.01)
than their controls. There was no interaction between onset of heat challenge
and sex (p>0.05). There was no difference in relative weight gains
between T2 and control (T1). At day 42, T2
showed 63% improvement in relative weight gains compared to T3.
These results showed that birds which received heat challenge at day 28
were able to recover their body weight by improvement in relative weight
gain. Birds from T2 showed lower mortality (p<0.01) than
T3 from 35-42 days. Among the T3, females had higher
mortality (37%) than males (12.5%) (Table 2). As shown
in Table 3, T2 (heat acclimated) chickens
could maintain their Tb at a lower level than T3 (non-heat
acclimated) chickens. However, it was not very long and heat acclimated
birds lose their ability to maintain lower Tb and gradually their Tb reached
to the point of non-heat acclimated group. Tb of T2 birds were
lower (p<0.01) than those of T3 at day 35 and 37 (Table
3). All treatments showed increase in Tb at day 39 which was not significantly
different between treatment groups.
||ANOVA and means for sex and onset of heat challenge
on body weight and relative weight gain of broiler chickens at 35
to 42 days of age
|NS- Not significant; * p<0.05; ** p<0.01, a, b
Means with no common superscripts within a column-subgroup differ
significantly, 1T1: Control; T2:
Heat stress from day 28 onwards; T3: Heat stress from day
||Mean mortality rate (%) of broiler chickens1
by sex and onset of heat challenge at different age period
|1Equal number of birds from each heat challenge
duration and sex treatment groups (3 replicate pens, 4 birds per pen)
were exposed to heat stress (35°C) or No-heat stress (21°C),
x,yMeans for sex within each period with no common superscript
in a same row differ significantly (p<0.05), a,bMeans
for treatments with no common superscript in a column differ significantly
There was no interaction between sex and heat challenge duration on Tb
for both T2 and T3. Trend analysis was carried out
to investigate the regression between time and Tb; it was revealed that
except for male birds of T3, there were no significant regressions
during 35-39 days of age (Fig. 1).
||ANOVA and means of sex and heat challenge duration on
deep body temperature (°C) of broiler chickens at 35-39 days of
| NS: Not Significant; *p<0.05; **p<0.01,
a,bMeans with no common superscripts within a column-subgroup
||Quadratic regression curve and equation between time
(age) and Tb for T3 male birds during 35-39 days of age
The significant linear and quadratic regression equations (p<0.01)
of Tb with time in T3 male birds are shown in Fig.
1. Mean Tb was recorded as 41.05°C for control group in both male
and female birds.
Acclimation is a more extensive adaptation than thermal conditioning,
which involves hypothalamic thermoregulatory threshold changes that enable
chickens, within certain limits, to cope with acute exposure to unanticipated
high ambient temperature. The obstacle of thermal conditioning lies in
its inefficiency to raise thermotolerance close to the level achieved
through acclimation (Yahav, 2001). The present study showed that subjecting
birds to acclimation (T2) reduced Tb, suggesting better ability
to cope with heat stress. This result confirmed earlier findings that
acclimated chickens were better able to maintain body temperatures at
a point which was lower than that of non-heat acclimated chickens following
heat exposure (May et al., 1987; Teeter et al., 1992; Yahav
and Hurwitz, 1996; Yahav and Plavnik, 1999). After 3 h of heat stress
the non-heat acclimated chickens (T3) exhibited a significant
rise in Tb by almost 3.5°C, while the response in heat acclimated
group (T2) was only 2.7°C. In the present study, heat acclimated
birds did not exhibit a greater reduction of Tb at day 39 than the non-heat
acclimated chickens. However, May et al. (1987) practiced acclimation
at an older age and observed reduction in the Tb of acclimated chickens
during heat challenge.
Guerreiro et al. (2004) reported that broilers raised at high
temperature showed steep colonic temperatures increase during the first
2 h of heat challenge (1.06°C h-1) and more slowly thereafter
(0.59°C h-1). Broilers reared at thermoneutral temperature
showed a small increase in the first 4 h of heat stress (0.18°C h-1)
and then colonic temperature increased sharply (0.72°C h-1).
Nevertheless, both groups presented similar final colonic temperature
by the end of the heat challenge period. This inconsistency may be due
to procedural difference for acclimation and heat challenge duration.
Guerreiro et al. (2004) used a 420 min heat challenge period and
gradually increased temperature from 28 to 42°C while May et al.
(1987) exposed the birds to 41°C during a 210 min time. The rate of
increase in colonic temperature during the heat challenge period in the
heat-acclimated birds might be associated with the process of reducing
water loss in order to prevent dehydration.
Previously Yahav and Hurwitz (1996) demonstrated that as the phase of
thermal conditioning being extended, the compensatory growth response
deteriorated. This development of positive gain in Tb maybe occurred because
of the ongoing trend of bird`s metabolic rate and subsequent heat production.
Cooper and Washburn (1998) indicated that broiler chickens lose their
ability to thermoregulate efficiently under extreme conditions because
of their dramatically increased growth rate. Therefore, it seems that
metabolic rate follows the same developmental pattern like dramatic growth
rate during 6th week of age. However, the data on relative weight gain
indicated that although heat acclimated chickens lost their ability to
keep low Tb, they succeeded to recover their productive potential to the
level of control birds. Moreover, significant differences in mortality
rate of heat acclimated and non-heat acclimated birds confirmed the above
It was suggested that the feed consumed by the acclimated birds created
a delay in nutrient metabolism or carryover of nutrients for utilization
during the next hours of heat stress or thermoneutral period (Wiernusz
and Teeter, 1996). Thus, it may be concluded that decreased feed intake
not only allows the birds to reduce heat production but, also enables
maximum acclimation expressions such as increased respiration efficiency
and therefore lower relative weight gain in acclimated birds than non-heat
In conclusion, both male and female broilers could acclimate to heat
stress during last two weeks before marketing age, but this adaptation
was not stable and long lasting, however they compensated for relative
weight gain at the end of 6th week of age. In another word, acclimation
after thermoregulatory system maturation during within first two weeks
of life is not seems to be persistent throughout the life.
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