The air temperature exceeded the annual average of Brasilia 25.9 ºC (INMET, 2020) with values above 27.30 ºC at 12h. On scalding days, air temperature ranged from 27 to 34.10 ºC (Table 2). The air temperature does not very much, with a temperature range of only 7ºC, which shows us that air temperature alone is not enough to define the level of stress that laying hens may be submitted throughout the day.
Therefore, the importance of measuring the amount of shortwave radiation, which in the first hours of the day at 8 is around 33 W m−2 and peaked at 2 pm with radiation of 756.98 W m−2 (Figure 1). The Sun emits the maximum value of shortwave radiation that reaches the planet Earth is around 1,350 W m−2, but the range that reaches the Earth's surface is around 800 to 1000 W m−2 at sea level (Da Silva, 2008). In this study this statement can be confirmed with the solar radiation survey, which showed that for all months of the year the shortwave radiation was above 700 W m−2 and that this situation worsens in the months of January, February, March, September, and November when the radiation exceeds 870 W m−2 (Figure 2).
The Brazilian Savanna (or Cerrado biomes) is located in the tropical zone and occupies the second largest Brazilian biomes with an extension of 2,036,448 km2 which is equivalent to occupying 23.9% of the national territory (Penereiro et al., 2018). Therefore, it becomes necessary to provide good environmental conditions for animals, considering that they can absorb heat from the environment, mainly by exposure to solar radiation and high air temperature (Maia et. al., 2020). These meteorological factors directly affect animals raised on pasture because it is a type of farming that does not provide favorable conditions related to thermal comfort (Mós et al., 2020).
Through the temperature of the black globe (in the sun and shade), we can establish the thermal sensation that the animal feels throughout the day. In this experiment, it ranged from 29.40 to 50.20 W m−2 with a range of 20.80 W m−2 with the globe in the sun, and between 28 to 38 W m−2 with a range of 10 W m−2 with the globe in the shade. This thermal sensation was what influenced the frequency of the hens' behaviour of staying inside or outside the rearing housing (Broom, 2010).
When relating solar radiation with the hens' behaviour, it was observed that the radiation is 33 W m−2 at 08:00am (Table 2), it was observed that the hens show greater dispersion over the paddocks and better use was made of the outside area of the housing, especially in the paddock using the pastures (Figure 1). Shortwave radiation, also known as solar radiation, is characterized as a heat gain mechanism for animals and surfaces exposed to the sun's rays (Da Silva, 2008; Da Silva and Maia, 2013; Gosling et al., 2014; Teixeira et al., 2021).
Regardless of the day evaluated, between 12h and 14h (Figure 1), times that the radiation reached 756.98 and black globe temperature at 50 ºC, the laying hens remained inside the housing at the hottest times of the day, to protect themselves from the high incidence of solar radiation and high thermal sensation, for not finding shading areas (both natural and artificial) in the pen.
It was also observed that with and as the sun began to lower and form shade from the structure of the housing the hens at the exit door of the housing, that the hens began to crowd in the available shade strip, demonstrate the desire to leave the housing. When radiation begins to decrease at 4 pm to 348.37 W m−2 on the hottest days (Table 2), consequently, the black globe temperature is lowered to 30.40ºC, is the time that all hens leave the housing toward the paddocks to use the pasture areas.
It is important to note that by survival instinct, as the hens are animals considered "prey" and because they feel vulnerable when the pasture is higher than their size, they rarely use this area and do not go into dense vegetation. Just as was observed at 4pm, where despite being loose outside the housing, most stayed outside the grass areas, so we should keep the grass trimmed at all times, in addition to the other regulations for grazing areas stipulated by Humane Farm Animal Care (HFAC, 2018).
However, it is remarkable how much direct interference solar radiation has on the Free-range system for laying hens. Between the period from 10 am to 4 pm is 6 hours without any use of the hens to the paddocks, which shows the inefficiency of the purpose of this system since it is mandatory that the hens have access to the outdoor area for at least 6 hours of the day (HFAC, 2018). Therefore, it is necessary to provide shaded areas near the barn, either artificially, such as shading screens (Tainika and Şekeroğlu, 2021; Mós et al., 2021), or naturally, through the use of trees (Teixeira and Nascimento, 2020), so that the hens can use the outdoor area for longer and not suffer from direct radiation interference.
The same were cited in an article by Certified Humane Brazil, for the maintenance of productivity to hens and based on knowledge about physiology and animal behaviour one should always consider that they should have access to shade in the hottest periods of the day/year, even for species and breeds adapted to tropical regions (Cattani, 2020). Improving the environmental conditions meeting domain 2 of Animal Welfare (Mellor and Beausoleil, 2015), and consequently improve the health and quality of life of the hens.
The variations of solar radiation are direct, diffuse, and reflected radiation, which are subdivided according to their characteristics. The direct radiation is radiation that does not undergo mirroring as it travels to Earth, i.e., radiation descends in a straight line and directly reaches all surfaces. The diffuse radiation is caused by the deflection of direct solar radiation by some particles or molecules in the atmosphere, such as clouds, so that it reaches the ground at different angles. The reflected radiation is the radiation that reaches the ground and has a part of its energy reflected in other surfaces or animals (Da Silva, 2008; Da Silva and Maia, 2013).
Each radiation component interferes in different ways in poultry rearing systems. Only hens housed in fully enclosed and climate-controlled housings are free of shortwave radiation. Hens housed in conventional housings with open sides receive reflected radiation, and in the free-range system where hens have access to areas outside the housing, the hens are susceptible to all types of radiation, direct, diffuse, and reflected (Maia et al, 2020).
Letting the hens outdoors does not provide thermal comfort. Solar radiation, black globe temperature, and air temperature when they are above the hens' surface temperature are also considered additional sources of heat gain for the animal (Da Silva and Maia, 2013; Gosling et al, 2014; Teixeira et al., 2021). It is for this reason, that at the times of day with the highest incidence of radiation, and high air and shell temperatures, that the hens seek shelter inside the housing to get away from direct solar radiation. However, it is necessary to provide shading areas in the paddocks, and natural shade from trees is the most suitable in this situation, bringing the interaction of sustainable and durable systems (Teixeira and Nascimento, 2020).