Cattle use of microclimates on a northern latitude
winter range
G. A. Houseall and B. E. Olson2
24
Department of Animatand Range Sciences, Montana Statg tJniyersity, Bo,zeman, MT 59717, USA. Received
March 1995, accepted 31 August 1995'
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-
Can. J Anim. Sci.75:
Houseal, G. A. and Olson, B. E. 1995. Cattle use of microclimates on a northern latitude winter range.
can negatively affect
and
wind
cold
extreme
However,
501-507. Grazingnative range can help lower costs of wintering livestock.
cattle (Bos tau'.
Free-ranging
low.
is
forage
the
of
value
nutritive
the
when
demand
energy
balance,-increasin!
thermal
an animal's
high
winds and
as
such
stressors
environmental
avoid
or
rus) may exploit diit'erences in topograihy aii microclimate to minimize
period for two consecutive
6-wk
a
over
studied
was
stress
cold
to
in
response
of
microclimates
Cattle
selection
cold temperatures.
moderate microcliwinters. The objective of this study was to determine if cows respond to exfieme wind and cold by selecting
cold temperatures'
and
winds
higfi
to
avoid
and
resting
grazing
for
microclimates
moderate
selected
foraging.
Cattle
mates for
reference climate
They also tended io remain in microclimates above their lower criiical temperature (LCT) of -23"C, even though
to contrnue grazcows
allow
pasture
may
a
in
microclimates
moderate
of
The
availability
LCT.
below
their
were
conditions often
gazrng.
from
to
defer
them
cause
ing, thus maintaining intake, even when general condiiions might otherwise
Key words: Behavior, winter ecology , Bos taurus, microclimates, thermoneutral
des zones ir hiver
Houseal, G. A. et Olson, B. E. 1995. Utilisation des microclimats par les bovins dans les parcours d'hiver
peut contribuer ir
parcours
naturels
des
I'exploitation
Si
75:
501-507.
sci.
Anim.
can.
J.
des
Etats-unis.
nord-ouest
froid du
sur le bilan
n6gatifs
effets
des
peuvent
avoir
le
vent
et
abaisser les co1ts d'hivemage du b6tail, en revanche les froids intenses
Des bovins (Bos
nutritive.
de
faible-valeur
sont
fourrages
les
lorsque
6nerg6tiques
besoins
ses
accriissant
I'animal,
de
calorifique
ou
minimiser
taurus)inparcours libies sont capables d'exploiter Ies diff6t"tt""i de topographie et de micro-climat de fagon i
des bovins h
comportement
Le
tempdratures.
les
bisses
ei
forts
les
vents
cofilme
de
strJss
agents
I'action
des
d s'y soustraire
pendant deux hivers
l'6gard des micro-climats en r6po'nse d des temp6ratures ffes basses a fait l'objet d'une 6tude de six-semaines
en recherchant les microextrcmes
et
thermiques
an6mom6triques
conditions
aux
r6agissent
vaches
si
les
voulait
savoir
de suite. on
aires de pdture et
climats plus temp6r6s pour y pdturer. Les bovins choisissaient effectivemeniles microclimats temp6r6s corlme
thermique critique
leur
seuil
de
d
celle
sup6rieure
6tait
la
temp6rature
dont
microclimats
des
en
outre
Ils
recherchaient
repos.
