Animal’s Heat Tolerance It’s Concept and Measurement Presented by SAMARA E. M. A. B.V.M.&S.,...

Animal’s Heat Tolerance It’s Concept and Measurement

Presented by

SAMARA E. M. A.B.V.M.&S., R.A., and M.Sc. Student

Introduction

• Heat– Form of Energy – Byproduct of all metabolic processes

– Measured as Temperature – Expressed in units called calories or joules

– Thermal vs. Heat

Introduction

• Body Temperature – Rectal vs. Core Body Temperature– Balance between heat input and output

• Thermal Steady State • Heat input vs. Heat output

HP + HG = HL + ∆C

• Homeotherms vs. Poikilotherms

Introduction

• Heat Stress – Animals stress response

• Homeostatic mechanisms counteract stresses• Genetic vs. Physiological mechanisms

– Thermo-physiological responses » Behavioral vs. Non-Behavioral response.

• Homeostasis vs. Homeorhetic mechanisms

(Shearer and Beede, 1991)

Introduction

• Adaptation vs. Heat tolerance

– Phenotypic vs. Genotypic adaptation

– Trait (character), Process, and Compensation Changes

• Acclimation vs. acclimatization» Body Function vs. Structure

Introduction

• Adaptation vs. Heat tolerance

– The basic concepts of natural selection• Fitness and environment.

– Every animal process is a function of heredity and environment. • Heat tolerance process as an aid to selection

Heat tolerance (HT)

• The ability of the body to endure the impact of a hot environment without suffering ill-effects.

– "ill-effects"• Animal Husbandry Man

– Poor performance with respect to grazing, thrift and milk yield

• Veterinary Practitioner – Low fertility and high proneness to disease

• Physiologist– Displacement from the normal of body functions

» Body temperature, sweating, and respiratory activity.

Heat tolerance (HT)

• Different people, different criteria,

different angles of view– which makes any comparison accompanied with

difficulties both fundamental and practical.

• It would appear therefore to … – Find a way of assessing an animal's innate overall

capacity to cope successfully with a hot environment

– Determine the animal's that inherent heat tolerance trait.

HTT ‘tests’ or HTI ‘indices’

• To design a HTI, three main factors have

to be considered …

– The hot climate (the stress producing agent)– The animal body (the target) – A numerical scale for expressing the effect of

the hot climate on the animal body.• In form of test or index

Heat tolerance (HT)

• Heat tolerance is a complex phenomenon; – The impact of one complex system “the thermal

environment” on another complex system “the animal's body”.

• The Numerical Scale in form of test cannot take into consideration – All climatic elements and All animal characters.

Only the most important/suitable factors has to be selected.

The Hot Climate

• The question is

– Which of the many climatic factors have to be taken into consideration for a heat tolerance test?

The Hot Climate

• HTI must be carried out in a standard thermal environment. – In order to give reproducible and comparable results– This is difficult to achieve in the field

– Environmental Chambers• Controlled atmosphere of a hot room

• Preferable• Has its limitations

The Hot Climate

• Animals tested simultaneously in the same geographical area ranked according to their heat tolerance – Adv. Allows selection of those animals – DisAdv. Not comparable with another breed in another

climate

• The total heat load imposed on an animal should be large enough to evoke a pronounced response, but not so large as to cause damage.

The Hot Climate

• The question is

– Which of the many climatic factors have to be taken into consideration for a heat tolerance test?

The Hot Climate

• The principal component factors – Temperature, humidity, wind and solar radiation.

• Impose strain on animal's internal environment – Affect their health, reproductivity, and eventually their

productivity

• They should all be included in a HTI either in single or in combination.

• Each climatic factor affects body in a different way, – Specific structures and mechanisms of the body.

The Hot Climate

• High environmental temperature,– Interferes with the dissipation of body heat by

convection and radiation if does not exceed body temperature

• Which emphasizes the importance of factors – Ratio of body surface to body mass,

– Circulation of the blood

– Body thermal gradient.

