where EI is expressed as °C/Mcal/m2, WIND is wind speed (kph), and HAIR is effective hair depth (cm). MUD and HIDE are adjustments for mud and hide thickness. Total insulation (IN) is
and LCT may be calculated as (National Research Council, 1981b):
where LCT, IN, and HE/SA are as described previously. The term He represents the minimal total evaporative heat loss and is estimated (Ehrlemark, 1991) as:
The animal can receive or lose heat by solar or long-wave radiation. The net impact of thermal radiation on the animal depends on the difference between the combined solar and long-wave radiation received by the animal and the long-wave radiation emitted by the animal. For animals in bright sunlight, a net gain of heat by thermal radiation usually exists, resulting in an increased effective ambient temperature (EAT) of 3° to 5° C (National Research Council, 1981b). In bright sunlight, this effect lowers LCT by 3° to 5° C. Conversely, CSIRO (1990) have indicated that the rate of heat loss by long-wave radiation increases on cold clear nights resulting in an increase in the LCT. Within the temperature range of -10° to 10° C this effect is about 5°C.
The increase in energy required to maintain productivity in an environment colder than the animal’s LCT may be estimated as
where MEc is the increase in maintenance energy requirement (Mcal/day), SA is surface area (m2), LCT is lower critical temperature (°C), EAT is effective ambient temperature (°C) adjusted for thermal radiation, and IN is total insulation (°C/Mcal/m2/day).
Total net energy for maintenance under conditions of cold stress (NEmc) becomes
Heat Stress If ambient temperature and thermal radiation exceed the temperature of the skin surface, the animal cannot lose heat by sensible means (conduction, convection, and radiation) and will gain heat by these routes. Evaporative heat loss occurs from the skin (cutaneous) or through respiration. The effectiveness of both cutaneous and respiratory evaporative heat loss diminishes as relative humidity (RH) of the air increases and is totally ineffective when RH=100. Animals can store some heat in their bodies during the day and dissipate the stored heat during cooler daytime periods or at night, if the animal’s heat production exceeds its ability to dissipate heat; but if hyperthermia persists, animals cannot survive.
There has been much study of the various aspects of heat stress on animal performance, but there are no established bases for quantitative description of effects. Ehrlemark (1991), for example, developed a regression of respiratory heat loss on the ratio of ambient temperature minus LCT to body temperature minus LCT but did not include cutaneous evaporative heat loss or the influence of RH. It is generally agreed that adjustments to maintenance energy requirement for heat stress should be based on the severity of heat stress; however, severity can vary considerably among animals, depending on animal behavior, acclimatization, diet, level of productivity, radiant heat load, or genotype. The type and intensity of panting by an animal can provide an index for appropriate adjustment in maintenance requirement—an increase of 7 percent when there is rapid shallow breathing and 11 to 25 percent when there is deep, open-mouth panting (National Research Council, 1981b). With severe heat, feed consumption is reduced and consequently metabolic heat production and productivity are reduced.
Total heat production increases during gestation (Brody, 1945). Although indirect evidence is available to suggest maintenance requirements of cows increase during gestation (Brody, 1945; Kleiber, 1961; Ferrell and Reynolds, 1985), an increase has not been directly measurable by comparative slaughter evaluations (Ferrell et al., 1976). Increased heat production associated with pregnancy, for the purpose of estimating energy requirements, may be assumed to be attributable to the productive process of pregnancy.
In contrast, Moe et al. (1970) estimated ME requirements for maintenance to be 22 percent higher in lactating than in nonlactating cows (primarily Holstein). A similar difference (23 percent) was reported by Flatt et al. (1969), whereas Ritzman and Benedict (1938) reported a larger (49 percent) difference. Neville and McCullough (1969) and Neville (1974) using Hereford cows and different approaches, estimated the maintenance requirement of lactating cows to be more than 30 percent higher than nonlactating cows. The reports of Patle and Mudgal (1975, 1977) agree with those observations, whereas data of Ferrell and Jenkins (1985b, 1987; and unpublished data) suggest a difference of 10 to 20 percent. Taken in total, available data indicate maintenance requirements of lactating cows to be about 20 percent higher than those of nonlactating cows.