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Animal Environment Interactions Livestock live within an environment complicated by a multitude of factors encompassing both physical and psychological aspects of the animal's sur- roundings. The thermal environment has a strong influence on farm animals with air temperature having the primary effect, but altered by wind, precipi- tation, humidity, and radiation. Ideally, the impact of the thermal environ- ment can be described in terms of effective ambient temperature (EAT), which combines the various climatic events. Animals compensate within limits for variations in EAT by altering food intake, metabolism, and heat dis- sipation, which in turn alter the partition of dietary energy by the animal. The net result is an altered energetic efficiency, which can require dietary changes in nutrient-to-energy ratios. THERMAL BALANCE Homeothermic animals maintain a relatively constant core temperature by balancing the heat gained from metabolism against that gained from or given up to the environment. This heat balance is achieved through the concerted effects of physiological, morphological, and behavioral thermoregulatory mechanisms (Monteith, 1974; Robertshaw, 19744. Too rapid a rate of heat loss leads to hypothermia; too slow a loss to hyperthermia. Neither can be to- lerated for an extended time. Under most conditions there is a continual net loss of sensible heat from the body surface by conduction, convection, and radiation, and under all conditions there is a continual loss of insensible (evaporative) heat from the respiratory tract and skin surface. The net rate of
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6 FARM ANIMALS AND TlIE ENVIRONMENT heat loss depends upon the thermal demand of the surrounding environment and the resistance to heat flow of the tissue, skin, and its cover (pelage or plumage). This environmental heat demand is a function of meteorological factors and reflects the cooling power of the surroundings. (Under unusual circumstances where environmental temperature exceeds core temperature, animals may gain net heat from the environment, but then expend energy to rid themselves of heat via evaporation.) Environmental heat demand equals the rate of heat flow from an animal to a particular environment. EFFECTIVE AMBIENT TEMPERATURE Because animals are always exposed to and affected by several components of the climatic environment, there are advantages to evaluating responses of the animals to an index value representing the collective thermal impact of the animal's total environment. EAT iS one such index described in terms of environmental heat demand: the temperature of an isothermal environment without appreciable air move- ment or radiation gain that results in the same heat demand as the environ- ment in question. Several attempts have been made to formulate a means of quantifying EAT. Most have fallen short of expectations, usually because of the resourcefulness of animals in combatting thermal stress by physiological and behavioral reactions, which in turn influence the environmental heat de- mand. Specific formulas for calculating EAT for each species have not been developed, although the combined effect of selected environmental variables have been reported, e.g., wind-chill factors and the temperature-humidity index ETHIC. EAT iS, however, a useful concept when predicting the effect of the thermal environment on animals. Several factors, in addition to air temperature, in- fluence environmental heat demand. Examples that have been documented for livestock include: 1. Thermal radiation. Thermal radiation received by an animal has two primary sources: solar radiation (direct, or reflected from clouds and sur- rounding surfaces) and terrestrial or long-wave radiation (emitted from all surfaces constituting the surroundings). The net impact of thermal radiation on an animal depends on the difference between the combined solar and long-wave radiation received and the long-wave radiation emitted by the ani- mal. Shades, nearby structures and other animals, ground cover, clouds, sur- face characteristics of the animal, and insulation along with interior surfaces of housing are examples of factors influencing the net impact of thermal radi- ation. For animals in sunlight, a net gain of heat by thermal radiation usually ex