The Hot Climate

• High air humidity – Interferes with dissipation of body heat by

evaporation. • It thus plays a role in evaluating an animal's capacity

for evaporative cooling by sweating and panting.

– DisAdv. • Minor importance in a non-sweating or non-

panting animals

The Hot Climate

• Wind – Promotes dissipation of body heat by convection– Assists evaporation.

• Large surface area of an animal – Higher its rate of skin evaporation

– More benefit from the cooling power of wind.

The Hot Climate

• Solar radiation– Imposes a heat load on the body by

transferring energy to it from outside. • in contrast to the other three

– Allows a determination the animal with capacity for absorbing\reflecting radiation,

• Largely dependent on the characteristics of its skin and hair coat.

– A glossy, light colored coat Vs dull, dark coat

The Hot Climate

• Since all these climatic factors can affect the animal's thermal balance, it would appear desirable to express them jointly as a single figure,

– Adv.• Different climates would be compared

• Various attempts have been made to combine two or more climatic factors into a single index.

◄ Heat Load Index (HLI) ►

– Incorporate the effect of all four climatic variable – Gaughan et al. (2002) calculated the HLI using these

three equations …

[BGT = 1.33Ta – 2.65 √Ta + 3.21 log(SR + 1) + 3.5] ………..…... (1)[HLI = 10.66 + 0.28RH + 1.3BGT – WS] …………………………(2)[HLI = 8.62 + 0.38RH + 1.55BGT – 0.5WS + exp(-WS + 2.4)]…..(3)

Where; BGT = black globe temperature; Ta = dry bulb temperature (°C); SR = solar radiation (W/ m2); RH = relative humidity (%); WS = wind speed (m /s)

Animal Body

• The questions is,

– What is the relation of some selected body structures to heat tolerance?

– Which body functions are eligible for use as criteria in a heat tolerance test?

Animal Body Structure

• Surface/mass ratio • Texture and Color of the hair coat

• In general …– A large relative body surface, in particular large

body appendages (dewlap, navel fold, ears, tail), with movable skin

– Short glossy coat and a well pigmented

• Associated with a high heat tolerance.


• HT and animal body correlations have great practical value for several reasons. – Determined very easily– High degree of heritability which facilitates their

genetic fixation by breeding– Structural features correlated well with physiological

responses indicative of an animal's heat tolerance – The knowledge of these features may enable a breeder

to predict the future heat tolerance of a young animal


• Surface/mass ratio – McDowell, Lee & Fohrman (1953)

• Did not find significant differences in the surface/mass ratio between European and Zebu cattle, which otherwise differed in heat tolerance.

– McDowell (1958) • Surgical removal of hump and dewlap in Red Sindhi bulls

did not depress their heat tolerance.


• Texture and Color of the hair coat– Bonssma (1949)

• Established a clear relationship between coat texture and heat tolerance

– Animals with a sleek coat withstand heat better than animals with a woolly coat.

– Dowling (1959) • A high proportion of modulated hair fibers in the coat favors heat

tolerance

– Turner & Schleger (1960)• Described a subjective method for scoring of cattle coats which

seems to afford a good prediction of cattle performance in a hot climate.

– “Felting test”


• However,– It should be realized that any such structure is

not eligible as a criterion in a heat tolerance test

• Because it does not change measurably during exposure to an acute standard heat stress.

• It just, possess prognostic value

Animal Body


– What is the relation of some selected body structures to heat tolerance?

– Which body functions are eligible for use as criteria in a heat tolerance test?

Animal Body Function

• A hot environment affects the entire animal. – This means that a great number of physiological

activities undergo specific changes.

– The information driven from a heat tolerance test should thus be expected to become more complete the larger the number of activities observed.


• However, the use of a large number of criteria in a heat tolerance test has the disadvantage of complicating the performance of a test. – large the number of animal characters undesirable,

more difficult and less effective in selection

• It is necessary to make an optimal choice among the various body functions.


