Poultry thermoregulation in the heat : seasonal acclimatization and partitioning of evaporative water losses

Abstract

Abstract

At high environmental temperatures (Ta), birds maintain homeostasis by dissipating heat via evaporative water loss (EWL). EWL is an important mechanism of heat dissipation by animals exposed to intense solar radiation and can be partitioned into respiratory evaporative water loss (REWL) and cutaneous evaporative water loss (CEWL) and the sum of both representing the total rates of evaporative water loss (TEWL) of an organism. Another important aspect of avian thermal physiology is variation within individuals via phenotypic plasticity, one category of which is seasonal acclimatization. Seasonal acclimatisation of physiological responses gives insight into the value of phenotypic adjustment and has been well studied in wild birds in thermoneutral and low temperatures. However, some physiological mechanisms such as resting metabolic rate (RMR) and total evaporative water loss (TEWL) are lacking in poultry science in particular the variations that exists among chicken breeds in part of the different genetic make-up and productive systems in which they are reared in. The study aimed to evaluate the thermoregulation of Boschveld chickens specifically the major pathways of heat dissipation at high temperatures by partitioning EWL into REWL and CEWL. Also adaptive changes in resting metabolic rate (RMR), body mass, and TEWL, in response to seasonal acclimatisation were investigated in this study. 19 female Boschveld chickens were bought and kept in outdoor aviaries where they were fed a commercial diet and provided water ad libitum. EWL was partitioned using a glass metabolic chamber and rates of REWL, CEWL and TEWL were measured over a range of 20-43 °C including Tb using a flow-through respirometry system from (November 2019 – January 2020) and (January 2020-February 2020). For seasonal acclimatisation, parameters such as body mass, TEWL, thermal conductance, evaporative heat loss/metabolic heat production, and RMR were measured using a flow-through respirometry system but maintained a Ta range between 20 and 35 °C in winter and summer from (June 2020- January 2021) measurements of body temperature (Tb) were taken using a passive integrated transponder (PIT) tag and weighed the birds. Body temperature increased with increasing Ta in experiment 1 but there was a sudden decrease at Ta≈ 37 °C of 42.18±1.19 °C, n=19 and a further decrease to the following Ta =39 °C mean Tb of 41.96±0.67 °C. Seasonally, Tb did not vary significantly between seasons (F1:115 = 0.023, P˂ 0.05), with winter Tb (40.12 ± 1.44 °C, n =19) being similar to summer Tb (40.38 ± 1.22 °C, n =19). Seasonally, there were significant differences in body mass (F1:116 = 132.73, P˂ 0.0001) with winter birds (2101.57 ± 268.00 g) being heavier than summer birds (1948.14 ± 257.03 g). RMR varied over the experimental Ta range and minimum RMR (4.58 ± 1.67 W) was observed at Ta~ 35 °C in experiment 1. The average winter RMR (3.94 ± 1.42 W, n =19) was significantly lower (F1:115 = 75.64, P= 0.0001) than the summer RMR (5.54 ± 2.25 W, n =19). There were significant differences in the thermal conductance of Boschveld chickens between seasons (F1:115 =47.68, P˂ 0001) with average thermal conductance being high in summer (0.54 ± 0.28 W °C-1, n= 19) than in winter (0.36 ± 0.17 W °C-1, n= 19). CEWL was the major route of heat dissipation at 35 °C≥ Ta≤41 °C'; moreover, CEWL represented more than 60% of heat dissipation to the total rates of evaporative water loss and was for the most part a dominant route of heat dissipation except at 25 and 30 °C. Total evaporative water loss increased with increasing Ta at Ta above 25 °C and between Ta 35 and 39 °C. TEWL was closely similar, with mean TEWL 5.15± 1.8348 mg min-1 (Ta 35 °C). Seasonally, TEWL showed no significant differences between seasons (F1:115= 1.98, P> 0.05). Overall, evaporative cooling efficiency (i.e., EHL/MHP) was significantly affected by season (F1:114=15.93, P˂0.001); with the ratio of EHL/MHP being higher in winter (0.90 ± 0.65) than in summer (0.75 ± 0.47). CEWL represents an important avenue of heat dissipation in Boschveld chickens and based on the climate change scenario, this chicken breed is suitable for extensive farm systems and is capable of adapting to the natural environment.