Impact of Hair Color on Thermoregulation of Dairy Cows to Direct Sunlight

Abstract

The effects of hair coat color on the thermoregulatory responses of heat-stressed cows exposed to direct solar radiation were observed. During August 2000, three white and three black lactating Holstein cows in Hawaii were exposed to 1.5 hr of sunlight for four trials in the morning and four trials in the afternoon with or without spray cooling. Solar radiation ranged between 495 and 971 watts/m 2 and THI ranged between 78 and 81 during the experimental trials. Cows with white hair coat absorb about 66% of the short wave radiation, while cows with black hair coat absorb about 89% of the short-wave radiation. Placing the cows under direct sunlight increased the surface temperature of the black cows by 4.8°C and the white cows by 0.7°C. Rectal temperatures of black cows increased 1.3°C/hr, while the rectal temperatures of white cows increased 0.8°C/hr. Increasing solar load on the skin surface increases the local rate of sweating. Spray wetting alone is not sufficient to cool cows exposed to high solar loads.

Full text

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This is not a peer-reviewed paper.
Paper Number 014031
An ASAE Meeting Presentation
Impact of Hair Color on Thermoregulation OF Dairy
Cows to Direct Sunlight
P. E. Hillman , Senior Lecturer
Biological and Environmental Engineering, Cornell University, , Ithaca,
NY 14853
C. N. Lee , Professor
Animal Sciences, University of Hawaii at Manoa, , Honolulu, Hawaii
96822
J. R. Carpenter , Associate Professor
Animal Sciences, University of Hawaii at Manoa, , Honolulu, Hawaii
96822
K. S. Baek , Postdoc
Animal Sciences, University of Hawaii at Manoa, , Honolulu, Hawaii
96822
A. Parkhurst , Professor
Biometry, University of Nebraska, , Lincoln, NE 68583
Written for presentation at the
2001 ASAE Annual International Meeting
Sponsored by ASAE
Sacramento Convention Center
Sacramento, California, USA
July 30-August 1, 2001
Summary:The effects of hair coat color on the thermoregulatory responses of heat-stressed cows
exposed to direct solar radiation were observed. During August 2000, three white and three black
lactating Holstein cows in Hawaii were exposed to 1.5 hr of sunlight for four trials in the
morning and four trials in the afternoon with or without spray cooling. Solar radiation ranged
between 495 and 971 watts/m 2 and THI ranged between 78 and 81 during the experimental
trials. Cows with white hair coat absorb about 66% of the short wave radiation, while cows with
black hair coat absorb about 89% of the short-wave radiation. Placing the cows under direct
sunlight increased the surface temperature of the black cows by 4.8°C and the white cows by
0.7°C. Rectal temperatures of black cows increased 1.3°C/hr, while the rectal temperatures of
white cows increased 0.8°C/hr. Increasing solar load on the skin surface increases the local rate
of sweating. Spray wetting alone is not sufficient to cool cows exposed to high solar loads.

