The SLICK hair locus derived from Senepol cattle confers thermotolerance to intensively managed lactating Holstein cows

Abstract

The SLICK haplotype (http://omia.angis.org.au/OMIA001372/9913/) in cattle confers animals with a short and sleek hair coat. Originally identified in Senepol cattle, the gene has been introduced into Holsteins. The objectives of the current study were to determine (1) whether lactating Holsteins with the slick hair phenotype have superior ability for thermoregulation compared with wild-type cows or relatives not inheriting the SLICK haplotype, and (2) whether seasonal depression in milk yield would be reduced in SLICK cows. In experiment 1, diurnal variation in vaginal temperature in the summer was monitored for cows housed in a freestall barn with fans and sprinklers. Vaginal temperatures were lower in slick-haired cows than in relatives and wild-type cows. In experiment 2, acute responses to heat stress were monitored after cows were moved to a dry lot in which the only heat abatement was shade cloth. The increases in rectal temperature and respiration rate caused by heat stress during the day were lower for slick cows than for relatives or wild-type cows. Moreover, sweating rate was higher for slick cows than for cows of the other 2 types. In experiment 3, effects of season of calving (summer vs. winter) on milk yield and composition were determined. Compared with milk yield of cows calving in winter, milk yield during the first 90 d in milk was lower for cows calving in the summer. However, this reduction was less pronounced for slick cows than for wild-type cows. In conclusion, Holsteins with slick hair have superior thermoregulatory ability compared with non-slick animals and experience a less drastic depression in milk yield during the summer.

Full text

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J. Dairy Sci. 97 :5508–5520
http://dx.doi.org/ 10.3168/jds.2014-8087
© American Dairy Science Association
®
, 2014 .
ABSTRACT
The SLICK haplotype (http://omia.angis.org.au/
OMIA001372/9913/) in cattle confers animals with a
short and sleek hair coat. Originally identified in Sene-
pol cattle, the gene has been introduced into Holsteins.
The objectives of the current study were to determine
(1) whether lactating Holsteins with the slick hair
phenotype have superior ability for thermoregulation
compared with wild-type cows or relatives not inherit-
ing the SLICK haplotype, and (2) whether seasonal
depression in milk yield would be reduced in SLICK
cows. In experiment 1, diurnal variation in vaginal tem-
perature in the summer was monitored for cows housed
in a freestall barn with fans and sprinklers. Vaginal
temperatures were lower in slick-haired cows than in
relatives and wild-type cows. In experiment 2, acute
responses to heat stress were monitored after cows were
moved to a dry lot in which the only heat abatement
was shade cloth. The increases in rectal temperature
and respiration rate caused by heat stress during the
day were lower for slick cows than for relatives or wild-
type cows. Moreover, sweating rate was higher for slick
cows than for cows of the other 2 types. In experiment
3, effects of season of calving (summer vs. winter) on
milk yield and composition were determined. Com-
pared with milk yield of cows calving in winter, milk
yield during the first 90 d in milk was lower for cows
calving in the summer. However, this reduction was less
pronounced for slick cows than for wild-type cows. In
conclusion, Holsteins with slick hair have superior ther-
moregulatory ability compared with non-slick animals
and experience a less drastic depression in milk yield
during the summer.
Key words: heat stress , SLICK locus , body tempera-
ture , milk yield
INTRODUCTION
A largely unexplored strategy to reduce the severity
of heat stress effects on dairy cattle is to select cattle
genetically for increased thermotolerance. There is ge-
netic variation for heat tolerance in Holsteins (Dikmen
et al., 2012) and specific SNP have been identified that
are associated with that genetic variation (Hayes et al.,
2009; Dikmen et al., 2013). One genetic strategy is to
introgress specific genes that confer thermotolerance
from cattle breeds that are genetically more resistant to
heat stress into dairy breeds that arose in cool climates.
Only one such gene has been identified to date—at
the SLICK locus. Originally described in the Senepol
breed that arose in the Caribbean island of St. Croix
and inherited as a single dominant gene, the SLICK
haplotype confers animals with a short and sleek hair
coat (Olson et al., 2003). The locus has been mapped
to bovine chromosome (Chr) 20 (Mariasegaram et al.,
2007; Flori et al., 2012). Cows of the Carora breed and
Carora × Holstein crossbreds possessing a phenotype
consistent with inheritance of the SLICK haplotype
experience lower body temperatures and increased milk
yield compared with animals with the wild-type hair
coat (Olson et al., 2003). The SLICK haplotype has
also been introduced into Holstein cattle: lactating Hol-
steins with slick hair have superior thermoregulatory
ability compared with wild-type Holsteins (Dikmen et
al., 2008). In part, this difference reflects an increased
rate of thermal sweating (Dikmen et al., 2008).
Herein we describe the results of studies that confirm
the superior thermoregulatory ability of Holsteins with
slick hair compared with non-slick animals and show
that inheritance of the SLICK haplotype reduces the
depression in milk yield during the summer in high-
producing Holsteins reared in confinement conditions.
Results indicate the utility of introgression of the
SLICK haplotype for reducing the effect of heat stress
on dairy production.
The SLICK hair locus derived from Senepol cattle confers
thermotolerance to intensively managed lactating Holstein cows
S. Dikmen ,* F. A. Khan ,†
1
H. J. Huson ,‡
2
T. S. Sonstegard ,‡ J. I. Moss ,† G. E. Dahl ,† and P. J . Hansen †
3
* University of Uludag, Faculty of Veterinary Medicine, Department of Animal Science, Bursa 16059, Turkey
† University of Florida, Department of Animal Sciences, Gainesville 32611-0910
‡ USDA, Agricultural Research Service, Animal Genomics and Improvement Laboratory, Beltsville, MD 20705-2350
Received February 27, 2014.
Accepted May 20, 2014.
1
Present address: Department of Population Medicine, Ontario
Veterinary College, University of Guelph, Guelph, ON, N1G 2W1,
Canada.
2
Present address: Department of Animal Science, Cornell University,
Ithaca, NY 14853.
3
Corresponding author: Hansen@animal.ufl.edu