de
(STe) de -23"C, m€me si les conditions climatiques de r6f6rence 6taient souvent inf6rieures dL leur STQ. L'existence de microcliune prise alimentaire
mats plus temp6r6s dans les parcours peut perm;ttre aux vaches de continuer d pdturer et ainsi de maintenir
de pdturer'
emp6cher
les
devoir
sembleraient
satisfaisante, m6me lorsque Gs conditions macro-climatiques de la r6gion
Mots cl6s: Comportement, ecologie d'hiver, Bos taurus, microclimat
prevailing winds by the surrounding uplands. South-facing
Grazing native range can help lower the costs of wintering
livestock. In northern latitudes, forage quality is low in winter,
and severe weather may further stress livestock. Extreme
cold and wind can negatively affect an animal's thermal bal-
ispects receive more solar radiation than north-facing
utp""tt. There are also temperature differences associated
with changes in elevation. Free-ranging cattle may exploit
these differences in topography and microclimate to minimize or avoid environmental stressors such as high winds
ance, increasing energy demand (Webster l97l). Partly
of this increased demand for energy, cattle lose
weight or have lower weight gains when exposed to cold
(Webster 1970; Hidiroglou and Lessard 1971). Cold and
wind have been correlated to a decrease in grazing time and
forage intake, which may result in a loss of condition and
reproductive potential of range cows (Malechek and Smith
1976: Adams 1989). These effects can be minimized if cattle
are provided with adequate shelter, either natural or man
made (Webster 1970).
In the relatively open grasslands of the west, topography
because
and cold temperatures.
Foraging takes priority over thermoregulatory behaviour
in
free-ranging animals (Ingram and Dauncy 1985)'
However, in extreme weather, thermoregulatory behavior
may override foraging' Cattle will defer from grazing when
cold stressed until it becomes warmer (Malechek and Smith
over a landscape. LowJying areas are sheltered from the
1916), and select resting sites to avoid extreme wind in winter
(Senft and Rittenhouse 1985a)'
Cattle use of microclimates in response to cold stress was
studied over a 6-wk period for two consecutive winters. The
objective of this study was to determine if cows respond to
rPresent Address (G.A.H.): 562 Green Gable Court,
Camilln. GA 31730 USA.
2To whom correspondence and reprint requests should be
Abbreviations: DGT, daily grazing time; LCT, lower
critical temperature; MHL, metabolic heat loss; MIIP,
metabolic hiat production; MR, metabolic rate; OMI,
is a primary factor influencing microclimatic
differences
organic matter intake; STTS, short-term thermal stress;
addressed.
501
502
CANADIAN JOURNAL OF ANIMAL SCIENCE
extreme wind and cold by selecting moderate microclimates
for foraging.
THEORY AND METHODS
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Thermoneutral Zone and Lower Gritical
Temperature
Animals exchange heat with their environment through a
dynamic process of balancing heat gain with heat loss
(Moen 1973). Maintaining this balance results in a relatively
constant body temperature in endotherms. Metabolic heat
production (MHP) or heat loss (MHL) are used to maintain
constant body temperatures. However, there is a range of
environmental temperatures whereby MHp and MHL are
unaffected by ambient temperatures (young 1985). This
range of temperatures, or thermoneuttal zone, varies among
species and even among individuals, depending on size, sex,
age, level
of nutrition and previous acclimatization
(Webster 1910,l97l1, Christopherson er al. 1979; Senft and
Rittenhouse 1985b; Young 1985). The LCT of this range is
the temperature below which animals become cold stressed
whereby they must increase MHP to maintain homeostasis
(Moen l9'73).It the ambient temperature remains below the
individual's LCT for an extended period of time, and MHp
can no longer compensate for heat loss, hypothermia may
compromise the health, reproductive potential, and even the
life of the individual.
Standard Operative Temperature
Environmental factors other than ambient remDerature
influence the thermal environment of an animal. Extreme
fluctuations in temperature, solar radiation, wind, and precipitation can either help or hinder thermoregulaiion
(Webster l97O,I97l; Moen 1973; Ames and Insley 1975;
Senft and Rittenhouse l9S5b).
Parker and Gillingham (1990) developed a model to
estimate critical thermal environments for mule deer
(Odocoileus hemionus hemionus). They initially used an
equation presented by Campbell
tive temperature (2"):
(I97j) to estimate
opera-
T"=To+frr\Robs_es6Ta4)llpcp
(1)
Operative temperature includes ambient temperafure (2.)
