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Heat stress in dairy cattle : physiological responses and variations in milk composition and equilibrium


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Rees, HV 1964 , 'Heat stress in dairy cattle : physiological responses and variations in milk composition and equilibrium', Research Master thesis, University of Tasmania.

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Dairy cattle were exposed to air temperatures from comfort
to heat stress levels within the range 70°F to 105°F in which relative
humidity changed from 71.0 ± 2.0 percent at 70°F to 36.0 ± 1.0 percent
at 105°F.
The following reactions were studied under conditions of
constant and alternating day/night temperatures in which different
feed treatments were used (dry feeds, balanced and unbalanced for milk
production) :- rectal temperature; respiration rate; pulse rate;
rumenal movement; animal behaviour; milk production and composition
(solids-not-fat and butterfat content); inherent acidity and freezing
point of milk; water and feed consumption; body weight.
Temperature increase above an apparent threshold (85°F)
resulted in rectal temperature increase and severe disturbance in
respiratory and cardiac function. The respiration rate increased prior
to body temperature and both showed a significant correlation with air
temperature. The pulse rate was initially stimulated but declined to
minimum rates when body fever was maximum.
The cycle of rumination was not influenced by ambient
Distress and nervous tension was evident at temperatures
greater than 95°F but sweat-gland activity was strictly limited. Changes
in behaviour (restlessness, panting, tongue protrusion, frequent visits
to water, salivation, sliming of nostrils and change in faecal texture)
were most apparent in cows which experienced greater body temperature
During active heat dissipation, important differences
between cows were recorded in the relative emphasis placed on the
different physiological responses involved in maintenance of homeostasis.
Milk composition and equilibrium changes were related to
increase in rectal temperature. There were decreases in solids-not-fat,
acidity and the freezing point depression which were unrelated to feed
treatment. Changes in butterfat were extremely variable and did not
conform to a common pattern.
The degree of animal response was influenced by the intensity
and duration of heat stress. Constant heat stress conditions effected
a progressive decrease in milk yield and production of fatty and nonfatty
solids. Animals under heat stress for shorter periods (alternating
day/night temperatures) did not show this trend.
In each trial, the cow with the higher milk production level
was least able to cope with a hot environment as shown by greater body
temperature increase, more intense depression of solids-not-fat and
acidity and higher elevation of the freezing point. This response was
independent of feed treatment and suggested a probable relationship to
a higher heat increment resulting from greater mammary gland metabolism
and the difficulty of dissipating extra heat.
Decline of feed intake paralleled and reflected decline in
milk production and was associated with body weight loss when heat stress
effected a significant increase in body temperature. (3° to 4°F rise
above normal). Conditions in which feed intake and milk production
remained relatively constant were associated with lower body temperatures
and body weight gains.
Body weight decreased under constant temperature exposure
and increased under alternating day/night temperatures. In each case
the balanced diet minimised weight loss and increased weight gain.
Water consumption greatly increased with air temperature
increase but in one cow was reduced to a level below maximum intake
when there was a drastic decrease in milk production and feed intake.
The chance occurrence of "natural" body fever in the field
permitted observation of concurrent changes in milk and body temperature.
Changes in milk production, composition, acid-base balance and osmotic
pressure were similar to those associated with increase in body
temperature induced by ambient air temperature increase. The pulse rate
but not the respiration rate was similarly affected.
Improved animal performance at higher temperatures within
the comfort zone (indicated by stability of rectal temperature) was
considered to be due to greater efficiency of feed utilisation and the
lower energy requirement for body heat maintenance and general metabolism.
This trend was independent of the effect of feed treatment.
Marked milk changes occurred during controlled conversion from
field to trial feeding of the unbalanced ration which were similar to
those effected under heat stress conditions. This suggested that in
the field environment, the effect of change in the qualitative character
of seasonal feed could either be supplementary or complementary to that
of the direct effect of temperature on the dairy cow.
The contribution by solar radiation and humidity to the heat
load placed on the lactating cow is discussed. The evidence suggested
that in the field, a much lower shade-temperature than that established in this study would initiate and sustain similar changes in milk composition and equilibrium.
The changes recorded in milk when cows were in heat stress,
may be related to changes in blood composition and acid-base balance.

Item Type: Thesis - Research Master
Authors/Creators:Rees, HV
Keywords: Cows, Dairying, Milk
Copyright Holders: The Author
Copyright Information:

Copyright 1964 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (M.Agr.Sc.) - University of Tasmania, 1966. Includes A study of the mechanism of solids not fat and freezing point variation with progression of the lactation period of the dairy cow, by H.V. Rees (63 p.) in pocket at back. Includes bibliography

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