HEAT STRESS EFFECTS: ISSUES & QUESTIONS DON SPIERS UNIVERSITY OF MISSOURI-COLUMBIA.

HEAT STRESS EFFECTS:

ISSUES & QUESTIONS

DON SPIERS

UNIVERSITY OF MISSOURI-COLUMBIA

WHAT IS STRESS?(Initial Question)

Stress = the environment or stimulus

(same for everyone)

Strain = the effect or response (different for everyone)

“It’s not stress that kills us, it is our reaction to it.”

“I cannot, and should not,

be cured of my stress

but merely taught to enjoy it”

Hans Selye, l950

What is heat

strain?

Impaired Functions (the short list)

1. Drop in production

2. Increased days open

3. Depressed immune system

4. Decreased fertility

THE COST

Annual losses in the USdue to heat stress equal $1.7 billion.

Dairy = $897 million

Beef = $369 million

PHYSIOLOGICAL CHANGES

1. Thermal Status

2. General Effects

3. Immune Condition

4. Nutritional Status

5. Behavior

6. Endocrine

7. Reproductive Status

Heat Stress - Physiological Strain

Thermal Status

1. ↑ Core Body Temperature – rumen – tympanic – intraperitonealA. Total Body Heat Content

2. ↑ Respiration Rate and Respiratory Evaporative Heat Loss

3. ↑ Skin Temperature, Blood Flow, and Sweat RateA. ↓ Blood Flow to Internal Organs

4. ↑ Salivation, Drooling, and Panting Rates

5. ↓ Metabolic and Heat Production Rates

6. ↓ Heat Loss via Radiant, Conductive, and Convective Avenues

7. ↑ Dehydration

Heat Stress - Physiological Strain

General

1. ↑ Impact Other Stressors

2. ↑ Heart and Pulse Rates

3. ↑ HyperventilationA. ↓ Blood Carbon DioxideB. ↓ Blood BicarbonateC. ↓ Blood Buffering CapacityD. ↑ Respiratory Alkalosis

4. ↑ Urinary Sodium and Bicarbonate Excretion

5. ↓ Hepatic Portal Blood Flow

6. ↑ Hepatic Vitamin A Storage

7. ↑ General Vitamin E Deficiency

Heat Stress - Physiological Strain

Immune Status

1. ↓ Immune Function

2. ↑ Susceptibility to Parasitic and Nonparasitic Diseases

3. ↑ Mastitis

4. ↑ Somatic Cell Count

5. ↑ Plasma Antibody - Immunoglobulin Concentration

6. ↑ Death

Heat Stress - Physiological Strain

Nutritional Status

1. ↓ DMI, Weight Gain or Growth, Condition Score, and Blood Glucose Level

2. ↑ Energy Requirement for Maintenance

3. ↑ SalivationA. ↓ Saliva to RumenB. ↓ Salivary Bicarbonate Pool for Rumenal BufferingC. ↓ Rumen pHD. ↑ Acidosis

4. ↑ Potassium Loss from Skin

5. ↑ Dietary Requirements for Potassium and Sodium

6. ↑ Urinary Nitrogen Loss

7. ↑ Water Intake

Heat Stress - Physiological Strain

Nutritional Status - continued

8. ↓ RuminationA. ↓ Gut and Rumen MotilityB. ↓ Gut Passage RateC. ↑ Gut FillD. ↓ Rumen Volatile Fatty Acid ConcentrationE. ↑ Acetate to Propionate Ratio

Milk Production

1. ↓ Milk Production

2. ↓ Mammary Blood Flow

3. ↓ Mammogenesis

4. ↓ Lactation Peaks

5. ↓ Milk Component Levels

Heat Stress - Physiological Strain

Behavior

1. ↓ Grazing Time

2. ↑ Lethargy

3. ↑ Shadow or Shade Seeking

4. ↑ Body Alignment with Solar Radiation

5. ↑ Standing Time

6. ↑ Crowding Water Trough and Splashing

7. ↑ Agitation and Restlessness

Heat Stress - Physiological Strain

Endocrine

1. ↓ Hormones Linked to Metabolism – Thyroxine, Somatotropin,

Cortisol

2. ↑ Hormones Linked to Water and Electrolyte Metabolism –Antidiuretic Hormone, Aldosterone

3. ↑ Catecholamines – Epinephrine and Norepinephrine

4. ↑ Prolactin and ↓ Prolactin Receptor Numbers

5. ↑ Leptin

6. ↑ Insulin >> ↓ Blood Glucose

Heat Stress - Physiological Strain

Reproductive Status

1. ↓ Breeding Efficiency and Conception Rate

2. ↑ Fetal and Postnatal Mortalities + ↓ Calf Birth Weight

3. ↓ Semen QualityA. ↓ SpermatogenesisB. ↓ Sperm MotilityC. ↑ Percent Abnormal and Aged Sperm

4. ↓ Estrous ActivityA. ↓ Estrous DurationB. ↓ Heat Detection

5. ↓ Uterine Blood FlowA. ↓ Placental Weight and Growth + ↑ Retained PlacentaB. ↓ Gestation PeriodC. ↑ Labor and Delivery Difficulties

Heat Stress - Physiological Strain

Reproductive Status - continued

6. ↓ Follicular DevelopmentA. ↓ Oocyte qualityB. ↑ Multiple Ovulations and TwinningC. ↓ Corporea Lutea Size

7. Biochemical ChangesA. ↓ Plasma LHB. ↑ Ketone and NEFA Levels at CalvingC. ↓ Thyroxine During PregnancyD. ↑ Plasma Progesterone During Late GestationE. ↑ Prostaglandin Synthesis Rate and Level During Early

Postpartum Period

CAN WE REDUCE THE PROBLEM

FROM THE THERMAL STATUS

PERSPECTIVE?

