167 Int. J . B i o m e t e o r . 1973, vol. 17, number 2, pp. 167-176

Critical Temperatures in Lactating Dairy Cattle: A New Approach to an Old Problem

by A. B e r m a n a n d A. M e l t z e r * INTRODUCTION Ambient t e m p e r a t u r e is known to affect the r a t e of metabolic heat production. In the h o m o e o t h e r m i c animals heat production in the cold g e n e r a l l y equals the heat l o s s of the animal. This loss of heat d e t e r m i n e s the energy r e q u i r e d for maintenance of homoeothermy. Metabolic heat production is regulated to this r e quired r a t e by the body t e m p e r a t u r e control s y s t e m . The ambient t e m p e r a t u r e at which the m i n i m a l metabolic r a t e equals the heat r e q u i r e m e n t is defined as the lower c r i t i c a l t e m p e r a t u r e . At higher ambient t e m p e r a t u r e s , metabolic heat production r e m a i n s constant o v e r the t h e r m o n e u t r a l t e m p e r a t u r e zone. Within this zone, the combined convective and radiative heat l o s s accounts for a dec r e a s i n g p a r t of the metabolic heat production; evaporative heat loss i n c r e a s e s over this range of ambient t e m p e r a t u r e s . Over this range t h e r m a l heat s t r e s s gradually develops. A zone of ambient t e m p e r a t u r e s , within which t h e r m a l balance is maintained and productivity is not affected, would thus be delimited by a lower and an upper c r i t i c a l t e m p e r a t u r e . Since metabolic heat v a r i e s with the l e v e l of milk production, the c r i t i c a l t e m p e r a t u r e s would v a r y accordingly. The definition of these c r i t i c a l t e m p e r a t u r e s is of substantial p r a c t i c a l i m p o r tance in dairy husbandry. This paper aims to analyze the existing information on the c r i t i c a l t e m p e r a t u r e s in dairy cattle; a new approach to this p r o b l e m is presented. The lower c r i t i c a l t e m p e r a t u r e s in the dairy cow w e r e e s t i m a t e d f r o m c a l o r i m e t r i c studies at 2°C at maintenance, - 4 ° C when producing 10 kg FCM (Fat C o r r e c t e d Milk), and - 1 0 o c when producing 20 kg FCM (Hamada, 1971). S i m i l a r figures w e r e a r r i v e d at f r o m c a l o r i m e t r i c studies in s t e e r s (Blaxter and Wainmann, 1961). T h e s e are e s t i m a t e s of the ambient t e m p e r a t u r e s at which the animals produce with the l e a s t t h e r m a l s t r e s s whilst p e r i p h e r a l heat l o s s is m i n i m i z e d by v a s o c o n s t r i c t i o n . These t e m p e r a t u r e s a r e v e r y low, however, when c o m p a r e d to the actual t e m p e r a t u r e s at which the animals produce l a r g e milk yields: 21oc in the B e l t s v i l l e e x p e r i m e n t s (Moe, Flatt and T y r e l l , 1972), and the m a r k e d l y higher mean s u m m e r t e m p e r a t u r e in the hot d e s e r t a r e a in the south of I s r a e l (Kali, A m i r and Morag, 1970). These c r i t i c a l t e m p e r a t u r e s imply a v e r y wide t h e r m o n e u t r a l zone, about 30 ° to 4 0 o c wide. In the c a l o r i m e t r i c studies individual animals w e r e exposed to each of the ambient t e m p e r a t u r e s for up to one week. However, in animals living in a group behavioural adjustment by huddling together considerably reduced the i n c r e m e n t in heat production at low ambient t e m p e r a t u r e s (Mount and Holmes, 1969). In prolonged exposures to high t e m p e r a t u r e s the t h e r m a l conductance of the coat i n c r e a s e d o v e r a period of 9 weeks, at the end of which it had not attained a steady level (Kibler et a l . , 1965). In h e i f e r s exposed to a constant 32°C t e m p e r ature, n o r m a l growth r a t e was r e s t o r e d at the end of 12 weeks of exposure, con*) The Hebrew University, Faculty of Agriculture, Rehovot, I s r a e l . R e c e i v e d 12 March 1973

