Cardiovascular Parameters in Shorn Sheep Prior to and During Their Exposure to a Cold, Wet Environment

Abstract

When shorn sheep were exposed to ambient temperatures of 30C and the residual fleece was wetted, heart rate and cardiac output increased substantially above levels measured during a control period at 250C. With continued cold exposure and wetting the sheep become hypothermic and heart rate and cardiac output decreased. In most sheep at rectal temperature <32°C cardiac output had fallen below control values while heart rate remained above control. In a minority of hypothermic sheep cardiac output remained above control rates.Our interest in physiological events that occur during cold-stress in sheep originated from deaths of sheep from this cause after shearing in Australia. Hutchinson [1968] reviewed losses suffered by newly-shorn sheep that are exposed to cold, wet conditions and he concluded that approximately one million sheep died annually from cold exposure.Few data on cardiac output during cold exposure and hypothermia have been collected in conscious animals. The majority of results are confounded by anaesthesia of the subjects [Thauer, 1965] and the rates at which anaesthetized subjects were cooled [Brendel et al., 1958]. Data of other indices of cardiac physiology in cold-stressed sheep are also fragmentary. Hutchinson et al. [1960] reported elevated heart rates in sheep exposed to a cold, windy environment that reduced their rectal temperature (Tre) to approximately 37*4°C. In many other studies on newly-shorn sheep, cold exposure has been transient and measurements of body temperature and heart rate have not been concurrent [Slee, 1966; Slee and Sykes, 1967;Sykes and Slee, 1969].We have, therefore, studied cardiovascular parameters in newly-shorn sheep held at a control temperature of 250C and repeated the measurements when the sheep were exposed to a cold (3°C), wet environment. Endocrinological aspects of these experiments are reported elsewhere [Panaretto and Vickery, 1970]. METHODSThe sheep and their pre-experimental treatment. Six adult Merino ewes and one wether (no. 4) each weighing about 30 kg were used; they were accustomed to the presence of the observers. Each animal was given 400 g daily of a mixture of equal parts of lucerne chaff and oats which was sufficient to maintain body weight during the 4-8 weeks prior to the experiments. The same quantity of feed was given during cold exposure.The sheep were shorn and a femoral arterial catheter (clear vinyl, i.d. 1*5 mm, 101

The Oxygen Consumption, Fatty Acid and Glycerol Uptake of the Liver in Fed and Fasted Sheep During Cold Exposure

“…The increased heart rate, cardiac output and blood pressure in sheep during exposure to cold confirms previous observations on sheep exposed to 3°C, shorn and with the residual fleece wetted [Panaretto and Vickery, 1971]. In these published experiments, stroke volume changed little whereas in the present experiments and similar experiments with young steers cold exposure reduced the stroke volume.…”

Shearing at the end of summer affects body temperature of free-living Angora goats (Capra aegagrus) more than does shearing at the end of winter

“…Similar results were found for sheep in cold weather, in that daily movement patterns remained unaltered or even decreased after shearing (Webster and Lynch, 1966). These results are likely to correspond to the depression in food consumption reported for newly shorn sheep exposed to extreme cold (Webster and Lynch, 1966;Panaretto and Vickery, 1971;Donnelly et al, 1974), which is proposed to be a consequence of acute cold stress (Sykes and Slee, 1969b). The reduction in food intake coupled with the increased metabolic Hetem, de Witt, Fick, Fuller, Kerley, Maloney, Meyer and Mitchell costs of thermoregulation may place the animals into negative energy balance, putting Angora goats at a greater risk of lethal hypothermia (McGregor and Butler, 2008b).…”

“…These nighttime effects hold even, as we have shown, under relatively mild summer conditions. Like our goats, sheep have difficulty with heat conservation after shearing, as evident in the decrease in rectal temperature when shorn sheep are exposed to cold environments (Slee, 1966;Webster and Lynch, 1966;Phillips and Raghavan, 1970;Panaretto and Vickery, 1971;Bennett, 1972;Hofman and Riegle, 1977).…”

“…The routes for water influx available were only pre-formed water in forage or metabolic water, since we prevented access to drinking water. Shearing increases the food intake (Wheeler et al, 1962;Webster and Lynch, 1966;Hutchinson and McRae, 1969;Avondo et al, 2000) and metabolic rates (Hutchinson and Bennett, 1962;Panaretto and Vickery, 1971;Bennett, 1972;Hofman and Riegle, 1977;Symonds et al, 1986) of sheep, and goats (Holmes and Moore, 1981;Holmes and Clark, 1989), particularly when exposed to cool environmental conditions. Similarly, cold exposure increases the metabolic rate of Angora goats (Wentzel et al, 1979;Fourie, 1984), as indicated by an increased heart rate and blood glucose concentration.…”