Thermoregulation of horses in cold, winter weather: A review

LIVESTOCK PRODUCTION SCIENCE LivestockProductionScience40 (1994) 65-71

ELSEVIER

Thermoregulation of horses in cold, winter weather: a review Nadia F. Cymbaluk Department of Animal and Poultry Science Universityof Saskatchewan, Saskatoon, Sask., Canada

Accepted27 April 1994

Abstract Horses respond to chronic cold exposure by acclimatization or habituation. Acclimatized, adult horses have a lower critical temperature (LCT) of - 15°C. Yearling horses have a LCT of - 11°C if fed ad lib or 0°C if limit fed energy for moderate growth. Newborns have a LCT around 20°C. Horses, unlike cattle, do not respond to chronic cold exposure by increasing thyroid hormone secretion. Thyroid hormone secretion responds more to a lengthening photoperiod. Yet, in newborns, thyroid hormones are likely thermogenic. Metabolic rates increase by 70% above resting in severely cold stressed horses. Maintenance digestible energy (DE) intakes should be increased 2.5% per Celsius degree decrease in effective ambient temperature for adult horses and by 1.3% per degree decrease for growing horses fed for moderate gain at temperatures below LCT. Keywords: Horse;Cold;Thermoregulation;Critical themperature; Acclimatization

1. Mechanisms of thermoregulation Animals react to chronic cold in three ways - acclimation, acclimatization or habituation- but not all features of these responses have been quantified in horses. Acclimation is the adaptation of animals in response to experimental change of a specific climatic factor such as ambient temperature (Bligh and Johnson, 1972). Acclimation chiefly measures constant, non-fluctuating temperatures in climatic chambers. Responses to cold acclimation are often considered classical responses to winter. Yet this is not an appropriate model for "winter" because acclimation does not account for the effects that interactions between all five climatic variables and photoperiod have on productivity and metabolism (McArthur, 1987). Acclimatization is the adaptive change in animals due to changes in natural climates (Bligh and Johnson, 1972) and is a valid comparison of winter cold thermoregulation. In horses and cattle, DE intake critically affects productivity and 0301-6226/94/$07.00 © 1994Elsevier ScienceB.V. All rights reserved SSD10301-6226 ( 94 ) 00040-E

the metabolic responses observed during acclimatization (Christopherson et al., 1979; Cymbaluk and Christison, 1989a; Cymbaluk, 1990; Miaron and Christopherson, 1992). Habituation, also known as voluntary hypothermia, occurs as an adaptive response to severe, intermittent cold and produces a lowered metabolic response to cold and typically is accompanied by a 0.5 to I°C reduction in body core temperature (Kuhnen and Jessen, 1990). Management systems in which horses are kept indoors in moderately cold, unheated barns for the majority of the winter day and then turned outdoors for exercise for 5-6 h produced a habituation type response in yearling horses (Cymbaluk and Christison, 1993). Husbandry practices of horses are unlike those for other domestic livestock. For example, exercise and restriction of DE intake to meet specific growth criteria may be needed to minimize disorders such as osteochondrosis and flexural limb deformities (Savage et al., 1992; Bruin et al. 1992). Hence, management meth-

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ods such as continuous housing or increasing energy intakes used to minimize cold effects in other growing animals (NRC, 1981 ) may be inapplicable for the husbandry of some classes of horses.

2. Microclimates and lower critical temperatures Equids are very adaptable to temperature variation, and thrive in tropical to subarctic climates. The five main climatic variables that affect an animal's microclimate are ambient temperature, global solar radiation, relative humidity, precipitation, and wind velocity (NRC, 1981 ). Ambient temperature has the main sole effect on insensible and sensible heat exchange but its effect is modified by the other climatic variables (Blaxter, 1989). Thus, the collective effect of the climatic factors has a different effect on productivity than each factor alone and is called the effective ambient temperature (NRC, 1981). No suitable five-factor index has been defined for animals but a two-variable index that is highly correlated to productivity of animals in winter is windchill (NRC, 1981). Yet, wind-chill was not better than ambient temperature alone in predicting DE intakes or ADG by yearling horses fed ad lib and sheltered by a 20%-porosity fence (Cymbaluk and Christison, 1989a). Relative humidity had no effect on productivity of horses during cold weather in the Canadian prairies but global solar radiation and precipitation

