Title: Influence of heat stress on dairy cattle performance
Full Citation
Permanent Link: http://ufdc.ufl.edu/UF00091682/00001
 Material Information
Title: Influence of heat stress on dairy cattle performance
Physical Description: Book
Language: English
Creator: Thatcher, W. W.
Publisher: Dairy Science Department, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville, Fla.
Copyright Date: 1971
 Record Information
Bibliographic ID: UF00091682
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: 313787727 - OCLC

Full Text

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Influence of Heat Stress on
Dairy Cattle Performance i a iV Of-

W. W. Thatcher

Dairy Science Department I '-'i 1 iv. o FI0rida
Institute of Food and Agricultural Scier "-'s
University of Florida, Gainesville 32601


The tropical and subtropical areas of the world are becoming more impor-
tant in contributing to the world's food supply. Considerable research has
been initiated to determine the role of heat stress on milk production and re-
production, and to develop methods of alleviating its effects. Climatic labora-
tory studies have demonstrated the effects of ambient temperature extremes
and environmental control on milk production of dairy cows. From these con-
trolled studies, climatological interrelationships have been defined which now
allows for the prediction of milk production losses during the summer within
certain geographical areas of the United States. However, there is a sparcity
of information relating controlled environmental conditions to reproductive
Dairy systems in Florida definitely fall under the classification of large
herds. Only about 20% of Florida dairymen are milking less than 200 cows; 40%
milk from 200 to 499 and 40% are above 500 cows. The average dairyman milks
about 430, and the average 1971 Florida cow produced 9427 lb of milk compared
to the national average of 10,264 lb. Large herd size undoubtedly intensifies
management problems since less attention is allotted to individual cows. In
addition to management problems inherent to large herd operations, environmental
conditions associated with a subtropical area contribute to the lower levels of
reproduction and milk production. Average Florida humidity is over 78% and the
average annual temperature is 68 to 75.2 F depending on location within the state.
Objectives of the present report are: to characterize effects of environ-
mental temperature on reproductive performance, to consider certain physiological
and hormonal factors responsive to environmental and management conditions that
may affect cow performance, and to document under practical conditions the bene-
fits of environmental control on lactational and reproductive performance.


Se ona epressions of fertility due to heat stress in the cow are attri-
butable primarily to the female. Poor reproductive performance associated
with th rmal stress may be due to high temperature acting directly on the deve-
loping embryo and/or through a maternal endocrine imbalance. At the Dairy Re-
search Unit we evaluated influences of humidity and environmental temperature
on uterine and rectal temperatures, plasma progesterone and corticoid concen-
trations, and determined interrelationships among these factors and conception
rate. Animals used in this study included all first-calf heifers and lactating
dairy cows in our Experiment Station dairy herd during the period September 1,
1970 to August 31, 1971. Rectal (RT) and uterine temperatures (UT) were recorded
at insemination (Tb) and approximately 14.5 + 4.6 h (,. SD) post-insemination

(Ti) for first, second and third services. Daily climatological measurements
utilized were: maximum temperature (MXT), minimum temperature (MNT), minimum rela-
tive humidity and temperature humidity index (THI). Breed, service date, service
no. inseminator, time of service, service sire, age, lactation number, days post-
partum, and conception were obtained from herd records. Conception was based on
pregnancy diagnosis per rectum at 45-52 days postinsemination. Blood samples were
collectsd at T1 from the subcutaneous abdominal vein, and plasma analysed for total
corticoids and progesterone. Conception rate was significantly influenced by service
sire, days postpartum, environmental temperature on the day after insemination
(maximum, minimum and average), rectal and uterine temperature at insemination.
In contrast we were unable to detect any relationship between plasma progesterone
and corticoid concentrations with fertility. Considering the four measures of
ambient temperature and humidity (maximum, minimum,average, THI) for each day as
a group, days were ranked as to their relative influence on conception. The grouped
climatological measurements the day after insemination were most important.
These results indicated that environmental temperature on the day after insemination
was related to fertility. An additional analysis was conducted with 5062 insemi-
nations over a 12 yr period in the same herd to relate various climatological
measurements, on both the day of insemination and day after insemination, to
fertility. The following climatological factors were evaluated: maximum and mini-
mum environmental temperatures, relative humidity, wind movement, solar radiation,
maximum and minimum atmospheric pressures, and precipitation. As a group the
climatological measurements accounted for a statistically significant portion of
the variability in conception rates. Each measurement was ranked according to
the apparent magnitude of its effect on conception. Maximum environmental temper-
ature the day after insemination (MXTI) was the climatological measurement having
the highest association with fertility. No differences in fertility were associated
with months and this is undoubtedly due to incorporation of temperature measurements
in the analysis. As maximum temperature increased from 70 to 950F conception rate
declined from 40 to 31%. The average maximum temperature for the months of May
through September, based on a 60 year monthly average for Gainesville, Florida, are:
87.3, 90.1, 90.5, 90.9 and 88.90F, respectively. These are monthly maximum tem-
peratures which would exert a thermal stress associated with decreased fertility.
Analyses from both studies also emphasize the well-known importance of such factors
as sires and inseminators in a reproductive management system.