168 eomitant with the attainment of a light hair coat (Weldy et a l . , 1964). The c h a r a c t e r i s t i c s of the hair coat in dairy cattle exposed to the natural climate w e r e affected by both the photoperiod and the ambient t e m p e r a t u r e (Berman and Volcani, 1961). The changes in p e r i p h e r a l insulation w e r e not accounted for in the above-mentioned c a l o r i m e t r i c studies on the c r i t i c a l t e m p e r a t u r e . In animals bearing a s u m m e r coat, the r e s p o n s e s of the a n i m a l s t o a 32oc constant t e m p e r a t u r e stabilized only after an acclimatization period lasting about 2 weeks (Bond et a l . , 1961). In cattle, sweating r a t e s at a given air t e m p e r a t u r e w e r e higher in s u m m e r than in winter (Sehleger and T u r n e r , 1965). R e c t a l - t o skin heat conductance was also activated at higher skin t e m p e r a t u r e s in s u m m e r than in winter in animals exposed to a n e a r - n a t u r a l climate (Berman, 1971). Such changes did also occur in lactating cows during a 9-week a c c l i m a t i z a t i o n to a constant 29°C t e m p e r a t u r e (Kibler et a l . , 1965). In the c a l o r i m e t r i c studies the animals w e r e exposed to each t e m p e r a t u r e for about one week only. It is l i k e ly t h e r e f o r e that this period does not enable the animals to r e a c h complete acclimatization. A n y c h t h e r m e r a l 24-hr cycle (Bligh and Harthoorn, 1965) in the body t e m p e r a t u r e was shown to p r e v a i l in many ungulates in both the wild and the domesticated conditions (Berman, 1968; B e r m a n and Morag, 1971; Bligh and Harthoorn, 1965; Bligh et a l . , 1965; Mendel and Raghavan, 1964; Taylor, 1970). It has been suggested that the resulting heat storage may reduce the water r e q u i r e m e n t s for d i s sipation of heat (Taylor, 1970). It has also been shown that subsequent to adaptation to heat, heifers may r e g u l a t e their body t e m p e r a t u r e at a higher level ( B e t man and Kibler, 1959). However, in cattle exposed to a n e a r - n a t u r a l c l i m a t e (Berman, 1971), as well as in sheep and goats during short exposures to heat and cold (Johnson, 1971), p a s s i v e body t e m p e r a t u r e lability was not evident, although this may occur in the dehydrated animal (Bianca, 1965; Taylor, 1970). It is thus unlikely that adaptation to higher than n o r m a l body t e m p e r a t u r e s a l t e r s the c r i t i cal t e m p e r a t u r e s in n o r m a l l y producing lactating cows. These findings suggest that the available c a l o r i m e t r i c data as well as the e x i s tent constant t e m p e r a t u r e studies are not likely to provide information applicable to the evaluation of the t h e r m a l comfort t e m p e r a t u r e zone for cattle producing in the n e a r - n a t u r a l conditions. The lower and upper c r i t i c a l t e m p e r a t u r e s should thus be d e t e r m i n e d in animals exposed to the natural climate, as it is only in these conditions that the n o r m a l d e g r e e of acclimatization is acquired. Under these conditions, an experimental approach based on energy balance would not be feasible. On the other hand, t h e r m o r e g u l a t o r y reactions w e r e thought useful p a r a m e t e r s for the a s s e s s m e n t of the c r i t i c a l t e m p e r a t u r e s .

METHODS In o r d e r to e s t i m a t e the t h e r m a l comfort t e m p e r a t u r e zone, data of an e a r l i e r e x p e r i m e n t (Berman, 1971; B e r m a n and Morag, 1971) have been further analysed. This e x p e r i m e n t was c a r r i e d out on lactating cows (31 kg FCM/day) in the s u m m e r (23o-39°C) and in the winter (9o-24oc). Mean body weight of the animals was 600 kg. The animals w e r e stanchioned in an open-side shelter, protected f r o m d i r e c t s o l a r radiation. P a r a m e t e r s of t h e r m o r e g u l a t o r y functions w e r e m e a s u r e d at 3 - h r i n t e r v a l s during four consecutive 24-hr cycles in each season. Rectal, tympanic membrane, e a r pinnae and mean trunk skin t e m p e r a t u r e s w e r e m e a s u r e d simultaneously with skin water l o s s and n o n - e v a p o r a t i v e skin heat loss on t h r e e sites on the main body. Black globe t e m p e r a t u r e s (Tg) w e r e a s s e s s e d by standard m e t e o r o l o g i c a l black globe t h e r m o m e t e r s .

