Agriculture Reference
In-Depth Information
tion in the number of eosinophils), lymphopenia (reduc-
tion in the number of lymphocytes), polynucleosis (an
increase in polymorpho-nuclear leukocytes) and an
increased susceptibility to disease. Sustained adaptation
syndrome leads to reduced growth rate in young animals
and loss of weight in adult animals.
This non-specific response of the body, irrespective of
the origin of the stress, has been questioned and adapted
by Moberg (1987) and others. They suggest that corti-
costeroids may not be produced as a response to all
stressful stimuli, that different species may react differ-
ently to the same stressor and that in some cases other
endocrine pathways may be involved. It may be wrong to
assume, therefore, that a lack of adrenal response indi-
cates that there is no stress. There appears to be an
important role played by the brain, including the limbic
system, the pituitary and the hypothalamus, especially in
the mechanism by which animals cope with stress.
When
an animal puts substantial effort and resources into cop-
ing with a stressful situation, it can be considered to be
suffering distress
.
is subjected to a wide variety of stressors, many of which
have an adverse effect with subsequent deleterious
changes in the carcase. Even death may occur.
Stressors may include physical trauma and fear and
environmental excesses of noise, heat, cold, light, wind
chill or humidity. These may make excessive demands
on the animal and may result in handling problems
which may be reflected in abnormal bodily changes at
slaughter.
A husbandry system can be said to be stressful if it
makes abnormal demands on the animals. An individual
factor may be called a
stressor
if it contributes to the
stressful nature of a system of husbandry.
Seyle (1974) defined stress as a
non-specific response
in
an animal attempting to resist or adapt to maintain
homeostasis, that is, the tendency for the internal envi-
ronment of the body to be maintained constant and in
equilibrium. This suggests that, whatever the trigger for
the stress, the physiological response is identical.
There are two main reactions of an animal to stress:
the
alarm or emergency reaction
and the
general adaptive
syndrome
(together termed the
fight-or-flight syndrome
).
The alarm reaction is the result of a sudden adverse stim-
ulus and takes place immediately. It is reflected in an
increased activity of the sympathetic nervous system
which supplies the involuntary muscles, the secretory
glands and the heart. The result is an outpouring into the
bloodstream of the catecholamines, noradrenaline and
adrenaline by the medulla of the adrenal gland, leading
to increased heart rate and force of cardiac contraction,
constriction of peripheral blood vessels, elevated blood
pressure, dilation of bronchi, cessation of digestion and
mobilisation of liver glycogen with increases in blood
sugar. Glucagon, a peptide produced by the alpha cells of
the islets of Langerhans in the pancreas, is more power-
ful than adrenaline in the production of blood glucose
by the mobilisation of liver glycogen.
While the alarm reaction is immediate, the general
adaptive syndrome is the essential stress reaction and is
longer lasting. Adrenocorticotrophic hormone (ACTH)
is produced by the anterior pituitary gland and brings
about the production of corticosteroids such as cortisone
and hydrocortisone (cortisol), which regulate the gen-
eral metabolism of carbohydrates, proteins and fats on
a long-term basis. There is a decrease in carbohydrate
metabolism, and an increase in protein metabolism, the
amino acids being converted to glycogen in the liver. Fat
is metabolised from the fat deposits and is metabolised
in the liver, producing ketone bodies.
The overall result is
an increase in the level of blood glucose and ketones
. Other
changes in the general adaptation syndrome include
hypertrophy of the adrenal gland with reduction in its
ascorbic acid and cholesterol stores, eosinophilia (reduc-
Stress and meat quality
The physiological changes described previously which
occur when an animal is stressed can have a very signifi-
cant effect on the quality of the meat if the stress occurs
in the period prior to slaughter. In order to understand
these changes, it is necessary to have some knowledge of
the biochemical events which occur in muscle in the
period immediately after the animal's death.
After slaughter, the supply of oxygen to the muscle
ceases with the cessation of blood circulation. Normal
aerobic respiration in the muscle therefore stops, to be
replaced by anaerobic reactions. Anaerobic glycolysis,
the breakdown of hexose sugars, results in the produc-
tion and accumulation of lactic acid in muscle and a
characteristic fall in the pH from 7.0-7.2 to around 5.5.
This fall normally takes 4-8 hours in pigs, 12-24 hours
in sheep and 24-48 hours in cattle.
Anaerobic glycolysis results in the production of much
less energy, stored as adenosine triphosphate (ATP),
than its aerobic alternative. After death, the levels of ATP
therefore fall. Energy is required to keep muscle in its
relaxed state. When the levels of ATP fall to a critically
low level after death, the relaxed state can no longer be
maintained. The muscle's component molecules, actin
and myosin, combine irreversibly to form actomyosin,
and the muscle contracts slightly in what is known as
rigor mortis
- the carcase 'setting.
The rate of onset of rigor is therefore dependent on
the supply of ATP in the muscle at death. Any external
factor which depletes this supply of ATP will hasten the
onset of rigor mortis. This fact is demonstrated in