• Nominated body function

– Respiratory activity

– Cardio-vascular activity

– Cutanous evaporation

– Body temperature


• Body function requirements to be eligible as a criterion for a heat tolerance test;

– It should be meaningful

• Directly related to the body's thermoregulation

• Representative of the strain on the animal in a hot environment.

– It should respond to environmental heat in a ready and reproducible way

• Show a clear dose/response relationship.


• Body function requirements to be eligible as a criterion for a heat tolerance test;

– Its measurement should be accurate, quick, simple, and inexpensive.

– It should have a normal base line

• Show relatively little variation in hot vs. cold envi.

– It should be applicable also in the young animal• Allowing early prediction of an adult animal's heat tolerance.


• Body temperature is the best criterion of an animal's heat tolerance among the various body functions…

– It is physiologically meaningful; • Representing the resultant of all heat gain and heat loss

processes of the body

– It is relatively stable in a temperate environment

– It responds readily and in a repeatable way to heat stress

– It is easily and accurately measured.

Animal Economic Trait

• Productive performance– Milk vs. Beef production

• Related to heat tolerance in two ways: – They involve the generation of extra heat, which

reduces the capacity for heat tolerance.

– They become depressed when the environment is very hot, which favors heat tolerance.


• Cartwright (1955) – Found weight-gain in summer a more useful indicator

of heat tolerance than rectal temperature, respiratory rate and heart rate,

• Warwick & Kincaid (1959) – Postulated that the selection of beef cattle on the basis

of production automatically includes selection for heat tolerance.


• Although this approach has much to recommend it, it also has its limitations, especially with respect to milk yield.

– Cannot be tested in young animals. • Excludes prediction of an animal's future

performance from a calf hood index.


– Milk yield is affected by a hot environment in an indirect way

• Decline in milk yield is preceded by a decline in voluntary food intake.

– There is no immediate effect of hot weather on milk production as long as food consumption is not affected

• Food with a high fiber content vs. a high proportion of concentrates


• It may be said that – Productive performance seems to be useful as

an indicator to heat tolerance with beef cattle, • Less useful in these respects with dairy cattle.

The Numerical Scales

• Expressing the effect of the hot climate on the animal body.

• In form of test or index

• There are two ways of expressing an animal's heat tolerance.


• How much does a certain body function change when the animal is exposed to a standard hot environment?

– This is the commonly used way– Expresses heat tolerance in terms of animal

response– Relatively easy to perform – Give a precise numerical answer– Limited in its interpretation


• Which heat load must be imposed on an animal in order to evoke a standard change in body function?

– This way is less common with respect to climate

– Expresses heat tolerance in terms of a thermal environment

– Less easy to perform

– Less precise in its result

– It is more meaningful in its interpretation.

◄ Water Expenditure Test ►

• Rhoad (1940)

– Postulate that under field conditions measured parameters indicative of an animal's water loss through various channels, namely:

• Respiratory rate, as an index of moisture loss from the respiratory tract

• Loss of moisture from selected skin areas;

• Nitrogen concentration of the urine, as an index to water expenditure through the kidneys;

• Moisture content of the faeces, as an index to water expenditure through the excreta.


• Procedure

– Measured these parameters in 1 to 10 hours with an average of 3 hours.

– Water at atmospheric temperature was available at all times.


• Advantages ..

– This is a useful approach to an important side of the heat tolerance problem

• Water loss

– Rhoad has been able to demonstrate with his method clear breed differences.


• Disadvantages ..

– It is too complicated to be adopted as a heat tolerance test proper.

– It does not measure an animal’s water loss accurately enough.

– Rhoad did not condense the data into any formula.

– For judging the “Efficiency of heat disposal" Rhoad relied on the behaviour of rectal temperature,

• which he extended to the Iberia heat tolerance test.

◄ Iberia Heat Tolerance Test ►

• Rhoad (1944)

– Determines how much the rectal temperature of an animal exceeds 101°F (38.3°C)

when ……...?