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Animal-Environment Interactions ists, resulting in an increased EAT of 3 to 5°C. In winter, the increased EAT iS beneficial; in summer, it is detrimental. 2. Humidity. The air's moisture content influences an animal's heat bal- ance, particularly in warm or hot environments where evaporative heat loss is crucial to homeothermy. The higher the ambient vapor pressure, the lower the vapor-pressure gradient from the skin or respiratory tract to the air, and hence the lower the rate of evaporation. An increase in ambient vapor pres- sure generally has less impact on the heat balance of species that depend more on panting (and less on sweating) to lose heat during heat stress. Hence, different weightings are given dry-bulb and wet-bulb air temperatures in calculating temperature-humidity indices for different species. For cattle, which sweat in response to heat stress, one index is calculated as: t(0.35) (dry-bulb temperature) + (0.65) (wet-bulb temperature)], whereas, in an index for swine, a nonsweating species, wet-bulb temperature is given less weight and temperature-humidity index is calculated as: [~0.65) (dry-bulb temperature) + (0.35) (wet-bulb temperatures. 3. Air movement. Air movement affects rate of convective and evapora- tive heat exchange. However, the magnitude of this effect is moderated somewhat by the reduction in skin temperature because vasoconstriction re- duces the animal-environmental temperature gradient. The increase in rate of heat loss or gain per unit increase in air velocity is greatest at low air veloci- ties because disruption of the boundary layer of still air surrounding the body requires relatively little air movement. Above 6 km/in, increased air velocity results in little additional increase in convective heat transfer. By means of a wind-chill index, the combined effect of ambient tempera- ture and air speed on environmental heat demand is represented by a single value. Wind-chill indices have been developed for various species in cool and cold environments. In extremely hot environments (when ambient tem- perature exceeds animal surface temperature), animals gain heat convec- tively. 4. Contact surfaces. The nature and temperature of the floor or other con- tact surfaces determines rate of conductive heat flow from an animal. A1- though this is ordinarily a small part of total heat exchange, it can be signifi- cant in some situations such as piglets on a floor with high thermal conductivity, such as concrete. An animal may respond behaviorally to change its posture and thus its orientation to specific environmental compo- nents such as area of contact with a cool or warm floor, orientation to radia- tion sources and sinks, and orientation to drafts and winds.
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8 FARM ANIMALS AND THE ENVIRONMENT 5. Precipitation. Animals are sometimes exposed to inclement weather. A combination of low temperature, wind, and rain or wet snow can adversely affect an animal's heat balance. Water accumulates in an animal's pelage, displacing still air, thereby reducing external insulation. In addition, rain may flatten the pelage, thereby reducing its depth and thus insulative value. Snow or cold rain increase conductive heat loss, and drying of the potage cools the animal by evaporative heat loss. The continued effort to improve and develop criteria for determining EAT should be a goal of continued research even though it presently has limita- tions for practical application as discussed by McDowell (1972~. Although this report occasionally includes the use of EAT as described above for discus- sion purposes, the reader is expected to use the best description of the envi- ronment available in terms of environmental heat demand. In some in- stances, that may be limited to mean daily or monthly dry-bulb temperature. THERMAL ZONES salt 2. The range of EAT over which the body temperature remains normal, sweating and panting do not occur, and heat production remains at a mini- mum. (This is sometimes referred to as the zone of minimum thermal regula- tory effort.) 3. The range that provides a sensation of maximum comfort. (This is also defined as the thermal-comfort zone.) 4. The EAT selected by an animal offered an unrestricted range in environ- ments. (This is also called the preferred thermal environment.) 5. The optimum thermal environment from the standpoint of the animal, which is the environment that promotes maximum performance and least stress (including disease) for the animal. Evaluation of the relationship between animals and their thermal environ- ment begins with the thermoneutral zone (TNZ). The concept of thermoneu- trality may have varied meanings depending on the viewpoint of the de- scriber. For farm animals, this topic was reviewed by Mount (1974), where the following definitions evolved: 1. The range of EAT* over which metabolic heat production remains ba While these definitions are not totally synonymous, they are in general agreement. The preferred definition is based upon one's interest or reason for * Mount (1974) actually used the term, "operative environmental temperature," which is de- fined similarly to our use of EAT in this report.
