Measurement of Heat Stress in Animals
28.12.1 TemperatureHumidityIndex
It is essential to evaluate the impact of environmental factors on animal production. Livestock weather safety index (LWSI) is one of the methods for estimation of level of heat stress in livestock species.
The LWSI is based on the temperature humidity index (THI) which is uncomplicated, reliable and easy approach to assess the heat load in stressed animals. THI is a combination of Ta and RH into a single value to reveal the degree of stress and to establish its influence on animal production. The THI formulas differ according to region and parameters obtained to measure it. Thom (1959) established THI based on Ta and RH as follows: THI = 9/ 5 ? [(Ta ? 17.778) - (0.55 - (0.55 ? RH/100)) ? (T - 14.444)], value of THI - v + 10.5) ? (33 - Ta) where v is the wind velocity (m/s) and Ta is air temperature (°C). The WCI (kcal/ m2/h) was classified in to five levels, no chill (25 and ≤30), moderate (>30 and ≤35), severe (>35 and ≤40), extreme (>40 and ≤45) and extreme danger (CCI >45).28.12.6 TunicaDartosIndex
The tunica dartos index (TDI) was developed to judge the ability of ram’s heat tolerance during higher ambient temperature. The TDI was predicted by three ways as TDIa, TDIb and TDIc. TDIa referred as percentage of change in scrotal length in association with maximum scrotal length minus the testis length. TDIb indicated the variation between rectal temperature (RT) and scrotal skin temperature in percentage of RT, and TDIc is the combination of TDIb and TDIa. The three formulae are as follows: TDIa = [(Max. SCL — Min. SCL)/(Max. SCL — TL)] ? 100; TDIb = (RT — SST)/ RT ? 100 and TDIc = [(RT — SST)/RT ? (Max. SCL — Min. SCL)/(Max. SCL — TL)] ? 100. Where RT—average rectal temperature in summer, SST—average scrotal skin temperature in summer, Max.
SCL—average maximum scrotal length in summer, Min. SCL—average minimum scrotal length in winter and TL—average testis length during the winter and summer seasons.28.12.7 InfraredThermography
Infrared thermography (IRT) is a non-invasive and non-contact method of measurement of temperature which is rapid without restraining stress and also provides an opportunity for automation. Infrared thermographic cameras measure temperature of animals that radiates heat from the external body surfaces. Heat emissions are displayed as a thermogram of pixels in varying colours or shades that indicate different infrared temperature. In IRT, emissivity of objects varies from 0 to 1 and the emissivity of cattle external body surfaces varies between 0.93 and 0.98 depending on skin and hair colour, and density of hair. The IRT of the eyes specifically the skin around the inner corner of the eye socket indicates core body temperature of animal. In fact, the eyes are positioned close to the hypothalamic thermo-sensitive area which facilitates quick response. The blood vessels of eyes are identical to the brain and choroid vessels which are similar to the small intestine and kidney. The IRT of eyes moderately depict the rectal and vaginal temperature. Therefore, changes in the eye and rectal temperature IRT may indicate similar physiological responses to stress.
The IRT of external body surfaces invariably specifies surface temperature which is highly associated with peripheral temperature. IRT data are more dependable than routine measurement of rectal temperature. The IRT of limbs indicates the peripheral temperature which is highly fluctuating due to the formation of buffer between the core body and the environmental temperature through convection. IRT of external body surface area differs based on distinct tissue metabolism, blood flow, conduction and the ability of an object to absorb and emit radiation. Limbs are major route of heat dissipation in animals. The fluctuations in peripheral IRT of limbs are highly noticeable at the coronary band due to muscular activity involved in the movement, weight bearing and more blood circulation to provide nutrition. Therefore, IRT of external body surfaces is highly important to assess the environment induced changes in the animals’ body temperature.
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