herbage production of annual and perennial 
grasses is estimated on the same plots. Tempo- 
rary plots are used to establish relationships 
between estimated and actual herbage yields. 
Individual plot sizes range from 9.6 square feet 
(where herbage production is relatively high) 
to 192 square feet, where production is low. In 
June utilization of perennial grasses is esti- 
mated at each herbage production plot, using 
the ungrazed plant method (Roach 1950). 
These records, together with records of actual 
numbers of cattle grazed, are used to compute 
regressions of the form: 
S=bA+bPta 
where §S is the number of cattle needed to uti- 
lize a given percentage (usually 40) of the pe- 
rennial grass herbage, that is, 
S = Stocking x utilization, 
40 
A = annual grass production/acre, 
P = perennial grass production/acre. 
With these regressions we can estimate in Oc- 
tober how many cattle are needed to utilize the 
given percentage of the perennial grass herb- 
age by the following June. Annual adjust- 
ments in stocking based on these estimates 
have resulted in the desired degree of utiliza- 
tion on experimental pastures. Let us now con- 
sider what these records tell us about when, 
what, and how much the cows ate. 
Suppose we wish to measure monthly forage 
consumption of 40 cows from vegetation meas- 
urements on an 800-acre unit on the Santa 
Rita Experimental Range. Average perennial 
grass herbage production is about 600 pounds 
per acre (90 percent is produced between July 
1 and September 30); average utilization on 
June 80 is 40 percent. If each of the 40 cows 
consumes 20 pounds of dry matter per day, 
24,000 pounds of dry matter is consumed each 
month. Since there are 800 acres in the unit, 
consumption per month of all species combined 
is 30 pounds per acre. We cannot estimate 30 
pounds per acre of utilization (3.0 grams per 
9.6-square-foot plot). However, since some 
plots may be grazed heavily and others not at 
all, we have a better chance to detect utiliza- 
tion than if all plots were grazed evenly. Thus, 
uneven grazing may be easier to detect but 
more difficult to quantify. 
Some measure of the variability of forage 
production and use on range that produces 600 
pounds of perennial grass per acre is indicated 
by the fact that standard deviations for indi- 
vidual species usually exceed the means (table 
1). Herbage disappearance data (October 1 to 
June 30) derived from production and utiliza- 
tion estimates are even more variable than 
those for production. These estimates are 
based on 20 randomly selected permanent tran- 
sects in an 800-acre pasture (1) where the 
mesquite has been removed and (2) where a 
fairly uniform grass cover has been estab- 
lished. Herbage yields on five permanent 9.6- 
square-foot plots at each transect are averaged 
to obtain the transect yield. The number of 
transects needed to sample within 20 percent 
of the mean at 90-percent probability ranges 
from 20 for total perennial grass herbage 
production to more than 1,800 for Rothrock 
grama. Disappearance estimates are products 
of herbage estimates after growth was com- 
pleted and after utilization observations were 
taken in June at the end of the full year of 
grazing. The number of plots needed to esti- 
mate disappearance for a short period would 
be enormous. 
TABLE 1.—Production and disappearance (Lb/A) of perennial grass herbage and numbers of 
plots needed to sample within 20 percent of mean at 90 percent probability (Based on 1966 data 
from the Santa Rita Experimental Range) 
Herbage Transects needed (No.) 
Production Disappearance 
Species Standard Standard Produc- Disap- Utiliza- 
Average Deviation Average Deviation tion pearance tion 
Tall three-awn 85.6 135.5 31.0 52.5 170 194 34 
(Aristida hamulosa and 
A. ternipes) 
Sprucetop grama 80.6 78.3 24.4 27.3 64 85 62 
(Bouteloua chondrosoides) 
Slender grama 111.8 153.8 41.4 66.5 129 174 42 
(B. filiformis) 
Rothrock grama 6.2 28.0 2.8 12.3 1,380 1,308 680 
(B. rothrockit) 
Arizona cottontop 35.0 61.4 22.3 42.7 308 248 73 
(Trichachne californica) 
All perennial grasses (include 5738.1 292.8 2007 142.3 20 34 14 
species not listed) 
97 