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SLICK HAIR LOCUS IMPROVES THERMOTOLERANCE
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MATERIALS AND METHODS
Animals
Animal use was approved by the University of
Florida Institutional Animal Care and Use Commit-
tee (approval no. 2009-03578). Lactating cows of 3
genotypes were used. Slick- and normal-haired relatives
(15/16 Holstein or greater) were produced from mat-
ings of Holsteins (7/8 or greater) possessing the slick
hair phenotype originally derived from matings with
Senepol cattle. The original matings were made around
1990, and offspring were subsequently mated with ei-
ther wild-type bulls (registered Holstein) or slick bulls
that were at least 75% Holstein.
Slick animals possessed a short and sleek hair coat
that was most obvious because of the very short hair
on the face and poll and a small switch on the tail (Fig-
ure 1). Animals with these phenotypic characteristics
were considered to have inherited the SLICK haplo-
type. Normal-haired offspring were also produced from
slick-haired Holsteins; they did not possess the SLICK
phenotype but rather had plentiful hair on face, poll,
and switch. These animals were categorized as rela-
tives. Classification as slick or relative was confirmed
in genotyping experiments for some animals (described
below). The third group of cows, classified as wild-type,
was composed of Holsteins produced by matings of nor-
mal-haired Holsteins. These cows possessed hair coats
similar in appearance to cows of the relative group.
Genotyping
Genomic DNA was isolated from whole blood using
the Gentra Puregene Blood Kit (Qiagen, Gaithersburg,
MD). Concentration of DNA was measured using a
Nanodrop 1000 (Thermo Fisher Scientific, Wilmington,
DE) and Quant-iT Picogreen assays from Invitrogen
(La Jolla, CA). Stock DNA was diluted to 75 ng/μL
and aliquoted for processing on a BovineHD BeadChip
assay (Illumina Inc., San Diego, CA) according to the
protocol supplied with the reagents. Processed chips
were analyzed on an Illumina iScan, and SNP geno-
types were called in Illumina’s Genome Studio software.
Animal samples with total SNP call rate >90% were
exported in PLINK format as AB allele calls. To deter-
mine haplotypes at the Senepol-derived SLICK locus,
the approximate set of 22,000 SNP on Chr 20 were
analyzed using fastphase (Scheet and Stephens, 2006)
with a setting of a 50-SNP window. Haplotype identity
by descent analysis was completed using an internal
PERL script. The SLICK haplotype for animals in this
study was confirmed based on association results from
a separate genome-wide association analysis using a
resource population of Senepol animals (Huson et al.,
2014).
Housing
Studies were conducted at the University of Florida
Dairy Unit, Hague, Florida (29.77°N, 82.42°W). Cows
were housed in sand-bedded freestall barns equipped
with sprinklers (Rain Bird Manufacturing, Glendale,
CA) and fans (J&D Manufacturing, Eau Claire, WI)
that were programmed to become activated when dry-
bulb temperature exceeded 21.1°C. When activated,
the fans operated continuously and sprinklers were ac-
tivated for 1.5 min at 6-min intervals. Feed and water
were available ad libitum for all cows at all times. Cows
were milked twice daily between 0800 and 1000 h and
between 2100 and 2300 h.
Experiment 1: Diurnal Variation in Vaginal
Temperature During Heat Stress
Effect of the SLICK haplotype on diurnal variation
in body temperature during heat stress was determined
for slick cows (n = 16), relatives (n = 9), and wild-type
cows (n = 13). Slick cows were produced from 6 wild-
type and 4 slick sires, whereas relatives were produced
from 6 wild-type and 1 slick sire. Genotypes of 7 slick
cows were tested, and all were heterozygous for the
SLICK haplotype. Wild-type cows were selected from
the herd to be similar in parity and stage of lacta-
tion as slick-haired and relative cows. We observed no
significant differences (P > 0.05) among slick, relative,
and wild-type cows in parity, DIM, or milk yield (least
squares means ± SEM; 1.6 ± 0.2, 2.0 ± 0.2, and 1.7 ±
0.2; 165 ± 28, 246 ± 37, and 189 ± 31 d; and 28.2 ±
2.3, 33.2 ± 3.0, and 33.7 ± 2.5 kg, for slick, relative,
and wild-type, respectively) at the start of the experi-
ment.
Vaginal temperature was measured at 15-min inter-
vals for 3 d using a blank (i.e., without progesterone)
controlled internal release device (CIDR; Zoetis, New
York, NY) to which were attached 2 temperature data
loggers: a Hobo water temperature Pro V2 (Onset Co.,
Bourne, MA; accuracy of ± 0.21°C from 0 to 50°C) and
iButton (model 1922T; Maxim Integrated, San Jose,
CA; accuracy of ± 0.0625°C at 11-bit resolution). The
Hobo logger was taped to the CIDR using surgical ad-
hesive tape, whereas the iButton was placed in a groove
cut into central arm of the CIDR and affixed with
silicone (Figure 2). Vaginal temperature was monitored
as an indication of core body temperature because of
the high correlation with rectal temperature (r = 0.81;
Vickers et al., 2010) and because data loggers could be
maintained in the vagina for several days.

