Living animals dif'f'er from inanimat e obj ects in be ing able t o d i s s ip at e heat by way of evaporat i on of moisture.
At high a ir temperatures when losses of heat by radi at ion , conduct ion and c onvect ion becomes difficult , the animal must
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, ' /i - l .J<o_ ' I I I ." I r : . I t 1 2 1 I I . ! ' - 'UMMER 1948 I I I I MAN 'y/ I ·, - ' , . I / / "' _ _ _ _y i . . 1 . u1r 1 ')r ' 'l ) �� , '1 1 &I 70 &· '?"::'· :eo ' r ( ki ll B 1 / M IH A Il l RJ ' F
l•'ic. l�.(n; Loss o f' mo i s ture by c a t t l e a t vm· i ous air t em · ) era tures.
(� The p e r c ent qge o f v n o o r ized mo i sture of the
t ot al he 3t a r o duc t i on .'_) f man lllld c at t l e.
rely on cool ing by way of evaporat io� Als o at very high t emperatures the b ody abs orbs heat from the environment by
radiat ion , etc. , plus that abs orped from s olar rad i at io� Thus , it has to diss ipate not only it s normal body heat , but al s o that abs orbed from the environment.
As the environmental t emperature approaches skin temper ature heat d i s s ipat ion i s shif'ted from radiat ion , conduct i ons , and conve c t ion , to vapouri z a t ion. However , a striking
difference is seen in th i s respect , between man , s ay , a profuse ly sweat ing spec ie s , and c at t l e which have a poor sweat ing
apparatus , as shown in F ig. 1 2.
A sharp break occurs for man at about 84°F. which ,
a ccording to Kuno ( 45 ) i s the sweat ing threshold in man. Up t o this t emperature evapourat ion of diffu s i on or o smotic
water increas e s unt il at 84°F. it represents 35% of the t ot al heat diss ipated. After thi s vaporizat i on rises s t eadily unt il
at 95°F. it r epresent s lOO% of the total heat d i s s ip at ed , while at 104°F. it represent s 200% becau se o f heat that i s
abs orbed from the environment.
The curve for heat diss ipat i on is quite different for poorly sweat ing spec i es. No sharp break occurs at any t emper ature. To compens ate s omewhat for the ine fficiency of sweat ing t he s e poorly swe at ing animal s increase their respirat ion
r at e s markedly. There i s gre at controversy in the l it erature a s to whether c e.ttle sweat. Evidence for vaporizat ion loss is l imit ed.
Freeborne et al ( 1 53 ) , 1934 , used inverted petri d ishes on the bott oms of which were fast ened filter papers impregnated w ith c e.lc ium chloride , and these were att ached to the rump and s ides of Jersey cattle. At 840F. the s e areas - 35. 67 s quare inches - gave off 333. 23 mlgms. of water per hour. Then the right rump was treat ed with commercial fly spray. After a f ew days the sprayed areas lost mo isture at the rate of 180 mlgms. per hour, due t o the oil in the spray. Transpos ing the d at a on these small areas to the t ot al are a , they arrived at a
t i re surface . C alc ulat i ng f ur t he r t hi s r a t e is re sp ons ible for t he
di s s i pa t i o n of 5699 c . a day at 84oF . of 60% R.H. Dat a ob tai ned in a resp i rat i o n chamber s howe d the t ot a l da i ly di ssip��io n of heat t o be 1 7 , 920 Cal ori e s . He nc e , los s of hea t by e vap ora t i o n is 32% of t he t ot a l hea t loss . Sp rayi ng reduc e s t h i s t o 1 7 . 2% . T h i s fi gure o f 3 2% is remarkedly c lo se t o the 35% of evap orat i o n loss for ma n, a t 840F .
Regan a nd R ic hard s o n ( 60 ) use d a s imi lar t e c hnique w i t h 2 Jersey cow s a nd ca l c ula ted t hat a t 840F . a nd 60% R . H . eac h c ow
transp ire d from her s urface 1 lb . or 454 gm . pe r hour.
R i e ck and Lee ( 61 ) found t hat t he obse rved wat e r loss was alway s mu ch �er tha n c ould be acc ount e d for by re sp irat ory evap
ora t i o n. _At t he lowe s t tempe rat ure used , 85°F . , t he no n-respirat ory evaporat ive loss was 454 g. per hour .
The re s ult s of t he se 3 group s of w or ke rs are i n very c lo se a gre eme nt . R ieck a nd Le e c la ime d t hat if , a s B cody ( 61 ) s t a t e s , the rate of wat er l o ss by diff us i o n i s c o nstant the n t he ma�or par t of t he e x tra wate r lost mus t be by sweat gland ac t i v i t y . H oweve r , o t he r w orke r s - Wh i t e ho use e t al , - g ive expe rime ntal e vi de nce t o the e ffec t t hat diff us i o n wat e r l os s r i ses w i th a i r temperature and thi s would a c co unt i n a large me a s ure for t he e x t ra mois t ure l o s s a t t he h i g h t emp era t ure s .
B r ody e tal ( 44) i n a re c e nt re f i ne d experime nt found no s harp b re ak i n t he curve of he at d i ss ipa t i o n by vap orizat i o n . T here i s a c hange i n t he s l ope of t he curve , a s see n i n F ig . 1 2 , a t 55-65 � . w h i ch may re lfe c t a ri se i n t he re sp irat i o n rat e . The grea tly ac ce le ra t e d evaporat ive c oo l i ng i n man ab ove 94°F . re fle c t s his a c ce lera t e d swe a t i ng rat e above th i s t emperature . Be low so0p . however , t he c ow l o s e s more heat by vaporiza t i o n t han d oe s man.
