Heat Tolerance Indices In Domestic Animals

 Ankita Rautela*, Richa Rautela, Pramod Singh Painkra

Tolerance to heat stress is an adaptive process ensured by animals to withstand the effects of increase in ambient temperature beyond the temperature humidity index (THI) limit of thermal neutral zone (TNZ).

A heat-tolerant animal is the one which maintains homeothermy under high environmental heat loads. However, from a livestock breeding point of view, maintaining productive and reproductive levels under heat stress conditions might be the target. Maintaining homeothermy under such condition depends on the animal’s ability to balance thermogenesis and heat dissipation.

To identify heat tolerance in animals several measures have been proposed as criteria; which include body temperature, respiration rate, heart rate, and sweating rate. The overall ability of the animal to cope with heat can be checked by performance of animal. Certain measures like hair and coat characteristics which includes hair shedding rate and body surface to mass ratio which are related to the animal’s ability to dissipate internal heat have also been proposed as heat tolerant traits

Several biomarkers such as blood parameters or diverse molecules associated with the heat stress response have also been put forward as indicators of heat stress in livestock

Physiological characteristics such as body temperature or respiration rate are considered as important measures for heat tolerance, but their utilization in large-scale selection programs is still limited because it is expensive to collect these measurements. Biomarkers for heat stress can be detected in milk spectra, serving as apotential inexpensive tool to identify heat tolerant animals.

Mid-infrared spectroscopy can also be used to assess profiles of milk fatty acids as possible biomarkers for heat stress in dairy cattle.

Following indices may be applied for the determination of heat tolerance in animals:

  1. Iberia Heat Tolerance Index (IHTI)

One of the earliest index developed by Rhoad, in 1944 to evaluate thermal stress in animals was the Iberian heat tolerance test, which uses the rectal temperature (RT) as a variable.

This method is performed in an open area between 10 AM to 3 PM when the ambient temperature ranges between 85-95 0 F, and rectal temperature and respiration rate of animals were recorded before and after exposure for three consecutive days.

The heat tolerance coefficient was calculated using formula-

                       HTC= 100-10 (RT-101)

           Where HTC is Heat tolerance Coefficient

           And RT is Rectal temperature

*HTC value exceeding 100 is indicative of heat tolerant animal

*If two animals are having same HTC value, then animal having less RR will be considered as more heat tolerant animal.

  • Gaalaa’s Heat Tolerance Index

This index was given by R.F.Gaalaas in 1947.Gaalaas equation is a modification of  Rhoad’s  equation (IBHC), and the study is performed under similar condition as in case of IBHC.

The value was calculated by using formula

           HT = 100-14 (RT-101)

           Where HT =Heat Tolerance

           RT = Rectal Temperature

*HTC value exceeding 100 is indicative of heat tolerant animal

*If two animals are having same HTC value, then animal having less RR will be considered as more heat tolerant animal.

  • Benezra’s Coefficient of Adaptability (BCA)

This index was given by M.V Benezra in 1954 using Rectal temperature and respirartion responses of animal after exposing them for 7 hrs for 3 consecutive days.

The value was calculated by using formula

           BCA = RT/38.3 + RR/23

           Where RT = Rectal Temperature

           And RR =     Respiration Rate

Normal RT (oC) = 38.33   Normal RR/min = 23

*BCA value of 2 is indicative of maximum adaptability and values more than 2 indicates lower adaptability

  • Dairy Search Index (DSI)

 It was given by CK Thomas and coworkers in 1973. Physiological parameters i.e rectal temperature and respiration rate and pulse rate is recorded and the value of DSI/DI is calculate d accordingly. For this the animals are exposed for 7 hrs on sunny coudy day.

The values are calculated by the formula

DSI = 0.5 X1/X + 0.3 Y1/Y + 0.2 Z1/Z

Where X1, Y1 and Z1 represents rectal temperature, pulse rate and respiration rate after exposure and X, Y and Z  asthe same parameters before exposure respectively.

* If the calculated value of DSI is more nearer to one than the animals is more heat tolerant.