and the effects of wind and radiation on the animal. Theie
effects incorporate solar and thermal radiation absorbed
(Rorr), thermal radiation emitted 1e,oTo4), the animal's
resistance (rr) to convective and radiative heat transfer, and
the density (p) and specific heat (c.) of the air. However,
Parker and Gillingham (1987) deterfirined that T" underestimated the influence of wind on coat resistance oi mule deer
at high wind speeds. Thus, they used standard operative
temperature ({rr) (Bakken 1981) to estimate the animal's
thermal environment:
T"r=Tu-frrrr+r"r)/(rru+r"))x(To-7")
(2)
where Zr" is the standard operative or wind-chill corrected
temperature experienced by the animal (parker and Robbins
Table 1. Mean ambient temperature ("C) by month for each winter
Winter
December
January
February
I
-r.6
- t.J
-1.9
2.2
-7.0
-2.6
2
-7.8
-3.4
Normz
-6.0
230-yr average (1951-1980), Montana
Stare Climate Center, Station 1044.
Bozeman, MT
1984) and
I,
is the body temperature of the animal. The ani-
mal's resistance to convective and radiative heat ffansfer is
expressed as a ratio of coat and tissue resistances (rrru") and
*;-l)
boundary layer resistance (rrr) without wind (p < I
to
these resistances (rru+rr) under natural outdoor wind speeds.
Ar I^ decreases, either in a response to increasing wind
speed or decreasing 7,, whole body thermal resistance
increases (Parker and Robbins 1984). This is mainly caused
by vasoconstriction of subcutaneous blood vessels in the
skin, reducing blood flow and thus convective heat loss to
external tissues and to the environment (Webster 1974).
The standard operative temperature at which whole body
thermal resistance attains a maximum value corresponds to
an animal's LCT (Parker and Robbins 1984).
The standard operative temperature that is equivalent to
an animal's LCT can be used as a reference point to determine which environmental conditions (combinations of Z.
and p) may thermally stress the animal, or cause it to altei
its behavior. Wind can be measured directly using an
anemometer, and T" may be estimated based on direct measures with a black globe thermometer (Bakken 1992).
Theory of the Blackglobe Thermometer
Radiant energy is an important source of heat for animals
(Walsberg 1992). Short-wave radiation from the sun and
long-wave radiation emitted from sky, earth and terrestrial
objects influence the radiant heat load at a site but are difficult to measure. A black globe thermometer is influenced by
these sources of radiation, and is in equilibrium with its
environment when heat gained (or lost) equals heat lost (or
gained). This equilibrium temperature is an estimate of
I
(Kuehn et al. 1970). The difference between T, and Tu
equals the net thermal gradient between an animal and its
environment (Walsberg 1992). Bakken (1992) stated that
hollow copper models of animals with centrally located
thermistors provide an approximate value for 7,, but should
be verified with freshly killed specimens or life-size taxidermic mounts, which were not possible in this study.
For this study, blackglobe thermometers were constructed
from 10.2-cm copper ball cocks, painted mat black. A thermo
meter was inserted through a black rubber stopper, which was
then inserted into the copper sphere so that the thermally sensitive end of the thermometer was in the center of the sphere
(Renecker and Hudson 1986; Yousef 1989). Given the small
size of the blackglobe it may gain heat more rapidly than a
cow. However, the smaller boundary layer of the globe
would also allow it to cool more rapidly by convection at
least partially off-setting heat gain Campbell pers. commun.). These thermometers were used to estimate T" of the
microclimate at cow location and at fixed monitoring points.
HOUSEAL AND OLSON
-
CATTLE USE OF MICROCLIMATES 503
xo
E
E
t
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I
*
lll
WntsrZ
6
c
E
Fig. 1. Cow use of different exposure classes rela-
E3
to blackglobe temperature (In) and windspeed
(U). and blackglobe temperature (7") and windchill
at a fixed reference point in the pasiure in Winter I
and Winter 2. Horizontal planes represent cows
exDosure to their environment based on their location in the landscape. Exposure index; | = protected
(draw), 2 = moderately protected (lower slopes),
and 3 = exposed (upper slopes and ridges).
tivi
It
o
*
ut
IttO"p""cf""1
Model of Lower Gritical Temperature
As a reference point for cold sffess, a model of LCT for cattle
relative ro T" and p was developed by adapting Parker and
Gillingham'i (1990) model of the thermal environment for
mule deer. We simplified their model to estimate the combinations of T, and p that result in a LCT-J"'=-23'C.