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Journal of Dairy Science Vol. 97 No. 9, 2014
The experiment was conducted between July 31 and
August 3, 2012 (replicate 1), August 6 and August
9, 2012 (replicate 2), and August 10 and August 13,
2012 (replicate 3). A different set of cows was used for
each replicate. On each experimental day, black globe
temperature (inside and outside of the barn), dry bulb
temperature, relative humidity (RH), and dew point
temperature were recorded every 15 min in the barn
at a height of 2 m. Dry bulb temperature, RH, and
dew point temperature were measured using a Hobo-U2
data logger (Onset Co.), and black globe temperature
was measured using a Hobo Pro V2 data logger. The
temperature-humidity index (THI) was calculated ac-
cording to the equation of NRC (1971):
THI = (1.8 × T + 32) –
[(0.55 – 0.0055 × RH) × (1.8 × T – 26)],
where T = dry bulb temperature (°C) and RH = rela-
tive humidity (%).
Experiment 2: Thermoregulatory
Responses to Acute Heat Stress
This experiment was conducted to determine whether
the superior ability of slick-haired cows to regulate body
temperature during heat stress was due to differences
in surface temperature or sweating rate compared with
cows without the slick-haired phenotype. Lactating slick
(n = 13), relative (n = 9), and wild-type (n = 9) cows
were used. Slick cows were produced from 5 wild-type
and 1 slick sires, whereas relatives were produced from
3 wild-type and 2 slick sires. Slick cows were genotyped:
10 were heterozygotes, 1 was a homozygote, and 2 were
not genotyped. The experiment was conducted in five
Figure 1. Representative examples of the slick phenotype in Holsteins. Panel A shows a slick (left) and non-slick cow; a close up of the face
of a slick cow is shown in panel B. Note the absence of long hairs on the poll. Shown in panels C and D are examples of the shaved areas of the
rump of slick (C) and wild-type (D) cows. Color version available in the online PDF.

Journal of Dairy Science Vol. 97 No. 9, 2014
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1-d replicates during a period from July 30 to August 5,
2013, using a different group of cows in each replicate.
Slick, relative, and wild-type cows were matched
according to milk yield, stage of lactation, coat color
(percentage of the surface that was black), and par-
ity. We found no significant effects of genetic type or
genetic type × replicate on parity, DIM, or milk yield
at the start of the trial. Parity was 2.0 ± 0.2, 2.1 ± 0.2,
and 2.0 ± 0.2, DIM was 131.5 ± 31.9, 156.5 ± 31.4,
and 132.7 ± 32.6 d, and milk yield was 33.4 ± 2.2,
38.0 ± 2.1, and 37.9 ± 2.0 kg/d for slick, relative, and
wild-type cows, respectively.
For each replicate, cows were housed in an outdoor
environment from 0800 to 1800 h in a 10- × 8-m pen.
The pen had a concrete surface and was covered by
shade cloth (73% reduction in solar radiation; Dono-
van Enterprises, Stuart, FL) suspended 3 m overhead.
There were no sprinklers or fans for cooling. Feed and
water were available ad libitum for all cows. Cows were
milked before (<0800 h) and after the experiment (2100
h). Before the experiment, 4 areas of hair (~5 × 5 cm)
were clipped on the right side of the cow using an Oster
PowerPro hair clipper with Opti-Block blade kit (Oster
Professional Products, McMinnville, TN). The clipped
areas were located on the lower third of the neck, loin
(at last thoracic vertebra), rump (at the middle of the
tuber coxae and tuber ischii), and upper hind leg (at
the femorotibial joint).
Rectal and surface temperatures and respiration and
sweating rates were measured on each cow at 0900,
1100, 1300, 1500, and 1700 h. Surface temperature and
sweating rate were measured at the clipped regions
Figure 2. Devices used to measure vaginal temperature. Two temperature sensors were affixed to a blank controlled internal drug releasing
(CIDR) device—an iButton (Maxim Integrated, San Jose, CA) and a Hobo water temperature Pro V2 (Onset Co., Bourne, MA). The iButton
was placed into a groove cut into the CIDR and secured with aquarium sealant. The Hobo logger was taped to the CIDR using surgical adhesive
tape. The CIDR was then loaded into an applicator for insertion into the vagina. Color version available in the online PDF.

