Thermoneutral zone for laying hens based on environmental conditions, enthalpy and thermal comfort indexes
Introduction
In laying hens production systems, farmers usually focus on animal productivity rather than welfare to infer thermal comfort status. Currently, environmental issues, food biosafety, and animal welfare are the three biggest challenges for poultry production. In this context, there is a gradual concern about the potential impacts of the environment on the behavior and performance of animals.
The synergism between thermal environment and heat production in laying hens can be explained through thermal comfort limits, which use physiological variables to characterize and evaluate poultry productivity. Several methodologies can be used for analyzing these thermal comfort ranges, including the thermoneutral zone (TNZ), which specifies the range of ambient temperatures (AT) where energy requirement is minimal and constant. Therefore, TNZ is an important parameter when adjusting the temperature inside poultry buildings. However, the TNZ of birds is associated with their body weight and age (Meltzer, 1987). Birds' ability to dissipate heat decreases as ambient temperature and relative humidity rise above the TNZ. As a result, the bird's body temperature rises, resulting in heat stress (Curto et al., 2007).
The maintenance of thermal balance inside commercial poultry facilities is directly related to the bird's productivity. In thermoneutral environments, in which temperature, relative humidity and air velocity are within the thermoneutral zone, a bird's productivity reaches its maximum. Under these conditions, the dietary energy is not diverted to compensate thermal deviations from the thermoneutral range (Ponciano et al., 2011).
Birds are homeothermic animals and produce heat to maintain relatively constant body temperature. Slight variations within their body temperature range are tolerated without significant disturbance (ST-Pierre et al., 2003).
In laying hens, the ideal ambient temperature ranges between 21 and 28 °C (Castilho et al., 2015). Ferreira (2016) reported that the productivity of laying hens increased under environments with relative humidity ranging from 40 to 70%.
Under conditions of high temperature, the normal respiratory rate of birds (23 mov. min−1) may increase by up to tenfold in response to thermal discomfort, reaching up to 273 mov. min−1 (Kassim and Sykes, 1982), indicating necessity the use of the evaporative mechanism of cooling (Santos et al., 2006). Under conditions of high air temperatures and high relative humidity, birds may develop respiratory alkalosis (Borges et al., 2003).
Due to the thick insulation provided by the feather coat on most of the body surface, sensible heat loss is more efficient in featherless areas, where blood flow increases when birds are exposed to heat stress. Laying hens do not have sweat glands and cannot perspire. Thus, birds lose excess heat mainly by evaporation (breathing) and through surfaces such as comb, wattle, shanks, and featherless areas below the wings.
Increased cloacal temperature is a physiological response to conditions of high air temperature and relative humidity, resulting from the storage of metabolic heat (Silva et al., 2013). For cloacal temperature, the thermal comfort may vary from 41 to 42 °C (Oliveira et al., 2006). In this way, sensible exchanges between the animal and the environment are highly efficient; thus, the larger the difference between air and the bird's surface temperatures, the more efficient these exchanges will be (Nascimento and Silva, 2010).
Tinôco (2001) stated that birds are continually exchanging heat with the environment, and this exchange is efficient if the ambient temperature is within limits. These limits are dependent on the thermal sensation, which encompasses temperature, humidity and wind speed inside the building. The thermal sensation is closely related to the air flow across the surface of the birds' body, facilitating heat dissipation to the environment (Miragliotta et al., 2006). Therefore, these physiological mechanisms of body heat loss will be activated depending on the thermal environment surrounding the animal; thus, they are directly related to environmental variables (air temperature, relative humidity and velocity).
The approach of this work has a practical and direct effect on the analysis of yield losses due to thermal fluctuations in commercial laying hens, with updating of the thermal comfort limits for some variables that model the microclimate inside the facilities. Thus, this study used the physiological responses of laying hens to evaluate the thermoneutral zones, depending on environmental conditions, thermodynamic properties and thermal comfort indexes.
Section snippets
Environmental conditions
The experiment was carried out in four thermal environment-controlled wind tunnels equipped with heating and air moistening function, housed in an experimental room with an area of 31.92 m2. This room was equipped with two air conditioning systems with a power of 5.275 kW each, which were used to maintain the temperature inside the room below the target value.
All experimental procedures involving animals were previously approved by the Ethics Committee on Animal Use of the Federal University of
Results
The values of thermal comfort indexes and enthalpy indicate a transition from a thermoneutral environment to a condition of heat stress. Under these circumstances, laying hens may have to adjust their metabolism, as evidenced by the interaction between tdb, RH, and V, as shown in Table 1.