Estimates of Let for a 500-kg beef cow range from -13"C
in early pregnancy to -26"C in late pregnancy to -47'C during lactation (Christopherson 1985).
Bakken (1980) simplified the equation for standard operative temperafure as:
T"r=76-(KrlK"r)(Tb-7")
(3)
where K/K", is the ratio of overall thermal conductance of
the animal in the reference environment (K. essentially no
wind) to the outdoor environment (Kr"), respectively. Wind
speed is the principal environmental factor affecting the ratio
K / K (BaYken 1992). Once the influence of wind on K / K
",
",
experimentally,7", can be calculated from
is"delermined
measured values of T, and wind speed (p) (Bakken 1992)'
Because 2", was set as a constant in the model and wind
speeds could not be controlled, eq. (3) was solved for Trby
setting Z^ equal ro -23'C, and varying wind speed in the
resistanc6-term (KetK). Solving fot T"inEq. (3) results in:
T"=To+(K,lK")(Tes-T)
@)
LCT=-Z3"C was then plotted relative to T" and p and used
as a reference for cold stress.
Body temperature (T) for cattle was assumed to be constant at 38.6"C. Body iemperature may be elevated as a
response to high operative temperature but is relatively con,tunt ut low temperatures in wild ungulates (Parker and
Robbins 1 984). Chdstopherson et al. (197 9) found that rectal
temperatures in yearling Herefords did 19t change more
than a few tenths of a degree from 38.6'C when ambient
temperatures dropped from 10"C to -30"C.
Rlesistance raiios (K"lK.) were calculated-using Parker
and Gillingham's (1996) equations for whole body thermal
resistance of mule deer (rm/rn*) in winter over a range of
wind speeds from 0 to 15 m s-1. Boundary layer resistance
relative to coat and tissue resistance
(iampbell 19':.7), particularly in windy conditions, and
was considered to be negligible' Thermal resistances
(r,^+r") for mule deer and elk were similar within a range
(r-) iJ minimal
i 38'C (Parker and Robbins 1984). Resistance val"i'1ZO
ues for cattle are similar to those for elk (Monteith and
Unsworth 1990). Therefore, this simplification of Parker
and Gillingham's (1990) model should approximate T"rfot
cattle.
Windchill
Windchill incorporates the combined effects of ambient
temperature andwind, and influences the thermal balance
of an animal (Ames and Insley 1975)' Windchill was calculated specifically for cattle hides with a winter haircoat
using aniquation developed in a model system by Ames
and Insley (1975):
504
CANADIAN JOURNAL OF ANIMAL SC/,ENCE
3
o
10
tgo
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F
rtr
1z
t
1l
g
gts
t
a,
I
I
!
REFERENCE
!l
10
Fig. 2. Blackglobe temperature 1I".) and wind speed (p) at
cow location at (a) 0730, (b) l200,"tct 1600, and (d) ofreference climate during Winter l Curved line represents the
LCT for cattle set at -23"C. See "Model of lower critical
temperature" in Theory and Methods for explanation of
how the line representing LCT was derived from I- and u.
Exposure index: I = protected,2 = moderately protEcted, 3
-go
Q9!
g
= exposed.
Windchill ("C) = O.996To - 0.81lp +0.028p2 - 0.0077p3 (5)
where Tois in 'C, p is in miles h-l (0.45 m s-1).
Windchill is accurately calculated using wind run (Ames,
pers. commun.), or total distance of wind Dassins a fixed
point per unit time. In this study, values were 6ased on
average wind speeds recorded over l0-s intervals.