• Average rectal temperature of cattle = 101°F


• Procedure

– Animals exposed in the open field to the thermal conditions of a calm clear day with a shade temperature between 85 and 95°F (29°and 35°C)

– The rectal temperature measurements are taken at 10.00 and 15.00 hours on three consecutive days and the results averaged.

– Using the formula:

{ 100 - [10*(mean rectal temperature – 101) }


• Procedure

– Respiratory rate, • Counted at the same time,

• Differentiate between two animals showing the same rise in rectal temperature.

– The result is expressed as a percentage of maximal efficiency in maintaining rectal temperature at 101°F.

• The animal with the least rise in rectal temperature (and the lowest respiratory rate) is considered the most heat tolerant.


• Advantages ..

– This is a most useful approach to an important side of the heat tolerance problem

– This test is based on a sound principle and is easily performed.

– It has been used extensively for differentiating between the heat tolerances of various breeds of cattle.



– The two main limitations are:

• Environmental conditions are not sufficiently standardized

• 101 F, being an average figure for rectal temperature of cattle, does not make allowance for normal variation of this parameter due to age, breed, level of feeding, level of production, etc.

– Give falsely low figures for young animals

– Give falsely high figures for animals whose normal body temperature is below 101.00F.

◄ R-Value ► Heat Tolerance Test of Lee

• Lee (Phillips & Lee, 1948) – Laboratory analogue to the Iberia heat

tolerance test.


• Procedure

– It consists of a 7 hr exposure of the animal to a series of atmospheres at different known temperatures and humidity.

– The rectal temperature is measured periodically and an expression of its mean behaviour, called R-value is calculated.

– The R-value represents the area between the abscissa and the curve in a time-rectal temperature diagram.


• From the R-value for the different combinations of temperature and humidity the value of the constants a, b and c were calculated

• Using the equation:

[ Log R = aT + bH + c ]Where;

• T is temperature and H is humidity

• The values of a, b and c– Obtained upon two different animals

– Indicate the relative fashion in which the rectal temperature rises in response to atmospheric temperature for a given humidity.


• Advantages ..

– This test has the merit of a controlled environment and an integrated response of the animal.

– Similar procedures can be adopted with respect to respiratory rate, pulse rate, etc.



– If the animal was in a steady or in arising phase cannot be derived from an isolated R-value.

– As a result of a too rapidly rising body temperature, an animal has to be removed from the test atmosphere before the 7 hours of exposure have elapsed

◄ Benezra’s Coefficient of adaptability ►

• Benezra (1954)

– Postulates a summation of the body temperature response and the respiratory rate response

– Using the formula:[ body temperature (in °C)/38.33 + respirations per min/23 = 2]

– Interpretation:

• Value of 2 represent the norm or a state of maximal adaptability

• Value above 2 a state of a lower adaptability.


– While it seems a reasonable idea to take into consideration also an animal’s respiratory response,

it is not correct to give it equal weight with the body temperature response.

• 10% increases– Body temperature would eventually cause death,

– Respiratory rate is physiologically meaningless and lies within the normal limits.

◄ Benezra’s Coefficient of adaptability ►

◄ Lines of Equal Effect ►

• Barrada (1957) – Constructed empirically lines of equal effect

by • Charting separately the reactions of rectal

temperature, respiratory rate and respiratory volume of cattle to different combinations of wet and dry bulb temperature in a temperature-vapor pressure diagram.

◄ Lines of Equal Effect ►

• He found that these lines change their slope as one passes from cooler conditions to hot conditions in much the. same way as those obtained for humans.

• Adv.

– This is a very useful concept for the description of the combined effect of temperature and humidity on physiological reactions.

• DisAdv.

– The procedure is too elaborate to be used as a test of heat tolerance.

◄ Dowling "Cooling Efficiency" Test ►

• Dowling (1956)

– Considers the Iberia heat tolerance test as unsuitable in the hot dry inland climate of Australia,

• Body temperature would not be a meaningful criterion of adaptability to heat.