Fig. 2, Fig. 3 illustrate the clustering process in which treatments were classified based on similarity or distance. Environmental-related variables (Fig. 2) and thermal comfort indexes were assigned to each
Discussion
Values of BGHI ranging from 69 to 77 does not affect the performance of laying hens at peak production, indicating thermal comfort (Teixeira, 1991; Armstrong, 1994; Medeiros et al., 2005; Botelho et al., 2016). Therefore, values of BGHI within the thermoneutral zone (69–77) were also obtained in this study for hens exposed to temperatures of 24 °C/40% RH and 28 °C/60% RH, with wind speeds of 0.2; 0.7 and 1.4 ms−1. Based on the findings of Tinôco (1998), in which the upper limit for laying hen
Conclusions
Respiratory rate was the most appropriate physiological response to establish ranges of thermoneutrality for laying hens, since its response precedes the response of other variables, making it the most prudent choice.
The method for obtaining lower and upper comfort limits can be improved by increasing the number of replicates for each thermal challenge, as well as by using mathematical or computational models that allow obtaining more accurate statistical indices. The number of repetitions and
CRediT authorship contribution statement
Bruna Pontara Vilas Boas Ribeiro: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing, Visualization, Project administration. Tadayuki Yanagi Junior: Methodology, Resources, Data curation, Writing - original draft, Supervision, Project administration, Funding acquisition. Daniela Duarte de Oliveira: Resources. Renato Ribeiro de Lima: Software, Formal analysis. Márcio Gilberto Zangeronimo: Supervision.
Acknowledgements
This study was partially funded by the Coordination for the Improvement of Higher Education Personnel - Brazil (CAPES) - Financial Code 001 and with the support of the National Council for Scientific and Technological Development (CNPq) (Process 310729 / 2018-1).
Bruna Pontara Vilas Boas Ribeiro: Graduated in Animal Science from the Federal Institute of Minas Gerais - Bambuí Campus (2014), Master in Production and Nutrition of non-ruminants (2016) and current is a Ph.D. student in Construction, Environment and Waste Treatment by the Federal University of Lavras (2016). Has experience in Animal Science, focusing on Monogastric Nutrition, working on the following subjects: environment /welfare of pigs and poultry, alternative food, agroindustrial
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Bruna Pontara Vilas Boas Ribeiro: Graduated in Animal Science from the Federal Institute of Minas Gerais - Bambuí Campus (2014), Master in Production and Nutrition of non-ruminants (2016) and current is a Ph.D. student in Construction, Environment and Waste Treatment by the Federal University of Lavras (2016). Has experience in Animal Science, focusing on Monogastric Nutrition, working on the following subjects: environment /welfare of pigs and poultry, alternative food, agroindustrial by-products.
Tadayuki Yanagi Junior: Ph.D. in Agricultural Engineering (2002), concentration area in Rural Buildings and Ambience at the Federal University of Viçosa with sandwich training at Iowa State University of Science and Technology, where he served as a visiting researcher. He is a professor at the Engineering Department of the Federal University of Lavras (UFLA), participating in postgraduate programs in Agricultural Engineering and Systems Engineering and Automation. He has experience in Agricultural and Systems Engineering, especially in the area of construction and ambiance, where he guides undergraduate and postgraduate students in the development of basic and applied research, involving evaluation and mathematical modeling of biosystems.
Daniela Duarte de Oliveira: Graduated in Veterinary Medicine from José dos, Master's degree in Veterinary Medicine from Federal University of Minas Gerais (2004) and Ph.D. in Animal Science from Federal University of Minas Gerais (2008). She is currently a veterinary doctor, technical manager of the Santo Antônio e Granja São Jorge Aviary, Member of AVICOLA - COESA STATE HEALTH COMMITTEE and advisor of the Eggs Brazil Institute - IOB. Experience in the area of Veterinary Medicine, with emphasis on Preventive Veterinary Medicine, an inspection of animal products and quality control of eggs and egg products.
Renato Ribeiro de Lima: Graduated in Animal Science from the Federal University of Lavras (1993), Master in Genetics and Breeding from the Federal University of Viçosa (1997), Ph.D. in Agronomy / Statistics and Agronomic Experimentation from the Luiz de Queiroz College of Agriculture (2005), with sandwich training at the University of Kent at Canterbury, England, and a postdoc at the University of Wisconsin, Madison, WI, USA, in Genomic Statistics. Associate professor at the Federal University of Lavras. A reviewer of the journals Bragantia, Brazilian Journal of Biometrics and Rural Science. He has experience in Probability and Statistics, focusing on Probability and Applied Statistics, acting on the following subjects: Spatial Statistics, Agricultural Experimentation and Genomic Statistics.
Márcio Gilberto Zangerônimo: Veterinary Doctor graduated from the Federal University of Lavras in August 2002, started his master's degree in monogastric nutrition at this same university in February 2003, completing his dissertation in February 2004 (1 year). In March of that same year, he started his doctorate via level change by CAPES, consolidating his thesis defense in March 2006 (2 years). He was a postdoctoral fellow at FAPEMIG for two years (2006/07), developing work using crystalline amino acids for growing pigs and phytase. He is currently a productivity fellow at CNPq level 1C, an Adjunct Professor at the Federal University of Lavras - UFLA. It operates in the following areas: Biotechnology of porcine reproduction, monogastric nutrition, metabolism and animal physiology.