Study Site
The study site was
a 150-ha pasture on the Montana
Agricultural Experiment Station Red Bluff Research Ranch
(latitude 45"35',N, longitude 111.39,W) near Norris,
Montana. Elevation ranged from approximately 1520 m to
l77O m. The pasture had predominantely southwest-facing
slopes which typically blew free of snow by prevailing
I
,rf
t.- dr%t al
,r
t
1t
tt7.
t
I
-to
o51o15
o5to13
Wrd rp.cd (mr-t)
Wrdrpcod(mf)
pasture. Point 1 of each transect was on a bench on the west
side of the draw; point 2 was in the bottom of the draw;
points 3 and 4 were mid-slope and on an exposed ridge,
respectively, and on the east side of the draw. These fixed
points were placed at topographic exftemes to measure the
range of microclimates available in the pasture. The number
of fixed points (12) was constrained by the amount of time
necessary to monitor them.
One blackglobe and one dry-bulb thermometer were
attached with a buret clamp to a steel fence post at each
point, and were placed approximately at shoulder height
(1.3 m) of a mature cow (Yousef 1989). The dry-bulb
southwest winds, making forage accessible year round. The
topography also provided a range of microclimates. The
pastue was dominated by a Festuca idahoensislAgropyron
spicatum habitat type with Rhus trilobatalFestuca idahoensis
habitat type limited to southwest slopes of the major draws
(Mueggler and Stewart 1980). Scattered Rocky Mountain
juniper (Juniperus scopulorum) occurred on lower slopes
whereas limber pine (Pinus flexilis) occuned on the upper
slopes. Springs in three separate draws, two low and one
higher up in the pasture, provided water. Geothermal
processes kept these heated just enough to remain free-flowing
through most of the winter.
thermometer was inserted through a rubber stopper to which
was attached a heavy aluminum foil guard. The aluminum
guard reflects direct solar radiation away from the thermallysensitive end of the thermometer. A hand-held anemometer
was used to record average wind speed over a 10-s interval
at each point.
Pregnant Angus x Hereford range cows from 4 to 72 yt
of age grazed the pasture from mid-December to earlyFebruary. All cows had had previous experience with these
winter conditions. The cows were in moderate body condition and acclimated to winter conditions. Six cows grazed
the pasture in 199l-1992 (Winter 1) whereas 77 grazeditin
1992-1993 (Winter 2). In Winter 1, cows were fed a pellered
protein supplement (0.9 kg d-l) at the end of the last observation period. Supplement was fed at the cows current location to avoid disrupting their grazing pattern. Cows were not
Study Design
globe temperature, and windspeed were recorded at each
Three transects of four permanent monitoring points were
established across a major northwest-southeast draw in the
supplemented
in Winter 2. Ambient temperature,
black-
point and at the concurrent cow location three times per day
at approximately 0800 h, 1200 h, and 1530 h, 3 d per week
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Ho|'sEALANDoLsoN-cATTLEUSEoFMIoR0oLIMATESS0S
o3lo16
UH+rd(tnr)
Fig. 3. Blackglobe temperature (Q) and wind speed (p) at
coivlocationit (a)0730,(b) 1200:(c) 1600. and (d) ofreference climate during Winter 2. Curved line represents the
LCT for cattle set at-23"C. Exposure Index: 1 = protected,
otlo
Smrrrd(ml)
2 = moderately protected, 3 = exPosed.
over a 6-wk period from late December to early February in
1991-1992 and 1992-1993.
perature during the study period was -12'C (Table 1)' Most
iuy, *"." above 0'C. fnelggz-tgg3 (Winter 2) study period
(grazing, resting) were recorded after microclimates were
at the fixed points. Locating six cows in a rugged
150-ha pasture often required extra time. In Winter 2, cow
location and microclimate were recorded first, and fixed
points were read immediately thereafter, minimizing total
monitoring time. Elapsed time for measuring and recording
fixed points and cow location averaged 45 min to t h'
of the site.
In Winter 1, cow microclimate, location, and activity
ti"utot"d
Microilimate data from an exposed fixed monitoring point
(1620-m elevation) in the approximate center of the pasture
was used as a reference for general weather conditions in the
pasture. Cow location was classified as protected (draw)'
moderately protected (lower slopes), or exposed (bench,
upper slopes and ridgetops) based on topography, prevailing
wina Airection, and environmental conditions compared
with the reference climate.