– because the excessive solar radiation may cause loss of control of body temperature regulation in the animals.


• Dowling (1956)

– Proposes a heat tolerance test which should be based

• Solely on an animal's capacity for dissipating excess body heat

• Without the complication of a simultaneous heat load from solar radiation.

• Procedure ..

– The animal is exercised on a warm day for about half an hour until the rectal temperature reaches about 40°C (104°F)

– After that, animal allowed to stand in shade to cool down.

– The rate of cooling is followed by periodic measurements of rectal temperature.


• Interpretation …

– The animal which shows the highest rate of decrease of body temperature is considered to have the highest "cooling efficiency".


• Disadv.

– When devising a heat tolerance test for animals living in a region where solar radiation is excessive, it would seem to be more realistic to include this very factor in the test than to exclude it.

– In order to avoid the development in the animal of serious hyperthermia, the duration of each test would, of course, have to be restricted in time.

– Although in a test based on a short exposure to an extreme heat load, the animal is in a state of acute thermal imbalance.


◄ Predicted Heat Tolerance from Calf Total Moisture Vaporization Rates ►

• Yeck & Kibler (1958) – Reported that

[ Ratio of total evaporation in an environment of 27°C /

Total evaporation in an environment of 10°C ]

Correlated very well with the relative heat tolerance of calves

of 6 different breeds (Brahman; Shorthorn; Santa Gertrudis, Brown Swiss, Jersey, and Holstein).

• P.S.

– Total evaporation (skin plus respiratory)


• The highest ratio (2.75) and the highest heat tolerance were found in the Brahman;

• The lowest ratio (1.35) and the lowest heat tolerance in the Shorthorn

• With the other breeds in between in the following order of falling heat tolerance: Santa Gertrudis, Brown Swiss, Jersey, and Holstein.– These ratios changed very little for each breed with advancing

age.


• Adv.

– This is an impressive demonstration of the role played by evaporative cooling in the thermoregulation of cattle.

• DisAdv.

– As the accurate measurement of total evaporation is a fairly elaborate procedure

– It simpler to assess heat tolerance by a more easily measurable character such as body temperature.

◄ Average Relative Deviations (ARD) ►

• Marai and Habeeb (1998)

– Calculating the average relative deviations (ARD) from normal (regardless either positive or negative), due to exposure to hot climates, in thermal, water and/or nitrogen balances of the animals (or in all traits measured) …

Could be used in the estimation of parameters for detection of adaptability to a hot climate

– Using the equation:

[ Adaptability (%) = [100 − ARD] × 100 ]

◄ Tunica Dartos Indices (TDI) ►

• El-Darawany (1999) and Marai et al. (2006)

– Used tunica dartos indices (TDI) to measure the ability of the ram to tolerate increased ambient temperatures.


• Using the equations:

1) TDIa = ((Max. SCL - Min. SCL)/(Max. SCL - TL))*100

2) TDIb = ((RT - SST)/RT)*100

3) 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; TL = average testis length during the winter and summer seasons.


• El-Darawany (1999) and Marai et al. (2006)

– The scrotum actively controls its own temperature through the function of the tunica dartos muscle

• Distance between the testes and the abdominal wall.

• Reflects the magnitude of vascular heat exchange

• It’s function at high and low temperature.

◄ Various Tests of Adaptability of Cattle in Hot Countries►

• Bonsma (1949) – Described a number of procedures which are not heat

tolerance tests, but tests which give valuable information on an animal's general capacity to live in a hot country under ordinary field conditions.

• “Felting test“

• “Walking tests“

• “Water deprivation tests"


• “Felting test" – Sample of hair is taken from the animal, damped with

water and rubbed between the hands.

• If the hair forms a firm mass, i.e. felts, this indicates a smooth coat, and if the hair rubs away, this indicates a woolly coat.

• Woolly coat is associated with a low heat tolerance and a smooth coat with a high heat tolerance.


