Effect of Dietary Energy and Stocking Density on the Performance and Sensible Heat Loss of Broilers Reared under Tropical Winter Conditions

The objective of the present study was to evaluate the effects of different dietary energy levels and stocking densities on the thermoregulating parameters, live performance, and carcass traits of broilers reared under tropical winter conditions at different times of the day. In total, 1,312 one-d-old male broilers were used. Birds were allotted to three different stocking densities (10, 14 or 18 birds/ m2) and two dietary energy levels (2900 or 3200 kcal ME/kg). The following parameters were evaluated:radiant heat load (RHL), rectal temperature (RT), feed intake (FI), weight gain (WG), feed conversion ratio (FCR), livability (L), production of live weight per area (WA), and carcass yield. Stocking density did not affect sensible heat loss (SRL) or rectal temperature (RT); however, as expected, sensible heat loss (SRL) and RT were influenced by time of the day, with higher values in the morning and in the afternoon, respectively. There was no effect of treatment (p>0.05) on carcass or parts yield. Feed intake was reduced in 3%, whereas weight gain and feed conversion ratio improved in 8 and 10%, respectively, as dietary energy level increased. On the other hand, stocking density did not influence live performance or carcass traits. Based on the present results, it is concluded that sensible heat loss depends on dietary energy levels and particularly on time of the day. Therefore, environmental house management is suggested during tropical winters in order to reduce differences between broiler skin and environmental temperatures in the morning and in the afternoon.


InTRoduCTIon
Advances in genetics, nutrition, management, housing environment, etc., have been essential to promote better broiler performance.However, increasing production capacity in terms of live weight per area is still a challenge.
Another relevant aspect in broiler production is the manipulation of dietary energy levels.Goldfus et al. (1997), studying dietary energy levels (2,800; 3,000 or 3,200 kcal ME/kg) and two stocking densities (10 or 22 birds/m 2 ) observed reduced feed intake, better feed conversion ratio, and higher abdominal fat content in the broilers fed the highest energy level.In another study, Oliveira Neto et al. (2000) showed that feed conversion ratio improved as dietary energy level increased.However,

Effect of Dietary Energy and Stocking Density on the Performance and Sensible Heat Loss of Broilers Reared under Tropical Winter Conditions
it was found that increasing dietary energy levels in different feeding programs did not result in significant carcass quality differences (Duarte et al., 2007).
The environment directly influences poultry survival and performance.Adequate housing facilities should provide adequate temperature, radiation, relative humidity, and air velocity inside the poultry houses in order to allow birds to maintain thermal balance at the lowest metabolic cost, thereby increasing their productivity.There are few studies relating thermoregulating mechanisms to productivity in broilers, such as on heat production, sensible or latent heat loss, surface and body temperature of broilers reared at different stocking densities, particularly under field conditions.
It is important to consider the relations between environmental temperature, stocking density and dietary energy level in order to maximize production.The objective of the present study was to evaluate the effects of different dietary energy levels and stocking densities on the thermoregulation, live performance, and carcass parameters of broilers reared under tropical winter conditions at different times of the day.

MATeRIAlS And MeThodS
The experiment was carried out in a facility located at 21 o 15'22" S, 48 o 18'58" W and 595 m altitude.According to Koppen's classification, the climate is Awa, described as dry tropical during winter, with a defined dry season (April to September), and rainy during the summer months.The environmental parameters air temperature and sun radiation were measured during the entire experimental period and are shown in Figures 1 and 2, respectively.Air relative humidity during the experimental period ranged between 70 and 90%.This characterizes the so-called "Indian summer" during tropical winters, i.e., hot day and cold nights.
A total of 1,312 one-d-old male Ross broilers were distributed into 24 (2.8x1.4x3.5m)pens in a clay-roof broilers house with East-West orientation.Birds were submitted to the conventional management used in commercial farms, and received feed and water ad libitum.The floor was covered with 10-cm high woodshavings litter.
Feeds were based on corn and soybean meal, containing 22% crude protein (CP) in the starter (1-21 days) and grower (22-42 days) phases and supplied the nutritional requirements of each phase, according to Rostagno et al. (2005).A completely randomized experimental design in a 3 x 2 factorial arrangement, consisting of three stocking densities (10, 14 or 18 birds/m 2 ) and two dietary energy levels (2,900 or 3,200 kcal ME/kg), was applied.
Bird surface temperature was measured using an infrared thermometer (IT-330-Horiba, Japan) in five body areas: back, base of the comb, tip of the right wing, left leg, and head.Rectal temperature was measured with the aid of a fine-tip probe (model 402 LN-K73517, Yellow Spring Instruments, YSI, USA) connected to a six-channel telethermometer (model