169 R E S U L T S A N D DISCUSSION

The ear pinnae t e m p e r a t u r e s and the mean trunk skin t e m p e r a t u r e s (Ts) were in the main very s i m i l a r l y related to the ambient t e m p e r a t u r e s (Tables 1, 2). TABLE 1. R e g r e s s i o n of ear pinnae skin t e m p e r a t u r e (Te) on mean trunk skin t e m p e r a t u r e s (Ts) in lactating cows in a hot-dry d e s e r t Tg range

(°C) 9.5-24

Mean Ts (°C)

Mean Te (°C)

Regression equation (Te= a+b. Ts)

SE of estimate (s:~.x)

Corr. Coef.

SE of

Regr. coef. (Sb)

33.9

34.5

7.5 + 0.79x

0.8

0.90

0.1

-39

36.1

36.5

5.4 + 0.85x

0.6

0.79

0.1

9.5-39

34.9

35.4

5.7 + 0.85x

1.0

0.89

0.1

23

TABLE 2. R e g r e s s i o n of ear pinnae skin t e m p e r a t u r e (Te) on black globe t e m p e r a t u r e (Tg) in lactating cows in a hot-dry d e s e r t Tg range (°C) 9.5-24

Mean Te (°C)

I

Mean I R e g r e s s i o n Tu t equation (°K) (T e = a+b. Tg)

SE of estimate

(s~.x)

Corr.

SE of

Coef.

Regr. coef.

(Sb)

34.5

17.4

29.0 + 0.13x

1.5

0.58

0.05

-39

36.5

31.6

32.2 + 0.24x

0.6

0.89

0.03

9.5-39

35.4

23.9

31.6 + 0.16x

1.2

0.78

0.02

23

P e r i p h e r a l vasoconstriction was evident in only very few eases and in winter only. This and the s i m i l a r i t y of ear and Ts t e m p e r a t u r e s strongly suggests that in these lactating animals p e r i p h e r a l vasodilation prevailed as a rule over a wide range of ambient t e m p e r a t u r e s . It was also observed to p r e v a i l in s e x u a l l y m a t u r e female calves in either s u m m e r and winter (Berman and Samsky, unpublished data). The nychthemeral cycle in body t e m p e r a t u r e may however a l t e r the r e l a t i o n ship between skin and ambient t e m p e r a t u r e s relative to that in animals exposed to constant ambient t e m p e r a t u r e s . The t e m p e r a t u r e gradient between the animal and its environment in the cycling climate may thus differ from that in constant t e m p e r a t u r e studies. Mean trunk skin t e m p e r a t u r e could thus be satisfactorily predicted from Tg (Table 3). The equations were significantly different from that obtained in constgnt t e m p e r a t u r e studies by Thompson, Worstell and Brody, (1951). The c o r r e l a t i o n s between either rectal t e m p e r a t u r e or tympanic t e m p e r a t u r e and Tg were lower than that between Ts and T~. This is due to the large time lag between peak Tg and body t e m p e r a t u r e (Fig. 1~. This situation is typical for the nychthemerally cycling n a t u r a l climate; it is due to the a n i m a l s body acting as a heat sink and the mode of action of the t h e r m o r e g u l a t o r y s y s t e m . It indicates that t h e r m a l balances derived from steady states in constant t e m p e r a t u r e studies a r e not applicable to the natural cycling state. This was found true also for the r e s p i r a t o r y responses (Berman, 1967).

170 T A B L E 3.

R e g r e s s i o n of m e a n t r u n k s k i n t e m p e r a t u r e (Ts) on black globe t e m p e r a t u r e (Tg) Tg

range (oc) 9.5-24

B e r m a n (1971)

Regression equation (T s = a+b. Tg)

SE of estimate

(s:~.x)

Corr. Coef.

SE of b

(Sb)

33.9

30.8 + 0.18x

1.28

0.752

0.04

-39

36.1

27.6 + 0.27x

0.58

0.919

0.02

9.5-39

35.0

28.8 + 0.18x

1.06

0.834

0.02

23

T h o m p s o n et al.

(1951)

-17

-18.3

-

2 3 . 3 + 0.29x

1.30

0.930

18.3-40.6

-

2 5 . 5 + 0.23x

0.60

0.911

1

I

]

I

I

I

I

I

A-Rectal. temperature 39.~- A-Tympanic membrane / \ temperature / \ 39.6- O-BLack g!,obe // \ temperature / \

m 40Eo

8 39.4 o

39.2 +_,

o 39.0

E "" 38.8 "0

o

38.6

o 30 ~

38.4 38.2 T

F i g . 1.