• “Felting test" – Adv.

• Indirect test for animal's heat tolerance. • Simple • Applicable to very young animals


• “Walking tests" and “Water deprivation tests" – Bonsma demonstrated the superiority of

African cattle over European type cattle in

• Covering long distances

• Going without water over certain periods of time, in a semi-arid region.

◄ Human Heat Stress Indices ►

• The best known of these indices are:

– Effective temperature, Houghten & Yaglou (1923).

– Standard operative temperature, Gagge (1940).

Index of physiological effect, Robinson, Turrell & Gerking (1945).

– Corrected effective temperature, Bedford (1946).

– Predicted four hour sweat rate, McArdle, Dunham, Holling, Ladell, Scott, Thomson & Weiiner (1947).

– Heat stress index, Belding & Hatch (1955), and

– Singapore index, Webb (1959).


• From this description of various methods for grading heat tolerance it would appear that

– Many of them provide useful information

– None of them is entirely satisfactory.

– There is a definite need for an improved method to increase animal production in hot countries

• Rhoad's "classical" Iberia 'test probably is still one of the best.

Heat Tolerance (HT)


– Which factors influence animal heat tolerance ?

– How strong are these influences?

Factors Which Modify Heat Tolerance

• Inherent heat tolerance by animals is modified by a number of factors which vary during an animal's individual life.

• The most important of these are age, nutrition, activity, reproduction, level of production, acclimatization, management, state of health, and emotional balance.

Factors Which Modify Heat Tolerance

• It is important to know how and to what extent these factors influence heat tolerance– Then they may be eliminated, standardized or

adjusted

– Unless these factors are taken into consideration, heat tolerance tests will give misleading results.

Recommendation

• Considering the problem of heat tolerance of cattle in its widest context, a general four-step procedure may be envisaged:

– Young animals are selected for heat tolerance on the basis of structural characters, the most promising characters to be used in this way.

– At an age not less than about half a year the animals so selected are subjected to a suitable heat treatment

• To allow their temperature regulating mechanisms to attain maximal efficiency

• Become fully acclimatized.

Recommendation


– These pre-selected and pre-treated animals are then subjected to one or several suitable heat tolerance tests which include, stress producing climatic factors, and body temperature occupies a central position among the physiological variables measured.

Recommendation


– Once the animals selected in this way have reached their productive stage or living / transferred to a warm environment, they are finally selected on the basis of their productive level.

Conclusions

• Heat tolerance is a complex phenomenon.

• Its study involves three factors: – The thermal environment– The animal body – A suitable scale for expressing numerically

the effect of the thermal environment on the animal body.

Conclusions

• Since each of the principal component elements of a thermal environment makes specific demands on thermoregulation, it is desirable in a heat tolerance test to consider all of them.

– Heat Load Index (HLI)

– The total heat load imposed on an animal should be large enough to evoke a pronounced response, but not so large as to cause damage.

Conclusions

• Various morphological characters such as the texture and the color of the hair coat may have prognostic value

– Allow a certain prediction of a young animal's future heat tolerance.

– This is of practical importance for the breeder.

Conclusions

• Among the many physiological variables body temperature is considered the best criterion of heat tolerance.

• Productive performance as an indicator to heat tolerance seems to be more useful in beef cattle than in dairy cattle.

Conclusions

• From a description of the various methods which have been used for assessing heat tolerance, it is concluded that

– None of them is entirely satisfactory – There is a need for an improved method.

• Rhoad Iberia Heat Tolerance Test

Conclusions

• An animal's capacity for tolerating heat is influenced by a number of modifying factors

– Unless these factors are taken into consideration, heat tolerance tests will give misleading results.

References, at request

The End

Animal’s Heat Tolerance It’s Concept and Measurement Presented by SAMARA E. M. A. B.V.M.&S.,...

Documents

Transcript of Animal’s Heat Tolerance It’s Concept and Measurement Presented by SAMARA E. M. A. B.V.M.&S.,...