Cold Stress After Shearing
with a Snow-Comb
l
J. C. D. HUTCHINSON”, J. W. BENNETT” and
MANIKA WODZICKA TOMASZEWSKA:‘:
SUMMARY
The reactions of 32 Merino sheep were examined in 6 hr. and
18-20 hr. exposures to cold winds after shearing. Half were shorn
normally and half with a snow-comb. Those shorn with the snowcomb had higher rectal and skin temperatures, but lower heart rates
than the other group. In the 18-20 hr. exposures 7 of the 8 normally
shorn animals, but only 1 shorn with a snow-comb, had to be removed before the end, on account of hypothermia or weakness.
The thermal insulation of Merino fleece, half inch and 3/16 in.
in length, was measured with a hot-plate apparatus, mounted on a
life-size model of a sheep, which was placed at various angles to
winds ranging from 4.3 to 10.7 m.p.h. Over this range of conditions,
the ratio between the insulation of the two lengths of fleece (including the insulation of the air) averaged 2.2.
From these experiments, it is thought that the use of a snowcomb would be effective in reducing mortality off shears.
INTRODUCTION
Losses of sheep off shears are an important economic problem
in the sheep industry. In New Zealand the use of a snow-comb,
which leaves up to half an inch of wool on the sheep, has been common
in recent years. There seems at present to be no published work
on the efficiency of snow-combs in alleviating cold stress after shearing. We have performed experiments on the reactions of sheep to
cold winds, when shorn with a snow-comb or with a normal comb. We
have also measured the thermal insulation in various winds of the
wool left after shearing. The latter series of experiments is still in
progress.
METHODS
( i ) Animal Experiments.- Sixteen Merino ewes were shorn at
8 a.m., half of them in the normal way and half with a snow-comb.
Four only, two of each kind, were handled on any one day. The
animals were tethered in four pens in which arrangements had been
made to blow a cold blast from behind. In two pens the air came
from ducting fixed at the entrance. For the other two pens, which
had less air movement, a pair of 2 ft. fans drove the air. The sheep
were exposed to a low temperature (Table 1) for 6 hours, from
about 10.30 a.m. to 4.30 p.m., and had not been fed since the afternoon of the previous day. During the experiment the rectal temperature, skin temperature (in the greater wind) at 5 positions on the
trunk, and the heart rate were measured about every 45 minutes.
The length of fleece remaining after shearing was measured in 19
positions.
A further sixteen Merino sheep (14 ewes and 2 wethers) were
treated in the same way, except that the experiments lasted 18-20
hours. The observations were made about every hour. If the rectal
temperature of an animal fell below 96.5”F., -or if the sheep was
unable to stand, it was removed from the climate room as a “nonsurvivor”.
* C.S.I.R.O., Division of Animal Physiology, The Ian Clunies Ross Animal
Research Laboratory, P.O. Box 144, Parramatta, N.S.W.
199
(ii) Thermal Insulation of the Fleece.-The heat flow through
the shorn fleece was measured by a conventional thermostatic hotplate apparatus. This was mounted on a life-size model of a sheep
in a wind tunnel, which was placed in a climate room. The plate
temperature was IOSOF., the skin temperature around IOI’F., and
the air temperature about 53OF. So far, the insulation has been
measured of a piece of medium wool Merino fleece, mounted in a
high mid-side position. The lengths of fleece were 3/16 in. to represent normal shearing, and 4 in. to represent shearing with a snowcomb. The model sheep was placed at a number of angles ranging
from side-ways on (90” ) to facing away from the wind (0” ). The
air movements studied were still air (about 20 ft./min.), 4.3, 6.6 and
10.7 m.p.h.
The insulation is expressed in the clo unit. This is the amount
of thermal insulation which will maintain a resting sitting man,
whose metabolism is 50 kcal/ sq.m./ hr., indefinitely comfortable in
an environment of 70”F., R.H. less than 50 per cent. and air movement 20 ft./min. It is defined in physical units as 0.18
“C
(Burton and Edholm 1955).
kcal/sq.m./hr.
RESULTS
( i ) Animal Experiments.- The results of these are given in
Table I. Measurement of air movement could be only approximate
in the turbulent conditions prevailing.
In the 6 hr. exposures the animals shorn with a snow-comb were
able to maintain a higher rectal temperature than those shorn normally (PL 0.05). The skin temperature of both groups was low;
it was higher for the animals shorn with a snow-comb than for the
other group (PL 0.01). It has been found that cold increases the
heart rate of pigs and cows (Irving, Peyton and Monson 1956;
Kibler and Brody 1949, 1950). The normally shorn sheep had a
higher heart rate than the other group (PL 0.05). None of the
animals had to be withdrawn before the end of the exposure.