Activity was expressed as a percentage of the total observations in which all or at least half of the cows were
involved in the activity. The activity of individual animals
was not recorded because beef cattle are gregarious, and
thus do not behave independently of one another. Daily
grazing or resting times were not quantified' Forage intake
was also not determined.
An automated weather station approximately 1.6 km
north of the study site recorded ambient temperature, solar
radiation, and wind speed on a 24-h basis. Data from the station were used to supplement the manually-collected data'
RESULTS
Mean monthly temperatures were above normal during the
winter of 1991-1992 (Winter 1), the lowest recorded tem-
typical of winter in this region. A low of -24'C
was recorded and most days were below 0"C during the
study period. Wind speeds ranged from 0 to 13 m s-r both
yeari,indicating that the southerly wind is a stable feature
-u, -ot"
Microclimates at cow locations were usually within the
range of microclimates of fixed points with few exceptions'
In frinter 1, cow microclimates were outside this range by
1.5"C for 167o of the observations, and by 2'5"C for only
observations. These exceptions were mostly
delays associated with locating the cows'
time
by
caused
67o
of the
Only on one occasion did wind speed at cow-location vary
appieciably (> 2.0 m s-1) outside the range of wind speeds,
were in an aspen grove. In Winter 2, cows were
*h"n
"o*t
range on only one occasion, when-cow Z,
this
of
outside
was 5oC cooler than the range of temperatures because
conditions changed rapidly from overcast to full sunshine
at midday.
Microclimates, as affected by topography, influenced
winter use pattems (Fig. 1). At higher wind speeds and colder tempera-tures (f-), cows tended to seek protected and
moderately protect6d areas (Fig' 1; planes 1 and 2)' This
became even more apparent when windchill was considered
(Fig. 1). When cows were observed in protected and moderateiy protected areas, the mean windchill temperature of the
reference climate was lower than the mean windchill temperature when they were observed in exposed areas (data
not shown).
The reference climate was often below the lower critical
temperature (LCT=-Z3'C), particularly in Winter 2 (Figs' 2d
anO :O). Cows tended to avoid these extremes, using micro-
506
CANADIAN JOURNAL OF ANIMAL SCIENCE
Table 2. Observations (%) in which all or at least half of cows were
grazing or resting by level of exposure (EI)
Winter lJ
Resting
Graztng
Resting
3
45
LJ
37
13
2
14
10
4
l2
1
A
19
t2
I
conditions were warm in winter (-4 to 2"C). Under relatively
in exposed
areas would partially destroy the insulation of the hair coat
and increase convective heat loss (Ames and Insley 1975),
reducing heat stress. In contrast, in Winter two cows selected
Winter 2x
Grazing
warm winter conditions, higher wind speeds
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zExposure index,
3 = exposed, 2 = moderately protected, 1 protected
=
vn = 50 observations, I split activity (6
cows).
xn 53 observations,
15 split activity (11 cows)
=
climates which allowed them to remain above their LCT
most of the time (Figs. 2a-c and 3a-c). When the reference
climate was below LCT, cows were predominantly in pro_
tected or moderately protected areas ofthe pasture (nigr. Za
and 3d).
Cattle use was well dishibuted throughout the pasture over
both study periods, except that they tended to avoid using the
higher slopes in Winter 2, probably because of the colder
conditions (Table 1). Cows were observed grazing approxi_
mately 707o
of the
sampling time
of both winters.
In
Winter_l, most of the grazingand resting occurred in exposed
areas (Table 2). In Winter 2, grazing was more evenly dis_
tributed among the three levels of exposure, but cattle rested
more ln protected and moderately protected areas.
DISCUSSION
Cattle selected moderate microclimates for grazing as a
response to extreme wind and cold. This may partially
explain inconsistent results in studies of DGT and Otr.ll as i
response to cold stress. When cold stressed, cattle rest to
conserve energy and may defer grazing until ambient tem_
peratures increase (Malechek and Smith 1976). Cold ambi_
ent temperatures have been correlated with a decrease in
DGT and OMI (Malechek and Smith 1976; Adams et al.