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Abstract.Three black and three white lactating Holstein cows were exposed to 90 minutes of
direct sunlight during August 2000 in the Waianae district, Oahu, Hawaii. Prior to solar exposure
the cows were heat-stressed, having rectal temperatures of about 39.5°C, dorsal skin
temperatures of about 35°C and respiration rates of about 92 breaths/min while under shade with
THI values of 78.5 to 80.8. Upon exposure to solar loads of 495 to 820 watts/m, the dorsal skin
temperatures of the black cows rose about 4.8°C, while white cows only rose about 0.7°C. Rectal
temperatures of the black cows rose 1.3°C/hr, compared to a 0.7°C/hr rise for white cows. The
rate of sweating increases with solar load, nearly doubled from a solar load of 150 watts/m 2 to a
solar load of 1100 watts/m 2 . At a solar load of about 1000 watts/m 2 , evaporative heat loss by
sweating of the irradiated coat was about 750 watts/m 2 for black cows and about 600 watts/m 2
for white cows. This difference is probably attributed to black coats absorbing more energy and
heating the skin more than the white coats for the same solar load. Wetting the hair coat of cows
exposed to direct sunlight failed to stop the rise in rectal temperature, although it did reduce the
rate of temperature rise by about 50%. Respiration rates increased about 8 to 13 breaths/min,
after the cows were exposed to direct sunlight. Neither coat color nor wetting appeared to have
any effect on the observed increase in respiration rate. This study recommends that dairy cows
always have shade available to help keep them cool. Water spray alone is insufficient to keep
them cool under high solar loads.
Keywords: Solar radiation, Hair coat, Spray cooling, Heat stress relief, Dairy cows
INTRODUCTION
Dark colored cows absorb more solar radiation than light colored cows (Fitch et al., 1984). This
difference becomes important when cows are heat-stressed and are exposed to direct sunlight, at
which time coat color impacts both physiological and behavioral thermoregulation. For example,
white Holsteins have lower surface temperatures, body temperatures, and respiration rates than
black Holsteins (Hansen, 1990). Dark colored cows and steers seek shade more than do the light
colored cows and steers (Goodwin, et al., 1997a and Fitch, et al., 1984). White cows require
fewer services per conception in the summer months, than black cows (King, et al., 1988). White
cows have also been shown to have higher milk yields than black cows, when both white and
black cows had access to shade only or to shade and water spray during periods of excessive heat
load (Goodwin, et al., 1977b).
Coat color, sweating, and spray cooling modify the impact of solar load on the cow's surface.
The following field study attempts to elucidate these factors in heat-stressed lactating cows. A
better understanding of the benefits of lighter hair coat, shade, sweating and wetting in the face
of severe solar loads are needed to optimize diary management during periods of severe heat
stress.
The objectives of this study are:
(1) To characterize the impact of solar radiation on surface temperatures, body temperatures,
sweating rates and respiration rates of heat-stressed cows with either black coats or white coats.
(2) To assess the benefit of water spray cooling on heat-stressed cows exposed to direct sunlight.

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EXPERIMENTAL DESIGN
Three black and three white lactating Holsteins cows of approximately equal body weights and
milk production were used for this study (Table 1). In the experimental design, the six cows were
divided into two groups (Table 2). Half the experimental trials were without any wetting ("not
wetted"). For the remaining experimental trials the cows were sprayed water ("wetted") every 10
minutes to keep the hair coat wet. For the wetting trials, the dorsal area was wetted with about 3
liters using a low-pressure single nozzle a period of 1.5 min, which saturated the hair coat
enough that excess water would drip off the side of the cow. One group had two white cows and
one black cow and the other group had two black cows and one white cow. Each group was
exposed to one morning not wetted, one morning wetted, one afternoon not wetted and one
afternoon wetted. Each group had no more than one treatment on a given day. During each
treatment, the cows were restrained in stalls that are about 0.8 m wide and were lined up head-to-
tail. The experiments were conducted within the holding area next to a milking parlor at the
Pacific Dairy farm located in the Waianae district, Oahu, Hawaii.
Each day a cow was used in a trial it was placed in the same stall exposed to direct sunlight
(August 15 to August 20, 2000) with similar environmental conditions (Table 2). The cows were
held in full shade for at least 20 minutes before they were moved to full sunlight exposure lasting
90 minutes.
Rectal temperatures were measured with a digital thermometer (model M525, GLA Agricultural
Electronics, San Luis Obispo, CA) at 10 min intervals, starting at 20 min before each
experiment. Respiration rates were recorded every 5 min by counting the thoracic movement per
unit time. Dorsal surface temperatures, ground and sky temperatures were all measured every 5
min using a handheld infrared thermometer (model 9JM08, Raytek, Santa Cruz, CA) and short
wave radiation was measured every 10 minutes using a pyranometer (model CM 6B, Kipp &
Zonen, Delft, Netherlands). This same pyranometer was used to estimate percent absorption of
the hair coat by measuring the incident and the reflected short-wave radiation. Prevailing wind
was measured with a three-cup anemometer (Totalizer 2100, NRG Systems, Hinesburg, VT).
Ambient air temperature and relative humidity were recorded at 10 min intervals (ten 1 min
readings were averaged) with a data logger (HOBO H8 Pro, Onset Computer Corporation,
Bourne, MA). Evaporative heat loss from the skin surface was measured using a portable
calorimeter over a 10-minute period. Details of the methods for using the portable calorimeter
are identical to those described in Hillman, et al., (2001), except that the temperature of the outlet
is based on two RTD chips, rather than the sensing grid because it was difficult to zero the
sensing grid under direct sunlight. Air velocity over the hair coat surface was set at 1.0 to 1.1 m/s
for all measurements.
RESULTS AND DISCUSSION
Not Wetted Responses
For all the experimental trials, the cows were heat-stressed before they were exposed to direct
sunlight, with pre-treatment rectal temperatures 39.4 to 39.5°C (Table 6) and respiration rates 88
to 95 breaths/min (Table 7). Pre-treatment THI values ranged from 78.5 to 81.9 (Table 3) and
were in the "Alert" to "Danger" zones (Hahn, 1999) and differed little from THI values during
the treatments (Table 4).