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Journal of Dairy Science Vol. 97 No. 9, 2014
and at an adjacent nonclipped area. Rectal tempera-
ture was measured by means of a digital thermometer
(GLA M750; GLA Agricultural Electronics, San Luis
Obispo, CA). Surface temperature was measured with
an infrared thermometer (Sixth Sense LT300 Infrared
Thermometer, TTI Instruments, Williston, VT) held
at an angle of 90° at a position 5 cm from the skin
surface. Sweating rate was measured using the Vapom-
eter device (Delfin Tech. Ltd., Kuopio, Finland), and
respiration rate was determined by visual observations
of flank movements for 1 min.
On each experimental day, black globe temperature
(inside and outside of the barn), dry bulb temperature,
RH, and dew point temperature were recorded every 1
min at a height of 2 m. Dry bulb temperature, RH, and
dew point temperature were measured using a Hobo-U2
data logger (Onset Co.), and black globe temperature
was measured using a Hobo Pro V2 data logger (Onset
Co.).
Experiment 3: Seasonal Variation in Milk Yield
Daily milk yields for the first 90 DIM were obtained
from a total of 11 slick and 274 wild-type cows calving
between 2009 and 2011 at the University of Florida
Dairy Unit (Hague). Data from 1, 2, or 3 lactations
were available for each cow. Cows either calved in May,
June, or July (warm season; n = 9 lactations from slick
cows and 256 lactations from wild-type cows) or Octo-
ber, November, or December (cool season; n = 9 lacta-
tions from slick cows and 174 lactations from wild-type
cows). Slick cows were produced from 5 wild-type sires
and 3 slick sires. Only one slick cow was genotyped and
she was a heterozygote. Calving months were chosen so
that cows calving in the warm season would experience
the first 90 DIM while exposed to hot weather, whereas
cows calving in the cool season would experience the
first 90 DIM while exposed to cool weather. Daily milk
yield was measured at each milking by the AfiLab real
time milk analyzer (S.A.E. Afikim, Kibbutz Afikim,
Israel).
Statistical Analysis
Data from experiment 1 were analyzed by least-
squares ANOVA with the PROC GLIMMIX procedure
of SAS (v. 9.3; SAS Institute Inc., Cary, NC). The sta-
tistical model included genetic type (slick, relative, and
wild-type), replicate (1, 2, and 3), time of day, stage
of lactation (<100 DIM, 100–200 DIM, >200 DIM),
parity (primiparous vs. multiparous), genetic type ×
time, genetic type × replicate, and cow (genetic type ×
replicate). Orthogonal contrasts were used to compare
slick cows to relatives and wild-type cows and to com-
pare relatives to wild-type cows. Cow was considered
random and other main effects fixed. The relationship
between vaginal temperatures recorded by the 2 devices
was determined by linear regression analysis using the
PROC GLM procedure of SAS.
Data from experiment 2 were analyzed by least-
squares ANOVA with the PROC GLIMMIX procedure
of SAS (SAS Institute Inc.). The statistical model for
sweating rate and surface temperature included genetic
type (slick, relative and wild-type), replicate (1–5),
region of body, clipping (shaved vs. unshaved), time of
day, cow within genetic type by replicate, and all in-
teractions. The statistical model for rectal temperature
and respiration rate was similar except without the ef-
fect of region of body and clipping. Cow was considered
random and other main effects fixed.
For experiment 3, data were analyzed by least-squares
ANOVA using the GLIMMIX procedure of SAS (SAS
Institute Inc.). The model included main effects of ge-
netic type (slick vs. wild-type), season of calving, year
of birth, cow-year nested within genetic type by season
× year, DIM, and all interactions. Tests of significance
were calculated using expected mean squares; cow-year
was considered random and other main effects were
considered fixed.
RESULTS
Experiment 1: Diurnal Variation
in Vaginal Temperature
Environmental conditions are shown in Figure 3.
Dry bulb temperature and black globe temperature in-
creased from a nadir at about 0700 h to a peak between
1100 and 1700 h before declining again. Relative hu-
midity declined during the afternoon but THI followed
the pattern of dry bulb and black globe temperatures.
Vaginal temperatures of cows were measured with 2
different data loggers. Data are only shown for measure-
ments using the Hobo device because the correlation
between measurements using the 2 different devices
was 0.97 and statistical effects were similar. The linear
regression equation for the relationship between vaginal
temperatures using the 2 devices was as follows:
VT
iBut
= 0.840 + 0.983VT
Hobo
,
where VT
iBut
= vaginal temperature recorded by iBut-
ton, and VT
Hobo
= vaginal temperature recorded by the
Hobo device.
Effect of genetic type on diurnal variation in vaginal
temperature is shown in Figure 4. Lowest temperatures
occurred between 0800 and 0900 h. Temperatures then
increased until about 1300 to 1400 h, increased again

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between 0000 and 0500 h (except in slick cows), and
then declined commencing at 0500 to 0600 h. Vaginal
temperature varied between genetic types (P < 0.0001),
with slick cows having lower vaginal temperatures than
relatives and wild-type cows. We observed no significant
difference in vaginal temperature between relatives and
wild-type cows. The least squares means (±SEM) for
vaginal temperature were 38.5 ± 0.04°C for slick cows,
39.1 ± 0.05°C for wild-type cows, and 39.1 ± 0.05°C
for relatives. Genetic type × time interactions were not
significant, indicating that differences between genetic
types occurred throughout the day.
We detected effects of stage of lactation (P < 0.0001)
and an interaction between stage and genetic type (P <
0.0001) that reflect reduced effects of stage of lactation
for slick cows. For wild-type Holsteins, vaginal tem-
perature was greater for cows <100 DIM (least squares
means ± SEM = 39.8 ± 0.12°C) than for cows between
100 and 200 DIM (38.8 ± 0.06°C) or >200 DIM (38.7 ±
0.04°C). For relatives, rectal temperatures varied from
39.4 ± 0.13°C for <100 DIM, 39.3 ± 0.06°C for 100 to
200 DIM, and 38.7 ± 0.05°C for >200 DIM, respec-
tively). For slick cows, vaginal temperatures were 38.7
± 0.05, 38.4 ± 0.07, and 38.4 ± 0.07°C for <100, 100 to
200, and >200 DIM, respectively.
Experiment 2: Thermoregulation
During Acute Heat Stress
Environmental conditions during the experiment are
summarized in Figure 5. The magnitude of heat stress
Figure 3. Thermal environment during the experiment to measure
diurnal variation in vaginal temperature during heat stress in slick and
wild-type Holstein cattle (experiment 1). Data represent least squares
means ± SEM.
Figure 4. Diurnal variation in vaginal temperature during heat
stress as affected by genetic type (experiment 1). Temperature was
measured using a Hobo device (Onset Co., Bourne, MA). Data rep-
resent least squares means ± SEM of results for wild-type (closed
circles), relatives (open circles) and slick cows (closed triangles).