1986) in relatively open country with little natural or artifi_
cial shelter from prevailing winds. Senft and Rittenhouse
(1985b) predicted that cows would have an effective acclimation period of 9-14 d and would reduce forage intake as
a response to STTS. In contrast, in a pasture adjacent to the
study site, DGT was not reduced as a response to STTS
(Dunn et al. 1988) with temperature extrernes similar to
Adams et al. (1986). Prescott et al. (1994) found that DGT
and to a lesser extent OMI of pregnant cows were affected
by fluctuating ambient temperatures in the fall, but were
insensitive to the consistently low ambient temperatures
during winter.
For free-ranging ani'mals, foraging takes priority over
thermoregulatory behavior except under extreme conditions
(Ingram and Dauncey 1985). In Dunn et al.'s (19gg) and
Prescott et al.'s (1994) studies, cows may have been able to
continue grazing in moderate microclimates created by
topographic relief, thus maintainins DGT and OMI.
Microclimates also affected sJection
2.5'C. Beall (1916) found that elk preferred daytime bedding sites on north aspects in dense stands of timber when
of restins sites.
Preference for exposed resting sites when conditiois were
mild, particularly in Winter l, may have been a response to
heat stress. Parker and Robbins (1984) determined that the
upper critical temperature for mule deer in winter is about
protected and moderately protected areas presumably to
avoid high wind speeds combined with low temperatures
(high windchill). Senft and Rittenhouse (1985a) correlated
resting behavior to topographic variables that influence
microclimate. They noted that daytime use of southfacing
slopes peaked in winter, and observed cattle resting in protected sites on cold, windy davs. Malechek and Smith
(1916) observed cattle standing broadside to the sun on cold,
sunny days.
Thermoregulatory behavior of cattle seemed to vary more
with regard to windchill than with wind or temperature
alone. Cows tended to avoid extreme winds, particularly
when temperatures were low. They are probably less sensitive to differences of a few degrees in temperature. Webster
(1970) found that pregnant range cows exposed to an ambient temperature of -27"C without wind (p < 0.16 m s-l) were
not cold stressed, but increased MHP with wind (3.6-5.3 m
s-r) at the same temperature. Sakurai and Dohi (Igg2)
observed cows lying down in compact groups when the
wind was only 2-3 m s-r and correlated this with a 2-6.C
drop in skin temperature compared with skin temperature
without wind. When confined to the timberline zone by
lower snow cover than lowland areas, red deer move just
below timberline when wind velocities exceed 45 krn h-l
(temperatures between -5 and -20'C) where wind velocities
are dampened by the trees (Schmidt 1993). Wind of 4.7 km
h-l (1.3 m s-1) at 0'C ambient did not influence MR of either
bison or cattle, but elevated MR of both at -30"C
(Chnstopherson et al. 1979). On most days in this study,
particularly on sunny days, cattle were probably not coldstressed and could have remained comfortable
in any por-
tion of the pastue (Figs. 2d and 3d).
Cows tended to gtaze on the upper slopes of the pasture
(exposed) both winters when conditions were mild. periodic
sffong winds kept ridges and windward-slopes free of snow,
making forage more available to cattle, and travel easier.
Ryder and Irwin (1987) noted that pronghom preferred
wind-swept ridges for foraging when snow was deep.
The effect of cold stress on caltle grazing native range
may be minimized by selecting a pasture which provides
nafural shelter from extreme wind and cold. Cows may then
be able to continue grazing in microclimates where ambient
conditions allow the animals to remain within their thermoneutral zone.
ACKNOWLEDGMENT
We thank R. Ansotegui, C. Marlow, and B. Sowell for
reviewing the manuscript. This contribution is published
with the approval of the Director, Montana Agricultural
Experiment Station, Journal Series J2907. It was funded by
the Montana Agricultural Experiment Station, Project
rot776.
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