I

3

L

6

I

I

1

9 12 15 Time of day

K

18

I

21

I

24

N y c h t h e m e r a l changes in a m b i e n t and r e c t a l t e m p e r a t u r e in lactating cows.

N o n - e v a p o r a t i v e l o s s of heat f r o m the s k i n is p r o p o r t i o n a l to the t e m p e r a t u r e g r a d i e n t b e t w e e n the s u r f a c e of the a n i m a l and the e n v i r o n m e n t . The c o e f f i c i e n t r e l a t i n g n o n - e v a p o r a t i v e heat l o s s to the t e m p e r a t u r e g r a d i e n t in this e x p e r i m e n t ( B e r m a n , 1971) was s i m i l a r to that c i t e d by Burton and E d h o l m (1955) and to that d e t e r m i n e d in the s t e e r by B l a x t e r and W a i n m a n (1961). Since in the n a t u r a l c l i m a t e the g r a d i e n t i s dependent upon the t h e r m a l lag of the s u r f a c e t e m p e r a t u r e , the data w e r e a c c o r d i n g l y c o r r e c t e d . The c o r r e c t e d v a l u e s a r e v e r y s i m i l a r to

171

those found in calves by Holmes (1970). F r o m these data non-evaporative heat loss at various ambient t e m p e r a t u r e s was calculated (Fig. 2). Non-evaporative heat loss i n c r e a s e d at ambient t e m p e r a t u r e s below 12oc. This suggests i n c r e a s ed metabolic r a t e s to compensate for the r i s i n g heat l o s s . Minimal metabolic heat production varies, however, with milk production r a t e .

--non-evaporative

h e a t Loss

. . . . . . . e v a p o r a t i v e heat

l.oss

300 / /'

/

/

ffl /

o 20( / I) I/

e-"

/ / /

i0( ~. . /

//' 1"

--

.

"

" ~ "N

I 10

I 20

30

40

Btack globe temperature(°C) Fig. 2. Non-evaporative and evaporative heat loss from the skin surface at a i r t e m p e r a t u r e s 9 ° to 39oc in lactating cows. The levels of heat production for the different levels of milk production were calculated using the data of Moe, Flatt and T y r e l l (1972). The ambient t e m p e r a t u r e s at which metabolic rate will i n c r e a s e in the absence of vasoconstriction can thus be found (Fig. 3). These experimental data were compared with seasonal changes in the efficiency of feed utilization for milk production in lactating cows yielding about 21 kg FCM/day in a c o m m e r c i a l herd. The efficiency of feed utulization declined when ambient a i r t e m p e r a t u r e fell below 15°C (Fig. 4). These seasonal trends are y e a r l y repeated. This lower c r i t i c a l t e m p e r a t u r e defines the lower ambient t e m p e r a t u r e s at which the animal is not dependent upon vasoconstriction to attain maximal efficiency of feed utilization. The depressing effects of high t e m p e r a t u r e s on the level of milk production are well known. A typical situation is given in Fig. 5. The need thus a r i s e s for the definition of the upper level ambient t e m p e r a t u r e at which the animals may m a i n tain t h e r m a l balance. Non-evaporative heat loss declines to zero when Tg reached 38°C. Sweating r a t e s increased as Ts rose above 33°C. The equation r e l a t i n g skin evaporative heat loss to T s was 27 + 50 (Ts-33) W / m 2 . This equation was combined with that relating T s to Tg, yielding the changes in skin evaporative heat loss with black globe t e m p e r a t u r e . The total skin heat loss at different T g ' s was calculated, a s s u m i n g sweating at either the maximal rate or at 50% o f the m a x i m a l r a t e .

172

--

Maximal. vctsoditation Minima[ heat production

.....

30C

"L~I

E o

............................

.40 kg..F.CMl.dpy..

I ......................

30kcj.FCMl.doY..

...................

;~O.kg..F.CIyl,~dqy..

20( --

...............

1.0k9..FCM(d.ay.. Mointeno.nce

'1~

1oo-

I

l

10

Fig. 3.

~

20 30 BLack gl.obe temperature (°C)

I

40

Non-evaporative heat l o s s f r o m the skin surface at a i r t e m p e r a t u r e s 9 ° to 39°C and heat production at different m i l k y i e l d s .