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Solar radiation ranged from 495 to 971 watts/m 2 during the experimental trials, with an overall
average of 792 watts/m 2 (Table 4). Cows with white hair coat absorb about 66% of the short
wave radiation, while cows with black hair coat absorb about 89% of the short-wave radiation
(Table 1). Exposing cows under direct sunlight increased the surface temperature of the black
cows by 4.8°C and the white cows by 0.7°C (Table 5). During the first 50 minutes of solar
exposure, rectal temperatures of black cows increased 1.3°C/hr, while the rectal temperatures of
white cows increased 0.8°C/hr (Table 6). Respiration rates for both white and black cows
increased slightly (10 to 15%) when moved to direct sunlight (Table 7).
The rate of sweating increases with higher solar loads on the skin surface (Figure 1), where
evaporation is higher from the black hair coat than from the white hair coat for a given solar
load. Evaporation does increase with an increase in dorsal skin temperature (Figure 2),
suggesting that the higher rate of sweating of black skin at a given solar load is due to greater
local warming of a black coat compared to a white coat (Table 5). Finch, et al. (1982) observed
that the rate of evaporation is strongly related to mean rectal temperature in cattle. Our data
suggests that skin temperature might be the physiological triggering mechanism for high rates of
evaporation (i.e., greater than 500 w/m 2 ), rather than body temperature (Figure 2). Gatenby
(1980) observations support the suggestion that local skin temperature affects the rate of
evaporation at the site of local heating, where the rates of local sweating were greater on the right
back of the steer facing the sun, rather than the shaded, left back of the steer.
The maximum sweating rate of heat-stressed Holsteins in shade under similar ambient air
temperatures and humidity was about 240 watts/m 2 (Hillman, et al., 2001), which is similar to
the sweating rates of cows with a low solar load in this study (Figure 1). Local heating of the
skin may be required to recruit maximum sweating from the skin surface.
Wetted Responses
As expected, wetting cools the dorsal hair coat exposed to direct sunlight, when compared to
temperature of pre-treatment coat in the shade (Table 5). Wetting cooled the white hair coat more
than the black hair coat. Spray cooling did not cool the heat-stressed cows after they were
exposed to direct sunlight (Table 6). This is especially true of the cows with black hair coat.
Respiration rates increased when the cows were moved to direct sunlight with no apparent
differences with white or black cows or whether they were wetted or not (Table 7).
Wetting the hair coat surface resulted in the highest evaporation rates from the skin surface
(Figure 3) with an average evaporation heat loss of 5.9±0.9 watts (12) from the sample area (i.e.,
760±119 watts/m 2 or 1116 gm water/m 2 hr). These rates are similar to the evaporation rates
from the coats of wetted cows in shade (Hillman, et al., 2001). Maximum sweating rates at the
greatest solar loads are similar to the evaporation rates of the wetted coats (Figure 3). This
observation suggests that wetting cows under high solar loads will not enhance natural
evaporative cooling as much as wetting the shaded cow, which underscores the importance of
providing shade when cooling cows.
CONCLUSIONS
When exposed to direct sunlight, the surface of heat-stressed, lactating Holsteins with black
coats rises about 4.8°C compared to about 0.7°C for white Holsteins. This is due to the higher

