fermentation in the digestive tract. Animals dissipate heat by evaporation, radiation, convection, and conduction. Both heat production and dissipation are regulated to maintain a nearly constant body temperature. Within the zone of thermoneutrality, HE is essentially independent of temperature and is determined by feed intake and the efficiency of use; body temperature control is primarily via regulation of heat dissipation. When effective ambient temperature increases above the zone of thermoneutrality—that is, higher than the upper critical temperature (UCT)—productivity decreases, primarily as a result of reduced feed intake. In addition, elevated body temperature results in increased tissue metabolic rate and increased “work” of dissipating heat (for example, increased respiration and heart rates); consequently, energy requirements for maintenance increase. Conversely, when effective ambient temperature decreases below the zone of thermoneutrality—that is, below the lower critical temperature (LCT)—HE produced from “normal” tissue metabolism and fermentation is inadequate to maintain body temperature. As a result, animal metabolism must increase to provide adequate heat to maintain body temperature. Consequently energy requirements for maintenance increase. Both UCT and LCT vary with the rate of heat production in thermoneutral conditions and the animals ability to dissipate or conserve heat. As noted in other sections of this report, heat production of animals in thermoneutral conditions may differ substantially as functions of feed intake, physiological state, genotype, sex, and activity.

The word acclimatization is used to describe adaptive changes in response to changes in the climatic conditions and include behavioral as well as physiological changes. Behavioral modification includes using variation in terrain or other topographical features such as windbreaks, huddling in groups, or changing posture to minimize heat loss in cold and during decreased activity, seeking shade to decrease exposure to radiant heat, seeking a hill to increase exposure to wind, or wading in water to increase heat dissipation in high temperatures. Physiological adaptations include changes in basal metabolism, respiration rate, distribution of blood flow to skin and lungs, feed and water consumption, rate of passage of feed through the digestive tract, hair coat, and body composition. Physiological changes usually associated with acute temperature changes include shivering and sweating as well as acute changes in feed and water consumption, respiration rate, heart rate, and activity. It should also be noted that animals differ greatly in their behavioral responses and in their ability to physiologically adapt to the thermal environment. Genotype differences are particularly evident in this regard.

Recognizing the importance of adaptation, the National Research Council committee (1981b), relying primarily on the results of Young (1975a,b), concluded that required NEm of cattle adapted to the thermal environment is related to the previous ambient (air) temperature (Tp, °C) in the following manner:

This equation indicates that the NEm requirement of cattle changes by 0.0007 Mcal/BW0.75 for each degree that previous ambient temperature differed from 20° C. It should be noted that these corrections for previous temperature are largely opposite the photoperiod effect discussed previously.

Heat or cold stress occur when effective ambient temperature is higher than UCT or less than LCT. UCT and LCT are functions of how much heat the animal produces and how much heat is lost to the environment. HE of the animal may be calculated as shown previously:

where ME is ME intake and RE is retained energy, which may include NEg, NEl, NEy, etc. (all expressed relative to BW0.75).

Cold Stress Both environmental and animal factors contribute to differences in heat loss from the animal. Environmental factors include air movement, precipitation, humidity, contact surfaces, and thermal radiation. Although results are not totally satisfactory, numerous efforts have been made to integrate these effects with animal responses.

Factors contributing to differences in animal heat loss from conduction, convection, and radiation are surface area (SA), which includes surface or external insulation (EI), and internal or tissue insulation (TI). Evaporative losses are affected by respiration volume as well as SA, EI, and TI. Respiratory losses, although not quantified by National Research Council (1981b), represent 5 to 25 percent and total evaporative heat losses represent 20 to 80 percent of total heat losses (Ehrlemark, 1991).

Surface area is related to body weight by the equation

thus,

TI (°C/Mcal/m2/day) is primarily a function of subcutaneous fat and skin thicknesses. Typical values are 2.5 for a newborn calf, 6.5 for a 1-month old calf, 5.5 to 8.0 for yearling cattle and 6.0 to 12 for adult cattle. EI is provided by hair coat plus the layer of air surrounding the body. Thus, external insulation is related to hair depth. However, the effectiveness of hair as external insulation is influenced by wind, precipitation, mud, and hide thickness. These effects have been described as follows:



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