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Animal-Environment Interactions 9 describing the TNZ. It must be emphasized that thermal comfort for the stock- man may be different from the TNZ of the animal; therefore, selection or as- sessment of animal environments must not be based on human comfort. In this report, TNZ is defined as the range of effective ambient tempera- tures (EAT) within which the heat from normal maintenance and productive functions of the animal in nonstressful situations offsets the heat loss to the environment without requiring an increase in rate of metabolic heat produc- tion (Figure 11. Figure 2 shows expected zones of thermoneutrality for sev- eral species; however, it should be noted that shifts in the TNZ occur as a result of acclimation by the animal to cold or hot environments (e.g., for the cow, the TNZ can be shifted downward as much as 15°C through cold accli- mation during a winter season). At temperatures immediately below optimum, but still within the TNZ, there is a cool zone (Figure 1) where animals invoke mechanisms to conserve body heat. These are mainly postural adjustments, changes in hair or feathers, and vasoconstriction of peripheral blood vessels. As EAT declines within this zone, metabolic rate of the fed animal remains at the thermoneu- tral level. The effectiveness of various insulative and behavioral responses to cold stress are maximal at the lower boundary of the TNZ, a point called the lower critical temperature (LCT). Below this point is the cold zone (Figure 1) where the animal must increase its rate of metabolic heat production to maintain ho- meothermy. Increases in metabolic heat production parallel increased envi Lower Critical Temperature Cold Stress Cool 1 TH E RMON EUTRA ZON E i ' Optimum for Performance and I Warm Health (Jpper Critical Temperature Heat Stress _ .1 1 ' '- - -'' 1 1 Low E FF ECTI VE AM Bl ENT TEMPERATU R E High FIGURE 1. Schematic representation showing relationship of thermal zones and temperatures.
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10 FARM ANIMALS AND THE ENVIRONMENT Chick 0 1° LHen 10° | _ Piglet _ 1° L sow 15V ~ L Calf 1 2u 1 r cow 16 1 Newborn Lamb 01° _5 0 5 10 15 20 25 30 35 EFFECTIVE AMBIENT TEMPERATURE (°C) FIGURE 2. Estimated range in thermoneutral temperature for newborn and mature animals of different species (adapted from Bianca, 1970). ronmental heat demand in this zone for animals capable of maintaining con- stant body temperature. In general, initial responses of animals to cold stress rely more on increas- ing metabolic heat production, but long-term exposure to cold gradually results in adaptive responses through physiological and morphological change. Increased insulation, for example, is an added barrier to heat flow in animals and influences the rate at which sensible heat is exchanged with the environment. Insulation includes tissue insulation (fat, skin), external insula- tion (hair coat, wool, feathers), and insulative value of the air surrounding the animal. These insulative barriers are additive and are a major factor in es- tablishing LCT and rate of heat loss below LCT. Of course, as an animal's in- sulation changes, so do the limits of its thermal zones described in Figures 1 and 2. Lower critical temperature can be predicted from the thermoneutral heat production end thermal insulation (Blaxter, 1962; Monteith, 1974; Webster et al., 1970~. A summary of estimated LCT'S for typical classes of livestock is found in Table 1. These values should be considered only as indicators of cold-susceptibility as, in practice, the actual LCT may vary considerably de- pending upon specific housing and pen conditions, age, breed type, lacta- tional state, nutrition, time after feeding, history of thermal acclimation, hair or wool coat, and behavior; estimated effects for some of these are shown in Table 1. For example, a group of pigs has an LCT several degrees less than a single pig (Close et al., 1971), because huddling behavior of the pig in a