contributed 10 and 1%, respectively, to the equation which predicted DE intake by overwintered young horses (Cymbaluk and Christison, 1988). Precipitation that wets the skin increases evaporative cooling (Curtis, 1983). The extent of cooling can be inferred from data that showed when pony foals were wet with amniotic fluid, metabolic rates exceeded 200 W m - 2 compared to 110-130 W m - 2 when foals had dried (Ousey et al., 1991). By contrast, dry, granular snowfall in severe cold ( < - 1 0 ° C ) was non-wetting to mature ponies with dense winter haircoats (Speed, 1960). Horses are homeotherms and maintain a nearly constant body core temperature. The thermo-neutral zone (TNZ) is the temperature range when metabolic heat production (Hp) does not need to be increased to maintain internal thermostability (Curtis, 1983). The LCT is the lower limit of the TNZ and is the temperature below which metabolic Hp is increased to maintain body core temperature whereas the upper critical temperature (UCT) is the upper end of the TNZ and is the temperature at which evaporative heat loss must be increased to lower body temperature (Curtis, 1983). UCT is often disregarded in winter or cold conditions but this is the heat stress point for cold-acclimatized horses brought into heated barns or veterinary clinics (McBride et al., 1985). LCT is dynamic and in horses shifts gradually over 10 to 11 days of cold exposure (Senft and Rittenhouse, 1985). However, the TNZ and LCT for horses also

Table 1 Lower critical temperatures for horses Age

Lower critical temperature, °C Average Range

Upper critical temperature, °C

DE intake

Exposure t y p e

Reference

36

Suckle

Acute cold

Ousey et al., 1992

~ 40

Suckle

Acute cold

Ousey et al., 1992

Limited to DE needs at thermoneutrality Ad lib.

Acclimatized

Cymbaluk,1990

Acclimatized

Restricted

Acclimatized/ acute cold

Cymbaluk and Christison, 1989 McBrideet al., 1985; Youngand Coote, 1973

24 days 7-9 days Yearling

22

16-26

19

13-23.5

0

Unknown

Unknown

Yearling

- 11

Unknown

Unknown

Mature

- 15

- 20 to - 9.4

10

N.F. Cymbaluk/ Livestock Production Science 40 (1994) 65-71

vary with age, body condition, breed, season, climate, and importantly, dietary DE intake (Table 1 ). Neonatal pony foals (2-4 days) had an average LCT of 22. I°C but by 7-9 days of age the LCT had decreased to an average of 18.6°C (Ousey et al. 1992). By comparison, maintenance-fed adult horses had a LCT of - 1 5 ° C (McBride et al., 1985). In fattened ruminants, a high DE intake increases metabolic Hp and thereby lowers the LCT (Webster, 1970). The same effect can be inferred for horses. Ad lib-fed, acclimatized yearling horses were estimated to have a LCT of - 1 I°C (Cymbaluk and Christison, 1989a) but those limit-fed for "normal" growth had an estimated LCT of0°C (Cymbaluk, 1990).

3. Thermoregulation in cold weather Thermoregulation is achieved by changes in physiology, morphology and behavior (NRC, 1981; Curtis, 1983). Morphological regulation is poorly documented in horses but Speed (1960) suggested that Exmoor ponies resisted cold through morphologic change. Cold weather was concluded to cause inferior skeletal development in yearling horses (Rooney, 1984), but controlled trials with growing horses show that growth rates other than weight gain were not affected by cold (Cymbaluk, 1990; Cymbaluk and Christison, 1993). 3.1. B e h a v i o u r

Shelter-seeking and huddling are an early reaction to acute cold. Foraging and movement by Camargue horses decreased 20 min per Celsius degree temperature decrease, and could conserve 17% of daily energy expenditure (Duncan, 1985). Rainfall also reduced standing, resting and lying by horses (Duncan, 1985). Commonly, horses exposed to cold winds stand with their heads away from the wind, their tails set low and into the wind (Speed, 1960). Stimulation of areas innervated by the trigeminal nerve in humans suppresses shivering (Mekjavic and Eiken, 1985). Thus, postural position during cold weather may control shivering. Birth into a cold extrauterine environment or acute cold exposure in climatic chambers cause foals and mature horses to shiver (McBride et al., 1985; Ousey et al., 1991). In cold-acclimatized yearlings, increased play activity occurred during cold weather

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(Cymbaluk and Christison, 1988). Moderate muscular activity uses 30-40 1 oxygen min- ~, generates 628 kJ heat min - ~ (Carlson, 1982) and increases muscle temperature. Thus, horses exercised at temperatures as low as - 25°C had muscle temperatures of 40°C (Dahl et al., 1986). 3.2. Insulation

