3. Results and Discussion
In winter, the aviaries’ average interior daily temperature and relative humidity were not significantly different, nor were the summertime average daily temperatures (
p > 0.05). However, the mean daily summer relative air humidity was different (
p < 0.05), as
Table 1 points out, although with a range of only 4% in mean daily values between Aviary 43W and Aviaries 69W and 86W.
The significant but small magnitude variation in relative humidity among aviaries during the summer may be due to the predominance of rain and northwesterly winds, which directed humid air into the interior of the aviary, mainly the leeward region (
Figure 1b). The winds were generally from a northerly direction in winter, falling directly on the aviaries with exposed sides (
Figure 1a). These aviaries prevented the wind from flowing to the adjacent aviaries, generating homogeneity in the interior relative humidity.
Due to the interference of prevailing winds, the geographical position of the aviaries, and the statistical difference highlighted in
Table 1, the 43W aviary was chosen to assess the spatial variability of the thermal conditions during sampling for external egg quality (
Figure 2). As a result, the average values of temperature and relative humidity in winter and summer varied according to levels (N) and lines (L), and the N × L interaction was significant (
p < 0.001), as shown in
Table 2.
The interaction between levels and lines was visualized through the spatial distribution of temperature and relative humidity values obtained inside the 43W house in cross-section (lines × levels), as shown in
Figure 3.
During winter, inside the house, the level N1 air temperature varied between 19 and 21 °C, with the highest values in lines L2 and L3. At level N2, there was an increase in temperature in the center lines of the house (L2, L3, and L4), with values around 22 °C. At level N3, the heat concentrated around the L4 line, with a mean of 23 °C.
Interior relative humidity in winter was low and varied between 34% and 56%. The dry air concentrated in the center of the aviary with values around 35%, where the air temperature was greatest. The sides of the aviary, being open and exposed to the external environment, presented higher relative humidity values, between 44% and 56%.
During summer, the lowest air temperature values were measured in lines L1 and L5 at all levels. The hot air was concentrated in the center lines of the aviary, suggesting a need for greater ventilation, with the highest temperature values, between 27% and 28 °C, observed at level N3. The relative humidity of the air was around 50% to 65%; as in winter, the dry air was concentrated in the center of the aviary, ranging between 50% and 57%.
In mechanically ventilated caged aviaries, Zheng et al. [
24] found interior barn temperatures ranging from 26.5 ± 1.1 to 28.1 ± 1.1 °C during winter periods with outside temperatures of 13.0 ± 4.8 °C; and 29.1 ± 1.1 to 29.6 ± 1.2 during summer periods with outside temperatures of 26.0 ± 3.1°C. The naturally ventilated aviaries in this study were generally more uniform in temperature than the US facility studied by Zheng et al. [
24] and also in the three different systems (conventional cage, natural mating colony, cage-free aviary) studied by Li et al. [
25]. However, the more uniform and benign tropical climate in the present study also affected these differences in interior environment variability [
26].
Under heat stress conditions, there is a reduction in the concentrations of vitamins, minerals, and insulin available for metabolism and an increase in mineral excretion [
13], which causes laying hens to produce poor-quality eggs [
1].
The variability of light intensity in the 43W house was substantial in both winter and summer. There was a significant effect of the lines, levels, and the interaction between them (
p < 0.001), as shown in
Table 3.
The lower light intensity in the center (L2, L3, and L4) was generated predominantly by artificial lighting, whereas the higher intensity on the sides (L1, L5) of the house, both in winter and summer, was from direct exposure to the sky. In winter, only the L3-level N1 line presented an average light intensity value close to 10 lux, as recommended by Cotta [
20] and Jácome et al. [
21].
The high values of light intensity in winter on the L1 line can be explained by the direct incidence of the sun’s rays due to the latitude of the farm (22°17′45″ S and 44°56′05″ W). In summer, there is no direct incidence of radiation, and the light intensity values in the L1 line were below those measured in winter.
Egg weight was significantly affected by the factors “season of the year″ and “age of hens″ (
p < 0.001) and their interaction (
p < 0.01). On the other hand, egg shape index and specific gravity were only affected by the season of the year factor (
p < 0.05). Regarding the quality of the shell, there was a significant effect of the seasonal factor on shell weight (
p < 0.001) and shell thickness (
p < 0.05), and, for the age factor, on the shell percentage (
p < 0.01).
Table 4 lists the significant effects of the factors and the interactions between them on the egg quality parameters. The external egg quality parameters that were not affected by age hens and season of the year (
p > 0.05) are presented in
Table 4 and their means and standard deviation were followed by “ns”.
