High protein diets for heat-exposed broilers
D.E. FARIA FILHO1 K.A. ALFONSO-TORRES1 D.M.B. CAMPOS1 P.S. ROSA1,2 M.
MACARI1 and R.L. FURLAN1*
1Faculdade de Ciências Agrárias e Veterinárias/Departamento de Morfologia e Fisiologia Animal,
Universidade Estadual Paulista, Via de Acesso Prof. Paulo Donato Castellane s/n 14884-900
Jaboticabal – SP, 2 Researcher Embrapa Suínos e Aves (CNPSA), Professor, Universidade do
Contestado (UnC) – Concórdia, SC, Brazil.
* Corresponding author: firstname.lastname@example.org
This study was carried out to verify de effects of high protein diets for heat-exposed broilers. Seventy
hundred and twenty male broilers Cobb® 500, from 21 to 42 days of age, were randomly distributed in
a 3 x 3 factorial arrangement, as follow: temperature/feeding schedule (32ºC/Ad libitum, 22º/Ad
libitum, and 22ºC/Restricted to the level of 32ºC/Ad libitum) and protein levels (20.0, 21.5, and
23.0%). Four replicates of 20 birds were used. The temperature/feeding schedule were used to
determine the direct effect of temperature (comparison 32ºC/Ad libitum vs 22ºC/Restricted) and the
effect of low feed intake associated with heat exposure (comparison 22ºC/Ad libitum vs
22ºC/Restricted) on broilers body weight gain and feed conversion rate. For this, the feed consumption
of broilers reared under 32ºC/Ad libitum were daily assessed and provided to the broilers reared under
22ºC/Ad libitum. After confirmation of normality and homogeneity of variances, analysis of variance
were performed and when it was significant Tukey´s test was applied. There was no significant
interaction between temperature/feeding schedule and protein levels on broilers performance. The
results showed that the direct effect of temperature accounts for 39% of body weight decrease and for
100% of poor feed conversion rate of heat-exposed broilers. The protein levels did not influence body
weight gain, but feed conversion rate was gradually improved as protein level increases. In conclusion,
heat-exposure impairs broilers performance and the use of high-protein diets are technically viable for
broilers submitted to 22 or 32ºC.
Keywords: amino acids; environmental temperature; heat increment; heat production; pair-feeding
Broilers exposed to high temperatures show poor performance, lower percentage of breast meat and
greater fat deposition (Ain Baziz et al., 1996; Geraert et al., 1996). Considering the high heat
increment of proteins (Musharaf & Latshaw, 1999), modifications in dietary protein levels have been
studied as a means to minimize the deleterious effects of heat stress in broilers. Although it has been
suggested that protein should not be increased for heat-exposed broilers (Hruby et al., 1995; Cheng et
al., 1999), low-protein diets have impaired the performance of broilers reared under high
environmental temperatures (Alleman & Leclerq, 1997; Faria Filho, 2003; Faria Filho et al., 2006). On
the other hand, Temim et al. (1999), Temim et al. (2000 a) and Gonzalez-Esquerra & Leeson (2005)
reported better performance of broilers fed high-protein diets and reared under high temperatures.
This experiment was carried out to evaluate the effect of high-protein diets on performance of heat-
Material and methods
In the pre-experimental period (1-21 days of age), the birds were fed a single diet (Table 1) and
were reared under thermoneutral temperatures: 30.2 ± 2.8ºC (1 to 7 d), 27.3 ± 2.6ºC (8 to 14 d) and
26.3 ± 3.0ºC (15 to 21 d). Relative air humidity was kept at 56 ± 12%. At 21 days of age, birds with
similar body weight (882 ± 6 g) were distributed in the experimental units. Seven hundred and twenty
male Cobb-500® broilers were distributed in a completely randomized experimental design according
to a 3 x 3 factorial, as follow: crude protein levels (20, 21.5 and 23%) and temperature/feeding
programs (32ºC/ad libitum, 22ºC/ad libitum and 22ºC/restricted). There were four repetitions with 20
birds per treatment. The 22ºC/restricted group was pair-fed with the 32ºC/ad libitum group. Therefore,
the feed intake of 32ºC/ad libitum birds was determined daily and the same amount of food was given
to the 22ºC/restricted birds on the following day. The pair-feeding system (Geraert et al., 1996)
enables to separate the direct effect of the environmental temperature from the effect of heat exposure
on the decrease in feed intake. The birds were reared in environmentally controlled rooms with wood
shavings as bedding material. Mean temperatures during the experimental period were 23.2 ± 2.4ºC,
23.5 ± 2.9ºC and 31.9 ± 2.4ºC for the groups 22ºC/restricted, 22ºC/ad libitum and 32ºC/ad libitum,
respectively, and relative humidity was 64 ± 13%.