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Animal-Environment Interactions 11 TABLE 1 Estimates of Lower Critical Temperatures for Sheep, Cattle, Swine, and Poultry Lower Cubical Species . Temperature (°C) Souse . . . . Sheep . Shorn, maintenance feeding `. 25 Shorn, full feed : 13 5 mm-fleece, maintenance . 25 5 mm fleece, fasting 31 5 mm fleece, full feed 18 1 mm fleece, maintenance 28 10 m.m fleece, maintenance 22 50 mm fleece, maintenance 100 mm fleece, maintenance -3 Ames, 1969 Brink and Ames, 1975 Blaxter, lg67^ Blaster, 1967 Blaxter, 1967.',' Blaxter, 19~' BIaxter, 1967'' Blaxter, 1967 Blaster, 1967 Ca'tt~:-. 8 mm hair, -fasting 18 . Blaxter, 1967 8 Rim hair, maintenance 7 Bleater', 1967 8 inm~ hair full feed.-' - 1 ~Blaster' 1967 Newborn.~ves . 9' Webster, 1974 One-month-old calves; 0 Webster, 1974 Fat stock, 0.8 kg gain/day -Ad.` Webster, 1974 Fat stock, 1.5 kg gain/day -36~-! Webster, 1974 Beef cow, maintenance - 21 Webster, 1974 Dairy cow, dry and pregnant -14 Webster, 1974 Dairy cow, 2 gal/day - 24 Webster, 1974 Dairy cow, 8 gal/day -40 Webster, 1974 Swine 45 kg 23.3 Heitman et al., 1958 100 kg 20.2 Heitman et al., 1958 25-50 kg, fasting 25 Close and Mount, 1975 2 kg, maintenance (single weight) 31 Holmes and Close, 1977 2 kg, maintenance (group of pigs) 27 Holmes and Close, 1977 20 kg maintenance 26 Holmes and Close, 1977 6.0 kg maintenance 24 Holmes and Close, 1977 100 kg, maintenance 23 Holmes and Close, 1977 2 kg, 3X maintenance 29 Holmes and Close, 1977 20 kg, 3X maintenance 17 Holmes and Close, 1977 60 kg, 3X maintenance 16 Holmes and Close, 1977 100 kg, 3X maintenance 14 Holmes and Close, 1977 Poultry Chick 34 Richards, 1971; Five-week-old chick 32 Sturkie, 1965' i Adult 18 St~irkie, 1965 -'
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12 FARM ANIMALS AND THE ENVIRONMENT cold environment reduces the exposed surface and thus heat loss to the envi ronment. The predicted LCT for large ruminants on high feeding levels are consi- derably lower than for poultry, swine, and young animals. The extremely low values for the feedlot animal and dairy cow result from the large amounts of heat produced as an inevitable consequence of digestion and me- tabolism at high levels of production, from the small surface area to mass ra- tio of these relatively large animals, and from their large amount of insula- tive tissue. In contrast, the pig has a poorly developed hair coat and utilizes dietary energy more efficiently, thus producing less metabolic heat; hence it has a higher LCT. Measures of LCT have proven to be quite useful in determining nutrient re- quirements, in establishing design criteria for housing, and in guiding practi- cal husbandry decisions, particularly for cold-susceptible animals such as swine, sheep, and calves. However, the importance of LCT to cold- acclimated feedlot and dairy cattle is less direct. These animals have pre- dicted LCT'S that rarely occur in agricultural regions; for them, it appears that the influences of the thermal environment are largely through seasonal accli- mation and metabolic and digestive adjustments to the environment. As EAT rises above optimum, the animal is in the warm zone (Figure 1) where thermoregulatory reactions are limited. Decreasing tissue insulation by vasodilation and increasing effective surface area by changing posture are major mechanisms used to facilitate rate of heat loss. When EAT exceeds the upper critical temperature (UCT), animals must employ evaporative heat loss mechanisms such as sweating and panting. The animal is then considered heat stressed. In a hot environment, animals are faced with dissipating metabolic heat in a situation where there is a reduced thermal gradient between the body core and the environment, resulting in a reduced capacity for sensible heat loss. The immediate response of animals to heat stress is reduced feed intake, to attempt bringing metabolic heat production in line with heat dissipation capa- bilities. The higher producing animals with greater metabolic heat (from product synthesis) tend to be more susceptible to heat stress. This is different from cold conditions where high-producing animals with their higher meta- bolic heat production are in a more advantageous position than low or non- producing animals. In hot conditions, there may also be avenues of heat gain from the environment, such as direct or indirect solar radiation, long-wave radiation, conduction, and convection. (Gains from the latter three occur only if the temperature of the surroundings or air temperature is higher than animal surface temperature.) Evaporation of moisture from the skin surface or respiratory tract is the primary mechanism used by animals to lose excess body heat in a hot environment: this mechanism is limited by air vapor pres- sure but enhanced by air movement.
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