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absorption of solar radiation of black coats (89%) compared to white coats (66%). Local heating
of the skin surface by the sun appears to maximize sweating, but it is not enough to keep rectal
temperature from rising 1.3°C/hr in black cows or 0.8°C/hr in white cows. Evaporative heat loss
by sweating increases with increasing solar load, reaching over 800 watts/m 2 for black cows or
over 500 watts/m 2 for white cows with a solar load of about 1100 watts/m 2 . Wetting the dorsal
surface only cools the surface of black cows by about 0.6°C for black cows or about 3.1°C for
white cows. Rectal temperatures will still rise about 0.7°C/hr for black cows or about 0.3°C/hr
for white cows, even with the application of water spray. Only providing water spray to keep
cows cool during periods of high solar load is an incomplete strategy. To keep cows cool, shade
must be provided.
ACKNOWLEDGMENTS
This research is supported in part by Regional Hatch Funds and is a contributing project to the
Western Regional Project W-173 of the US Department of Agriculture. The authors sincerely
thank Ms. Monique Vanderstrom and Ms. Robin Dewalo of Pacific Dairy Inc. for the use of the
cows and facilities for these experimental trials.
REFERENCES
1. Hillman, P.E., K.G. Gebremedhin, A. Parkhurst, J. Fuquay, and S. Willard. 2001. Evaporative
and convective cooling of cows in a hot and humid environment. Livestock Environment
VI: Proceedings of the Sixth International Symposium, pp. 343-350, May 21-23, 2001,
Louisville, KY.
2. Finch, Virginia A., I.L. Bennett, and C.R. Homes. 1982. Sweating response in cattle and its
relation to rectal temperature, tolerance of sun and metabolic rate. J. Agric. Sci., Camb.
99:479-487.
3. Finch, Virginia A., I.L. Bennett, and C.R. Homes. 1984. Coat colour in cattle: effect on
thermal balance, behaviour and growth, and relationship with coat type. J. Agric. Sci.,
Camb. 102:141-147.
4. Goodwin, Peter J., John B. Gaughan, Trevor A. Schoorl, Bruce Young and Anita Hall. 1997a.
Shade type selection by Holstein-Friesan cows. Livestock Environment V: Proceedings of
the Fifth International Symposium, Volume II. p. 915-922, May 29-31, 1997,
Bloomington, Minnesota
5. Goodwin, Peter, John Gaughan, Patricia Skele, Maurie Josey, Anita Hall, and Bruce Young.
1997b. Coat color and alleviation of heat load in Holstein-Friesan cows. Livestock
Environment V: Proceedings of the Fifth International Symposium, Volume II. p. 923-927,
May 29-31, 1997, Bloomington, Minnesota
6. Hahn, G.L. 1999. Dynamic responses of cattle to thermal heat loads. J. Anim. Sci. 77 (Suppl.
2):10-20.
7. Hansen, P.J. 1990. Effects of coat colour on physiological responses to solar radiation in
Holsteins. Vet. Record 127:333-334.

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8. King, V.L., S.K. Denise, D.V. Armstrong, M. Torabi, and F. Wiersma. 1988. Effects of a hot
climate on the performance of first lactation Holstein cows grouped by color. J. Dairy Sci.
71:1093-1096.
Table 1. Properties of the lactating Holsteins. [All data are reported as mean±SD( n ).]
white cows cow #5335 cow #4300 cow #138 average
Color (>90% of surface) white white white white
Body weight* 626±5 kg (4) 694±8 kg (3) 677±21 kg
(4) 663±33kg
(11)
Milk production per milking** 11.6±1.1 kg
(15) 9.9±3.1 kg
(14) 12.4±2.7 kg
(15) 11.3±2.6 kg
(44)
Absorption of solar radiation 70 % 61 % 66 % 65.7±4.5 %
(3)
Area wetted 1.2 m 2 1.7 m
2 1.3 m
2 1.4±0.3 m 2
(3)
black cows cow #561 cow # 4769 cow #5112 average
Color (>90% of surface) black black black black
Body weight* 613±11 kg
(4) 678±6 kg (3) 641±11 kg
(4) 641±29 kg
(11)
Milk production per milking** 11.2±3.7 kg
(13) 12.2±5.1 kg
(15) 11.0±2.5 kg
(15) 11.5±3.9 kg
(43)
Absorption of solar radiation 89 % 88 % 90 % 89.0±1.0 %
(3)
Area wetted 1.1 m 2 1.6 m
2 1.5 m
2 1.4±0.3 m 2
(3)
*Estimated with a commercial
body weight tape
**3 milkings/day

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Table 2. Experimental design showing treatments and scheduling of white and black cows.
Date Start/Stop
Time Treatment Cow
#/color Cow
#/color Cow
#/color
8/15/2000 10:10 -
11:40 not
wetted #4769
(black) #4300
(white) #5112
(black)
8/15/2000* afternoon not
wetted #5535
(white) #561
(black) #138
(white)
8/16/2000 9:50 -
11:20 wetted #5535
(white) #561
(black) #138
(white)
8/16/2000 13:20 -
14:50 not
wetted #4769
(black) #4300
(white) #5112
(black)
8/17/2000* morning not
wetted #5535
(white) #561
(black) #138
(white)
8/17/2000 13:40 -
15:10 wetted #4769
(black) #4300
(white) #5112
(black)
8/18/2000 10:00 -
11:30 wetted #4769
(black) #4300
(white) #5112
(black)
8/18/2000 13:30 -
15:00 wetted #5535
(white) #561
(black) #138
(white)
8/19/2000 10:05 -
11:35 not
wetted #5535
(white) #561
(black) #138
(white)
8/20/2000 14:10-
16:20 not
wetted #5535
(white) #5112
(black)** #138
(white)
* Two of the originally scheduled
treatments (shown in italics) were not
included in the experimental design,
because of cloudy skies. They were
rescheduled to 8/19/2001 and 8/20/2001.
**On 8/20/2001 cow #5112 was
substituted for cow #561, because cow
#561 had mastitis.