Experimental diets were based on corn and soybean meal (Table 1), following the nutritional
requirements for the strain (Cobb, 2001). The composition of the ingredients was according to
Rostagno et al. (2000).
The following performance characteristics were evaluated from 21 to 42 days of age: feed intake
(FI), weight gain (WG), feed conversion (FC=FI/WG), rearing viability (RV = 100 – mortality rate)
and the index of productive efficiency [IPE = (DWG x RV) / (FC x 10)], where DWG is the mean
daily weight gain (g/d).
The normal distribution of studentized errors (Cramer-Von Mises test) and homogeneity of
variances (Brown-Forsythe test) were assessed. The presuppositions were violated for feed intake
because of the values from the 22ºC/restricted group, which was excluded from the analysis. After the
normal distribution had been confirmed, the data were submitted to analysis of variance using the
General Linear Model procedure of SAS® (LITTELL et al., 2002). Different means (p<0.05) were
compared by the test of Tukey at 5%.
Table 1. Composition of the pre-experimental (initial phase) and experimental diets (growing phase).
58.6 53.4 48.3
Soybean meal, 45
Choline chloride 60%
0.05 0.05 0.05
Zinc bacitracin 15%
0.10 0.10 0.10
100 100 100
Energy and Nutrients
Metabolizable energy (kcal/kg) 3,000
3,190 3,190 3,190
Crude protein (%)
0.90 0.90 0.90
Available phosphorus (%)
0.20 0.20 0.20
0.77 0.84 0.90
0.34 0.30 0.29
188 199 217
1.868 1.868 1.868
Digestible amino acids (%)
1.04 1.04 1.14
0.51 0.49 0.47
0.78 0.77 0.77
0.22 0.25 0.27
0.67 0.73 0.78
1.23 1.36 1.47
0.77 0.84 0.91
0.82 0.89 0.95
1.61 1.70 1.79
0.49 0.53 0.56
0.89 0.97 1.04
1.49 1.62 1.73
1CP = crude protein. 2Vitamin/Mineral supplement – Levels per kg diet: Vitamin A 1,500 IU; vitamin D3 500 IU; vitamin E
20 mg; vitamin K 0.5 mg; vitamin B1 2.0 mg; vitamin B2 6.6 mg; vitamin B12 20.0 mcg; Folic acid 0.1 mg; pantothenic
acid 10.00 mg; niacin 100.0 mg; antioxidant 125 mg; copper 10.0 mg; iron 50.0 mg; iodine 1.365 mg; manganese 88.00 mg;
selenium 0.25 mg; zinc 100 mg.
Results and discussion
There was no significant interaction between protein levels and the temperature/feeding program
on performance variables (Table 2).
Table 2. Means and analyses of variance for feed intake (FI, g), weight gain (WG, g), feed conversion (FC, g/g), rearing
viability (RV, %), and index of productive efficiency (IPE) in broiler chickens from 21 to 42 days of age.
Factors FI WG FC RV IEP
22ºC / Restricted
1,612 ± 10 b
1.64 ± 0.02 b
97.5 ± 1.1
456 ± 6 b
22ºC / Ad libitum
3,220 ± 26 a
1,986 ± 19 a
1.62 ± 0.01 b
96.1 ± 1.2
561 ± 9 a
32ºC / Ad libitum
2,644 ± 32 b
1,430 ± 14 c
1.85 ± 0.02 a
97.8 ± 0.7
360 ± 6 c
2,980 ± 106
1,665 ± 68
1.75 ± 0.04 a
97.5 ± 1.0
445 ± 24 b
2,918 ± 126
1,665 ± 78
1.70 ± 0.03 b
96.2 ± 1.2
453 ± 26 b
2,898 ± 109
1,698 ± 68
1.66 ± 0.03 c
97.6 ± 0.9
479 ± 26 a
TºC / feeding (T)
<0.0001 <0.0001 <0.0001 0.51 <0.0001
Crude Protein (P)
Interaction T x P
0.34 0.46 0.44 0.42 0.85
3.32 2.93 2.19 3.81 5.02
Means ± standard error followed by similar letters in the column, within each factor, are statistically similar according to the
Tukey’s test (p>0.05). *Mean feed intake = 2,132 g.