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Journal of Dairy Science Vol. 97 No. 9, 2014
increased over time, as indicated by changes in dry bulb
and black globe temperatures and THI. Note that THI
increased over time even though RH declined (Figure
5).
Rectal temperature also increased over time (P <
0.0001), peaking at 1300 h (Figure 6A). The increase
depended upon genetic type. We observed an effect of
genetic type (P < 0.0001) and genetic type × time (P
< 0.0001). Orthogonal contrasts indicated that rectal
temperature was lower (P < 0.0001) in slick Holsteins
(39.6 ± 0.05°C) than in wild-type cows (40.5 ± 0.05°C)
or relatives (40.3 ± 0.05°C). Moreover, rectal tempera-
ture was lower in relatives than in wild-type cows (P <
0.02). The interaction reflected the fact that differences
between genetic types were least at 0900 h compared
with later times in the day.
As shown in Figure 6B, respiration rate was also
affected by time (P < 0.0001), genetic type (P <
0.0001), and their interaction (P < 0.0001). Respiration
increased from 0900 to 1100 h and remained high there-
after. Orthogonal contrasts indicated that respiration
rate was lower (P < 0.0001) in slick Holsteins (93 ± 0.7
breaths per minute) than in wild-type cows (107 ± 0.8
breaths per minute) or relatives (101 ± 0.7 breaths per
minute) and lower in relatives than in wild-type cows
(P < 0.0001). The interaction was because differences
between genetic types were lowest in the morning.
Skin temperature and sweating rate were measured
at 4 body locations and on adjacent areas that were
either shaved or not shaved. Skin temperature was af-
fected by genetic type (P < 0.0005), time (P < 0.0001),
and genetic type × time (P < 0.05). Skin temperature
increased between 0900 and 1100 h and then decreased
by 1500 h (Figure 6C). Using orthogonal contrasts, skin
temperature was found to be lower for slick cows than
for wild-type cows or relatives (P < 0.0001); wild-type
cows and relatives did not differ (Figure 6C). Skin tem-
perature was also affected by location (P < 0.0001) and
region × time (P < 0.001): temperature was higher at
the 2 regions measured along the dorsal surface of the
animal (loin and rump) than at the 2 regions measured
lower on the animal (neck and hind leg). Skin tem-
peratures were 36.4 ± 0.12°C, 39.1 ± 0.12°C, 39.4 ±
0.12°C, and 36.0 ± 0.12°C for the neck, loin, rump, and
hind leg, respectively. Overall, we detected no effect of
shaved versus nonshaved on skin temperature but we
did observe a location × shave interaction (P < 0.0001).
The interaction occurred because skin temperature was
higher for the shaved area than the nonshaved area for
the hind leg (36.5 ± 0.16°C vs. 35.4 ± 0.16°C) but not
for other areas.
Sweating rate was affected by genetic type (P = 0.08),
time (P < 0.0001), and genetic type × time interac-
tion (P < 0.0003; Figure 6D). Sweating rate increased
from 0900 to 1300 h and then remained constant (wild-
type) or decreased slightly (slick and relatives). Using
orthogonal contrasts, sweating rate was found to be
higher for slick cows than for wild-type cows or rela-
tives (P < 0.003) and we found no difference between
wild-type cows and relatives. Sweating rate was also
affected by location (P < 0.001), region × time (P
< 0.04), shaved versus nonshaved (P < 0.0001), and
the interaction between location × shaving (P < 0.04;
Figure 7). Sweating rate was greatest for the neck and
loin, intermediate for the rump, and least for the hind
leg (Figure 7). Sweating rate was greater for the shaved
than the nonshaved area for all locations but the in-
teraction occurred because shaving increased sweating
rate to the greatest extent for the loin and rump and
to the least extent for the neck and hind leg (Figure 7).
Figure 5. Thermal environment during the experiment to measure
acute responses to heat stress (experiment 2). The top graph repre-
sents dry bulb temperature (solid line) and black globe temperature
(dotted line), and the bottom graph represents relative humidity (solid
line) and temperature-humidity index (dotted line).

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There were, however, no interactions of genetic type
with either location or shaved versus nonshaved with
genetic type.
Experiment 3: Seasonal Variation in Milk
Production Traits During the First 90 DIM
Milk yield was affected by a genetic type × season of
calving × DIM interaction (P < 0.0001). The interac-
tion resulted because calving in summer resulted in a
smaller decrease in milk yield for slick cows than for
wild-type cows, and these differences were more pro-
nounced after the first week of lactation than in the
first week (Figure 8). Across all DIM, the reduction
in daily milk yield in summer compared with winter
was 1.3 kg for slick cows and 3.7 kg for wild-type cows
(Table 1).
We detected 3-way interactions for percentage fat,
protein, and lactose (P < 0.0001). The interaction for
percentage fat represented the fact that the summer de-
pression in milk fat was apparent later in lactation for
slick cows than for wild-type cows but was of a greater
magnitude for slick cows during this time (Figure 9).
Across all DIM, the reduction in fat percentage in sum-
Figure 6. Acute response to heat stress in cows managed under shade cloth (experiment 2). Data represent least squares means ± SEM of
results for wild-type (closed circles), relatives (open circles), and slick cows (closed triangles).

