Influence of the environment on the physiological responses of isa brown layers under various production systems in hot/humid weather

The environment has always been a crucial factor for successful layer performance. In some Brazilian states, particularly Acre, located in the Northern Amazon, excess relative humidity (RH) and heat have resulted  in various alterations in the general physiology and the welfare of these birds, directly affecting egg end quality. This has led to the conclusion that production models considered "ideal" in the rest of the country are not always valid under the above-mentioned conditions. Animal ambient and welfare are not always compatible with the physiological needs of birds, resulting in increased susceptibility to various stressors, including heat stress (Furlan, 2005). The ideal temperature for egg production is 21 - 26°C for adult hens (Silva, 2000). Regarding the effect of ambient temperature on layers, numerous studies mention the existence of a thermal comfort zone. Nevertheless, determining such comfort zone implies the knowledge of the interactions of several variables that can impact this process, including moisture, management, ventilation, facilities, etc. The objective of this study was to evaluate the aspects related with the environment and the physiological behavior of Isa Brown layers in production, under a hot/humid environment, comparing the conventional cage production system, with that on the floor,  considered as "assuring the best animal welfare".

The experiment was conducted in Acre Federal University (UFAC), Multidisciplinary Center Centro of Cruzeiro do Sul, AC, Brazil, in the period from August to November 2010. In agreement with Koppen´s classification, climate in the region is humid-tropical _Af with rains well distributed throughout the year, with no dry season. Mean altitude is 170 meters, with yearly rains of 2,074 mm. Experimental units in commercial egg production farms, during production, were used and the study was divided into 4 different aviary models. A total of 200 Isa Brown layers in production were used. Considering that the experiment was conducted in commercial farms, birds between 23 and 42 weeks of age were included. The experimental feed was the same being used in the farm, formulated based on corn and soybean meal, in agreement with the nutritional requirements of the strain. Feed and water were offered ad libitum. The experimental treatments were represented by the production systems, with two systems (cages or floor) and two environments (with/without forced ventilation) and each treatment had 4 time repetitions (four 7-day times, one week apart). In each production model/environment, 50 representative birds were maintained. Data collections for all variables was done every day, regardless of rain, at pre-established times (09:00 - 10:00 and 15:00 -16:00 hours, local time). Forced ventilation was achieved using a fan with a capacity of 1 m³/second. The meteorological variables collected in and out the experimental models included: min/max temperatures, dry bulb/wet bulb temperatures, and RH inside/outside the house. Based on this meteorological information, the temperature/humidity index (THI), as recommended by Barbosa (2004), the ideal being within the 71 - 75 THI band. The physiological parameters collected included bird body surface area temperature, and rectal temperature (RT), and this data was used to calculate the mean skin temperature (MST). Using the TMP data and those of RT, the mean body temperature (MBT), and both MST and MBT were calculated using the equation proposed by Richards (1971). The experiment was designed in random blocks and for data analysis Tukey´s test was used at a 5% de probability.

Results of environmental conditions and physiological characteristics of these brown layers subjected to various production/environment systems are described in Table 1.

Table 1. Mean temperature inside/outside the house (IHT and OHT), mean relative humidity inside and outside the house (MRHIH, MRHOH), and temperature/humidity index (THI) in brown layers subjected to various production/ambient systems

IHT in the ventilated treatments were more efficient to maintain the temperature close to the birds' thermal comfort zone (P<0.05). Birds are able to tolerate extremely low temperatures, but not very much in excess to thermoneutrality, the ideal band being near an ambient temperature between 21 and 26°C, for adult hens.
Regarding the effect of ambient temperature on laying hens, several studies show the existence of a thermal comfort zone, which determination implies knowing the interactions of several variables potentially influencing this process, including humidity, management, ventilation, facilities, etc.
The thermal comfort zone is that where the animal response to the environment is positive, and environmental demand matches basal production, with increased heat production equivalent to normal activity and with the heat increment resulting from feed intake. In this zone, that varies for each phase and management type, the animal reached maximum potential and body temperature is maintained with a minimum use of temperature-regulating mechanisms (Filho, 2004).
Inn this experiment, the forced ventilation system contributed to eliminate the excess RH inside the houses. The systems with no ventilation showed internal temperatures higher than those in the ventilated houses which could explain - as stated above - the decreased performance of those birds housed in the non-ventilated cage system. It is worthy to mention that the two houses with forced ventilation had mean outer temperatures lower than those seen in the houses with no forced ventilation (P>0.05), despite of being located within a <4 km radius. This was not valid for mean outer relative humidity among all houses (P>0.05). This behavior can be explained by the fact that only those houses with forced ventilation were provided with a sharing system on the sides represented by tree species, decreasing the incidence of sun rays on the roof, and improving -together with the ventilation system - the internal house environment. Silva (1998) studied shading systems in layer houses aiming to improve the climate, and found improved bird performance when older trees existed in smaller areas, resulting in increased egg production of up to 12.5%.
Regarding the ideal RH for layers, no precise values exist (contrary to temperature). What can be stated with certainty is that, with extremely high RH values, birds are much more sensitive to heat stress. This is why it is important to maintain los RH levels inside the house, principally in summer. One additional serious problem resulting from high RH inside the houses with litter, as opposite to cages, is that in addition to being rious for bird health, can compromise the quality of eggs.
In this experiment, the mean RH values obtained with the forced ventilation models, generally  remained within the expected limits for egg-producing birds, in general (75%). Likewise, the models with no forced ventilation did not show RH levels in excess to 90%, which is related with the small dimensions (width and length) in these house models, in addition to the relative effectiveness of natural ventilation.
In this experiment, THI showed to be the best comfort indicator for all the production systems studied. It can be clearly seed that the comfort situation established reflects THI values within the above-mentioned band from 71 to 75. On the other hand, when  the stress condition is analyzed, THI values are higher in the forced ventilation system, approaching the thermal comfort zone upper limit, nearly meaning stress, and confirming the differences between the two conditions studied (with and without forced ventilation).
Mean skin temperature and mean body temperature values in these brown layers subjected to various production/environment systems are shown in Table 2.
Heat stress out stands among other stress factors. MST was statistically different and more adequate (P<0.05) in the floor/forced ventilation model, whilst both MBT and THI followed the same condition.

Table 2. Mean skin temperature Temperature (MST) and mean body temperature (MBT) of brown layers subjected to various production/environment systems

The cage with no forced ventilation system showed the highest temperature, resulting in decreased reduced bird performance, since this model provided the poorest thermal conditions. The floor/forced ventilation combination resulted in increased MBT. These results corroborate those reported by Welker et al. (2008).
Special attention should be paid to the facilities that provide the best possible temperature comfort conditions for the hens. This alerts the producers about the fundamental importance of a functional ventilation system in the houses, which must be oriented in such a way that hot air can readily be removed with the entrance of fresh air for birds to feel comfortable and egg production is not compromised (Filho, 2004).
Therefore, considering that in most poultry production systems in Latin America weather factors are poorly managed, and that the production micro-environment and animal welfare are not always compatible with bird physiological needs, this results in increased sensitivity to different stressors..

Under the conditions of this experiment, the systems with forced ventilation were efficient in maintaining bird temperature comfort inside the houses, contributing to animal thermal welfare, particularly in the suspended cage systems. Further studies are necessary to verify the economic feasibility of egg production under these production systems.

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Welker JS, Rosa AP, Moura DJ, Machado LP, Caterlan F, Uttpatel R. 2008. Temperatura corporal de frangos de corte em diferentes sistemas de climatização. Revista Brasileira de Zootecnia 37(8):1463-1467.