Total insulation in an animal is represented by tissue, haircoat and air insulation (Blaxter, 1989). Insulation is thus a property of muscle, fat, skin and haircoat and the physiological responses of these tissues including piloerection and vasoconstriction ( B laxter, 1989). Piloerection and vasoconstriction are acute mechanisms which reduce heat loss during cold weather whereas increased hair density is a chronic response to sustained cold exposure (Curtis, 1983; Blaxter, 1989). Piloerection during cold can raise effective haircoat depth by 16-32% (Young and Coote, 1973) or by a depth of 0.4 to 1.4 cm in newborn, pony foals (Ousey et al., 1992). Cutaneous vasoconstriction by horses exposed to acute or chronic cold can reduce distal limb temperature to 1.7°C yet maintain trunk temperatures at 12.8 to 15.6°C (Palmer, 1983). Chronic cold produced a dorso-distal gradient in peripheral skin temperature (Palmer, 1983; Mogg and Pollitt, 1992; Cymbaluk and Christison, 1993) whereas ponies exposed to fluctuating, chronic cold displayed unstable peripheral skin temperatures as a result of periodic cutaneous vasomotor activity (Mogg and Pollitt, 1992). At thermoneutrality, regional asymmetry in vasomotor activity allowed selective increases in surface temperatures at independent limb sites (Mogg and Pollitt, 1992). Vasoconstriction is an important mechanism in shunting blood through the hoof and this is facilitated by the dense population of arteriovenous anastamoses (500 cm -2) in the hoof lamina (Pollitt and Molyneaux, 1990). These authors speculated that the arteriovenous anastomoses, which display periodic vaso-dilation and vasoconstriction at cold ambient temperatures, were an adaptation to combat tissue damage in horses standing in snow and ice during winter. Haircoat density increases during winter (Young and Coote, 1973) and depends not only on ambient temperature and photoperiod (Kooistra and Ginther, 1975; Cymbaluk 1990) but also on horse breed (Cym-

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baluk and Christison, 1993). Adult horses acclimatized to temperatures of - 2.9°C showed no changes in body weight or haircoat density during winter (McBride et al., 1985). No changes are expected in haircoat density in mid-winter because decreasing photoperiod not ambient temperature is the main impetus to haircoat growth in horses (Kooistra and Ginther, 1975). Daylength decreases by August in Western Canada (Hare and Thomas, 1979) and maximal haircoat density occurs by December (Cymbaluk, 1990; Cymbaluk and Christison, 1993). Cold-acclimatized yearlings had heavier (30-45%) haircoat densities in the spring and shed their haircoats 3-4 wks later than warm-housed horses (Cymbaluk, 1990; Cymbaluk and Christison, 1993). 3.3. Vital signs

Rectal temperatures are constant, heart rates elevated, but respiratory rates lowered in cold-exposed neonates (Ousey et al., 1992). Heart rates were positively correlated to metabolic rate in neonates (Ousey et al. 1992). Unlike neonates, cold-habituated yearling horses showed a 27% reduction in respiratory rates and a 0.4°C decrease in rectal temperatures but no change in heart rate (Cymbaluk and Christison, 1993). 3.4. Metabolic rates

Neonates have high metabolic rates relative to adult horses but this is influenced by posture, environmental conditions, physical activity and breed (Ousey et al., 1991 ). Thoroughbred foals had 40% higher metabolic rates than pony foals at similar ages (Ousey et al., 1991). Metabolic rates of cold-exposed, 2-4 day-old foals were lower (73 W m -z) than for 7-9 day old foals (85 W m -2) (Ousey et al., 1992) although in an earlier study, newborn pony and Thoroughbred foals had higher metabolic rates than 1-2 week-old foals (Ousey et al., 1991). Metabolic rates doubled in acutely cold-exposed, pony foals (Ousey et al., 1992). Foals housed at temperatures of 20°C but wet with amniotic fluid had metabolic rates above 200 W m - 2 which declined to 110-130 W m -z as the foal dried (Ousey et al., 1991 ). Serum triiodothyronine (T 3) and thyroxine (T4) concentrations in neonates are 10 to 25fold greater than adult values and are likely thermogenic (Irvine, 1984). Brown adipose tissue and asso-

