FACTORS AFFECTING UTILIZATION 
The consumption of a given species per ani- 
mal on any part of a range unit depends on 
how accessible and desirable that species and 
part of the range are to the animal, and on the 
relative abundance of associated species. 
The degree of utilization on pine bunchgrass 
in northern Arizona was related inversely to 
distance from water and to steepness and 
length of slope, but positively related to prox- 
imity to trails and other access routes (Glen- 
dening 1944). Mueggler (1965) found a nega- 
tive exponential relation between accumulated 
relative use and distance upslope from the bot- 
tom of the slope. In an evaluation of 21 factors, 
Cook (1966) found that 11 were significantly 
related to utilization of mountain slopes by cat- 
tle. He concluded, however, that utilization on 
a given part of the range could not be pre- 
dicted from the relationships studied. On a des- 
ert range, Cook (1962) found that the aver- 
age percentage of utilization for each of seven 
species was related to such factors as: (1) Av- 
eraged weighted percent use for all species, 
(2) utilization on the more palatable species, 
(3) utilization on the less palatable species, 
and (4) the relative abundance of each forage 
species present. Clearly, the relationships that 
determine consumption of a given species by a 
given animal at a given time and given place 
are complex. 
SOME FACTORS AFFECTING UTILIZATION 
MEASUREMENTS 
Seasonal Variations in Forage Growth 
The problem of relating utilization of forage 
to animal consumption is complicated by spe- 
cies differences in seasonal rate of growth and 
in response to grazing. In Wyoming, Lang and 
Barnes (1942) found that short grasses and 
perennial forbs produced more forage when 
harvested frequently at ground level than 
when protected during the growing season and 
harvested after growth stopped. Midgrasses 
and annual forbs produced more forage if har- 
vested at the end of the growing season. In 
California, on annual range, Ratliff and Heady 
(1962) found that the periods of most rapid 
growth extended from March 28 to April 24. 
Early in the season, filaree and burclover were 
the most rapid growers, then wild oats and rip- 
gut, next soft chess, and finally ryegrass. In 
Arizona, on the Santa Rita Experimental 
Range native perennial grasses produced 93 
percent of the year’s growth in about 9 weeks, 
mainly in July and August (Culley 1943). 
Invisible Utilization 
Past utilization on rapidly growing plants 
may not be measurable because evidence of 
grazing or browsing has been obscured by re- 
growth. 
Another kind of invisible utilization is that 
of plant parts that are pulled out rather than 
cut off. Young basal leaves and their sheaths 
on grasses such as little bluestem often are 
pulled out, leaving the broken surfaces hidden 
by old sheaths. Immature seedstalks of many 
grasses disarticulate within the sheath, and 
flower stalks of some low-growing forbs have 
their weakest point at or below ground level. 
Flowers of these plants can be grazed without 
leaving visible stubble. 
A third category of invisible utilization is 
the consumption, in season, of deciduous mate- 
rials. These include many kinds of fruits, 
leaves of deciduous shrubs, and entire seed- 
heads of some grasses. Once the deciduous 
plant part has fallen, the amount taken by 
livestock cannot be estimated. 
Plants pulled up by the grazing animal con- 
stitute a fourth category of invisible utiliza- 
tion. However, steers on the Santa Rita Exper- 
imental Range usually discard annual grasses 
that are uprooted (Zemo 1968).’ 
Errors introduced by invisible utilization 
have not been studied extensively. At this 
point, about all we can say is that they do exist 
and should be measured if there is a chance 
that they will seriously reduce forage con- 
sumption estimates. 
Extraneous Utilization and Disappearance 
An opposing source of error, i.e., one that 
may bias forage consumption estimates up- 
ward, is that of distinguishing between forage 
consumed by study animals and losses to other 
influences. Most ranges support unmeasured 
populations of big game, small mammals, 
birds, and insects. It may be impossible to dis- 
tinguish between the grazing or browsing of 
domestic livestock and that of other animals. 
Apparent consumption also can result from: 
(1) Trampling that damages rapidly growing 
tender plants and reduces forage yields, (2) 
normal losses of deciduous plant parts, and (3) 
losses due to wind, hail, or the cutting action 
of sandstorms. Available forage declines as 
plants mature, cast seeds, shed leaves, and are 
broken up or flattened out by the weather. 
Some leaching of soluble materials also occurs. 
RELATING PLANT AND ANIMAL 
MEASUREMENTS 
In grazing studies, animals are used not only 
to obtain a desired experimental condition, but 
also to measure sward performance (Lucas 
1962). Animal weight gain, adjusted for en- 
ergy spent in gathering forage or added in sup- 
3 See footnote 2, page 94. 
95 