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Table 3. Ambient conditions during the 20 min, pre-treatment period.
not wetted
treatments
8/15/2000 AM 8/16/2000
PM 8/19/2000
AM 8/20/2000
PM
T ambient 30.6±0.4 ºC (3)
30.6±0.1 ºC
(3) 29.1±0.2 ºC
(3) 31.6±0.1 ºC
(3)
Relative humidity 48.8±1.3 % (3) 54.9±0.4 %
(3) 53.1±0.5 %
(3) 53.5±1.0 %
(3)
THI* 79.1±0.3 (3) 80.0±0.0 (3) 77.8±0.3 (3) 81.2±0.2 (3)
wetted
treatments
8/16/2000 AM 8/17/2000
PM 8/18/2000
AM 8/18/2000
AM
T ambient 29.4±0.4 ºC (3)
32.6±0.3 ºC
(3) 29.6±0.2 ºC
(3) 33.2±0.2 ºC
(3)
Relative humidity 55.5±1.7 % (3) 50.0±1.2 %
(3) 60.0±1.2 %
(3) 43.0±1.1 %
(3)
THI* 78.5±0.3 (3) 81.9±0.1 (3) 79.6±0.2 (3) 81.3±0.1 (3)
*Computed from the relationship
given by Hahn (1999).
Table 4. Environmental conditions during treatments.
not wetted
treatments
8/15/2000 AM 8/16/2000 PM 8/19/2000 AM 8/20/2000 PM
Solar radiation 820±326 809±79 758±236 495±156

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watts/m 2 (11) watts/m 2 (12) watts/m
2 (17) watts/m 2 (17)
T ground , low value 26.3±4.6 ºC
(10) 24.4±0.4 ºC
(5) 22.3±0.8 ºC
(13) 29.9±1.7 ºC
(10)
T ground , high value 57.3±7.5 ºC
(10) 61.1±2.6 ºC
(4) 52.5±7.9 ºC
(13) 47.9±2.6 ºC
(10)
T sky , low value -22.2±1.0 ºC
(10) -23.4±0.2 ºC
(5) -23.2±0.2 ºC
(13) -4.8±7.1 ºC (9)
T sky , high value 11.6±1.2 ºC
(10) 8.9±4.2 ºC (5) 14.4±3.2 ºC
(13) 17.3±1.1 ºC (9)
Wind speed 1.9±0.6 m/s
(11) 2.8±0.8 m/s
(11) 1.2±0.5 m/s
(17) 2.1±0.5 m/s
(18)
T ambient 29.9±1.2 ºC
(20) 30.6±0.2 ºC
(20) 30.1±1.2 ºC
(20) 31.4±0.5 ºC
(20)
Relative humidity 53.8±5.3 %
(20) 55.0±0.8 %
(20) 49.4±3.6 %
(20) 53.1±2.1 %
(20)
THI* 78.9±0.8 (20) 80.2±0.1 (20) 78.5±0.3 (20) 80.8±0.3 (20)
wetted
treatments
8/16/2000 AM 8/17/2000 PM 8/18/2000 AM 8/18/2000 AM
Solar radiation 648±343
watts/m 2 (11) 906±76
watts/m 2 (19) 927±203
watts/m 2 (17) 971±115
watts/m 2 (19)
T ground , low value 23.9±1.6 ºC (8) 23.6±1.2 ºC
(16) 23.3±0.9 ºC
(13) 23.8±1.2 ºC
(17)
T ground , high value 51.5±7.6 ºC (8) 61.0±5.2 ºC
(16) 54.1±6.6 ºC
(13) 64.0±4.5 ºC
(17)
T sky , low value -22.7±1.3 ºC
(8) -22.3±1.4 ºC
(16) -20.3±1.9 ºC
(15) -23.1±0.4 ºC
(15)