To (°C) 24-18 30--29 13--12

15-13 23--21 8--4

14-15 23--27 4--5

17--23 30--41 5--12

25-26 37--42 16--17

26--27 Mean 35-35 Max. 16--8 Min.

"13 (3 -13

o

b2

~ O.c.

D

"0

~ o.~ m

o o.~

0.6i 0.5~-- I 10-11

Fig. 4.

I

12-1

[

2-3 Month

I

of

4-5 the year

I

6-7

I

8-9

Bi-monthly changes in efficiency of feed utilization for milk production; mean m i l k y i e l d 21 kg F C M / d a y .

173

I dl

loo -

I

I

I

I

O 90-

o 0

el_

o

-a 8 0 -

O

'~., _~ 7 0 -

o__ SEP-OCT. CALVlNGSPEAK &I.0KG.

0

>. 6 0 n

• 0-

MARCH-APR. CALVlNGS PEAK &2.4 KG.

50-

i

0

I

I

1

2 3 Month of

I

I

I

4 5 tactation

I

6

I

Fig. 5. E f f e c t of s e a s o n of c a l v i n g on m i l k p r o d u c t i o n .

\\

- \~\~ \

~.~

Max. vasodiLation

---Max. vasoditation +50°Io of max. sweating - . . . . . . Max. vasoditation + max. sweating "-.

&-

\

30O .....

~.~,s_ ....

..... _'.:,~_

_."1_ >4_3 .......

.............

_~0~._FC_~!._d~

2-.3_i ..........

.....

2_0. k_~..EC_M#, d_~#. _ ._

u1

~200 •

3.5

t-

~ ~

" ' 2 g Maintenance

r

\

loo

F i g . 6.

I

1

10

20 Black

1 gtobe

30 temperature

40 (°C)

T o t a l s k i n h e a t l o s s at b l a c k g l o b e t e m p e r a t u r e s a n d h e a t p r o d u c t i o n at d i f f e r e n t m i l k y i e l d s .

of 9 t o 3 9 o c

174 If heat production at different daily milk yields is plotted, the black globe t e m p e r atures at which the animals may maintain t h e r m a l balance a r e obtained (Fig. 6). These t e m p e r a t u r e s e s t i m a t e the upper limit of the t h e r m a l comfort t e m p e r a t u r e zone. According to these e s t i m a t e s , each additional 10 kg FCM/day produced reduce by about 4 o c the ambient t e m p e r a t u r e at which the animal maintains t h e r m a l balance. Maximal sweating was found to r a i s e the t h e r m a l threshold by about 15oc. Maximal sweating is however g e n e r a l l y attained concomitant with an i n c r e a s e in the body t e m p e r a t u r e and in r e s p i r a t o r y frequency. In a r e c e n t study in b r o i l e r s , it was shown that growth r a t e is i n v e r s e l y r e l a t e d to the amplitude of the diurnal i n c r e a s e in body t e m p e r a t u r e (Berman, 1973). It s e e m s thus that the upper l i m i t of the t h e r m a l comfort t e m p e r a t u r e zone should probably be set at no m o r e than at 50% of the m a x i m a l activation of h e a t - d i s s i p a t i n g functions. This line, set in Fig. 6, indicates that skin evaporative heat loss widens the t h e r m a l comfort t e m p e r a t u r e zone by about 7oc. The n y e h t h e m e r a l amplitude in the ambient t e m p e r a t u r e is however wider than 7oc in most c a s e s . Plotting the n y c h t h e m e r a l cycle in ambient t e m p e r a t u r e t o gether with the upper limits of the t h e r m a l comfort t e m p e r a t u r e zones for the various l e v e l s of milk production enables the planning of the p r o p e r management p r o c e d u r e s in w a r m c l i m a t e s . This approach has the advantage that it can be applied to the n o r m a l conditions of management and that it is not influenced by the a r t i f i c i a l conditions in c l i m a t i c r o o m s .

REFERENCES BERMAN, A. (1967)

BERMAN, A. BERMAN, A. BERMAN, A. BERMAN, A. BERMAN, A. BERMAN, A.