Broilers reared under 32ºC/ad libitum showed feed intake 18% lower than the 22ºC/ad libitum
birds. Weight gain of 32ºC/ad libitum broilers was 28% smaller than that of birds reared at 22ºC/ad
libitum and approximately 11% lower than the birds reared at 22ºC/restricted. These findings suggest
that 39% (11/28) of the total loss of weight gain was due to the direct effect of temperature and that
the remaining 61% were due to the low feed intake caused by heat. Feed conversion in broilers reared
under 32ºC/ad libitum was 13% worse than that of broilers reared at 22ºC/ad libitum or
22ºC/restricted, but there was no difference between the birds kept at 22ºC. Therefore, feed conversion
impairment in broilers reared at 32ºC was only due to the direct effect of temperature, i.e., the
reduction in feed intake caused by heat exposure does not result in poorer feed conversion. Rearing
viability was not affected by environmental temperature. On the other hand, temperature was directly
responsible for approximately 61% of IPE impairment, whereas 39% was due to the decrease in feed
intake as a consequence of the hot environment.
Crude protein levels had no effect on feed intake, weight gain and rearing viability. Nevertheless,
feed conversion improved gradually with the increasing levels of dietary crude protein and IPE was
better in broilers fed 23% CP.
The lower feed intake in broilers reared under 32ºC could be explained by the prevention of heat
production associated with feed intake (Koh & Macleod, 1999). In the present study, feed intake was
approximately 18% lower in 32ºC/ad libitum birds, similar to the level of 15% previously reported by
Yalçin et al. (1997). Nevertheless, according to Ain Baziz et al. (1996), feed intake might decrease up
to 36% in broilers reared at 32ºC compared to birds reared at 22ºC.
Productive indexes are poorer because of heat exposure, in part due to the lower feed intake and the
consequent deficiency in energy and nutrients. Besides, production is also reduced due to direct effects
of temperature. In order to keep body temperature constant, evaporative cooling for heat dissipation is
triggered and energy expenses are greater (Furlan & Macari, 2002). The temperature/feeding programs
used in the present study permit to quantify the contribution of such components to production
impairment (temperature or feed intake). The findings indicate that the lower feed intake was
responsible for 61% of the decreased weight gain and for 39% of the poorer IPE. Therefore, it is
possible to increase the density of the diet as a means to prevent such losses in performance.
Nevertheless, the poorer feed conversion in heat-exposed broilers was solely due to the direct effects
of temperature and might not be reverted by nutritional management.
The increase in dietary crude protein levels improved feed conversion and the index of productive
efficiency in broilers reared at 22 or 32ºC. Nevertheless, the increase in dietary protein levels was not
recommended to birds reared in hot environments (Cheng et al., 1999) due to the high heat increment
of crude protein (Musharaf & Latshaw, 1999). On the other hand, Temim et al. (2000b) showed that
the diets with high protein levels (25%) had no effect on protein turnover in the muscles Pectoralis
major, Sartorius and Gastrocnemius of broilers reared at 22 or 32ºC. Since muscle turnover is highly
associated to heat production (Macleod, 1997), the results indicate that higher dietary protein levels
did not result in higher heat increment. The present findings showed that the use of diets with high
protein levels is a technically viable practice in broiler chickens reared under thermoneutral or high
temperature conditions, corroborating previous findings (Temim et al., 1999; Temim et al., 2000a;
Gonzalez-Esquerra & Leeson, 2005).
In conclusion, heat exposure impairs performance of broilers chickens, and the use of high-protein
diets are technically feasible for broiler chickens reared under thermoneutral or hot environmental
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