ciated non-shivering thermogenesis have not been observed in foals (Rossdale et al., 1985; Ousey et al., 1992). Resting and standing metabolic rates are similar in mature horses at thermoneutrality (Brody, 1964). Adult horses fed at 20% above maintenance metabolizable energy (ME) requirements had minimal (2%) increases in Hp (Martin-Rosset and Vermorel, 1991 ). These authors suggested that maintenance ME requirements vary with season. In summer (mid-June to midSeptember) at average temperatures of 19.4°C, the average ME requirements were 137_ 10.6 kcal/W °75 which were 9% greater than ME requirements of 127+ 10.8 kcal/W °75 for winter (mid-February to mid-May) at average temperatures of 7.5°C (MartinRosset and Vermorel 1991). Interestingly, individual variation was 8% which dilutes the relevance of the seasonal difference. No correlation was made between ambient temperature and maintenance ME requirements in the study. Maintenance energy needs in deer also decline in winter and are directly associated to lowered thyroid hormone secretion and feed intakes (Renecker and Hudson, 1986; Loudon et al., 1989). Horses, like wild ruminants, exhibit lower thyroid hormone concentrations during seasons with short photoperiods (see below; Johnson, 1986; Maenpaa et al. 1988). In the absence of acute cold stress, metabolic rates did not change during the winter months in acclimatized horses fed at maintenance DE intakes (McBride et ai., 1985 ). This was also reported in beef cattle whose fasting Hp decreased from summer to fall but were unchanged from fall through spring (Birkelo et al., 1991 ). Mature horses stressed by acute, severe cold in a metabolic chamber increased hourly metabolic rates from resting values of 437 kJ 100 kg B W - ~at temperatures above - 15°C to 759 kJ 100 kg B W - ~at - 40°C (McBride et al., 1985). 3.5. Nutrients and substrates

Dietary type determines the metabolic substrates available for energy utilization during cold stress. Respiratory quotients (RQ) were slightly negatively correlated to ambient temperatures in 2-4 day old neonates exposed to cold temperatures in a climatic chamber (Ousey et al., 1992). At thermoneutral exposures, RQ was about 0.7 and indicated fat was the principal metab-

N.F. Cymbaluk/ Livestock Production Science40 (1994) 65-71

olite (Ousey et al., 1992). As cold intensity was increased, the RQ increased to values near 0.9 suggesting carbohydrate metabolism which the authors attributed to elevated muscle glycogen use associated with shivering (Ousey et al., 1992). One foal fed a milk substitute and kept at thermoneutral conditions showed an increase in RQ and Hp in association with feeding (Vermorel et al. 1987). However, more experimental data using more foals is needed to confirm this effect. Substrate use can also be inferred from values of circulating metabolites. Limit-fed foals housed at - 5 ° C had higher serum calcium concentrations and alkaline phosphatase activities but lower serum glucose and phosphorus concentrations than those kept at 10°C (Cymbaluk and Christison, 1989b). Serum free fatty acid concentrations were not affected by cold ambient temperatures in yearling horses but were higher in animals fed high concentrate diets than those fed high forage diets (Cymbaluk, unpublished). Further study is needed to clarify the interactions of substrate and diet to cold thermoregulation in horses. Cold temperatures are estimated to reduce diet digestibility by 0.2% per degree below 20°C in cattle (NRC, 1981). However, when feed intake is controlled, the negative linear effect of cold exposure on digestibility was negated (Miaron and Christopherson, 1992). In mature horses, seasonal influences and a 20% increase in ME intake did not alter organic matter digestibility (Martin-Rosset and Vermorel, 1991). Similarly, in yearlings fed 65% grain or 65% alfalfa hay diets, neither cold exposure nor season affected dietary energy or protein utilization (Cymbaluk and Christison, 1993). In previous studies, cold-exposed Standardbred yearlings, fed a mixed grain - hay diet and limit fed to allow "normal" growth at thermoneutrality, showed a 9-26% reduction in phosphorus utilization and an 18-47% increase in neutral-detergent fibre utilization (Cymbaluk, 1990). Retention time also declined 13% between 8 and 12 mo of age but was unaffected by ambient temperature. 3.6. Hormonal effects o f cold

Thyroxine secretion increased with sudden cold exposure of adult horses, but this elevation was transient (Irvine, 1967). Mature and yearling horses adapted to cold do not invariably show elevations in