: Diurnal and seasonal changes in bovine r e s p i r a t o r y functions in a subtropical c l i m a t e . Austr. J. a g r i c . R e s . , 18 : 849-860. (1968) : N y c h t h e r m e r a l and seasonal patterns of t h e r m o r e g u lation in cattle. Austr. J. a g r i c . R e s . , 19: 181-189. (1971) : T h e r m o r e g u l a t i o n in intensively lactating cows in n e a r - n a t u r a l conditions. J. Physiol. (Lond.), 215: 477-489. (1973) : Homoeothezmy and growth rate in the fowl. B r i t . Poultry Sci., (in p r e s s ) . and KIBLER, H.H. (1959) : The effect of clipping the coat on the t h e r m o r e g u l a t o r y reactions of d a i r y h e i f e r s . Nature (Lond.), 179: 1256. and MORAG, M. (1971) : N y c h t h e r m e r a l patterns of t h e r m o r e g u l a tion in high producing cows in a hot-dry n e a r - n a t u r a l c l i m a t e . A u s t r . J. a g r i e . R e s . , 22: 671-680. and VOLCANI, R. (1961): Seasonal and regional variations in coat c h a r a c t e r i s t i c s of dairy cattle. A u s t r . J. a g r i c . R e s . , 12:528-538. 33-37.

BLAXTER, K . L . and WAINMAN, F . W . (1961) : Environmental t e m p e r a t u r e and the energy m e t a b o l i s m and heat e m i s s i o n of s t e e r s . J. a g r i c . Sci., 56: 81-90. BLIGH, J . and HARTHOORN, A . M . (1965) : Continuous r a d i o t e l e m e t r i c r e c o r d s of the deep body t e m p e r a t u r e of s o m e African m a m m a l s under n e a r - n a t u r a l conditions. J. Physiol (Lond.), 176: 145-162.

175 BLIGH, J . , INGRAM, D . L . , KEYNES, R.D. and ROBINSON, S.G. (1965): The deep body t e m p e r a t u r e of an u n r e s t r a i n e d Welsh mountain sheep recorded by a r a d i o t e l e m e t r i c technique during a 12-month period. J. Physiol. (Lond.), 176: 136-144. BOND, J., WELDY, J . R . , McDOWELL, R . E . and WARWICK, E . J . (1961): Responses of s u m m e r - c o n d i t i o n e d heifers to 90°F. J . A n i m . Sci., 20: 966. BURTON, A . C . and EDHOLM, O.G. (1955) : Man in a Cold Environment. Arnold, London. HAMADA, T. (1971) : Estimation of the lower c r i t i c a l t e m p e r a t u r e for dry and lactating cows. J. Dairy Sei., 51: 1704-5. HOLMES, C.W. (1970) : Effects of a i r t e m p e r a t u r e on body t e m p e r a t u r e s and sensible heat loss of F r i e s i a n and J e r s e y calves at 12 and 76 days of age. Anim. P r o d . , 12: 493-501. JOHNSON, K.G. (1971) : Body t e m p e r a t u r e lability in sheep and goats during s h o r t - t e r m exposures to heat and cold. J. agric. Sci., 77 : 267-272. KALI, J . , AMIR, S. and MORAG, M. (1970) : High yielding cows in a hot d e s e r t climate. Proc. 18th Int. Dairy Congr., Sydney, Vol. 1E, p.494. KIBLER, H.H., JOHNSON, H.D., SHANKLIN, M.D. and HAHN, L.R. (1965): Acclimation of Holstein cattle to 29°C t e m p e r a t u r e : changes in heat producing and heat dissipating functions. Univ. Mo. Agr. Exp. Sta., Res. Bull. No. 893. MENDEL, V.E. and RAGHAVAN, G.V. (1964): A study of diurnal t e m p e r a t u r e patterns in sheep. J. Physiol. (Lond.), 174: 206-216. MOE, P.W.~ FLATT, W . P . and TYRELL, H . F . (1972): Net energy value of feeds for lactation. J . D a i r y Sei., 55: 945-958. MOUNT, L . E . and HOLMES, C.W. (1969) : L o n g - t e r m c a l o r i m e t r i c m e a s u r e ments in groups of growing pigs. In EAAP Publ. No. 12 : Energy Metabolism of F a r m A n i m a l s . K. L. Blaxter, J. Kielanowski and G. Thorbek (ed.), Orrel Press. SCHLEGER, A.V. and TURNER, H.G. (1965) : Sweating r a t e s of cattle in the field and t h e i r reaction to diurnal and seasonal changes. Austr. J. a g r i c . R e s . , 16 : 92-106. TAYLOR, C.R. (1970) : Dehydration and heat: effects on t e m p e r a t u r e r e g u l a tion of East African ungulates. A m e r . J. Physiol., 219 : 1136-1139. THOMPSON, H . J . , WORSTELL, D.M. and BRODY, S. (1951): Influence of environmental t e m p e r a t u r e , 0° to 105°C, on hair and skin t e m p e r a t u r e s and the partition of heat dissipation between evaporative and non-evaporative cooling in J e r s e y and Holstein cattle. Univ. Mo. Agr. Exp. Sta., Res. Bull. No 481. WELDY, J . R . , McDOWELL, R . E . , BOND, J. and VAN SOEST, P . J . (1964): Responses of winter-conditioned heifers under p r o longed heat s t r e s s . J. Dairy Sci., 47: 47-691.