We have demonstrated that heat stress increases plasma progestin concen-
trations in heifers. None of the 22 heifers exposed to 90F maintained pregnancy
compared to 12 of 25 heifers at 700F. The fact that plasma progestins are elevated
certainly suggests the possibility that the intricate balance between progesterone,
estrogen and LH secretion may be altered during heat stress. Such alterations may
account for the observations that heat stress causes a shortening in duration of
heat, an increase in incidence of both silent heats and anestrus, and a reduction
in the intensity of estrus.
Any discussion of reproductive efficiency in dairy cattle must include
estrous detection. This is even more important in reference to large herds managed
under tropical or subtropical environments because high air temperatures shorten
the duration of estrus and lower its intensity. Our hormonal studies with dairy
cows indicate that distinct changes occur in heat responses of the reproductive
tract associated with transitory changes in endogenous estrogen secretion.

Consequently, the association between uterine temperature at insemination and
fertility may be due partially to timing of insemination to achieve maximum fer-
tility. In contrast, the uterine temperature association with fertility the day
after insemination appears to be influenced more by environmental temperature as
discussed previously. The association between fertility and uterine temperature
at insemination, possibly associated with timing of insemination, emphasizes the
importance of estrous detection and timing of artificial insemination. Additional
emphasis on estrous detection is warranted in large herds and should include:
frequent daily estrous checks, delegation of estrous detection to one or two respon-
sible persons, utilization of a chart for prediction of estrus and adoption of a
routine and well organized postpartum herd health program. Development of devices
for accurate estrous detection and utilization of effective compounds for estrous
synchronization should provide an effective and efficient system in which fer-
tility at the synchronized estrus may be normal.


Various studies have documented the benefits of air conditioning on milk
production but potential benefits on reproductive efficiency in cattle have re-
ceived less attention. In a cooperative study with the Graham Company in Southern
Florida near Lake Okeechobee, the effect of an air conditioned environment was
evaluated on yield of 4% fat corrected milk and reproductive performance. Four
groups of dairy cows (n=208), averaging 56.5 + 24.5 (' SD) days in lactation
and producing 46.0 + 1.1 lb milk per day, were used in a two-summer study. Treat-
ment I cows were kept in a shadeless lot approximately 492 feet from the air-
conditioned milking parlor 24 hr per day except for milking. Treatment II cattle
stayed in a similar lot from 5:30 a.m. to 7:30 p.m. and were housed in a free stall
air-conditioned structure from 7:30 p.m. to 5:30 a.m. Treatment III cows were air-
conditioned from 5:30 a.m. to 7:30 p.m. and remained outside from 7:30 p.m. to 5:30
a.m. Treatment IV cattle were housed 24 hr daily in one half of the 48 cow free
stall facility. All cows were fed a complete feed ration plus sudex green chop ad
libitum. The 1970 portion of the trial was initiated May I and continued for 148
days. The average maximum daily environmental temperature for the period was 90.50F.
During the summer of 1971, the trial was initiated on June 15 and continued for
79 days in which the average daily maximum temperature was 88.70F.
To estimate the relative periods of time each group spent at temperatures
above 700F, daily degree-hours above 70F were calculated for each of the treat-
ment groups (Table 1). A base of 700F was selected because environmental temper-
atures above this base, in the lactating bovine, cause deep body temperatures
to rise. Consequently, relative differences in degree-hours among the treatments
reflects differences in exposure to thermal stress. The lower number of degree-
hours during 1971 was due to milder weather conditions and maintenance of the air-
conditioned facility at 60oF rather than at 700F as was done in 1970.
A highly significant effect of treatment was detected on yield of 4% fat
corrected milk. Cows air-conditioned 24 hr per day (Treatment IV) produced 9.4%
more 4% fat corrected milk than non air-conditioned control cows (Treatment I;
Table 2). This increase in milk production is comparable to that reported by
Louisiana workers. The measured loss in production of 2.95 Ib of 4% fat corrected
milk per day (difference between Treatments IV and I) compared favorably with
the predicted losses estimated by Missouri workers for the Florida region.
All cows in this study were checked twice daily for estrus and bred by
artificial insemination at the first heat after 60 days postpartum. Pregnancies
were confirmed per rectum at approximately 45 days postinsemination. Treatment
group observations were not evenly distributed between the 2 years because of the