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Journal of Dairy Science Vol. 97 No. 9, 2014
mer compared with winter was 0.19% for slick cows
and 0.13% for wild-type cows (Table 1). The reduction
in percentage protein and percentage lactose during
the summer was also greater for slick cows than for
wild-type cows, with differences between genetic types
occurring after ~5 to 10 DIM (Figures 10 and 11). For
percentage lactose, the interaction also reflected that
differences between summer and winter were reduced
after about 35 to 40 DIM in slick cows and about 50
DIM in wild-type cows. Across all DIM, the reduction
in protein percentage in summer compared with winter
was 0.22% for slick cows and 0.12% for wild-type cows
(Table 1). The reduction in lactose percentage in sum-
mer compared with winter was 0.17% for slick cows and
0.12% for wild-type cows (Table 1).
Somatic cell score was also affected by a 3-way inter-
action (P < 0.0001): SCS was lower in summer than
in winter for slick cows on some days but not others,
whereas season had no effect in wild-type cows (Figure
12). The reduction in SCS in summer compared with
winter was 0.13% for slick cows and 0.05% for wild-type
cows (Table 1).
DISCUSSION
Results from the present study confirm earlier find-
ings that the slick-hair phenotype confers superior
thermoregulation in dairy cows exposed to heat stress
(Olson et al., 2003; Dikmen et al., 2008) and extends
the range of environments in which this advantage is
expressed to the freestall housing system. In addition
to improving capacity for body temperature regulation,
cows with slick hair were also resistant to heat stress
with respect to milk yield. Whereas wild-type cows
Figure 7. Interactions between body location and shaving on sweating rate for cows placed under shade cloth during heat stress (experiment
2). The approximate location of each region in which sweating was examined is indicated by the small box. Data represent least squares means
± SEM of results for shaved and unshaved areas in each body region.

Journal of Dairy Science Vol. 97 No. 9, 2014
SLICK HAIR LOCUS IMPROVES THERMOTOLERANCE
5517
calving in the summer had lower milk yields than those
calving in the winter, this difference was greatly reduced
in slick-haired cows. Thus, it is likely that increasing
the frequency of the SLICK haplotype in Holstein cows
can reduce the negative consequences of heat stress on
milk yield, even in situations where housing has been
modified to reduce the effect of heat stress.
The physiological basis for thermotolerance in slick
cows involves multiple modes of heat loss to the envi-
ronment. The short hair length caused by the SLICK
haplotype reduces the insulation to conductive and
convective heat loss in the hair coat (Berman, 2004)
and thereby increases sensible heat loss. Evaporative
heat loss is also heightened in slick cows because of the
Figure 8. Milk yield during the first 90 DIM as affected by genetic
type and season of calving (experiment 3). Winter is represented by
solid circles and summer by open circles. Data represent least squares
means. The pooled SEM was 0.31 kg/d for wild-type cows and 1.50
kg/d for slick cows.
Table 1. Interactions between genetic type (i.e., slick vs. wild-type) and season of calving (i.e., winter or summer) on milk yield and composition
during the first 90 DIM (experiment 3)
Item Milk yield (kg/d) Fat (%) Protein (%) Lactose (%) SCS
Wild-type–winter 37.4 ± 0.6 3.65 ± 0.02 3.03 ± 0.01 4.71 ± 0.01 3.46 ± 0.02
Wild-type–summer 33.7 ± 0.6 3.52 ± 0.02 2.91 ± 0.01 4.59 ± 0.01 3.41 ± 0.02
Slick–winter 39.0 ± 2.7 3.62 ± 0.10 3.11 ± 0.05 4.68 ± 0.05 3.48 ± 0.09
Slick–summer 37.7 ± 2.7 3.43 ± 0.10 2.89 ± 0.05 4.51 ± 0.05 3.35 ± 0.09
P-value
Genetic type >0.10 >0.10 >0.10 >0.10 >0.10
Season >0.10 0.02 0.0001 0.0001 >0.10
Genetic type × season >0.10 >0.10 >0.10 >0.10 >0.10
Genetic type × season × DIM 0.0001 0.0001 0.0001 0.0001 0.0045
Figure 9. Percentage fat in milk during the first 90 DIM as af-
fected by genetic type and season of calving (experiment 3). Winter is
represented by solid circles and summer by open circles. Data repre-
sent least squares means. The pooled SEM was 0.027% for wild-type
cows and 0.130% for slick cows.

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DIKMEN ET AL.
Journal of Dairy Science Vol. 97 No. 9, 2014
increased sweating rate during heat stress. In an earlier
study, slick cows were found to lose more water by sweat-
ing than wild-type cows (Dikmen et al., 2008). In that
study, in which sweating was measured at the shoulder,
differences between slick and wild-type cows could be
eliminated by shaving the hair and it was concluded
that the elevated sweating rate in slick cows reflected
reductions in trapping of humid air in the hair coat. In
experiment 2 of the current study, however, where sweat-
ing was measured at the neck, loin, rump, and hind leg,
differences in sweating rate between slick and wild-type
cows were apparent for both shaved and unshaved areas
of the skin. The interpretation is that either slick cows
have a greater density of sweat glands than wild-type
cows or individual glands of slick cows have increased
capacity for sweat production. Evaporative heat loss is
the major mode available to the cow for dissipation of
metabolic heat (Maia et al., 2005) so the increased ca-
pacity of slick cows for sweating could be an important
modification responsible for increased thermotolerance.
The fact that respiration rate increased to a lesser ex-
tent for slick cows than for wild-type cows is probably
largely a reflection of the increased sweating rate of
slick cows. Cows lose evaporative heat by both sweat-
ing and panting. Cutaneous water loss is greater than
via the respiratory tract, with the difference widening
with increasing environmental temperature (Kibler and
Brody, 1952). The increased heat loss due to sweat-
ing in slick cows would have obviated the need for an
increased respiration rate. One expected consequence
would be increased utilization of water and minerals
by slick cows to meet the demands of sweating, as well
as reduced likelihood of respiratory alkalosis caused by
alveolar ventilation in the lungs (Sanchez et al., 1994).
Another discrepancy between this study and that of
Dikmen et al. (2008) is that skin temperature was lower
in slick cows than in wild-type cows in experiment 2 of
the present study, whereas no such difference between
slick and wild-type cows was found in the earlier study.
Figure 10. Percentage protein in milk during the first 90 DIM as
affected by genetic type and season (experiment 3). Winter is repre-
sented by solid circles and summer by open circles. Data represent
least squares means. The pooled SEM was 0.013% for wild-type cows
and 0.062% for slick cows.
Figure 11. Percentage lactose in milk during the first 90 DIM as
affected by genetic type and season of calving (experiment 3). Winter
is represented by solid circles and summer by open circles. Data rep-
resent least squares means. The pooled SEM was 0.020% for wild-type
cows and 0.098% for slick cows.