176 A B S T R A C T . - T h e r m o r e g u l a t o r y r e a c t i o n s of lactating cows (33 Kg milk/day) have been m e a s u r e d in s u m m e r (Tg 25 ° - 39°C) and in winter ( T g g , 5 ° - 24°C) at 3 - h r intervals, during four 242hr c-ycles. The animals maintaine~ an almost continuous p e r i p h e r a l vasodilation throughout the e x p e r i m e n t a l period. The upper ambient t e m p e r a t u r e s at which a d a i r y cow maintains homoeothermy w e r e c a l culated for different metabolic r a t e s . At the maintenance level a dry cow may maintain homoeothermy at up to 24oc without sweating and up to 4 0 o c if sweating at 50% of the m a x i m a l sweating capacity. For a cow producing 30 kg milk/day, the r e s p e c t i v e figures w e r e 12oc and 24oc r e s p e c t i v e l y . These data indicate that the t h e r m o r e g u l a t o r y capacity of the animals in the natural climate considerably expands the t h e r m a l comfort t e m p e r a t u r e range. ZUSAMMENFASSUNG.- Die w~irmeregulatorischen Reaktionen milchgebender Ktlhe (33 kg Milch/Tag) wurden i m S o m m e r {T~ 25 ° - 39°C) und im Winter (T~9, 5 ° - 24°C) in 3-Stunden Intervallen w~ihr~nd 4 mal 24 Stunden p e r i o d i s e h ge.~nessen. Die T i e r e hatten eine beinahe kontinuierliche p e r i p h e r e G e f ~ s s e r w e i terung wiihrend der Versuchsperioden. Die obere Umgebungstemperatur, bei der Milchkllhe h o m o i o t h e r m i s e h bleiben, wurde aus v e r s c h i e d e n e n Stoffwechselraten b e r e c h n e t . Auf dem Erhaltungsniveau kaun eine trocken stehende Kuh die Homoiot h e r m i e bis 24oc ohne Schwitzen halten mad bis auf 4 0 o c bei Schwitzen bis 50% des m a x i m a l e n Schwitzverm~gens. FUr eine Kuh, die 30 kg M i l c h / T a g leistet sind die entsprechenden Werte 12oc beziehungsweise 24°C. Diese Werte zeigen, dass das W'~irmeregulationsverm~gen der T i e r e im natUrlichen Klima den W~irmek o m f o r t - T e m p e r a t u r b e r e i c h e r h e b l i c h ausdehnt. R E S U M E . - On a m e s u r ~ l'aptitude de thermor~gulation de vaches eu lactation (33 kg de lait par jour) toutes les 3 h e u r e s durant 4 cycles de 24 heures et cela aussi bien en ~t~ (Tg 25 ° ~ 39°C) qu'en h i v e r (Tg 9, 5 ° ~ 24°C). Les animaux out pr~sent~ une vasodffatation p~riph~rique a s s e z constante durant toute la p6riode des e s s a i s . On a en outre calcul6, en partant de diff~rents taux de m~tabolisme, la t e m p e r a t u r e ambiante m a x i m u m pour laquelle une vache laiti~re peut m a i n t e n i t son hom~othermie. Pour une vache s~che, ce m a x i m u m est de 24°C sans t r a n s p i r e r et de 4 0 o c si elle t r a n s p i r e le 50% de sa capaeit~ m a x i m u m . Pour une vache donnant 30 kg de lait p a r jour, ces chiffres s ' a b a i s s e n t / ~ 12oc dans le p r e m i e r eas, ~ 2 4 o c dans le second. Ces chiffres montrent que la capacit~ de thermor6gulation du b~tail bovin augmente consid~rablement l'amplitude de la zone de eonfort t h e r m i q u e en e l i m a t naturel.