K uno , previously s t a ted t hat , at 840p . , man loses 35% of hi s
he a t l oa d by mea ns of diffus i o n wat e r los s . Becaus e h i s
re sp ira t io n ra t e var ie s l i t t le w i th t e mpe ra ture (Baze t t ( 7 ) ) t he l o s s per me di um· of t he re sp irat ory pas sage s w o uld no t be very great . Brody ' s f igure , howe ve r , s how s t ha t at 84� . t he cow lo se s ab out 72% of i t s heat load by vap ori za t i o n . The wo rk
g ive 35% o f t he t o t al heat d i s s ipat ion at 84°F. as b e ing d iffuF> ion wat er l o s s from the skin and does not include
mo i sture l o s s from the resp irat ory tract. Thus , unit ing with Brody ' s d at a it app ears that , at 84°F. the c ow loses 72% of
i t s heat by vapor i z at ion of mo i stur e , 35% of this from the skin and the b al anc e - 37% from the resp ir at ory t ract .
R ichards on ( 1 54) found that i n man two thirds o f the
i ns ens ibl e loss c ame from the skin , i from the resp ir at o ry
p as s age. W ith c at t l e i t app ears otherw ise. However , this conclu s ion has been reached by deduct i on only. No true e xp er iment al bas i s ex i s t s .
Mitchell an d Hamil t on ( 1 5 5 ) repor t ed that in steers , heat l o s s by vapor i zat i o n r anged from ab out 1 5% o f t ot al heat l o s s at 1 301". t o about 4�% at 69°F. and that following
s he aring o f the hair , ins ens ib l e loss decreased by about 20%. Transpo s i ng t hese result s t o Brody ' s graph shows a fa �r agree-
ment.
The c ow then , i s relying very much on heat l o s s by
e vap orat i on l ong before its skin t emperature e qual s air temper-
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atur e ( 1020F. - Brody). At 1020F. al l heat must b e lost by
evaporat i on in c ows , where as , im man , 94oF. i s the t emperature when air and skin t emperatur e s are e qu al and all heat must be l o s t by evaporat i on. Howeve r , well before 84°F. the c ow l o s e s 70% o f i t s he at load by evaporat i on.
Why do es the cow h ave to rely t o such a l arge e xt ent
on heat l o s s by evaporat ion at c omp arat ively early t emper ature s comp ared with man? In the c as e of man ther e i s no hairy c o at and he at l o s s i s not hindered by an envelop ing airlayer as i t i s in c at t l e. Als o , pulse rat e in man r i s e s with a i r t emper- atur e ind i c s t i ng that vasodilat ion is act ive. Skin temperature c hanges and a fine re-adjustment of the c i rcul at o ry system
and the use of the e xtremi t i e s for d i s s ip at ing heat allows man t o make good use of t he heat l o ss avenues o f rad i at i o n ,
c onduc t ion and c onvect i on unt i l skin and a ir t emperatur e ne ar
e qu i l ibr ium. C at t l e , on the other hand , with their hai ry c oat , decrease in pul s e r at e , p o s s ibly no vas o d i l at i on o r
circulatory ad justment in the ext r emit i e s , h ave pooe methods
of r a i s i ng skin t emp er ature and a l t e r ing the d i s t ribut i on o f
heat d i s s ips.t i on. Henc e , t hey s .r e not ab l e t o make u s e of the
u s ual a venu e s o f h e a.t l o s s , e ven at c omparatively l ow tempe r atur e s and have t o r e s o rt t o evaporat ion.
� orke r s d ep l ore the fact th at c at t l e are p o o rly sweating animal s and b e c au s e of this, have p oor powers o f adaptation t o high a i r t emp e r a t ur e s . Thc:.t c at t l e are poorly SV'leat ing animals
may prove t o be true , but rath e r should one d epl ore the c ow • s
poor thermoregu l at o ry ad j us tment t o c;.u it e mod e r at e rises
i n a i r temperature, thus C �ll.l S ing t'he animal t o r e s ort t o
evap orat i o n methods at a very e ar ly s t ag e so that when higher t emp er atur e s ar e app r o ach ed , the cow i s already relying t o a very gre at ext ent on evaporat i on , whe r e as , man i s st ill u s ing
the u sual avenu e s o f heat los s and s t i l l has the greater part
of the evap o r at i on �athod o f h e at loss i n r e s e rve. Thi s
e xpl a ins h i s sup e r i o r i ty over c at t l e at h igher a ir t empe r ature s .
Work will h ave t o b e c ar r i ed out t o find just what c i rcul
at o ry adjustment s do t ake p l ac e i n c at t l e ; wh ether vasodil at ion
i s a fact or and what i s the thermal co nduct anc e of the t issues. Although s ome w orkers h ave s t r e s s ed the imp o rt anc�f coat
colour and qual i t y i t i s probable that the character o f the
c o at i s t he c h i e f d i s advant age o f cat t l e in hot weather and
more work needs t o b e undert a ken t o s t u�y the role of t he hairy c o at i n thermoregulat i on.
Brody show ecl that when unit s urface are a i s used as a b asE
for c ompar ison , differences b etween Hol s t e ins a nd Jerseys are smal l , wh en size d i fferenc e s in e v aporat i ve l o s s are elimi nated.
On t he basis o f body w e ight , however , (per 1000 lb. L. W. ) small
cow s evaporat e more moisture t han l arger c ow s . Between 90 and 1 0C °F. a 1000 lb . c ow evaporate s 2 lb. o f mo i s ture p e r hour , n o t much ab ove t h at o f man , whose we ight is about one
t enth that o f the c ow At 70°F. man d i s s ip at e s 1 8%
of the h e at pro duced by evaporati on, c ows - so%.