shorter duration of the trial the second year. Also, more cows were pregnant at the
start of the trial in 1971. Conception rates for all services during the 2 yr trial
and for 1970 alone are shown in Table 3. We obtained a higher conception rate for
cows cooled continually or during the day (Treatment IV and III) than non air-
conditioned cows or cows cooled at night but not during the day (Treatments I and
II). Thus, reducing environmental thermal stress apparently prevented the expected
decrease in fertility encountered during the summer months in Southern Florida.
The similar levels of fertility for Treatment III and IV suggest that maintenance
of reproductive efficiency can be obtained without total 24 hrcooling. This study,
conducted with large groups of cows under practical management conditions, does
not allow an evaluation of the reproductive critical period of thermal sensitivity,
pre or post-ovulation. Promotion of heat loss through air conditioning requires
expensive facilities. However, net expense diminishes if consideration is given
to both reproductive and lactational improvements. Only part of the herd may need
to be cooled to maximize returns efficiently. For example, cooling might be con-
sidered for only those cows in heavy production that are open. In addition less
expensive cooling systems involving shades, cooled shades, evaporative cooling,
sprinklers, etc., warrant consideration.


Environmental conditions associated with subtropical areas contribute to
lower levels of reproduction and milk production. Fertility is inversely related
to the maximum environmental temperature the day after insemination and to uterine
temperature both at insemination and the day after insemination. No differences
in fertility were associated with month effects when daily climatic parameters were
included in the statistical model. Heat stressful environments were associated with
alterations in hormonal balance such as an elevation in plasma progestins. The
association between uterine temperatures at insemination and fertility may be re-
lated to optimal timing of insemination to achieve maximal fertility. Air condi-
tioning dairy cows for 24 harper day caused a 9.4% increase in daily yield of 4%
fat corrected, and either continued or day time air conditioning reduced expected
summer time decreases in fertility.


Table 1. Daily degree hours above 700F

Treatment 1970 1971

1: Outside 205 156
11: Out A.M., In P.M. 133 88
111: In A.M., Out P.M. 91 45
1V: Inside 18 3

Treatments (1, 11, 111,
n*** ^^TC^^^'* ** ** ** ** *** *** ***

1V) described in

Table 2. Least squares treatment means for 4% fat
corrected milk (4% FCM) and fat percent

b Responsesa
Treatment Daily yield of Fat percent
4% FCM (Ibs)

1 31.4 3.08
11 31.9 2.92
111 33.0 3.27
IV 34.3 3.14

Least squares means 32.7 3.10

aDifferences in daily yield of 4% FCM and fat percent-

bage among treatments were highly significant (P/.01).
Treatments (1, 11, 111, 1V) described in text
Table 3. Percent cows conceiving to all services
while on trial
Treatment Total Services % Conceptionb

1 89 28.1 (22.6)c
11 81 28.4 (19.0)
111 55 40.0 (41.9)
1V 80 38.8 (40.0)

Treatments (1, 11, 111, IV) described in text
bX X4.15; .10 cParenthesis contain % conception for 1970,
X=10.5; .01


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