Journal of Dairy Science Vol. 97 No. 9, 2014
SLICK HAIR LOCUS IMPROVES THERMOTOLERANCE
5519
Present results are what would be expected given the
increased rates of heat loss at the skin for slick cows.
The observation that there was little seasonal varia-
tion in milk yield in slick cows is consistent with the
lower body temperatures experienced by slick cows in
experiments 1 and 2. The difference in vaginal tempera-
ture between slick and wild-type cows in experiment 1
was 0.6°C, a value greater than the difference in rectal
temperature between cooled and control cows in some
studies in which cooling during the summer improved
milk yield (Igono et al., 1985; Chen et al., 1993). Dif-
ferences in milk composition between cows calving in
winter versus summer were also modified in slick cows,
with seasonal differences in percentage fat, protein, and
lactose as well as SCS being exacerbated in slick cows.
Analysis of seasonal data in Florida indicates that heat
stress is associated with decreases in percentage fat and
protein (Rodriquez et al., 1985). However, seasonal vari-
ation in milk composition can be difficult to interpret
with respect to heat stress because of possible variation
in diet and other management factors. Few experimen-
tal studies have determined effects of heat stress on
milk composition. Heat stress decreased percentage fat
and protein but not lactose in one experiment involv-
ing only 4 cows (Bandaranayaka and Holmes, 1976).
Cooling cows in summer in Argentina did not affect
percentage protein or lactose (Gallardo et al., 2005). In
that same study, cooling increased fat percentage for
cows fed a diet with a 80:20 forage:concentrate ratio
but reduced fat percentage for cows fed a diet with a
70:30 forage:concentrate ratio. More work is required to
understand how the slick phenotype affects changes in
milk composition. The observation that seasonal effects
on milk fat, protein, and lactose were altered in slick
cows suggests the possibility that the SLICK haplotype
is associated in a complex way with milk composition.
Moreover, some of the economic advantage associated
with the SLICK haplotype in the summer associated
with reduced effect of heat stress on milk yield would
be offset if seasonal effects on composition are exagger-
ated in cows of this genotype.
Interestingly, cows that were relatives of slick cows
without inheriting the SLICK haplotype showed im-
proved capacity for regulation of body temperature
compared with wild-type Holsteins in experiment 2.
These cows were intermediate between wild-type and
slick cows with respect to diurnal variation in rectal
temperature. Regulation of body temperature is heri-
table in Holsteins (Dikmen et al., 2012, 2013). The
improved thermoregulation of relatives could represent
random variation in genetic merit for body temperature
regulation among the parents of the relative cows or
heterosis from the small percentage of Senepol remain-
ing in these animals (some evidence exists for heterosis
in body temperature regulation; Dikmen et al., 2009);
alternatively, the SLICK haplotype is not the only gene
conferring thermotolerance that was introduced from
the founder Senepol animals. Indeed, the small num-
bers of animals categorized as slick and relatives means
that genetic characteristics of founder animals could
have affected physiological and production responses
to heat stress.
In conclusion, Holstein cows inheriting the SLICK
haplotype from Senepol cattle have superior ability to
regulate body temperature, at least in part because of
increased capacity for sweating, and they experience
less-pronounced reductions in milk yield during the
summer in a hot environment.
ACKNOWLEDGMENTS
This research was supported by Agriculture and
Food Research Initiative Competitive Grants no. 2010-
Figure 12. Somatic cell score in milk during the first 90 DIM as
affected by genetic type and season of calving (experiment 3). Winter
is represented by solid circles and summer by open circles. Data rep-
resent least squares means. The pooled SEM was 0.057 for wild-type
cows and 0.278 for slick cows.