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T sky , high value 10.7±3.8 ºC (9) 15.3±4.3 ºC
(16) 15.3±1.6 ºC
(15) 15.4±1.2 ºC
(15)
Wind speed 1.6±0.6 m/s
(12) 1.8±0.4 m/s
(19) 1.7±0.4 m/s
(18) 1.7±0.5 m/s
(19)
T ambient 29.9±1.0 ºC
(20) 33.7±0.4 ºC
(20) 30.7±0.7 ºC
(20) 34.3±0.6 ºC
(20)
Relative humidity 53.9±5.1 %
(20) 42.4±3.4 %
(20) 55.0±2.5 %
(20) 39.4±1.5 %
(20)
THI* 78.9±0.7 (20) 81.8±0.2 (20) 80.2±0.6 (20) 82.0±0.5 (20)
*Computed from the
relationship given by Hahn
(1999).
Table 5. Changes of dorsal skin temperature to direct sunlight.
not wetted treatments black cows white cows
T dorsal , pre-treatment in shade 35.3±0.7 ºC (6) 34.4±0.7 ºC (6)
T dorsal , during 90 min exposure to direct sunlight 40.0±1.9 ºC (6) 35.1±1.4 ºC (6)
? T dorsal , due to exposure to direct sunlight 4.8±1.9 ºC (6) 0.7±1.0 ºC (6)
wetted treatments black cows white cows
T dorsal , pre-treatment in shade 35.9±0.9 ºC (6) 33.9±0.3 ºC (6)
T dorsal , during 90 min exposure to direct sunlight 35.3±1.8 ºC (6) 30.8±1.6 ºC (6)
? T dorsal , due to exposure to direct sunlight -0.6±2.4 ºC (6) -3.1±1.5 ºC (6)
Table 6. Changes of rectal temperatures to direct sunlight.
not wetted treatments black cows white cows

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T rectal , pre-treatment in shade 39.5±0.5 ºC (6) 39.4±0.6 ºC (6)
T rectal , last 30 min of 90 min of direct sunlight 41.0±0.3 ºC (6) 40.4±0.6 ºC (6)
? T rectal , first 50 min of exposure to direct sunlight 1.3±0.2 ºC/hr (6) 0.8±0.2 ºC/hr (6)
wetted treatments black cows white cows
T rectal , pre-treatment in shade 39.5±0.6 ºC (6) 39.5±0.5 ºC (6)
T rectal , last 30 min of 90 min of direct sunlight 40.3±0.5 ºC (6) 39.8±0.4 ºC (6)
? T rectal , first 50 min of exposure to direct sunlight 0.7±0.4 ºC/hr (6) 0.3±0.3 ºC/hr (6)
Table 7. Changes in respiration rate (RR) to direct sunlight.
not wetted treatments black cows white cows
RR, pre-treatment in shade 95.3±20.9 breaths/min (6) 88.0±7.3 breaths/min (6)
RR, during 90 min of sun 105.2±10.7 breaths/min (6) 100.9±8.6 breaths/min (6)
? RR, due to exposure to sun 9.9±19.9 breaths/min (6) 12.9±7.2 breaths/min (6)
wetted treatment black cows white cows
RR, pre-treatment in shade 93.0±21.8 breaths/min (6) 78.5±11.6 breaths/min (6)
RR, during 90 min of sun 106.1±17.2 breaths/min (6) 86.9±9.7 breaths/min (6)
? RR, due to exposure to sun 13.1±10.9 breaths/min (6) 8.4±10.8 breaths/min (6)

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Figure 1. Heat loss by sweating from black and white hair coats exposed to different levels of
solar radiation. The low levels of solar radiation were observed on the cloudy periods on
8/15/2000 PM and 8/17/2000 AM. These two treatments were not included for the rest of this
study because of cloudy skies (see Table 2).
Figure 2. Rectal and dorsal skin temperatures for different levels of evaporative heat loss from
the skin surface. Dorsal skin temperatures were recorded by the portable calorimeter while
evaporation was being measured.

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Figure 3. Evaporation from the skin surface by sweating alone compared to the wetted the hair
coat. The straight line represents the linear regression of the sweating alone data (R2 = 0.589) for
both the white and the black cows. The average heat loss from the wetted skin was 5.9 watts.
Absorption of solar radiation by sample area is corrected for the percent absorption by the hair
coat.
Copyright © 2005. American Society of Agricultural and Biological Engineers. All rights reserved.