DETERMINING ANIMAL CONSUMPTION 
Relating Vegetation Measurements to Forage Consumption by Animals 
S. CLARK MARTIN! 
The productivity of range animals depends 
largely on the quality and quantity of the for- 
age they consume. The focus of this conference 
is on habitat evaluation—with emphasis on 
how well range forage meets the needs of 
range animals. Therefore, proper evaluations 
must include animal performance. We can 
weigh livestock at intervals to determine rate 
of gain. However, if the animals have had ac- 
cess to several kinds of forage, we cannot 
know, from weights alone, which kinds of for- 
age were responsible for the observed gain. 
Several methods of observing animals, their 
stomach contents, or their excreta—to deter- 
mine kinds and relative quantities of forage 
eaten—are evaluated elsewhere in this publica- 
tion. However, data points are so difficult to ob- 
tain by these methods that the scope of such 
measurements usually is severely limited. The 
purpose of this paper is to explore the possibil- 
ities of determining the quality and quantity of 
the forage animals eat by observing or measur- 
ing forage plants. 
METHODS OF MEASURING FORAGE 
PRODUCTION AND UTILIZATION 
Most methods for estimating production and 
utilization of range forage were developed to 
Improve range management decisions. These 
methods have provided workable guides for 
stocking the range. Their relative merits as 
management tools have been discussed in pre- 
vious meetings and will not be reviewed here. 
Rather, the question is whether vegetation 
measurements can be used to determine when, 
what, and how much a grazing animal ate. 
Methods of estimating herbage production 
were reviewed by Reppert et al. (1962). Forage 
production on range units may be measured by 
the weight estimate method (Pechanec and 
* Principal Range Scientist, Rocky Mt. Forest and 
Range Exp. Sta., USDA Forest Serv., located at Tucson, 
Ariz., in cooperation with Univ. Ariz. Central head- 
quarters for the station is maintained at Fort Collins, 
Colo., in cooperation with Colo. State Univ. 
Pickford 1937), some modification including 
doubie sampling (Wilm et al. 1944), or by 
clipping and weighing forage from sample plots 
or transects. 
Smith et al. (1962) classified utilization 
methods as: (1) Methods based on differences 
before and after grazing or between grazed 
and ungrazed plots or plants; (2) methods 
based on measurement, correlation, and regres- 
sion of factors related to utilization; and (3) 
methods based on general observations and 
comparisons with predetermined standards of 
use. For administrative use, ocular estimates 
(Reid and Pickford 1941), methods based on 
height-weight relationship, as recommended by 
Lommosson and Jensen (1948) and various 
grazed-plant-count methods (Roach 1950; 
Springfield 1961; Gierisch 1967) have been rea- 
sonably satisfactory. Actual weight methods 
that require movable cages have been popular 
in pasture and intensive range studies. 
Difference Methods 
One approach to estimating consumption is 
to estimate or measure the amount of herbage 
on marked plots at the beginning and end of 
each grazing period. The major problems are 
to account for nonrelevant utilization and for 
growth. If these limitations are recognized, 
ocular estimates on the same plots before and 
after grazing can provide clues to animal con- 
sumption in range and wildlife habitat re- 
search. 
Cages are commonly used in estimations of 
production and utilization of forage on small, 
intensively managed, improved pastures where 
forage yields of 3,000 to 5,000 pounds of dry 
matter per acre are common. One objection to 
the use of cages is that they modify the micro- 
climate. However, changes in the microclimate 
due to caging were fairly small, with the use of 
open-mesh wire cages on annual range in Cali- 
fornia (Heady 1957). 
Using three variations of the clipped-plot 
method, where yields averaged 4,000 pounds 
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