Older birds produced heavier eggs, as expected. With increasing age, hens produce larger follicles [
27] and have a greater capacity to transfer lipids to the yolk [
28], stimulating heavier egg production. This relationship between hens’ age and egg weight is well documented (e.g., Dirkmen et al. [
29], Samiullah et al. [
7], and Onbaşilar et al. [
6]).
The influence of thermal conditions inside the aviaries was more accentuated for laying hens aged 69, 79, and 86 weeks. At these ages, there was a significant decrease in egg weight in the summer compared to the winter. Birds in environments with high temperatures reduce food intake, leading to metabolic changes [
14] and, consequently, reduced egg quality.
Seasonal and location effects in the aviary had little influence on specific gravity. The average value of specific gravity was 1.086 g·mL
−1 and 1.090 g·mL
−1 in winter and summer, respectively. These values are higher than those reported by Torki et al. [
13] for laying hens subjected to cold (17 °C) and heat stress (32 °C).
The shape index is directly affected by egg weight and indirectly by winter and summer thermal conditions. At cooler temperatures, hens tend to ingest and metabolize food better, producing heavier eggs. These eggs concentrate albumen (the dense part of the egg) in the central equatorial region, which gives a greater shape index [
2].
Shell thickness values varied between seasons, with thinner shells in the summer than the winter. Pulmonary hyperventilation occurs due to hen’s increased respiratory rate when exposed to high-temperature environments. Pulmonary hyperventilation causes a reduction in the blood HCO
3 and CO
2 levels, which increases the blood pH and causes respiratory alkalosis. Consequently, there is a reduction in the synthesis of calcium carbonate, which is necessary for the formation of the shell [
20,
30], which makes them thinner and lighter in weight.
Eggs from hens aged 56 weeks had a higher shell percentage than eggs from hens aged 86 weeks. Castro et al. [
31] subjected laying hens to thermal comfort (21 °C) and heat stress (32 °C) conditions during the period of 19 to 45 weeks of age and observed that heat stress conditions did not affect the shell percentage; Hu et al. [
1] found no difference in egg weight, shell weight or thickness, or shell percentage for 16–32-week-old hens subjected to a short-term heat stress event. In this study, hens exposed to heat stress for 45 weeks produced eggs with a shell percentage of 9.13%, corroborating the present work because no statistical differences in shell percentage values were found between winter and summer.
With advancing age, there is less intestinal calcium absorption and a higher removal of calcium from the bones. As a consequence, the calcium content available for the formation of the shell decreases, reducing its weight [
20]. Therefore, older birds tend to produce heavier eggs with lighter shells, resulting in a low shell percentage, as verified in the present work and by Dirkmen et al. [
29] and Samiullah et al. [
7].
The variability of thermal conditions in the aviaries in winter and summer affected external egg quality. The spatial distribution of egg quality was studied in aviaries housing laying hens aged 43 and 86 weeks (Aviary 43W and Aviary 86W). These two aviaries were chosen to analyze the spatial distribution due to the significant bird age effect on the parameters of egg weight and shell percentage, as observed in
Table 4.
In the 43W aviary (43 weeks old hens), there was a significant difference between the weight of eggs obtained from the lines (
p < 0.1) and levels (
p < 0.1) in winter and between levels (
p < 0.001) in summer. In the 86W house, the line factors (
p < 0.01) and level (
p < 0.1) and the line × level interaction (
p < 0.01) were significant in winter, as well as the line (
p < 0.01) and level (
p < 0.05) and the line × level interaction (
p < 0.05) in summer.
Table 5 displays the effect of the level (N) and line (L) factors on the mean egg weight (g) value obtained in 43W aviaries in winter and summer and the effect of the interaction N × L average egg weight (g) obtained in 86W aviaries in winter and summer. There was no significant effect (
p > 0.05) of the line factor (L) on the average value of egg weight (g) obtained in 43W aviaries in the summer, therefore, in
Table 5 the means and standard deviation were followed by “ns”.
Level N1 eggs in aviary 43W were approximately 2 g heavier in both winter and summer. This was also noted in the 86W aviary, with a difference in weight of approximately 6 g. The variability of egg weight values was more accentuated in 86W due to thermal conditions and bird age. In the winter, the highest-weight eggs were obtained in lines L1 and L5, and in the summer, in line L1. The difference between the egg weight values, in the lines and levels, can be spatially visualized as shown in
Figure 4. This clearly shows where attention by engineering and management to improve interior thermal conditions would be most effective.
