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

2. WHAT IS STRESS?
(Initial Question)

3. Stress = the environment or
stimulus
(same for everyone)
Strain = the effect or response
(different for everyone)

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

5. “I cannot, and
should not,
be cured of my
stress
but merely
taught to enjoy it”
Hans Selye, l950

6. What is
heat
strain?

7. Impaired Functions (the short list)
1. Drop in production
2. Increased days open
3. Depressed immune system
4. Decreased fertility

8. THE COST
Annual losses in the US
due to heat stress equal
$1.7 billion.
Dairy = $897 million
Beef = $369 million

9. PHYSIOLOGICAL CHANGES
1. Thermal Status
2. General Effects
3. Immune Condition
4. Nutritional Status
5. Behavior
6. Endocrine
7. Reproductive Status

10. Heat Stress - Physiological Strain
Thermal Status
1. ↑ Core Body Temperature – rumen – tympanic – intraperitoneal
A. Total Body Heat Content
2. ↑ Respiration Rate and Respiratory Evaporative Heat Loss
3. ↑ Skin Temperature, Blood Flow, and Sweat Rate
A. ↓ 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

11. Heat Stress - Physiological Strain
General
1. ↑ Impact Other Stressors
2. ↑ Heart and Pulse Rates
3. ↑ Hyperventilation
A. ↓ Blood Carbon Dioxide
B. ↓ Blood Bicarbonate
C. ↓ Blood Buffering Capacity
D. ↑ Respiratory Alkalosis
4. ↑ Urinary Sodium and Bicarbonate Excretion
5. ↓ Hepatic Portal Blood Flow
6. ↑ Hepatic Vitamin A Storage
7. ↑ General Vitamin E Deficiency

12. 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

13. Heat Stress - Physiological Strain
Nutritional Status
1. ↓ DMI, Weight Gain or Growth, Condition Score, and Blood
Glucose Level
2. ↑ Energy Requirement for Maintenance
3. ↑ Salivation
A. ↓ Saliva to Rumen
B. ↓ Salivary Bicarbonate Pool for Rumenal Buffering
C. ↓ Rumen pH
D. ↑ Acidosis
4. ↑ Potassium Loss from Skin
5. ↑ Dietary Requirements for Potassium and Sodium
6. ↑ Urinary Nitrogen Loss
7. ↑ Water Intake

14. Heat Stress - Physiological Strain
Nutritional Status - continued
8. ↓ Rumination
A. ↓ Gut and Rumen Motility
B. ↓ Gut Passage Rate
C. ↑ Gut Fill
D. ↓ Rumen Volatile Fatty Acid Concentration
E. ↑ Acetate to Propionate Ratio
Milk Production
1. ↓ Milk Production
2. ↓ Mammary Blood Flow
3. ↓ Mammogenesis
4. ↓ Lactation Peaks
5. ↓ Milk Component Levels

15. 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

16. 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

17. Heat Stress - Physiological Strain
Reproductive Status
1. ↓ Breeding Efficiency and Conception Rate
2. ↑ Fetal and Postnatal Mortalities + ↓ Calf Birth Weight
3. ↓ Semen Quality
A. ↓ Spermatogenesis
B. ↓ Sperm Motility
C. ↑ Percent Abnormal and Aged Sperm
4. ↓ Estrous Activity
A. ↓ Estrous Duration
B. ↓ Heat Detection
5. ↓ Uterine Blood Flow
A. ↓ Placental Weight and Growth + ↑ Retained Placenta
B. ↓ Gestation Period
C. ↑ Labor and Delivery Difficulties

18. Heat Stress - Physiological Strain
Reproductive Status - continued
6. ↓ Follicular Development
A. ↓ Oocyte quality
B. ↑ Multiple Ovulations and Twinning
C. ↓ Corporea Lutea Size
7. Biochemical Changes
A. ↓ Plasma LH
B. ↑ Ketone and NEFA Levels at Calving
C. ↓ Thyroxine During Pregnancy
D. ↑ Plasma Progesterone During Late Gestation
E. ↑ Prostaglandin Synthesis Rate and Level During Early
Postpartum Period

19. CAN WE REDUCE THE PROBLEM
FROM THE THERMAL STATUS
PERSPECTIVE?

20. Influenced by
Body Surface Area
Body Coverings Influenced by Sources
Water Exchange
Blood Flow Calorigenic
Hormones Food
Environment:
Temperature
Production: Body Reserves
Wind
Humidity Milk
Meat Rumen or Cecum
Wool Fermentation
Non-Evap. Evaporative Environment
Cooling Muscular
Cooling
Radiation Respiration Activity
Maintenance
Convection Skin
Conduction
Light
HEAT LOSS HEAT GAIN Bulbs
Hypothermia Hyperthermia
Normal
Body Temperature

21. BIG PROBLEM
METABOLISM
OR
HEAT PRODUCTION

22. 1 ADULT HUMAN = 1 - 100 WATT
LIGHT BULB
1 LACTATING DAIRY
COW = 16 - 100
WATT LIGHT
BULBS
BIG BOVINE PROBLEM = HUGE HEAT PRODUCTION

23. Occurs with reduction in feed intake.

24. AVENUES
OF HEAT LOSS

25. Conductive, Convective, and Radiative Exchanges
Require a Temperature Gradient

26. CONDUCTION
RADIATION
CONVECTION
LESS EFFECTIVE AS AIR
TEMPERATURE APPROACHES SKIN
TEMPERATURE

27. EVAPORATIVE
EXCHANGE

28. Only Solution is Increase Evaporative Heat Loss

29. Heat Dissipation (kcal/m/hr)
Surface Vaporization
140
Non-Evaporative Process
2
120 Respiratory Vaporization
100
80
60
40
20
0
0 20 40 60 80 100
Ambient Temperature ( oF)
Redrawn from Yeck and Kibler (1956) and
Kibler and Yeck (1959)

31. BUT
A CHALLENGE TO
CHANGE
ESPECIALLY FOR BEEF
CATTLE AND GRAZING
DAIRY COWS

32. MANY DIFFERENT LEVELS OF SENSITIVITY
COMPLICATING FACTORS:
1. Breed
2. Age
3. Health
4. Gender
5. Geographic Location and
Climate
6. Acclimation Duration

33. IS IT POSSIBLE TO PREDICT ANIMAL
RESPONSE TO HEAT STRESS?
1. Remove or provide extra care for “sensitive” animals
A. Change environment
B. Change animal
1. Reduce heat production
2. Increase heat loss
2. Identify heat extremes in advance
A. Change environment
B. Change animal
1. Reduce heat production
2. Increase heat loss
3. Acclimation Program

34. AMBIENT
CONDITION
ANIMAL
Present
THERMAL
STATUS
ANIMAL
PRODUCTIVITY

35. AMBIENT
Extremely difficult
CONDITION
Why?
Ambient condition = indirect
Present
stressor
Does not account for
“Complicating Factors”
Temperature Humidity Index
ANIMAL is an example
PRODUCTIVITY

36. AMBIENT
CONDITION
ANIMAL
Improvement
THERMAL
Why? STATUS
Account s for
“Complicating Factors”
Physiological Strain Index is an
example

37. AMBIENT
CONDITION
ANIMAL
Present
THERMAL
STATUS
ANIMAL
PRODUCTIVITY

38. Critical
Zone ?
Performance or Health
HYPERTHERMIA
WEIGHT LOSS
DYSFUNCTION
DISEASE
DEATH
Stress or Strain

39. 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

40. “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