hour standing than lying, and that each time 
he stands and reclines he expends 12 more 
keal. Young animals at play may require en- 
ergy at a rate 10 percent above the fasting 
level, i.e., the basal metabolism requirements. 
Short and Golley (1968) reported that cattle 
may require 15 percent more energy for nor- 
mal range activities than for fasting condi- 
tions, and that the energy demands of wild 
herbivores under usual range and climatic con- 
ditions may be at similar percentages above 
the fasting level. 
The NAS-NRC (1964) reported that the en- 
ergy required for maintenance is 24 to 77 per- 
cent higher for grazing sheep than for stall- 
fed sheep. Grazing sheep may require as much 
as 77 percent more energy than those main- 
tained in pens or shelters (Grimes 1966). A 
foraging sheep that walks 6,400 m. and as- 
cends 100 m. in a day’s activity may need en- 
ergy at a rate 20 percent above the basal level 
(Blaxter 1962). 
Energy requirements are calculated from es- 
tablished metabolic constants. The basal me- 
tabolism of animals varying in size from mouse 
to elephant is proportional to body weight 
(BW) raised to the 34, power (Byerly 1967). 
There is relatively little information on the en- 
ergy metabolism of domestic ruminants at dif- 
ferent levels of productivity (Flatt 1966; Flatt 
and Coppock 1965). Few, if any, energy metab- 
olism studies have been undertaken with range 
livestock and wild herbivores; their energy re- 
quirements are often extrapolated from stan- 
dards developed in livestock feeding trials. 
In general, maintenance requirements of 
ruminants can be estimated as 1.36 kcal. of 
metabolizable energy per keal. of fasting meta- 
bolism, or as about 1.7 kcal. of apparently 
digested energy pel’ kcal. of fasting energy 
metabolism (Blaxter 1962). Using the inter- 
species mean of 70 keal./BW (kg.)%/24 hours 
and Blaxter’s conversion factors, the estimated 
maintenance requirements for ruminants are 
95.2 keal. ME/BW (kg.)%/24 hours or 119.0 
keal. DE/BW (kg.)%/24 hours. Some caution 
should be exercised in using these standards, 
because it has been pointed out that mainte- 
nance requirements of sheep (NAS-NRC 1964), 
for example, are lower than those for other 
kinds of livestock (Byerly 1967). Digestible 
energy requirements for various animals are 
given in table 1. 
Severe physical activity expends much en- 
ergy, and sometimes more energy is spent in 
the search for food than can be gained from its 
digestion. Both body heat loss and coat insula- 
tion varies throughout the year. Wind, hair 
wetness, air temperature, and humidity modify 
heat loss and influence the metabolic activity 
required for necessary body warmth (Blaxter 
12 
1962). In very cold weather, conservation of 
heat may offer the best chance for survival of 
deer (Silver and Colovos 1957). Emaciated ani- 
mals are sensitive to cold, but when they have 
access to roughage—even straw or dry, 
weathered range grass—the heat increment at- 
tained by its digestion will keep them warm 
and alive at —40°C. Without a source of feed 
energy, animals may die of cold at —23° C. 
(Byerly 1967). At —20° C. a 50-kg. deer can 
withstand more than twice as much wind ve- 
locity when on a full diet as when on a mainte- 
nance diet (Moen 1968). 
The time and conditions under which feed is 
deficient in energy are greatly significant. In 
white-tailed deer, prenatal malnutrition re- 
tarded length and weight growth of the fetus. 
At birth the average weight of the fawns was 
46 percent less and they tended to be stunted 
skeletally when compared to fawns from well- 
fed does (Verme 1963). In another study 
(Verme 1967), white-tails that were inade- 
quately nourished in summer or autumn repro- 
duced only one-third as rapidly as deer receiv- 
ing good rations. Yearling white-tailed deer on 
limited amounts of feed for 10 weeks never at- 
tained the average body weight of deer on un- 
restricted feed or of deer on restricted feed for 
only 5 weeks (Long et al. 1959). 
Short periods of feed deficiencies are not 
necessarily harmful. For example, calves from 
3 to 8 months old can maintain their weight 
on rations that meet their nutritional needs, 
without later loss in efficiency of feed utiliza- 
tion or quality of meat (Winchester and Ellis 
1957). In a study by Winchester and Howe 
(1955), a reduced feed intake by steers for 180 
days did not change the amount of energy later 
required to reach a weight of steers fed ad libi- 
tum. Apparently the long-established practice 
of maintaining cattle and sheep through win- 
ters or droughts at low planes of nutrition is 
not harmful. 
EFFICIENCY OF FOOD UTILIZATION 
Fattening, growth, pregnancy, and lactation 
affect food and energy consumption and the ef- 
ficiency of energy utilization above the mainte- 
nance level. The efficiency with which ingested 
materials are converted into fat is lowest for 
feeds with a high roughage content (Blaxter 
1962). During lactation, the way in which food 
energy is utilized changes. Body fat is fre- 
quently converted to milk production by high- 
producing dairy cattle fed unlimited amounts 
early in lactation (Flatt 1966). At later stages 
of lactation, less of the ingested gross energy is 
utilized for milk production, and more is used 
to restore the depleted body reserves. Young 
animals are more efficient than adults in using 
food energy for growth. 