The spatial distribution of temperature, relative humidity, and egg weight in
Figure 3 and
Figure 4 highlights that greater egg weight values were in those regions of aviaries with temperatures within the thermal comfort zone as defined by Ferreira [
8], amongst others. In winter, the air temperature inside the aviaries was between 19 and 23.5 °C, that is, within the comfort zone.
The lowest egg weight values ranged between 59 and 61 g in the 43W aviary and 62 and 66 in the 86W aviary. Akbari et al. [
32] subjected laying hens aged 42 weeks to the cold stress condition (6.8 ± 3.0 °C) and found eggs weighing close to those found in the present study in the 43W aviary, where the housed birds were 43 weeks-of-age, and the thermal condition was characterized as thermoneutral. Star et al. [
33] submitted 67 to 78-week old laying hens to a temperature of 20 °C and obtained eggs with an average weight of 63.1 g. This value is close to that obtained in the upper central region of the 86W aviary in winter (
Figure 4a), where the air temperature was between 21 and 23 °C (
Figure 3a).
In summer, the lowest egg weight values ranged between 56 and 60 g (
Figure 4b) and were concentrated in the N3 level of the aviaries. At this level, temperature values between 27 and 28 °C were measured, which exceed thermal comfort (
Figure 3b). Karami et al. [
8] raised hens from 42 to 45 weeks in a 32 °C environment and obtained eggs with an average weight of 58 g, a value close to that found in the 43W aviary of the present study; Hu et al. [
1] reported egg weights of 53–55 g for 16–32-week-old hens raised under mild heat stress and subjected to one acute heat stress event.
The area for lighter-weight eggs in the 86W aviary in summer was smaller than the area observed in the 43W aviary (
Figure 4b). The hens’ heat acclimatization can explain this observation. Mashaly et al. [
34] submitted 31-week-old birds to an environment with an air temperature of 35 °C for five weeks and found a decrease in egg weight only in the first week and weight maintenance thereafter. Therefore, when suffering heat stress for a longer period, older hens undergo an acclimatization process, which reduces the effect of high temperatures on egg quality compared to young birds [
35]. However, even under long-term or chronic heat stress, small improvements in the thermal environment have been reported to improve egg production and quality. For example, Hu et al. [
36,
37] reported greater egg production, fewer cracked eggs, heavier eggs with greater breaking force, and especially shell thickness and eggshell percentage, for hens subjected to two years of heat stress but with cooled perches [
38], compared to non-cooled or no perches.
In winter, the specific gravity inside the houses had a slight but significant variation between levels (p < 0.1). Level N1 had the highest values of specific gravity, averaging 1091 g·mL−1, and level N3 had the lowest values, averaging 1085 g·mL−1. In summer, the spatial distribution was homogeneous, and there were no significant variations between levels, lines, and sections. The mean specific gravity was 1092 g·mL−1 at level N1 and 1089 g·mL−1 at level N3.
Castro et al. [
31] verified eggs with a specific gravity of 1087 g·mL
−1 and 1090 g·mL
−1 in 45-week-old hens subjected to thermal comfort (21 °C) or thermal stress (32 °C), respectively,. Khatibi et al. [
39] studied the quality of eggs from 52, 56, and 60-week-old laying hens housed in aviaries with an average internal temperature of 27.41 ± 2.54 °C and an average relative humidity of 35 ± 5%, during a subtropical summer and verified that for 50-week-old hens, specific gravity ranged between 1079 and 1090 g·mL
−1. For hens at 56 weeks of age, the specific gravity found was between 1084 and 1092 g·mL
−1, and for hens at 60 weeks of age, between 1081 and 1096 g·mL
−1. Thus, the specific gravity values obtained in winter and summer agree with the results obtained by the authors above.
The influence of winter and summer thermal conditions on the geometric characteristics of eggs was analyzed through the spatial distribution of the egg shape index values. There was a significant difference between the egg shape index values obtained between the levels in winter (p < 0.05) and summer (p < 0.01).
In winter, eggs from the N2 level had higher shape index values (an average of 76.4 ± 1.0%), with eggs from the N1 and N3 levels having the lowest values (76.0 ± 1.2% and 75.9 ± 1.2%, respectively). In summer, N1-level eggs had the highest shape index values (76.2 ± 1.3%), and N3-level eggs had the lowest (75.4 ± 4.3%). Therefore, the spatial distribution of the egg shape index can be useful for depicting the difference between the average egg shape index values inside the aviaries, as shown in
Figure 5.
The observed highest shape index values from the N2 level in the winter, around 76%, occurred with a mean air temperature of about 23 °C (
Figure 3a and
Figure 5a). By contrast, during summer, the lowest shape index values, 75%, were verified in the aviary region with air temperature values between 27 and 28 °C, outside the thermal comfort range for hens (
Figure 3b and
Figure 5b).
