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 US
due 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 – 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
Heat Stress - Physiological Strain
General
1.
↑ Impact Other Stressors
2.
↑ Heart and Pulse Rates
3.
↑ Hyperventilation
A.
B.
C.
D.
↓
↓
↓
↑
Blood Carbon Dioxide
Blood Bicarbonate
Blood Buffering Capacity
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.
↑ Salivation
A.
B.
C.
D.
↓
↓
↓
↑
Saliva to Rumen
Salivary Bicarbonate Pool for Rumenal Buffering
Rumen pH
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.
↓ Rumination
A.
B.
C.
D.
E.
↓ Gut and Rumen Motility
↓ Gut Passage Rate
↑ Gut Fill
↓ Rumen Volatile Fatty Acid Concentration
↑ 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 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
Heat Stress - Physiological Strain
Reproductive Status - continued
6.
↓ Follicular Development
A.
B.
C.
7.
↓ Oocyte quality
↑ Multiple Ovulations and Twinning
↓ Corporea Lutea Size
Biochemical Changes
A.
B.
C.
D.
E.
↓ Plasma LH
↑ Ketone and NEFA Levels at Calving
↓ Thyroxine During Pregnancy
↑ Plasma Progesterone During Late Gestation
↑ Prostaglandin Synthesis Rate and Level During Early
Postpartum Period
CAN WE REDUCE THE PROBLEM
FROM THE THERMAL STATUS
PERSPECTIVE?
Influenced by
Body Surface Area
Body Coverings
Water Exchange
Blood Flow
Environment:
Temperature
Wind
Humidity
Evaporative
Cooling
Non-Evap.
Cooling
Radiation
Respiration
Convection
Skin
Influenced by
Sources
Calorigenic
Hormones
Food
Production:
Milk
Meat
Wool
Body Reserves
Muscular
Environment
Rumen or Cecum
Fermentation
Activity
Maintenance
Conduction
HEAT GAIN
HEAT LOSS
Hypothermia
Light
Bulbs
Hyperthermia
Normal
Body Temperature
BIG PROBLEM
METABOLISM
OR
HEAT PRODUCTION
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
Non-Evaporative Process
2
Heat Dissipation (kcal/m/hr)
Surface Vaporization
140
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)
BUT
A CHALLENGE TO
CHANGE
ESPECIALLY FOR BEEF
CATTLE AND GRAZING
DAIRY COWS
MANY DIFFERENT LEVELS OF SENSITIVITY
COMPLICATING FACTORS:
1. Breed
2. Age
3. Health
4. Gender
5. Geographic Location and
Climate
6. Acclimation Duration
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
Present
AMBIENT
CONDITION
ANIMAL
PRODUCTIVITY
ANIMAL
THERMAL
STATUS
AMBIENT
CONDITION
Extremely difficult
Present
Why?
ANIMAL
PRODUCTIVITY
Ambient condition = indirect
stressor
Does not account for
“Complicating Factors”
Temperature Humidity Index
is an example
AMBIENT
CONDITION
Improvement
Why?
Account s for
“Complicating Factors”
Physiological Strain Index is an
example
ANIMAL
THERMAL
STATUS
Present
AMBIENT
CONDITION
ANIMAL
PRODUCTIVITY
ANIMAL
THERMAL
STATUS
Performance or Health
Critical
Zone ?
HYPERTHERMIA
WEIGHT LOSS
DYSFUNCTION
DISEASE
DEATH
Stress or Strain
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