Effect of Dietary Energy and Stocking Density on the Performance and Sensible Heat Loss of Broilers Reared under Tropical Winter Conditions
46 TUC, YSI, USA), which was inserted 4-7cm inside the rectum.Data were collected from day 10, daily, in the morning (before sunrise, 06:30h) and when the sun passed by the meridian (zenith, approximately 12:30h).
In order to calculate sensible heat loss (SRL, in watts), the following equation was used: SRL = ECA (Ts 4 -Tg 4 ), where: E -surface emissivity (feathers = 0.94), C -Stefan-Bolzman's constant (5.67 x 10 -8 W/m 2 ), Ts -average broiler surface temperature, Tg -average environmental temperature, and A = broiler surface area in m 2 , where: A = 8.19 W 0.67 , where W = body weight (g).The following performance parameters were evaluated:weight gain, feed intake, feed conversion ratio, livability, and meat production/area.At the end of the experiment, two birds per replicate were sacrificed and their carcasses evaluated for breast, drumstick+thighs (leg), wing, and abdominal fat yields.
Data were submitted to analysis of variance (ANOVA) using the General Linear Models procedure of SAS software program (SAS, 2000), and means were compared by the test of Tukey (p ≤ 0.05).

ReSulTS And dISCuSSIon
Table 1 presents performance results as a function of stocking density and dietary energy level.Weight gain, feed intake, and feed conversion ratio were not influenced (P>0.05) by stocking density.However, it must be mentioned that, as stocking density increased, feed intake of broilers housed at 14 and 18 birds/ m 2 was, in average, 30 and 40 g lower, respectively, compared with those housed at a density of 10 birds/ m 2 .This feed intake difference may be explained by the restricted access to the feeders, as suggested by Sorensen et al. (2000) or by the restricted movement of broilers housed at higher densities (Estevez et al., 2007;Simsek et al., 2009).Consistent results were described by Moreira et al. (2004), who observed that stocking density did not influence feed conversion ratio in their experiment.The authors worked with stocking densities of 10, 13, and 16 birds/m 2 and detected weight gain differences among birds.On the other hand, livability was significantly influenced (p ≤ 0.05) by stocking density, and presented 5.5 and 10.1% reduction in broilers housed at 14 and 18 birds/m 2 compared with those housed at 10 birds/m 2 .As expected, meat production per area increased (p ≤ 0.05) with stocking density.

