Chemical Indicator Techniques for Determining Range Forage Consumption * 
BRENT THEURER ? 
The problem of measuring forage consump- 
tion on native range is both important and dif- 
ficult. The system devised by Garrigus and 
Rusk (1939) is often used in estimating for- 
age consumption of the grazing animal: calcu- 
lation of dry matter consumption is based on 
the feces collected during the test period. This 
procedure involves determining the digestibil- 
ity of the forage and measuring fecal excre- 
tion. With these parameters, dry matter con- 
sumption may be computed from the formula: 
Dry matter intake (kg.) = 
Fecal excretion (kg.) 
100 — % dry matter digestibility ne 
The determination of range forage consump- 
tion by chemical indicator techniques is an in- 
direct method which depends upon the reliabil- 
ity of the indicators to accurately estimate di- 
gestibility and fecal output. 
The determination of the digestibility of 
grazed forage depends on a knowledge of the 
composition of both the fecal material and the 
ingested forage. Using a naturally occurring 
indicator in the forage, digestibility may be de- 
termined indirectly by the fecal-index method 
or by the indicator-ratio technique. Fecal ex- 
cretion may be determined by total fecal collec- 
tion or by the use of external indicators. Re- 
views on the use of indicator methods for de- 
termining consumption and digestibility of for- 
age by grazing animals have been prepared by 
Harris et al. (1959), Weir et al. (1959), and 
Bohman and Lesperance (1967). 
DETERMINATION OF DIGESTIBILITY 
WITH INTERNAL INDICATORS 
The fecal index technique is based on the re- 
lation of some fecal chemical constituent to di- 
gestible organic or to digestible dry matter. 
With this method, the internal indicator does 
not need to be indigestible, and is measured 
only in the feces after representative dietary 
samples have been fed to determine the index 
relationships. This technique may be used if 
the forage is relatively uniform, and it is palat- 
able when it is hand clipped and fed. Since 
much range forage is not only heterogeneous 
but also unpalatable when hand fed, the indi- 
cator-ratio techniques appear more promising 
*Arizona Agricultural Experiment Station Technical 
Paper 1380. 
* Associate Professor, Department of Animal Science, 
University of Arizona, Tucson, Ariz. 
for estimating digestibility. The ratio tech- 
nique involves the use of a naturally occurring 
substance in the forage, and determination of 
this indicator in forage and feces. The relation 
is as follows: 
Digestibility (%) = 100 — 100 x 
% indicator in forage 7 nutrient in forage 
% indicator in feces §% nutrient in feces 
Collection of Forage Samples 
The validity of the ratio technique depends 
upon collection of forage samples representa- 
tive of the animal’s diet and of a representa- 
tive sampling of fecal material. Since grazing 
animals exhibit considerable selectivity, forage 
ingested by the animal differs in composition 
from herbage available (Weir and Torell 1959; 
Lesperance et al. 1960b; Galt et al. 1966). The 
inability of investigators to hand sample for- 
ages representative of that ingested by live- 
stock has led to the use of fistulated animals to 
collect naturally grazed samples. For this pur- 
pose Torell (1954) devised an esophageal fis- 
tula. A collection bag with a screen bottom is 
attached to the animal’s neck, and the animal 
is allowed to graze until a sufficient sample has 
been obtained (Van Dyne and Torell 1964). 
Lesperance et al. (1960a) developed a tech- 
nique to utilize rumen-fistulated animals as 
forage samplers. The rumen and reticulum are 
emptied thoroughly prior to releasing the ani- 
mals to select forage samples. 
McCann et al. (1967) * noted that both dry- 
matter intake and fecal output were signifi- 
cantly greater for intact than for rumen fistu- 
lated steers grazing a summer grassland range 
in northern Arizona (table 1). Average weight 
gains during the summer grazing period were 
also somewhat greater for the intact steers. 
The estimated digestibility of dry matter, gross 
energy, and crude protein of forage grazed 
were not different between fistulated and intact 
steers. Rumen evacuations were conducted at 
the time of the final fecal collection and there- 
fore did not influence digestibility; however, 
the rumen evacuations, fistulation per se, or 
both markedly influenced dry matter intake. 
This is an agreement with hand-feeding trials 
using the same steers (McCann and Theurer 
1967). These data suggest that the use of ru- 
men-fistulated steers to estimate forage intake, 
*McCann, C. P., Theurer, B., and Pearson, H. A. 
1967. Unpublished data. University of Arizona, Tucson, 
Arizona and Rocky Mountain Forest and Range Exper- 
iment Station, Flagstaff, Arizona. 
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