69

thyroid hormone concentrations (Irvine, 1967; McBride et al., 1985; Johnson, 1986). Increases in thyroid hormone secretion appear to depend on increasing photoperiod and thus, higher thryoid hormone concentrations are found in spring than in fall (Katovich et al., 1974; Johnson, 1986; Maenpaa et al., 1988). Moreover, diet did not affect baseline thyroid hormone concentrations. Yearling horses fed 65% grain or 65% alfalfa hay diets and kept at cold or at thermoneutral barn temperatures showed no change in T3 or T4 from autumn to spring (Cymbaluk and Christison, 1993). Catecholamine responses to cold are not documented in horses. 3. 7. Productivity

Yearlings acclimatized to cold ( < - 5 ° C ) but fed DE intakes designed to produce moderate growth rates at thermoneutrality (NRC, 1989) gained 23% less weight than horses housed at 10°C (Cymbaluk, 1990). Yet, draft-type foals fed to meet energy needs for moderate growth and kept at average winter temperatures of 0°C had nearly identical ADG (0.75 kg) to those kept at 18°C (Cymbaluk and Christison, 1993). Thus, yearling horses fed at energy intakes to meet NRC (1989) specifications for moderate growth can be estimated to have a LCT between - 5 and 0°C. Feed to gain ratios by ad lib-fed foals were unaffected at temperatures as low as - 11°C but ADG declined at temperatures below - 1 1 ° C suggesting redistribution of energy use from tissue accretion to maintenance (Cymbaluk and Christison, 1989a). Productivity and DE intake by foals was biphasic from fall to spring (Cymbaluk and Christison, 1988). ADG and DE intake responded to changes in ambient temperature from autumn to mid-winter but were not correlated thereafter despite severely cold ambient temperatures. This was attributed to general acclimatization produced by tissue insulation and seasonal adaptation (Cymbaluk and Christison, 1988). 4. Feeding and management of horses in cold weather 4.1. Feeding

Energy is the main nutrient needed by horses overwintered outdoors (Cymbaluk, 1990). Daily mainte-

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nance energy needs in mature, 500-kg horses overwintered outdoors were estimated to increase by 1534 kJ ME per degree Celsius decrease in effective ambient temperature below LCT (McBride et al., 1985). Using the DE system and a ratio of 0.90 between DE and ME (NRC, 1989), maintenance DE intake by adult horses must be increased by about 2.5% per degree below LCT. Likewise, the maintenance DE component for growing horses fed for moderate growth was estimated to increase 1.3% per degree below LCT (Cymbaluk, 1990). The LCT in these definitions are - 15°C for adult horses (McBride et al., 1985) and 0°C for growing horses (Cymbaluk, 1990). It is still unclear what dietary compositions are energetically advantageous to horses exposed to cold weather conditions. Heat increment which is associated with certain feeds in ruminants can contribute to maintenance of body core temperature (Blaxter, 1989). If heat production at maintenance can be taken to represent the sum of net energy and heat increment (Curtis, 1983), then minimal difference occurred in heat increment of horses fed either poor-quality hay and haygrain diets (Vermorel et al., 1991 ). Heat increment as a compc,nent of ME is poorly described for the wide range of feeds used for horses. Until heat increment data is collected experimentally rather than by inference, no specific recommendations as to feed type can be confidently stated. 4.2. Shelter

An unprotected, equine mechanical model was reported to have a 26%, 18% and 9% greater climatic energy demand, under British conditions, than a sheltered-rugged, exposed-rugged, and sheltered-unrugged model, respectively (MacCormack and Bruce, 1991 ). Unfortunately, mechanical models neither adapt nor grow hair. However, the mechanical model mathematically predicts a 20% improvement in heat conservation in horses if shelter is provided (MacCormack and Bruce, 1991 ). In the Canadian prairies, protracted cold exposure below the LCT of mature horses are uncommon. Three-sided sheds are suitable protection from cold winds and snow (Kidd et al., 1982). Adequate area for lying comfortably (7.5-9 m 2 per horse) and adequate bedding, preferably straw, should be provided (Agricultural Animal Care Guide, 1988). Heated housing may be desired for competitive or performance

horses. Heating barns under temperature conditions of Canadian winters necessitates a large external energy demand. To heat an uninsulated barn to 10°C when outdoor temperatures are between - 15°C to - 30°C may require heat input of 557 to 1230 watts for each horse (Kidd et al., 1982).

Acknowledgments The Saskatchewan Agricultural Development Fund is acknowledged for the continued support of the thermoregulatory studies conducted by the author. The critical review of the manuscript by Dr. G.I. Christison was appreciated.

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