Influenced by

Body Surface AreaBody CoveringsWater ExchangeBlood FlowEnvironment: Temperature Wind Humidity

Non-Evap.Cooling

Radiation

Convection

Conduction

EvaporativeCooling

Respiration

Skin

HEAT LOSS

Hypothermia HyperthermiaNormal

Body Temperature

Influenced by

CalorigenicHormones

Production: Milk Meat Wool

Muscular

ActivityMaintenance

Sources

Food

Body Reserves

Rumen or Cecum Fermentation

Environment

HEAT GAIN

Light Bulbs

METABOLISM

OR

HEAT PRODUCTION

BIG PROBLEM

1 ADULT HUMAN = 1 - 100 WATT LIGHT BULB

1 LACTATING DAIRY COW = 16 - 100

WATT LIGHT BULBS

BIG BOVINE PROBLEM = HUGE HEAT PRODUCTION

Occurs with reduction in feed intake.

AVENUES

OF HEAT LOSS

CONDUCTION

Conductive, Convective, and Radiative Exchanges Require a Temperature Gradient

CONDUCTION

RADIATION

CONVECTION

LESS EFFECTIVE AS AIR

TEMPERATURE APPROACHES SKIN

TEMPERATURE

EVAPORATIVE

EXCHANGE

Only Solution is Increase Evaporative Heat Loss

0

20

40

60

80

100

120

140

Hea

t D

issi

patio

n (

kcal

/m2/h

r)

0 20 40 60 80 100

Ambient Temperature ( oF)

Respiratory Vaporization

Non-Evaporative Process

Surface Vaporization

Redrawn from Yeck and Kibler (1956) and Kibler and Yeck (1959)

A CHALLENGE TO CHANGE

ESPECIALLY FOR BEEF CATTLE AND GRAZING DAIRY COWS

BUT

MANY DIFFERENT LEVELS OF SENSITIVITY

COMPLICATING FACTORS:

1. Breed

2. Age

3. Health

4. Gender

5. Geographic Location andClimate

6. Acclimation Duration

IS IT POSSIBLE TO PREDICT ANIMAL RESPONSE TO HEAT STRESS?

1. Remove or provide extra care for “sensitive” animalsA. Change environmentB. Change animal

1. Reduce heat production2. Increase heat loss

2. Identify heat extremes in advanceA. Change environmentB. Change animal

3. Reduce heat production4. Increase heat loss

3. Acclimation Program

ANIMALTHERMALSTATUS

ANIMALPRODUCTIVITY

Pre

sent

AMBIENT CONDITION Future

Future

ANIMALPRODUCTIVITY

Pre

sent

AMBIENT CONDITION

Extremely difficult

Why?

Ambient condition = indirect stressor

Does not account for “Complicating Factors”

Temperature Humidity Indexis an example

ANIMALTHERMALSTATUS

AMBIENT CONDITION Future

Improvement

Why?

Account s for “Complicating Factors”

Physiological Strain Index is an example

ANIMALTHERMALSTATUS

ANIMALPRODUCTIVITY

Pre

sent

AMBIENT CONDITION Future

Future

Per

form

ance

or

Hea

lth

EU

STR

ES

S G

OO

DS

WE

ET

SP

OT

Stress or Strain

DIS

TRE

SS

BA

D

DEATH

HYPERTHERMIAWEIGHT LOSSDYSFUNCTIONDISEASE

Critical Zone ?

Young’s Modulus of Elasticity = object or substance's tendency to be deformed elastically (i.e., non-permanently) when a force is applied to it.

The elastic modulus of an object is defined as the slope of its stress-strain curve in the elastic deformation region:

            Elasticity = Stress/Strain

Thomas Young

“If it were possible to measure quantitatively all the strains responding to a given stress, we could devise a type of Young’s Modulus of Elasticity (stress/strain). The reciprocal of this (strain/stress) would represent the additive compliance of animals and might be called an index of adaptation. This index could be of value both in determining the degree of adaptation achieved by an animal after a given period of exposure to the stress and in determining when adaptation is complete. It might also be useful in comparing the degree of adaptation achieved by different species under similar conditions.“

Melvin Fregly

Symposium Conducted by The National Academy of Sciences-National Research Council August, 1966

Being healthy means being able to adapt to and overcome obstacles. It allows"a margin of tolerance for the inconsistencies of the environment." Health is a "regulatory flywheel of the possibilities of reaction" .

Normative beings are able to deal with conflicts in a way that leaves open the possibility of future correction. Any normality limited to maintaining itself, hostile to any variation in the themes that express it, and incapable of adapting to new situations does not represent health.

An organism is healthy to the extent that it is capable of adjusting these norms in the face of changing circumstances.

Georges Canguilhem