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DIKMEN ET AL.
Journal of Dairy Science Vol. 97 No. 9, 2014
85122-20623 and 2013-68004-20365 from the USDA
National Institute of Food and Agriculture (Washing-
ton, DC) and a grant from Southeast Milk Inc. Milk
Check-off Program. The authors acknowledge the ef-
forts of Timothy A. Olson in establishing the SLICK
gene in Holsteins, data collection by Anna C. Denicol,
Sofia Ortega, and Paula Tribulo (all from Department
of Animal Sciences, University of Florida, Gainesville),
and the assistance of Eric Diepersloot and staff of the
University of Florida Dairy Unit.
REFERENCES
Bandaranayaka, D. D., and C. W. Holmes. 1976. Changes in the com-
position of milk and rumen contents in cows exposed to a high
ambient temperature with controlled feeding. Trop. Anim. Health
Prod. 8:38–46.
Berman, A. 2004. Tissue and external insulation estimates and their
effects on prediction of energy requirements and of heat stress. J.
Dairy Sci. 87:1400–1412.
Chen, K. H., J. T. Huber, C. B. Theurer, D. V. Armstrong, R. C. Wan-
derley, J. M. Simas, S. C. Chan, and J. L. Sullivan. 1993. Effect of
protein quality and evaporative cooling on lactational performance
of Holstein cows in hot weather. J. Dairy Sci. 76:819–825.
Dikmen, S., E. Alava, E. Pontes, J. M. Fear, B. Y. Dikmen, T. A.
Olson, and P. J. Hansen. 2008. Differences in thermoregulatory
ability between slick-haired and wild-type lactating Holstein cows
in response to acute heat stress. J. Dairy Sci. 91:3395–3402.
Dikmen, S., J. B. Cole, D. J. Null, and P. J. Hansen. 2012. Heritability
of rectal temperature and genetic correlations with production and
reproduction traits in dairy cattle. J. Dairy Sci. 95:3401–3405.
Dikmen, S., J. B. Cole, D. J. Null, and P. J. Hansen. 2013. Genome
wide association mapping for identification of quantitative trait
loci for rectal temperature during heat stress in Holstein cattle.
PLoS ONE 8:e69202.
Dikmen, S., L. Martins, E. Pontes, and P. J. Hansen. 2009. Genotype
effects on body temperature in dairy cows under grazing condi-
tions in a hot climate including evidence for heterosis. Int. J.
Biometeorol. 53:327–331.
Flori, L., M.I. Gonzatti, S. Thevenon, I. Chantal, J. Pinto, D. Berthi-
er, P. M. Aso, and M. A. Gautier. 2012. A quasi-exclusive Euro-
pean ancestry in the Senepol tropical cattle breed highlights the
importance of the slick locus in tropical adaptation. PLoS ONE
7:e36133.
Gallardo, M. R., S. E. Valtorta, P. E. Leva, M. C. Gaggiotti, G. A.
Conti, and R. F. Gregoret. 2005. Diet and cooling interactions on
physiological responses of grazing dairy cows, milk production and
composition. Int. J. Biometeorol. 50:90–95.
Hayes, B. J., P. J. Bowman, A. J. Chamberlain, K. Savin, C. P. van
Tassell, T. S. Sonstegard, and M. E. Goddard. 2009. A validated
genome wide association study to breed cattle adapted to an envi-
ronment altered by climate change. PLoS ONE 4:e6676.
Huson, H. J., E. S. Kim, R. W. Godfrey, T. A. Olson, M. C. McClure,
C. C. Chase, R. Rizzi, A. M. O’Brien, C. P. Van Tassell, J. F. Gar-
cia, and T. S. Sonstegard. 2014. Genome-wide association study
and ancestral origins of the slick-hair coat in tropically adapted
cattle. Front. Genet. 5:101.
Igono, M. O., B. J. Steevens, M. D. Shanklin, and H. D. Johnson.
1985. Spray cooling effects on milk production, milk, and rectal
temperatures of cows during a moderate temperate summer sea-
son. J. Dairy Sci. 68:979–985.
Kibler, H. H., and S. Brody. 1952. Environmental physiology, with
special reference to domestic animals. XIX. Relative efficiency of
surface evaporative, respiratory evaporative, and non-evaporative
cooling in relation to heat production in Jersey, Holstein, Brown
Swiss and Brahman cattle, 5° to 105° F. Mo. Agric. Exp. Sta. Res.
Bull. 497. Univ. Missouri Agric. Exp. Sta., Columbia.
Maia, A. S. C., R. G. daSilva, and C. M. Battiston Loureiro. 2005.
Sensible and latent heat loss from the body surface of Holstein
cows in a tropical environment. Int. J. Biometeorol. 50:17–22.
Mariasegaram, M., C. C. Chase Jr., J. X. Chaparro, T. A. Olson, R.
A. Brenneman, and R. P. Niedz. 2007. The slick hair coat locus
maps to chromosome 20 in Senepol-derived cattle. Anim. Genet.
38:54–59.
NRC. 1971. A Guide to Environmental Research on Animals. Natl.
Acad. Sci., Washington, DC.
Olson, T. A., C. Lucena, C. C. Chase Jr., and A. C. Hammond. 2003.
Evidence of a major gene influencing hair length and heat toler-
ance in Bos taurus cattle. J. Anim. Sci. 81:80–90.
Rodriquez, L. A., G. Mekonnen, C. J. Wilcox, F. G. Martin, and
W. A. Krienke. 1985. Effects of relative humidity, maximum and
minimum temperature, pregnancy, and stage of lactation on milk
composition and yield. J. Dairy Sci. 68:973–978.
Sanchez, W. K., D. K. Beede, and J. A. Cornell. 1994. Interactions
of sodium, potassium, and chloride on lactation, acid-base status,
and mineral concentrations. J. Dairy Sci. 77:1661–1675.
Scheet, P., and M. Stephens. 2006. A fast and flexible statistical model
for large-scale population genotype data: Applications to inferring
missing genotypes and haplotypic phase. Am. J. Hum. Genet.
78:629–644.
Vickers, L. A., O. Burfeind, M. A. G. von Keyserlingk, D. M. Veira, D.
M. Weary, and W. Heuwieser. 2010. Technical note: Comparison of
rectal and vaginal temperatures in lactating dairy cows. J. Dairy
Sci. 93:5246–5251.