Abstract:
CUTANEOUS WATER DISSIPATION IN TROPICAL MERINOS DURING HEAT TREATMENT G.I. KNIGHTS* and P.S. HOPKINS* Summary The water metabolism of shorn and unshorn tropical Merino ewes was studied during heat treatment. The dry bulb temperature (450C) and relative humidity (26%) of the climate chamber were adjusted to elicit a hyperthermic response similar to that measured in sheep exposed to the normal summer climate. Measurements of urinary, faecal and respiratory water loss, rectal temperature, and respiration rate were recorded during an 8 h period of hyperthermia while food and water were withheld. Animals were accurately weighed immediately before and after heat treatment. The non-respiratory evaporative water loss of both shorn and unshorn sheep was 4-6 ml/kg/h. The concomitant respiratory water loss was only 0.9-1.4 ml/kg/h. These data indicate the importance of cutaneous dissipation as an avenue of water loss in tropical Merinos. 1 . INTRODUCTION Programmes aimed at breeding-sheep adapted to the semi-arid conditions of tropical Australia are likely to have a desirable impact on the production performance of animals in this region. There is a pressing need to understand more about the environmental physiology of sheep exposed to heat stress. This would allow the identification of physiological indices which characterize adaptation. These indices could then be used to screen a gene pool to select adapted sheep for inclusion in breeding programmes. More than 30 years ago Lee and Robinson (1941) suggested that sweating played a role in the water loss mechanism of sheep. Since then, numerous climate chamber studies have reported a range of cutaneous water dissipation rates for this species. These studies have led to the belief that cutaneous water loss represents only a minor component of the total evaporative water loss (see Brown and Hutchinson 1973). These conclusions are based upon observations made on temperate sheep in relatively humid conditions. This paper examined the water metabolism and in particular the non-respiratory evaporative water loss of tropical Merinos under climate chamber conditions which simulated the ambient temperature and relative humidity levels of the tropical summer. II . MATERIALS AND METHODS (a) Location The experiment was conducted at the 'Toorak' Research Station near Julia Creek in N.W.. Queensland (long. 141'E; lat. 21'S). This environment offers semi-arid grazing conditions with little or no shade. Mean monthly maximum temperatures exceed 35OC for 6 months of the year. The relative humidity normally varies between 20% and 30% during the summer months. * Queensland Department of Primary Industries, 'Toorak' Research Station, Julia Creek, Queensland, 4823. I 165 (b) Animals and Procedure Six non-pregnant, locally bred, medium fine wool Peppin Merino ewes were selected at random, placed in metabolism cages in a climate chamber and offered an ad lib. ration of lucerne (Medicago sativa) pellets and water. The animadjusted to the experimental regimen during a 2 week period of thermoneutral conditions. Immediately before the sheep were subjected to heat stress three of .them were shorn. The unshorn ewes carried 10 months wool. Urinary catheters were used to drain the bladder contents before accurately weighing the sheep and subjecting them to the heat stress. The dry bulb temperature' (45'C) and the relative humidity (26%) of the climate-chamber were chosen to elicit a hyperthermic response similar to that measured in sheep exposed to the normal summer climate. These settings were predetermined from preliminary observations with other sheep. Measurements of rectal temperatures and respiratory rates were made on three occasion8 during an 8 hr period when rectal temperatures were elevated above 40 C and respiratory rates were 150-200 breaths per min. Urine and faeces were collected during this period. Evaporative losses were minimized by using plastic bags as urine receptacles. Faecal water loss was calculated from measurements of fresh and oven dried material. Face masks and associated tubing from an open circuit anaesthetic machine were used to measure respiratory water by passing the respiratory gases through two cannisters of anhydrous calcium chloride connected in series. Col.lections were made during 3 ten-min periods in the second, fourth and sixth h of exposure and the results were corrected accordingly. Animals were again accurately weighed at the conclusion of the experiment to allow calculation of evaporative and non-evaporative water loss. III. RESULTS The water output rates, rectal temperatures and respiratory rates of shorn and unshorn sheep during the 8 h period of heat treatment are shown in Table 1. These results indicate that between 76% and 83% of the decreased live weight of the three unshorn sheep (15009; 17509; 2250g) could not be attributed to the combined respiratory, urinary and faecal water loss. The corresponding values in the three shorn sheep were 77% to 80% of 1750, 2000 and 25009. These data indicate that the nonrespiratory evaporative water dissipation rate of both shorn and unshorn sheep was 4-6 ml/kg/h. The concomitant respiratory water loss was only 0.9-1.4 ml/kg/h. IV. DISCUSSION Apart from a minor contribution by untrapped gaseous losses (e.g. carbon compounds), the non-respiratory evaporative water loss calculated by difference must, by inference, represent cutaneous water dissipation. It is difficult to substantiate this finding by the quantitative measurement of cutaneous dissipation rate. The dessicated capsule method normally used for making these measurements alters the micro-environment and measures water uptake over a very small collection area. 166 TABLE 1 The rate of cutaneous water dissipation calculated in this experiment is far greater than the commonly reported 'sweating. rate' of sheep. Our collateral studies with sheep exposed to normal summer conditions (high ambient temperature, high solar radiation load, moderate wind movement and low relative humidity) support the contention that tropical Merinos have a cutaneous water dissipation rate of at least 4 ml/kg/h. The -fact that the sheep used in this experiment were both acclimated and adapted may be partly responsible for their relatively high levels of cutaneous water dissipation. A further reason may be the low relative humidity (26%) used in the current study. Previous research efforts have been undertaken in climate chambers where the relative humidity has been in excess of 40%. The fact that shorn and unshorn ewes had similar rates of cutaneous water dissipation indicates that the presence of a fleece offered little impedance to this avenue of water loss. It is not possible to ascertain the magnitude of the sweat and passive transpiration components of the total cutaneous water dissipation reported herein. Furthermore, it is not possible to determine the thermotaxic effects of this dissipation. Assuming a latent heat of evaporation of 2.4 J/ml then a cutaneous water dissipation rate of.200 ml/h may theoretically remove 4800 3 from the surface of a shorn sheep in the heat of a normal summer day. The nett loss of heat from an unshorn animal exhibiting this rate of cutaneous water dissipation while standing .in the sun may be considerably less if this evaporation occurs part of the way along the fibre. The results nevertheless serve to underline the importance of cutaneous dissipation as an avenue of water loss in tropical Merinos. In so doing they disagree with the commonly held belief that 'sheep depend mainly on panting for evaporative cooling'. A comparison of the anatomical configuration of the heat exchange areas of the head of the sheep and truly panting animals suggests that this disagreement should come as no surprise. The usefulness of this type of research finding to the animal producer will depend on the integrated nature of subsequent research efforts.- There is clearly a pressing need to extend studies of this discipline, since the current limits to adoption by industry stem from the lack of basic information on the environmental physiology of tropical sheep. V. ACKNOWLEDGEMENT Financial sup port from the Australian Wool Corporation for the servicing of this project is acknowledged. VI. REFERENCES BROWN, G.D. and HUTCHINSON, J.C.D. (1973). 'The Pastoral Industries of Australia'. (Ed. G. Alexander and O.B. Williams). 336. (Sydney University Press: Sydney). LEE, D.H.K. and ROBINSON, K.W. (1941). of Queensland. Z : 189. 53 Proceedings of the Royal Society 168