Effect of Dietary Energy and Stocking Density on the Performance and Sensible Heat Loss of Broilers Reared under Tropical Winter Conditions
Dietary energy levels determined significant differences (p ≤ 0.05) in weight gain, feed intake and feed conversion ratio, but did not affect livability.Feed intake was reduced in 3%, whereas weight gain and feed conversion ratio improved in 8 and 10%, respectively, as dietary energy level increased from 2,900 to 3,200 kcal ME/kg (Table 1).The findings of the present study are consistent with those reported by other authors, who showed that broiler performance improved with increasing dietary energy levels (Bertechini et al., 1991;Zanusso et al., 1999;Sakomura et al., 2004).Some authors associate the better performance observed when energy-rich diets are fed to management conditions and to genetic potential.
On the other hand, seasonal climatic conditions should be taken into consideration when using diets with different energy levels, as it is known that maintenance requirements change when broilers are submitted to conditions outside their thermal comfort zone, particularly at lower temperatures, when their requirements increase.In the present study, tropical winter climate conditions (Figures 1 and 2) were recorded, with hot temperatures during the day and cold nights, which characterizes the so-called "Indian summer".Therefore, environmental temperature variation range is relevant for broilers, which, in addition to the energy for the maintenance of homeothermia, also increase carcass energy reserves (increased fat deposition).
As expected, there was an increase in live weight production per area (kg/m 2 ) as a function of stocking density, which was also described by several other authors (Goldfus et al., 1997;Moreira et al., 2004;Tong et al., 2012).Relative to dietary energy level, production per area significantly increased with dietary energy level (Table 1); however, this effect was only observed in broilers reared at densities of 14 and 18 birds/m 2 (data not shown).
Carcass and parts yield was not influenced (p>0.05)neither by stocking density nor by dietary energy level (data not shown).This shows that the dietary energy levels were sufficient supply broilers' requirements for thermoregulation and growth, and did not result in any carcass or parts yield loss.Using different stocking densities and nutritional managements, other authors did not report any effect of stocking density (Tong et al., 2012) or dietary energy level Goldfus, 1997;Campos, 1999;Moreira et al., 2004) on carcass yield.It must be noted that, despite the supply of different dietary energy levels, no significant differences (p>0.05) were detected in abdominal fat carcass content (expressed as percentage of live weight) among broilers reared at different stocking densities (data not shown).However, Moreira et al. (2004) reported differences in abdominal fat content in female broilers, but not in males, as it is the case of the present study.
There was no effect of stocking density on sensible heat loss (SRL) or rectal temperature (Table 2).However, the broilers fed the high energy level (3,200 kcal ME/ kg), despite not presenting different body temperature compared with those fed 2,900 kcal ME/kg, presented higher sensible heat loss (5.42 and 4.95 W/bird for the high and the low energy level, respectively).These results suggest that broilers fed high energy levels present unfavorable energy balance compared with those fed low energy levels, particularly under the conditions of the present study (winter), when sensible heat loss becomes relevant for the maintenance of homeothermia.Considering the thermal variation of the environment, as expected, the results showed important sensible heat loss before dawn (06:30h) compared with zenith (12:30h), with 7.42 vs. 2.95 W/bird, respectively.Rectal temperatures were also different (p ≤ 0.05), being higher in the afternoon than in the morning (41.71 vs. 41.23 o C).These findings show that the lower sensible heat loss in the afternoon resulted in an increase in rectal temperature.These results also demonstrate the importance of environmental management at both times of the day for the maintenance of broiler energy balance, because the difference between skin and environmental temperatures was more important in the morning than in the afternoon.Consequently, Furlan RL, M Macari

Effect of Dietary Energy and Stocking Density on the Performance and Sensible Heat Loss of Broilers Reared under Tropical Winter Conditions
the heat flow between the bird and the environment is higher in the morning than in the afternoon.Havenstein et al. (2003a,b) showed that the cardiovascular and respiratory systems of fast-growing broilers are not well developed, and these systems are critical for the control of latent or sensible heat exchange mechanisms.Therefore, in addition to the genetic traits associated to fast growth, it is difficult to broilers to dissipate heat, thereby increasing their sensitivity to environmental stresses.Sensible heat loss capacity may be increased by early thermal conditioning, with lower radiation and convection losses, as shown by Yahav et al. (2005).However, in the present study, birds were no submitted to this early conditioning, and therefore, the higher or lower sensible heat loss observed is associated to temperature differences between the skin and the environment.

ConCluSIonS
Based on the present results, it is concluded that sensible heat loss depends on dietary energy levels and particularly on time of the day, with significant losses before dawn.Therefore, proper environmental house management is suggested in order to reduce temperature differences between broiler skin and the environment in the morning and in the afternoon.

Table 1 -
Feed intake (FI, g), weight gain (WG, g), feed conversion ratio (FCR), livability (L, %), and production of live weight per area (WA, kg LW/m 2 ) of broilers fed different dietary energy levels and housed at different stocking densities.
3 Data transformed in arcsine x 1/2 a-c -Means followed by the same letter in the same column are not different by the test of Tukey (5%).Nogueira WCL, Velásquez PAT, Furlan RL, M Macari

Table 2 -
Effect of dietary energy level, stocking density, and time of the day on broiler sensible heat loss (SRL, Watts) and rectal temperature (°C).