In the longer experiments all the four normally shorn animals,
exposed to the stronger wind, had to be removed before the end, at
7, 93, 11 and 18 hours. Three of the normally shorn animals in the
lesser wind had to be removed at 15B, 19 and 19% hours with rectal
temperatures below 96.5 OF. Only one of the group shorn with a
snow-comb failed to complete the experiment. It was removed from
the stronger wind after 15 hours, because it became unable to stand.
Its rectal temperature was 98.6’F. The rectal temperature of the 7
“survivors” of this group ranged from 98.0 to 101.8’F. at the end
of the experiment. As in the short experiments, the mean skin
temperature was higher, and the heart rate lower than those of the
normally shorn group.
The mean rectal temperature in the lesser wind was higher than
that of the normally shorn animals. In the stronger wind, the mean
rectal temperature for the normally shorn animals up to 7 hours was
higher than the means up to 7 hours (100.5”F.) and 15 hours for
those shorn with a snow-comb. As mentioned above, these normally
shorn animals failed to maintain their body temperatures during the
la.ter stages of the exposure.
(ii) Thermal Insulati,on of the Fleece.-This includes the insulation of the envelope of still air surrounding the fleece, as well
as that of the fleece itself. It does not include the insulation of the
skin, nor that of the subcutaneous tissues. The value for thermal
insulation of the fleece in still air was 2.88 clo for half an inch of
wool, and 1.50 clo for 3/16 in.; the ratio of these is 1.92, while that
of the lengths of wool is 2.67. If 0.7 clo is subtracted for the in200
::: i:. :::
::
:
::: :.:.
::: i:.
: : :
:: :: ::
: : : : :
:: :: i. i. ..:
:: ::
: :
::
:
.: ::
:
.
.
.
:
::: :::
:
:
i
:
.
:
l
-%
:
:
:
:
:
:
:
i
:
:
:
.
sulation of the air (Larose 1947), the insulation per inch of wool is
4.4 clo for the half inch length and 4.3 clo for 3/16 in. This is of
the same order as the values obtained by former workers (Scholander
et al. 1950; Hammel 1955; Moote 1955) for the pelage of many other
mammalian species. The values obtained at various angles in different winds are given in Table II. Although the insulation fell as
wind increased, the ratios of the insulation of the two lengths of
fleece, with the air surrounding them, remained constant. It did not
vary much between different angles. The average was 2.21, somewhat
above the value in still air.
DISCUSSION
In the second series of animal experiments 7 of the 8 normally
shorn sheep had to be removed before the end, whereas only one of
the group shorn with a snow-comb failed to complete the experiment.
All these 8 animals removed were either unable to stand or weak,
and it is unlikely that they would have survived under equivalent
climatic conditions in the field. Thus, there is evidence that the use
of a snow-comb would reduce mortality. It is not, however, possible from the animal experiments to assess the value of this comb
in quantitative terms.
The measurements of insulation indicate that with an efficient
snow-comb leaving half an inch of wool, the insulation of the fleece,
and its surrounding air, is in windy conditions more than twice that
of a fleece shorn in the normal way. It must be realised that at
present only one position on an animal has been studied, though
at a number of angles. In addition, the effects of ‘disturbance of the
fleece, such as occur on a live animal, have not yet been studied, nor
the effect of rain, which is thought to make an important contribution
to cold stress. However, if it should turn out that this ratio between
the insulations is general for all conditions, then it would seem that
the expense of developing a snow-comb, acceptable to Australian
shearers, would be well worthwhile.
ACKNOWLEDGMENT
We are grateful to Mr. W. A. Richardson for the design and
construction of the hot-plate apparatus. The Sunbeam Corporation
very kindly provided us with a snow-comb.
REFERENCES
Burton, A.C., and Edholm, 0. G. (1955).-“Man in a Cold Environment”. p. 36. (Edward Arnold: London).
Hammel, H. T. (1955) .-Amer. J. Physiol. 182: 369.
Irving, L., Peyton, L. J., and Monson, Mildred (1956).-J. Appl.
Physiol. 9:- 4211
Kibler, H. H., and Brody, S. (1949).-Res. Bull. MO. Agric. Exp. Sta.
No. 450.
Kibler, H. H., and Brody, S. (1950).-Res. Bull. MO. Agric. Exp. Sta.
No. 464.
Larose, P. (1947) .-Canad. c1. Res. (A). 25: 169.
Moote, Irene (1955) .-Text. Res. (J.). 25: 832.
Scholander, P. F., Walters, V., Hock, R., and Irving, L. (1950).Biol. Bull. 99: 225.
203
DISCUSSION
J. Robertson (Vie.) enquired about the nutritional status, appetite
and condition of the sheep used.
Answer.-The sheep were in forward store condition and had
been brought in from pastures the afternoon before the experiments.
They were not fed until the test was complete. The sheep which went
down would eat if feed was placed in front of them.
Professor T. K. Ewer (Qld.) suggested that the snow comb might
be useful in Queensland to minimise heat stress following summer
shearing.
204