FLORIDA AGRICULTURAL EXPERIMENT STATIONS
Dairy Science Mimeo Report DY65-6
June 30, 1965
EFFECTS OF HOT WEATHER ON DAIRY CATTLE
J. M. Wing
The effects of high temperature on productivity of dairy cattle have
been reported from numerous field studies. Most of these are somewhat
confusing, however, since the quality and amount of feed generally was
not held constant. Most studies indicate that the optimum temperature
for milk production is approximately 500F. Increasing the temperature
from 500 to 650F caused a slight increase in the production of milk but
a reduction in the butterfat content (10). Milk solids-not-fat and fat
often were reduced during the stress of hot weather. Fat per cent almost
always was affected. Reductions in temperature from 610F to 270F in the
field caused an increase in milk fat from 5.4 to 6.0 per cent (6). This
effect has been confirmed in various studies with cows which were confined
to climatic chambers. There is general agreement that stress resulting
from high environmental temperatures is detrimental to production and
reproduction, particularly if the humidity is high.
Twenty pairs of twin cows were divided between Fiji, where the temperature
averaged 850F, and New Zealand, where the average temperature was 650F.
Under these conditions the cows produced much better in New Zealand than
in Fiji and calves appeared to be healthier and grew faster in New
The effect of humidity appears to be very important (15); 950F with a
relative humidity of 45 per cent produced approximately the same effect
as 85F with a 90 per cent relative humidity. At temperatures of 800F,
wind had no effect (12), however, at 950F a wind of 1/2 mile per hour
caused an increase of approximately 10 per cent in milk production.
Faster winds produced somewhat better results (1).
Under these conditions radiation from heat lamps was detrimental (2).
One week of exposure to 180 BTU per square foot per hour caused marked
reductions in production with temperatures as low as 700F. Yet, shades
failed to increase milk production in recent work at the Georgia station
although they obviously promoted animal comfort (8).
In Louisiana (16) sprinkling plus shade was found to be superior to shade
alone, resulting in an improvement in production ranging from .8 to
5-1/2 pounds of 4 per cent fat corrected milk daily. Conditioning of the
cows may have been an important factor in both the Louisiana and Georgia
Another study in Louisiana (10) showed that keeping cattle inside for
eight hours daily at temperatures of 82?F resulted in higher production
than did keeping them outside and providing shades. The average outside
temperature was 920F. In this experiment the feed was kept constant and
humidity was not particularly severe.
Crossbred animals with some Indian breeding generally have been observed
to be more heat tolerant than the European breeds. It should be noted,
however, that these crossbreds generally are lower producers than pure-
bred European-type dairy animals, and that approximately 75 per cent of
the heat load in dairy cattle comes from metabolic processes associated
with maintenance and production (18). It has been recommended that in
the area of Beltsville, Maryland, protection from exposure be provided
for cattle when the temperature reaches 660F in open fields or 71F in
open barns (11).
Studies at the Louisiana Agricultural Experiment Station showed that as
outside temperature increased from 65.50 to 95.00F there was a drop of
2.14 pounds of 4 per cent fat corrected milk daily for each degree rise
in rectal temperature. The Holsteins increased from 1020 to 1060 in
rectal temperature whereas the Jerseys increased from 101.40 to 1060F.
Reproduction appears to be depressed by high temperatures. It is known
that heat is particularly detrimental to the reproductive processes of
bulls (4). Fertility of cows possibly is affected somewhat less severely
than that of the bulls, yet continued exposure to temperatures of 900F
caused the estrus cycles of some cows to stop. Since at least five weeks
of such severe constant exposure was required to produce the effect, it
appears that heat affects reproduction in the cow considerably less than
it affects semen quality. Thus since frozen semen is almost universally
available, heat stress may not pose an especially serious problem in
Hot weather usually causes a tremendous increase in the requirements of
animals for water, particularly as the temperature increases above 800F.
Water consumption almost doubles as temperatures increase from 800 to
Feed consumption usually is decreased as temperatures become particularly
high; moreover, pasture quality often is depressed by hot weather. This
probably accounts for some of the lost milk production. It is not the
entire answer, however, since it appears that the production of milk
usually decreases before feed consumption is depressed (1). Under some
conditions it has been observed also that milk production was depressed
by high temperature when the feed was constant (7). Feeds which are
high in net energy have been recommended as warm weather rations since
usually they result in less metabolic heat than do fibrous feeds which
are relatively low in productive energy (14). Thus, low-roughage,
high-concentrate rations may be helpful.
The daily range in temperature has been found to be rather unimportant
as compared with the average daily temperature in effect upon the
productive processes of dairy cattle (3). Ranges of temperature similar
to those found in the south have been used in various laboratory studies.
These would apply to any part of Florida since studies under a Southern
Regional Research Project showed different sections of Florida to be
comparable to those at various parts of the state, and with locations
in Louisiana, Arkansas, Georgia, Puerto Rico, South Carolina and Texas
(9). A range in temperature from 700 to 1000F, with an average daily
temperature of 850F, had the same effect on cows as did a constant
temperature of 850F.
Of interest was the fact that a 500 to 1100F daily range, with the average
780F, was less deleterious than a 700 to 1000F range, and that a 400 to
1100F range, where the average temperature was 660F, appeared to cause
very little drop in milk production.
It may be that conditioned cows can withstand weather as severe as any
Florida has to offer with only a minimum of protection. The questions
are how to condition them and what constitutes minimum protection.
Research being conducted cooperatively by the Dairy Science and Agricul-
tural Engineering Departments at the University of Florida will help
answer the most pertinent questions.
Cows which freshen close to or during the summer months are being used,
and the experimental animals are not allowed shade except when being
milked. Feed of the same quality is offered year around, and the amount
consumed is noted. Yield and composition of milk, feed consumption,
reproductive performance, and various other physiological responses of
the cow will be recorded.
Climatological measurements include temperature, humidity, wind speed
and direction, rainfall, and reduction of solar radiation by practical
At present, it seems advisable to provide shade on a free-choice basis.
More elaborate provisions seem to be of doubtful economic value, but firm
recommendations will not be practical until further research has been
accomplished. The project which is now in progress at the Florida Agri-
cultural Experiment Station will help pinpoint conditions which are
necessary to weather adaptation and protection in Florida.
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