On the lower half of the Santa Rita Experi- 
mental Range, we graze about 50 cows on 5,000 
acres. Again, if it is assumed that each cow 
consumes 20 pounds of dry matter per day, 
monthly consumption is 6 pounds per acre, or 
one-fifth as much as on better ranges. Obvi- 
ously, average consumption per unit area 
under normal rates of stocking is too small to 
estimate for short periods with the present 
level of sampling. 
INTERPRETING RESULTS 
As a starting point, let us assume that a for- 
age disappearance must be about 100 pounds 
per acre before we can measure it. Let us also 
assume that, with proper stocking, monthly 
consumption rates of 6 and 30 pounds per acre 
for low- and high-yielding pastures, respec- 
tively, are reasonable. Under these assump- 
tions, we would have to stock the high-yielding 
range at more than 3 times the usual rate and 
the low-yielding range at 17 times the normal 
rate to produce measurable consumption in a 
month. Regardless of whether we measure ani- 
mal response, forage output, or effects of graz- 
ing on vegetation, it will be difficult to derive 
practical management guides from results ob- 
tained by grazing at several times the intensity 
required for sustained forage production. 
We can probably estimate the amount and 
composition of forage consumed by animals if 
certain conditions exist. Range units must be 
small enough, grazing intensities must be heavy 
enough, and grazing periods must be long 
enough to provide differences that are large 
enough to measure or estimate. Where forage 
production and rates of forage removal are rel- 
atively high, the paired-plot cage methods offer 
good possibilities for determining forage com- 
position and consumption. Cage methods are 
especially useful during periods of rapid for- 
age growth. Hand plucking the caged plot to 
match use on the open plot seems to have ad- 
vantages over clipping both plots and deter- 
mining consumption by difference. 
On ranges of low productivity that are 
grazed at normal stocking rates, consumption 
during brief periods is too small to determine 
from vegetation measurements. Some clues as 
to the kinds of forage being taken can be ob- 
tained by checking favorite species and favor- 
ite areas, but quantities cannot be measured by 
methods in common use. General information 
on quantitative consumption can be accumu- 
lated by pooling data from several areas or 
several years, but the accuracy of such esti- 
mates is uncertain. 
Forage consumed can be estimated by meas- 
uring the vegetation, by weighing the animal, 
or by observing what the animal eats. The 
strengths and weaknesses of estimates of for- 
age consumption obtained from animals are ad- 
98 
equately discussed by other contributors to this 
symposium. If the “vegetation” estimates dif- 
fer from the “animal” estimates, which should 
we believe? Post-grazing vegetation measure- 
ments tell us very little about what a given an- 
imal ate on a given day. On the other hand, the 
activity of a given animal on a given day, or 
his weight change during a given period, tells 
us next to nothing about the impact of grazing 
on the range. Both kinds of measurements are 
necessary to properly evaluate vegetation-an- 
imal relationships. 
CONCLUSIONS 
Range science has developed several kinds of 
vegetation measurements that provide reliable 
guides for stocking the range. Such measure- 
ments can provide estimates of average forage 
consumption by livestock. For best results, 
sampling must be frequent and intensive, the 
rate of forage removal must be high, and unac- 
countable losses of forage must be low. 
Grazing intensities on  low-productivity 
ranges may have to be several times the level 
that is required for maintenance of the forage 
stand to produce a measurable effect on the 
vegetation in a short time. Consumption data 
so obtained may have little practical meaning. 
Serious problems exist in recognizing the 
difference between (1) forage consumption by 
domestic livestock, (2) forage consumption by 
other animals, and (3) forage lost or displaced 
by physical forces of the environment. 
Past forage use is not always visible. Invisi- 
ble utilization may include plants that are 
pulled up by the roots, plant parts that are 
pulled out leaving no visible stubble, deciduous 
fruits or leaves, and use that has been obscured 
by subsequent growth. 
Weathering and trampling can significantly 
reduce the amount of forage that is left at the 
end of the grazing period, thereby creating an 
impression either of low yield or high utiliza- 
tion, depending on the method of measurement 
used. 
The use of cage methods provides estimates 
of growth during the grazing period and may 
reveal some invisible grazing. Cage methods 
have been satisfactory for pasture studies 
where production was high, grazing periods 
short, and consumption rather complete for 
each grazing period. Intensive range studies 
undoubtedly can use cage methods effectively. 
Daily consumption of forage by animals var- 
les with the quantity and quality of forage 
available, and with other factors that affect 
the metabolism—the energy budget—of the ani- 
mal. Because of the many variables that influ- 
ence animal performance, animal weight 
changes are not necessarily a good measure of 
forage consumption on the range. 
a, 