The egg shape index value found in winter was about 1% higher than that found by Orguz et al. [
40] at an environmental temperature of 18 °C, but about 1% lower than that described by Akbari et al. [
32] for a cold stress condition (6.8 °C). On the other hand, the summer and winter shape indexes were 2% higher than those obtained by Torki et al. [
13], who subjected laying hens to thermoneutral environments (17 °C), and 4% higher than those obtained in heat stress (32 °C), which shows the influence of thermal conditions on the shape index.
The shape index is an external indicator of egg quality and is used to classify eggs based on their size. Eggs can be classified according to their shape into long, normal, and round. Long eggs have a shape index less than 72, normal eggs between 72 and 76, and round eggs greater than 76 [
41]. The present study found uniformly normal oval eggs with a mean shape index between 75% and 76%.
The influence of winter and summer thermal conditions on shell quality was also analyzed through the spatial distribution of shell weight and thickness inside the aviary. For shell weight, the effect of levels (
p < 0.001) and lines (
p < 0.001) was significant in both winter and summer. However, only the line effect (
p < 0.1) was significant in the summer for shell thickness.
Table 6 summarizes mean eggshell weight and thickness by level (N) and line (L) factors for winter and summer. There was no significant effect (
p > 0.05) of the level (N) and line (L) factors in winter and the level (N) factor in summer on the average value of eggshell thickness (mm), therefore, in
Table 6 the means and standard deviation were followed by “ns”.
The overall mean eggshell weight was 5.93 g and 5.57 g in summer and winter, respectively (not shown in
Table 6). Level N1 and lines L1 and L5 displayed the highest values. In the summer, shell thickness values varied among lines, with the L1 line displaying the highest values. In winter, the mean value of shell thickness was 0.421 mm, and in summer, 0.381 mm. Measured variability between the mean values of shell weight and thickness inside the aviaries can be observed by a representative plot of spatial distribution in cross-section, as highlighted in
Figure 6.
Across both seasons, eggshell weight varied significantly with cage level, with weights being greatest in N1 and lowest in N2 (
p < 0.05). During the winter period, the highest shell weight values were obtained where the lowest air temperature values were measured (L1 and L5), as shown in
Figure 6a.
In summer, the lowest values of shell weight and thickness were obtained in the region of the aviaries where the highest air temperature values were measured, that is, at level N3 and lines L3–L4 (
Figure 6b). In these regions, the air temperature was typically between 27 and 28 °C, and the shell weight and thickness values were 5.3 g and 0.368 mm, respectively. In contrast, the spatial variation of shell thickness was not significant in winter (
Table 6,
p > 0.05). In general, greater shell thickness was observed in the region with the highest shell weight. Winter-time values of shell weight and thickness were higher than those described by Netto et al. [
42], Sahin et al. [
12], Torki et al. [
13], and Yan et al. [
43] and close to those obtained by Samiullah et al. [
7] in thermoneutral environmental conditions. The measured shell thickness values corroborate those obtained by Karami et al. [
11], Sahin et al. [
12], and Torki et al. [
13], measured in environments with elevated temperatures (32 °C and 34 °C). According to Kim et al. [
44], exposure of hens to high ambient temperature results in a significant decrease in shell weight and shell thickness as these parameters are directly associated with reduced feed intake and mineral metabolism; this was also confirmed in a long-term heat stress trial by Hu et al. [
1].
Regarding the effect of light intensity on the external egg quality, eggs from lines L1 and L5 (exposed to high levels of natural light,
Table 3) had greater weight and greater shell thickness (
Figure 4 and
Figure 6); this observation contradicts that of Renema et al. [
16]. In this study, within the interior lines of the aviaries (L2, L3, and L4), there was a reduction in egg weight and shell thickness with increasing light intensity from level N1 to level N3; Yildiz et al. [
14], who investigated the effects of cage location and different types of lighting on egg quality parameters in a semi-confined facility with a multilevel cage system, also reported this effect.
The egg weight values in the aviary region with a light intensity of 11 lux (
Table 3) were 62 g (
Figure 4), a value higher than those obtained by Yuri et al. [
19] and Yildirim et al. [
18] when subjecting hens to an intensity of 15 lux. On the other hand, Min et al. [
15] submitted laying hens to a 20-lux environment and obtained eggs with an average weight of 60.9 g, a value below that obtained in the region of the aviaries in this study with a light intensity of 20 lux, which was 64 g. These differences could be attributed to several factors, including age of laying in the other studies, breed of hen, and other uncontrolled factors.