plemental feed, is the usual standard against 
which vegetation measurements are evaluated. 
The amount of forage consumed per animal 
ean be a dominant factor controlling produc- 
tion per animal. In fact, Crampton et al. (1960) 
report that the relative importance of forage 
intake and forage digestibility in determining 
rate of gain are 70 and 30, respectively. Ani- 
mal performance always responds to changes 
in forage yield and quality, but it is a trust- 
worthy index only if other factors conditioning 
the relationship are standardized or can be 
properly evaluated. 
Observations of Hereford and Santa Gertru- 
dis cows in New Mexico showed that they ate 
some of all available species, and that there 
was no apparent difference between breeds 
(Herbel and Nelson 1966). Cook et al. 
(1962) found that the daily intake of for- 
age was less on poor ranges than on adja- 
cent good ranges, and that the daily intake 
from both good and poor ranges decreased as 
utilization reduced available forage. In a re- 
lated study, Cook, Kothmann, and Harris 
(1965) found that total protein, ash, lignin, 
and other carbohydrates were somewhat higher 
in forage selected by sheep from poor ranges; 
that ether extracts, cellulose, and gross energy 
were higher in forage from ranges in good con- 
dition; and that the digestibilities of cellulose 
and other carbohydrates and gross energy in 
the forage from both good and poor ranges de- 
creased as utilization increased. The amount 
and quality of forage available can influence 
forage intake more than the animals’ forage 
preference. Decreases in forage intake and 
quality as forage becomes scarce may also in- 
crease the ratio of forage consumption to ani- 
mal gain. Because of these factors, it is hazard- 
ous to estimate forage consumption from ani- 
mal gains. 
Relation of Apparent Forage Consumption 
to Animal Gain 
Evans et al. (1962) state that the meaning- 
fulness of herbage weights is questionable be- 
cause of the rapid changes in vegetation, and 
they stress the need for rapid weight inventory 
methods. Linehan and Lowe (1946) found that 
total output, as measured using the movable 
cage method, differed from that based on ani- 
mal weighings by only 6.8 percent. The coeffi- 
cient of correlation for the two kinds of yield 
estimates was 0.88. Grazing periods ranged 
from 7 to 28 days. Excessively frequent clip- 
ping in periods of rapid herbage growth was a 
major source of error in the movable-cage 
method. 
In a later study, Linehan (1952) reported 
that clipping procedures were accurate enough 
where the herbage on highly improved 2-acre 
study paddocks was grazed down in 8 days, the 
96 
cattle moved, and clippings taken of the uneat- 
en herbage. However, on pastures grazed sea- 
son long under the farmer’s control, the same 
clipping schedule gave low accuracy. On farm- 
er-controlled areas, the sward was overdefol- 
lated in the spring, it accumulated heavily in 
the flush period, and the accumulation was not 
consumed until near the end of the season. 
Morrison and Ely (1946) estimated the TDN 
(total digestible nutrients) per acre by clip- 
ping at 1,578 pounds, compared to 2,305 
pounds as computed from animal weight gains. 
Herbage was clipped with a lawnmower set at 
1 inch, and caged plots were mowed at the end 
of each grazing period. Wagner et al. (1950) 
compared the use of six 4- by 4-foot cages with 
four mower strips 3 by 30 feet, clipped at 2 
inches before the cattle were turned in. Aver- 
age TDN yields of 3,500 to 4,000 pounds per 
acre were recorded. They found that the strips 
gave yields of TDN that more closely approxi- 
mated computed TDN intake of animals 
grazed. When the cage method was used, yields 
of both bluegrass and orchardgrass were over- 
estimated. Grelen (1967), using plots 3.1 feet 
square protected by cages 4 feet square, also 
round that stationary cages overestimated 
yield. 
Most attempts to relate herbage measure- 
ments to forage consumption have involved 
areas of 10 acres or less and herbage yields of 
2,000 to 4,000 pounds per acre. Most attempts 
have also involved grazing periods of 3 days to 
6 weeks, and nearly complete harvesting of the 
forage during the grazing periods. In these 
tests, forage-consumption estimates derived 
from adjusted animal weight gains have been 
the standard against which consumption esti- 
mates based on vegetation measurements were 
evaluated. Actually, animal weight gains may 
provide very poor estimates of forage con- 
sumption. This is especially true under range 
conditions where animals may travel several 
miles from water to forage, where external 
and internal parasites and other factors have 
unmeasured impacts on the animal, where 
grazing periods range from a few weeks to sev- 
eral months, where many kinds of forage are 
available, and where the plane of nutrition 
varies seasonally from submaintenance to ade- 
quate. 
Herbage Production and Consumption on 
Semidesert Range 
Since 1954 records of herbage production 
and utilization have been maintained for the 16 
major pastures on the Santa Rita Experimen- 
tal Range.t On about October 1 of each year 
‘Maintained by the USDA Forest Serv., located 30 
miles south of Tucson, Ariz. 
